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Valorisation of post-sorption materials: Opportunities, strategies, and challenges
Valorisation of post-sorption materials: Opportunities, strategies, and challenges D. Harikishore Kumar Reddy, K. Vijayaraghavan, Jeong Ae Kim, YeoungSang Yun PII: DOI: Reference:
S0001-8686(16)30218-4 doi: 10.1016/j.cis.2016.12.002 CIS 1703
To appear in:
Advances in Colloid and Interface Science
Please cite this article as: Harikishore Kumar Reddy D, Vijayaraghavan K, Kim Jeong Ae, Yun Yeoung-Sang, Valorisation of post-sorption materials: Opportunities, strategies, and challenges, Advances in Colloid and Interface Science (2016), doi: 10.1016/j.cis.2016.12.002
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ACCEPTED MANUSCRIPT Valorisation of post-sorption materials: Opportunities, strategies, and challenges D. Harikishore Kumar Reddya, K. Vijayaraghavanc, Jeong Ae Kima, Yeoung-Sang Yuna,b* Division of Semiconductor and Chemical Engineering, Chonbuk National University,
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Department of Bioprocess Engineering, Chonbuk National University, Jeonbuk 561-756,
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b.
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Jeonbuk 561-756, Republic of Korea.
Republic of Korea.
Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai
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600036, India
*
Corresponding author:
Prof. Yeoung-Sang Yun E-mail address: [email protected] (Y.-S. Yun) Tel.: +82 63 270 2308; Fax: +82 63 270 2306
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ACCEPTED MANUSCRIPT Content 1. Introduction
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1.1. Adsorption
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1.2. Post-sorbents
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1.3. Objective of this review 2. Post-sorption applications
2.2. Carbon- metal nanoparticles
2.4. Catalysts
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2.5. Bioactive compounds
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2.3. Feed additives
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2.1. Fertilizers & Soil conditioners
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2.6. Miscellaneous applications
3. Conclusions and future perspectives
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4. References
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ACCEPTED MANUSCRIPT ABSTRACT Adsorption is a facile, economic, eco-friendly and low-energy requiring technology
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that aims to separate diverse compounds (ions and molecules) from one phase to another
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phase using a wide variety of adsorbent materials. To date, this technology has been used
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most often for removal/recovery of pollutants from aqueous solutions; however, emerging post-sorption technologies are now enabling the manufacture of value-added key adsorption products that can subsequently be used for (i) fertilizers, (ii) catalysis, (iii) carbonaceous
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metal nanoparticle synthesis, (iv) feed additives, and (v) biologically active compounds.
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These new strategies ensure the sustainable valorisation of post-sorption materials as an economically viable alternative to the engineering of other green chemical products because
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of the ecological affability, biocompatibility, and widespread accessibility of post-sorption
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materials. Fertilizers and feed additives manufactured using sorption technology contain elements such as N, P, Cu, Mn, and Zn, which improve soil fertility and provide essential
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nutrients to animals and humans. This green and effective approach to managing postsorption materials is an important step in reaching the global goals of sustainability and
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healthy human nutrition. Post-sorbents have also been utilized for the harvesting of metal nanoparticles via modern catalytic pyrolysis techniques. The resulting materials exhibited a high surface area (>1000 m2/g) and are further used as catalysts and adsorbents. Together with the above possibilities, energy production from post-sorbents is under exploration. Many of the vital 3E (energy, environment, and economy) problems can be addressed using postsorption materials. In this review, we summarize a new generation of applications of postadsorbents as value-added green chemical products. At the end of each section, scientific challenges, further opportunities, and issues related to toxicity are discussed. We believe this critical evaluation not only delivers essential contextual information to researchers in the field but also stimulates new ideas and applications to further advance post-sorbent applications. 3
ACCEPTED MANUSCRIPT Keywords: Post-adsorption; post-sorbent; exhausted sorbent; spent sorbent; adsorption; biosorption; fertilizers; feed-additives; catalysis; carbothermal synthesis; nanoparticles; water
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treatment; aqueous medium
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1. Introduction
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Development of sustainable materials and technologies is a currently a popular research area in many fields of science. The three major factors that drive this area of research are: (i) environment, (ii) economy, and (iii) depletion of natural reserves. Due to the
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rapid population growth worldwide, there is a high demand for various commercial goods and products. A large number of these products contain metals (Fe, Cd, Cu, Pb, etc.) and
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materials (Nano, plastic, etc.) that contribute to significant environmental pollution problems when they are discarded or damaged [1]. One notable example is the vast quantities of “e-
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waste” generated globally from discarded electric and electronic equipment, which will build
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up even more rapidly in the near future [2]. This waste contains various toxic metal ions, such
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as Al, Fe, Pb, Cd, Co, Cu, La, Ni, and Hg along with traces of rare earth and precious elements such as Au, Ag, Sn, Se, Te, Pt, Pd, Ta, Co, and In [2-6], which, when released into
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the environment, cause severe health problems and environmental issues either directly or indirectly. Additionally, nanomaterials/nanoparticles used in various commercial products have become a serious concern [7]. Nanomaterials are one of the key ingredients in products such as sunscreen, cosmetics, photovoltaics, liquid magnets, automotive coatings, and more [8-10]. There is an ongoing debate about the safety and toxicity of nanomaterials because their fate and behavior in air, water, and soil environments have not been entirely examined [11-20]. Iron (Fe) is one of the technologically important key elements with different valence states (0, II, III, IV, V, and VI) and its polymorphs such as Fe2O3 and FeO(OH) have been widely used around the globe for various industrial applications [21]. For instance, iron is commonly used in steel production by combining with other alloying elements [21, 22]. 4
ACCEPTED MANUSCRIPT Magnetic iron-based materials are of particular and emerging interest because of their facile separation property, and these materials have been used in various fields such as water
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treatment [23, 24], food analysis [25], photocatalysis [26], electrochemical biosensing [27],
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magnetic hyperthermia [28], drug delivery [29], sensors [30]. Nonetheless, there is still
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continuing discussion about the safety and toxicity of iron oxide nanoparticles [31-33]. Hightech materials are also of economic concern because the manufacturing of most of these products requires very costly raw materials. Furthermore, the cost of these materials is
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increasing annually due to high demand; thus, it is important to find sustainable ways to reuse
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or recover these materials from products after their use. Additionally, because most of these materials are manufactured using naturally occurring resources, reserves of these resources
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are limited, and are rapidly being depleted [34]. For example, natural reserves of metal ions
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widely used in technology manufacturing have measurably decreased [35]. Although there are still untapped reserves, such as sea water, that could be a source of various critical metal
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ions [36], environmental, technical and economic issues limit the practicality of mining from these sources [37-40]. The need for sustainable technologies and materials is more essential
issues.
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than ever in order to overcome these environmental, economic, and resource availability
Adsorption is one of the most successful techniques in the field of separation science. Diverse solid materials (adsorbents) have been used to remove or recover a wide range of contaminants, including metal ions, dyes, organics, and pharmaceuticals (adsorbates) (Fig. 1). Charcoal and activated carbon are the most popular and widely used adsorbents for removing various contaminants and remain at the forefront for commercial applications. These materials can be disposed of easily, or sent to a landfill after they are no longer useful. However, although carbon-based materials show a high efficiency of binding organic pollutants, they have a limited tendency to bind inorganics. To overcome this limitation and 5
ACCEPTED MANUSCRIPT to improve their performance, researchers have focused on the development of new and diverse adsorbent materials, such as zeolites, biosorbents, polymers, metal oxides,
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nanomaterials, chemically modified adsorbents, and others [41-48]. Most of the research into
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these adsorbent materials has focused on improved performance and increased removal rates,
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whereas less emphasis has been placed on the storage, disposal, and reuse of these adsorbents once they are spent. Some of the newer-generation adsorbent materials used in water treatment have been examined for possible toxic effects on living organisms [17, 49-51]. Also
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of concern is the high cost of manufacturing some of the adsorbents due to the use of
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expensive raw materials. Thus, in recent years, researchers have focused on the use of postsorbents, which are sorbents with bound materials, for various applications to meet
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environmental, energy, agriculture, and dietary needs [52-55].
Fig. 1. Numbers of publications focused on adsorption of metal ions, dyes and pharmaceuticals (data from Scopus® search date: 14-06-2016)
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ACCEPTED MANUSCRIPT 1.1.
Adsorption
Adsorption is a naturally occurring phase transfer process and is widely used for the
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removal of adsorbates (molecules or ions) from fluid phase (gases or liquids) using adsorbent
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materials (typically solid). Using adsorption to remove notorious toxic pollutants from the
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liquid (aqueous) phase has been one of the most widely employed techniques to obtain clean water for drinking and for industrial purposes. Although the term “adsorption” was not coined until 1881, the process itself has been used for many years. Egyptians and Sumerians used
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charcoal for the reduction of copper, zinc, and tin ores in the manufacturing of bronze around
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the year 3750 B.C. [56] and in 1550 B.C., the Egyptians applied charcoal for medicinal purposes [57]. In the year 1881, the term adsorption was introduced into the literature by H. Kayser, at the suggestion of E. du Bois-Reymond [58, 59]. Prior to this, both absorption and
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the adsorption phenomenon were referred to as “absorption” [60]. The tremendous interest in adsorption has led to numerous publications including research reports, reviews, monographs
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on historical evolution, and information about mechanisms and applications of adsorption many of which are referred herein [56, 61-66]. Although adsorption can be used for a variety
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of applications, it has historically been used primarily for wastewater treatment to address both strict environmental regulations and the growing level of contaminants (heavy metals, dyes, pharmaceuticals, micro pollutants, biological and plastics) in water. Progress in designing and developing novel adsorbents with multi-functional properties has made adsorption a key technology in wastewater treatment (Fig. 2). The goals for development of these materials have focused on achieving a high sorption capacity, a rapid sorption rate, and high affinity. Therefore, the new generation of adsorbents now includes nanomaterials, composites, metal-organic frames works, and layered-double hydroxides. Review articles published in the past year have described the removal of organics, metal ions and dyes, and the overall number of reviews on adsorption is indicative of the importance of this field [677
ACCEPTED MANUSCRIPT 87]. Individual reviews discuss the performance of particular adsorbent materials such as aerogels [76], biochar [68], biochar-based nano-composites [88], carbon nanotubes [71],
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chitosan [69], coal-based adsorbents [89], composite fibers [77], engineered nanomaterials
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[90], hydrogel [91], layered double hydroxides [67], magnetic chitosan composites [92],
palygorskite
[74],
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metal-organic frameworks [75, 85, 86], nanoadsorbents [70], nanocomposites [78, 79, 83], polymer-functionalized
nanocomposites
[83],
polypyrrole-based
adsorbents [73], and spinel ferrites [81]. These reviews primarily describe the efficiency and
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performance of adsorbent materials in water treatment, and none of these discuss post-sorbent
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applications. Biomaterials (plant, microbial and agricultural wastes), which when used for adsorption are known as biosorbents, are also widely used to address contaminants problems
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because they are abundant in nature, low-cost and easily available. The process of biosorption
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is a very popular technique and involves the use of solid biomass for the removal/recovery of various toxic pollutants through various physico-chemical mechanisms. Although there are
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slight differences between adsorption and biosorption, we will use the term adsorption to refer to both processes throughout this review because the intention is to focus on post-
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sorbent applications of both.
Fig. 2. The development of adsorbents
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ACCEPTED MANUSCRIPT 1.2.
Post-sorbents Two separate terms are used to describe sorbents after they have been used: post sorbents
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and spent (exhausted) sorbents. Post-sorbent is a term used to describe a solid adsorbent
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concentrated with adsorbate ions or molecules that have been removed from dilute aqueous
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phase through the adsorption process. Spent or exhausted sorbent is the solid waste generated after adsorbate has been regenerated or recovered from adsorbent. The major drawback of adsorption is that this is a nondestructive technology; the adsorbate does not degrade or
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disappear when the adsorbate is transferred from aqueous to solid phase. Thus, post-sorbent
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(adsorbate-laden sorbent) management is of utmost importance for the sustainability of adsorption technology. Current options for managing post-sorbents are limited to: (i) recovery
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of the sorbate, (ii) disposal in a landfill, and (iii) safe storage or disposal. Landfill disposal of
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post-sorbent (laden with toxic contaminants) leads to secondary pollution problems, such as soil, underground water or surface water contamination through natural leaching or
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desorption [54]. Recovery of the sorbate (contaminant) from the sorbent is possible; however, this process has several disadvantages. Regeneration of adsorbents is dependent on factors
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such as the type of adsorbent, the adsorbate itself, adsorbate-adsorbent interactions, the treatment process, and the cost of the regeneration process. Most of the techniques used for sorbent regeneration, including the use of thermal energy [93], microwave irradiation [94], ultra-sonication [95], dielectric barrier discharge plasma [96], and others, have notable limitations. For example, spent activated carbon can be regenerated through thermal treatment, which reduces the active surface area and decreases the adsorption efficiency in subsequent adsorption cycles. When a harsh chemical compound, such as an acid or alkali, is used in the regeneration process there are both economic and environmental consequences. For example, regeneration of a fixed-bed granular activated carbon operation comprises 75% of the total operation and maintenance cost [97]. Although adsorbents are recycled, 9
ACCEPTED MANUSCRIPT inadequate or incomplete removal of the toxic adsorbate can occur. Additionally, stable adsorbent-adsorbate interactions limit the effectiveness of the recovery process when
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conventional solvents (acids, bases and organic solvents) are used [98].
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The two methods used to manage both post-sorbents and spent (exhausted) sorbent wastes
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are landfill disposal and incineration [99]. The factors that influence these methods include: (i) cost of the adsorbent, (ii) pollutant type, (iii) operating cost, (iv) cost of the combustion and incineration plant, and (v) fees for disposal. Although landfills have typically been used
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for the disposal of sorbents, this method has a subsequent pollution risk when toxic
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compounds leach from adsorbents into the soil. With this in mind, strict environmental regulations have been put in place. Both Spanish and U.S. EPA waste disposal guidelines,
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prevent the disposal of metal- laden ashes in landfills [100]. Some of the newly developed
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adsorbents, such as nanoparticles, metal oxides, and composites, are themselves composed of toxic materials and metal ions. These have often been used as high-efficiency adsorbents for
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treating a variety of contaminants because of their high sorption capacities and rapid rates of adsorbent removal. The safety of these materials is a topic of debate because most cause
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problems when they enter air, soil, and water environments. Researchers have paid more attention to technology development and less attention to safe treatment and effective management of post-sorbents and spent adsorbents. Adsorbent cost is another important issue because most of the emerging adsorbents are in the form of composites and hybrid materials that comprise two or more distinct cost-effective materials. For example, nanomaterials such as graphite/graphene, single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWCNTs) have been used to fabricate composite adsorbents [101-104], and the cost of these materials is in the range of 0.10 to 1000$ per gram depending on their quality, grade and functionalization [90, 105]. The cost of some of the most commonly used adsorbents are listed in Table 1 and are estimated 10
ACCEPTED MANUSCRIPT primarily from the cost of raw materials alone. A true estimation of the overall cost of an adsorbent includes the cost of raw materials, energy, labor, and processing. Additionally, safe
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management (storage or disposal) costs should be taken into consideration as well. Adeleye et
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al. [90] thoroughly describe the costs and environmental issues regarding engineered
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nanomaterials used in water treatment processes. Adsorbent cost is one of the key factors that influences real-world industrial applications. As most of the emerging adsorbents are
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useful application of these materials (Fig. 3).
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expensive to produce and some are potentially toxic themselves, reutilization is essential for
Fig. 3. Schematic of pollutant removal using adsorbents and post-treatment options
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ACCEPTED MANUSCRIPT Table 1. Estimated costs of adsorbents Cost ($/kg)
Reference
Mag-PCMA
70a, ~4b
[90, 106]
Biochar
0.08-13.48
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Adsorbent
Activated carbon
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Granular activated carbon MWCNTs
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Graphene
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Modified Ficus carica fibers
PSA–GO
1-22
[108-110]
3.30-6.71
[111, 112]
6000
[111]
990
[111]
0.2
[113]
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Cost ($/t)
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Polyampholyte hydrogel
[107]
[114]
6.93 × 103
[115]
8.08 x 103
[115, 116]
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Amino siloxane oligomer-linked graphene oxide
6.3×103
*a=Laboratory scale, *b=factory scale
1.3.
Objective of this review A considerable amount of progress has been made in adsorption research with the aim of
removing toxic pollutants from wastewater using a wide variety of adsorbent materials [80, 102, 117-119]. For an adsorption method to be sustainable, regeneration of the post-sorbent is a particularly important step. However, most of the existing technologies require harsh chemicals, energy, and processing steps, and may limit the lifetime of the adsorbent itself. 12
ACCEPTED MANUSCRIPT Thus it is essential to identify alternative ways to utilize post-sorbent materials as a valueadded product. Numerous reviews have been published describing adsorption [68, 81, 92,
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120-124], but most focus on the performance of materials, sorption isotherm and kinetics, and
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mechanisms. Safety management and reuse or disposal problems associated with post-
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adsorbents or spent adsorbents have seldom been addressed. In fact, to date few studies are addressing the management of spent regenerated solutions, spent adsorbents and exhausted sorbent applications [54, 125, 126]. Feng et al. [54] discusses the disposal problems
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associated with spent adsorbents and their management. Gómez-Pastora et al. [125] reviews
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the management of spent magnetic adsorbents through regeneration and reuse. Dodson et al. [127] discusses the value-added applications of metal-loaded sorbents. It is necessary to find
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sustainable pathways for effective utilization of pollutant-loaded adsorbents as value-added
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products to resolve the economic, toxic and management issues associated with these materials. In this review, we propose alternative strategies for the effective utilization of post-
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sorbents based on their emerging applications (Fig. 4).
Fig. 4. Past and present status of post-sorbents
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ACCEPTED MANUSCRIPT 2. Post-sorbent applications 2.1. Fertilizers and soil conditioners
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World food resources depend primarily on agriculture, and it is predicted that global
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agricultural production must increase by 70% by the year 2050 [128] to meet the demands of
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a rapidly growing world population (9.7 billion by the year 2050) [129-132]. To confront this global food demand, modern farming techniques that maximize agricultural production have been employed; however, these lead to depletion of carbon and nutrients in the soil. Crops
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cultivated in these overused soils become less productive [133] and they cannot provide the
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essential elements required for a healthy human diet, and therefore lead to cases of microand macro malnutrition [131, 134, 135]. Dimpka et al. [136] comprehensively discusses the
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importance of micronutrients in effective agronomic production. Billions of people around
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the globe suffer from an insufficient intake of micronutrients due to mineral-deficient food crops [137]. Micro- and macro- malnutrition lead to diverse health effects, including mental
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retardation a weakened immune system and, an overall decrease in health [138]. A conference held in 2014 by the FAO/WHO (Rome, Italy) emphasized that dietary deficiencies in
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essential micronutrients affects more than two billion individuals around the world, with women and children particularly at risk [139, 140]. At the conference, a Rome Declaration on Nutrition was put formulated, with one goal being “the elimination of malnutrition in all its forms”. A reliable and appropriately balanced supply of micro and macronutrients is essential to enhance plant growth and to overcome malnutrition problems [141]. Fertilizers have been widely used to enhance crop yields. Fertilizers are typically comprised of six macronutrients, including calcium (Ca), magnesium (Mg), phosphorus (P), nitrogen (N), potassium (P), and sulfur (S), and eight micronutrients including boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), zinc (Zn) [142]. The nutrients (micro or macro) are applied in either organic or inorganic form to 14
ACCEPTED MANUSCRIPT soils that are deficient. The use of fertilizer in agriculture is likely to increase considerably in the future [143]. The IFA (International Fertilizer Association) [144] website reports that the
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global use of nutrient fertilizer (nitrogen, phosphate, and potash) is estimated at 186 million
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tons, and will reach around 200 million tons by the year 2018.
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Several studies demonstrate that excessive usage of fertilizers leads to the release of high concentrations of compounds such as nitrogen and phosphorus into the environment [143, 145-147]. The enrichment of these nutrients in particular ecosystems can cause
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eutrophication, or the excessive richness of nutrients in the water, likely due to land runoff.
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This is detrimental to the environment as it results in a lack of oxygen that promotes plant formation while increasing animal mortality [34, 146, 148]. The raw resources required for
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the production of N and P fertilizers are non-renewable. Furthermore, the limited natural
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reserves of these materials are being exhausted due to their widespread use as fertilizers [149151]. Thus effective utilization of fertilizers applied to the crops is essential.
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A limited supply of nutrients leads to low crop production, soil degradation, and poor human nutrition; in contrast, high concentrations lead to environmental pollution [147]. The
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nutrient use efficiency (NUE) of crops is poor under realistic conditions. According to the IFA, in the initial year of nutrient fertilizer application, the utilization rate of both N and K is about 50-60%, while only 10-25% for P [152]. The remaining unused nutrients are destined to degrade, release or leach, all of which may cause surface and groundwater pollution. The major disadvantages of using conventional fertilizers are the low efficiency and excessive waste, thereby resulting in numerous severe environmental and health complications. Thus, for the sustainable management of fertilizers, it is critical to develop slow and controlledrelease technologies (Fig. 5). The European Standardization Committee (CEN) Task Force stipulated that slow-release fertilizers should meet the following conditions at a temperature of 25 oC: (i) no more than 15% release in 24 h, (ii) no more than 75% release in 28 days, and 15
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(iii) at least 75% released by the stated release time [153].
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Fig. 5. Conceptual nutrient use efficiency of slow and rapid release fertilizers
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Various techniques have been explored for the purpose of regulating the quantity of nutrients that discharge into water. Of these, adsorption is one of the more proficient and economical practices and has the following benefits: ease of handling, a wide choice of adsorbents, minimal operation costs, less sludge release, and fewer disposal complications. Adsorption is the only technology for which it is feasible to use nutrient-laden adsorbents as fertilizer and soil conditioner in agricultural systems [154]. Compared to conventional fertilizers, sorption-based fertilizers offer several advantages: (i) soil-deliverability, (ii) controlled nutrient release, (iii) fewer adverse effects, (iv) biocompatibility, and (v) excellent nutrient use efficiency. Manufacturing fertilizers by sorption technology would be more sustainable and economic because, precursor materials used for nutrient loading are 16
ACCEPTED MANUSCRIPT renewable (biomass, zeolites, biochar and manure) and available at low or no cost. However, the successful application of adsorbent as fertilizer relies heavily upon the choice of
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adsorbent materials because these will define the relevant properties, such as nutrient loading
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capacity, cost, nutrient release rate, adaptability to soil conditions, and water
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retention/holding capacity. Several specific adsorbents could be fabricated to load both cationic (NH4+, K+), anionic (NO3-, PO43-), and trace mineral (Cu, Fe, Mn and Zn) nutrients in
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aqueous solutions, which could be successfully applied as fertilizer (Fig. 6).
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Fig. 6. Fertilizer applications of post-sorption technology
One of the most successful adsorbents used for fertilizer applications is biochar, a solid carbonaceous porous product acquired by partial combustion of organic materials using modern pyrolysis techniques [55, 155-157]. The biochar pore structure directly contributes to the soil porosity providing water, nutrient, and mineral storage and accessibility. Biochar exhibits typical pore structure which might be divided into three kinds such as macro, meso, and micropores as shown in Fig. 7. This division is based on pore sizes, large pore sizes > 50 nm are macropores, pore size in the range of 5-20 nm are mesopores, whereas smaller pore size in the range of <2 nm are micropores. The micropores in biochars have high sorption 17
ACCEPTED MANUSCRIPT ability for small molecules [158], thus during soil applications biochars with high micropores might reduce the nutrient loss during volatilization of organic molecules.
Mesopore (2-50 nm)
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Macropore (>50 nm)
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Micropore (<2 nm)
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Fig. 7. Representative image of biochar showing the macro, meso and micropores.
Biochar has been widely used to improve soil fertility and soil carbon sequestration
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in soil amendment fields, which generally improves soil quality and helps to mitigate processes associated with climate change [159-161]. Utilization of biochar has received
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global attention in the past few decades and several reviews on this application have been published [162-168]. The soil organic carbon (SOC) levels that are essential for plant growth have decreased around the world due to intensive agricultural activities. Biochar, derived from carbon-rich biomass, has a high organic C content that could enhance SOC levels. According to published reports, the carbon content of biochar can reach 70% depending on the feedstock material [169]. Soil containing biochar has been shown to have measurable improvements in water-holding capacity, structure, and porosity, cation exchange capacity, electrical conductivity, and leaching of nutrient and agrochemicals [170-174]. Not only can biochar improve the growth of plants through its natural mineral/nutrients composition, it also protects plants from the abiotic stress of toxic elements and organic pollutants by 18
ACCEPTED MANUSCRIPT adsorbing or precipitating toxic pollutants [175]. Biochar can be manufactured from a widerange of materials that are of low to no cost because they are typically discarded as waste.
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These include: agricultural wastes, animal manure, forest waste, and sewage sludge. For all
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of these reasons, several researchers have focused on the application of biochar as a sorbent
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for the recovery/enrichment of nutrients and its subsequent application as fertilizer. Although biochar is one of the strategic key resources that have been used to enhance soil fertility, it does not show the same performance and effectiveness under all conditions. This
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inconsistency is because the physicochemical characteristics of biochar depend upon the
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properties of the feedstock materials (available nutrients, carbon content, and porosity) and the pyrolysis conditions (time and temperature) that are used. The nutrient (both micro and
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macro) availability of biochar-amended soil depends on several physical and chemical
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properties such as pH, surface area, porosity, cation exchange capacity (CEC), and the transfer of nutrients into the amended soil. However, it is possible to prepare custom or
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engineered biochars in such a way that they function effectively under specified conditions. A major drawback of biochar is that it is ineffective at recovering nutrients because
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most nutrients (nitrate and phosphate) are anionic, while the surface of biochar is highly negatively charged. In one study, thirteen different biochars derived from various feedstocks were tested for nitrate or phosphate adsorption, and most exhibited little or no sorption ability [176]. Similar results were observed with nine other biochars prepared from different feedstock materials [177]. These two studies were demonstrated that nutrient sorption ability of biochar significantly differs with biochar and nutrient kind. To improve the anionic nutrient sorption ability of biochar, composites were manufactured by combining inorganics, such as Al, Ca, Mg, Fe, La, Ca-Mg, and Mg-Al, with biochar [178-188]. Efforts were focused on the development of Ca and Mg biochar nanocomposites because they have the attractive properties of biocompatibility, high surface area, high nutrient loading capacity, and 19
ACCEPTED MANUSCRIPT controlled release of nutrient. Mg-enriched tomato tissue has been used as an engineered biochar for phosphate recovery from aqueous solution and then repurposed as a soil P-
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fertilizer [55]. Post-sorption characterization results revealed that both precipitation and
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surface deposition are key mechanisms involved in P recovery. Bioassay results showed that
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the addition of P-loaded biochar nanocomposite improved seed germination rates from 53% to 85% (p < 0.001). To enhance the anionic sorption capacity and ease of separation of biochar from aqueous solutions, magnetic MgO-decorated biochar was manufactured using
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ferrous chloride, and magnesium chloride, and subsequently tested for phosphate recovery
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and reuse [156]. Mg content during the impregnation increased from ~0.44 to 20%, and at an Mg content of 20%, the biochar had a high phosphate sorption capacity. The sorbate-sorbent
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interactions were attributed to protonation, surface electrostatic attraction, surface inner-
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sphere complexation, and precipitation.
Biochar characteristics may significantly vary with pyrolysis temperature which
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further influences the sorption ability of biochar [189-191]. Considering this in a study the effect of temperature during pyrolysis (200-800 oC) on the characteristics and phosphate
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sorption ability on biochar prepared using Undaria Pinnatifida roots as feed-stock were investigated [192]. The results revealed that when pyrolysis temperature increased, the following were observed: (i) decreased yield, (ii) increased aromaticity, and (iii) decreased polarity. Phosphate sorption increased with pyrolysis temperature upto 400 oC, and then decreased as the temperature was further increased. When the fertilizer efficiency of phosphate-loaded biochar was studied using potted indoor lettuces, the pot with supplied biochar grew faster and had a larger leaf area and size in the first 21 days, and larger and wider sizes of leaves after 30 days when compared with control pot containing commercial compost [192]. Biochar as a soil conditioner was also examined in a study on ammonia-laden corn cobs [193]. The biochars were manufactured at two different pyrolysis temperatures, 20
ACCEPTED MANUSCRIPT 400 and 600 oC, and biochar prepared at the lower temperature had a higher sorption capacity than that prepared at the higher temperature.
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Zeolite and clays, which are abundant in nature, have also been used extensively for
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nutrient recovery and subsequently as fertilizers [194-197]. Citing the advantages of air
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permeability, good soil osmosis, and water storage efficiency, a recent study reported the use of Kanuma and Akadam clays, along with zeolites, to manufacture ceramic adsorbent for the recovery of ammonium that was then repurposed as a fertilizer [198]. A maximum nitrate
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sorption capacity of 75.5 mg/g was obtained using an initial concentration of 10,000 mg/L
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NH4+-N was used.
As a demonstration of the value of the high nutrient loading ability of composite
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materials, a 30%, palygorskite nanocomposite was manufactured using carboxymethyl
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chitosan, acrylic acid, and palygorskite and the resulting composite exhibited a high ammonium loading capacity of 237.6 mg/g [199]. The composite exhibited both slow-release
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of nitrogen, with 60% of the nitrogen released into the soil over ten days, and good waterretention properties. In addition to nitrogen, palygorskite composites also contain nutrients
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such as P, K, Ca, Fe, Mg, and Mn, which are useful for plant growth. In another study, six surfactants comprising functionalized and unmodified synthetic silicate were manufactured and used for the recovery and reuse of anionic nitrate [200]. Surfactant-loaded clinoptilolite had a higher sorption capacity, of 125 mg/g, compared with other unmodified and modified silicates. The results also showed that surfactant-modified silicates could be effectively used as fertilizer due to their slow release performance, wherein nitrate continued to be released after 15-20 days of observation. The same group also employed surfactant-modified silicates for phosphate removal and for application as a slow release fertilizer [201]. Phosphate sorption was efficient at pH 7 and a maximum sorption capacity of 93.46 mg/g was perceived for hexadecyltrimethylammonium bromide modified clinoptilolite. The sorption mechanisms 21
ACCEPTED MANUSCRIPT between phosphate and surfactant-modified silicates were identified as electrostatic attraction, Van der Waals interactions, and interlayer adsorption. Nutrient leaching studies in
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soil-column tests revealed phosphate leaching even after 15 days demonstrates the slow-
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release characteristics of surfactant modified silicates.
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In addition to the above materials, biomass such as wheat straw [202, 203], chicken feathers [204], waste biomass [205], seaweed [206], and spent mushrooms [207, 208] have been utilized for nutrient loading and then reused in fertilizer applications. In one study, straw
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cellulose adsorbent was prepared and used for the recovery of both cationic and anionic
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NH4+and H2PO4− nutrients from aqueous solutions [209]. Soil experiments were performed to evaluate the slow-release behavior and water retention capacity of nutrient loaded sorbent,
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and the results were positive, 95.1% N released within 20 days and 60.0% P released within
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30 days, with increasing fertilizer dosage from 0.5 to 2 wt% soil water-retention capacity was increased. In another application, recognizing the importance of Mg in phosphate
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sequestration Wang et al. [210] prepared a hybrid artemia shell–Mg adsorbent by impregnating magnesium nitrate into artemia shells in an in situ process. Phosphate sorption
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experiments revealed a maximum sorption capacity of 32.7 mg/g at pH 6.4-6.9. The post sorbent (shell-Mg-P) has a slow-release behavior; only 65.8% of the phosphate had been released after 30 days. In addition to nitrogen and phosphorous other micronutrients, such as Mn(II) and Zn(II), have also been recovered and repurposed as fertilizer. In one study, spent mushrooms were converted to fertilizer on a pilot plant scale by loading Zn(II) and Mn(II) elements using sorption-based technology [207]. The biomass exhibited a sorption capacity of 15.84 mg/g for Zn(II) and 11.81 mg/g for Mn(II). Another study incorporated, feather protein, extracted from chicken feathers, as the basic macromolecular skeletal material for manufacturing hydrogel sorbent, and used this sorbent for the removal of cationic ammonium and anionic phosphate [204]. The authors propose that the resulting nutrient-laden sorbent 22
ACCEPTED MANUSCRIPT could be utilized further as fertilizer. A summary of the adsorbents that have been used as
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fertilizers, the target nutrients recovered, and their applications are presented in Table 2.
23
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Nutrient
Sorption capacity (mg/g)
Post-sorption applications
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Adsorbent
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Table 2. Sorption based fertilizers (nutrient-loaded adsorbents) and their characteristics
Fertilizer
Reference
Nutrient release
P
116.600±26.590
Seed germination 7555.5 ± 10.5 mg [55] (85%) P/ kg
MgO decorated biochar
Phosphate
121
Ryegrass seedling 37.53% growth
[156]
Porous MgO-biochar nanocomposites
Phosphate
835
-
-
[184]
Nitrate
95
-
-
Proposed as fertilizer
-
[186]
327
-
1.66%
[187]
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81.8
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Mg/Al layered double Phosphate hydroxide functionalized biochar
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Engineered biochar
Ca-Mg biochar
Phosphorous
Undaria pinnatifida roots
Phosphate
32.6
Lactuca sativa plant growth
[192]
Corn cob
Ammonia
6.37
Soil conditioner
[193]
Ceramic adsorbent
NH4+–N
75.5
Soil conditioner 24
-
[198]
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NH4+
Soil fertility
60% in 10 days
[199]
125
Soil fertility
75% in 6th irrigation
[200]
93.5
Soil
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Surfactant modified Nitrate silicates (HDTMAB
238
-
[201]
Soil conditioner
69.2% in 10 days
[202]
Proposed as fertilizer
-
[207]
Soil
95.1% N (21 days)
[209]
Soil
60.0% P (30 days)
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Surfactant modified Phosphate silicates (HDTMAB
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loaded clinoptilolite)
Ammonium
7.15 mmol/g
Spent mushroom
Mn(II)
11.8
Zn(II)
15.8
NH4+
68
H2PO4–
38.6
Artemia egg shell
Phosphate
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PT ED
Wheat straw
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loaded clinoptilolite)
Straw cellulose
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Palygorskite nanocomposite
32.7
Mung bean seedling growth
69.41 ± 2.2 mg [210] P/kg
Ca impregnated ramie
Phosphate
105
-
-
[211]
P
252
Proposed
-
[212]
Biomass Fe-Mn binary oxide nanoparticles
25
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-
Starch stabilized Mn nanoparticles
313
-
Fe-
-
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298
-
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CMC stabilized Fe-Mn nanoparticles
Phosphorous
454.5 mmol/g
Soil conditioner, P release even after [213] P nutrient 1080 hrs
Natural zeolite
Ammonia
6.89± 0.64
Cultivation of A. platensis
35% ammonia
[214]
Surfactant-modified zeolite
Nitrate
91 mmol/Kg
Sweet corn (Zea mays) plant growth
-
[215]
Seaweed growth
-
[216]
Correct soil acidity and
-
[217]
Proposed as fertilizer
20.4 (±6.35)
[218]
Zn(II)
Anaerobically digested Phosphorus sewage sludge and ochre
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Manure/straw 1:3
49
225 mmok/Kg
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double Phosphate
6.70
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NiAl layered hydroxide
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Surfactant-Modified Zeolite
26
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Very little research has addressed the utilization of macronutrient (nitrogen, phosphorous)- loaded sorbents as fertilizers and soil conditioners, and a few studies have
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evaluated the efficiency of micronutrient-loaded sorbents as fertilizers. However, recent attention
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has turned toward health-related issues associated with malnutrition attributed to micronutrient
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deficiency [219-221]. For instance, a zinc deficiency in the diet of people in developing countries is one of the 10 key issues contributing to health concerns according to the World Health
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Organization (WHO) [222, 223]. This micronutrient deficiency can be overcome through
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supplementation with fertilizers. For example, in the 1970s, in Finland, an extremely low Se consumption of 0.025 mg/day was detected and to overcome this situation, in 1984, sodium
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selenite multi nutrient fertilizers were recommended [224]. Since 1985, most of the fertilizers in
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Finland have included Se, and currently, all crop fertilizers contain 15 mg Se/Kg. Thus, it is
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conceivable to pinpoint a nutrient deficiency in a particular geographic location and address the problem by developing customized, nutrient-loaded sorbent fertilizers that provide the optimal nutrient supplementation. In another example, anemia, which is a significant global health problem affecting 1.62 billion individuals worldwide [225], is frequently caused by an iron deficiency [226, 227]. Anand et al. [228] report that iron deficiency anemia is one of the chief public health complications in developing countries such as India. Insufficiencies of minerals, specifically iron (Fe), zinc (Zn), and calcium (Ca), are a major problem in developing countries where they lead to adverse effects on human health [229]. Again, the possibility of manufacturing custom-tailored post-adsorbents (laden with essential trace elements) that can then be used as multi nutrient fertilizers may be a solution to improving public health. Zeolites are ideal substrates because they carry nutrients and release them gradually, and they have 27
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hydration and dehydration capacities that are helpful for maintaining water balance [197]. Zeolites also possess selective adsorption efficiency towards ammonia (NH 4+), which is useful in
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manufacturing of nitrogen-based fertilizers [230]. The specific adsorbent-nutrient interactions
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and the nutrient release rate are both important properties to consider. The following strategic
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points are important to consider when manufacturing nutrient-loaded sorbent fertilizers: (i) affinity for both cations and anions (NH4+, K+, NO3-, and PO43-), (ii) stability under soil
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ability to function for long periods of time.
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environmental conditions, (iii) slow-release characteristics, (iv) water-holding capacity, and (v)
Although adding post-sorbent (nutrient-loaded sorbents) to soil improves agricultural
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yields and nutritional qualities (laboratory scale), socio-economic conditions will determine
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whether or not the materials can be applied to commercial agriculture. The key issues that
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influence the application of post-sorbent fertilizers are the availability of feedstocks, the technology to manufacture fertilizer, and investment capacity. Further studies should carry out economic analyses of post-sorbent fertilizers to provide the appropriate information about the application of these materials to agricultural systems. A recent cost analysis for manufacturing Zn(II) and Mn(II) loaded mushroom material was carried out [207]. A cost estimation analysis for manufacturing the fertilizer estimated $ 4.76/Kg for the production of 100 kg/day in the pilot plant. Raw materials cost significantly influences the overall post-sorbent fertilizer manufacturing cost; thus low-cost sources should be explored. Nutrients used to load onto adsorbent were of high-cost; however, these metal ions are available in various waters at low or no cost. Seawater and wastewater, which are rich in various trace metal ions, should be considered as sources of micronutrients for sorption technology because this will decrease the 28
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costs of manufacturing nutrient-laden sorbents [36, 231-233]. This strategy will be useful in twoways: one is to remove toxic metal ions from wastewater and second one is to reuse them, thus
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this strategy reduces the manufacturing cost of nutrient-laden sorbents. In one study, poultry
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litter (PL) containing P was used as a source and Fe-Mn binary oxide nanoparticles were used to
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control the leaching of P from PL; the treatment cost was estimated to be $0.0036–0.0056/kg-PL [212]. Composite materials have also been developed as sorbents using several types of metal
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oxide nanoparticles, such as MgO, which is widely used to fabricate biochar composites. For
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preparing MgO composite materials, alternative Mg resources should be explored. In one study, carbon materials with stabilized MgO nanoparticles (~17-18 nm) were manufactured using an
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MgCl2 solution by using average Mg concentration in seawater i.e. 1540 mg/L [234]. Utilization
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of untapped reserves to manufacture nutrient-loaded sorbent fertilizers will invariably reduce the
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overall product costs. A comprehensive assessment of the economic, technical, and environmental strength and weakness of post-nutrient laden fertilizer manufacturing technologies should be carried for a better understanding of the real-world applications of the developed materials.
It is important that post-nutrient laden sorbent fertilizers be examined for toxic characteristics, which would limit their validity and application. For example, to enhance both nutrient loading capacity and controlled release properties, several researchers have employed nanocomposite fertilizers. However, in recent years there has been much debate about the environmental safety and toxicity of nanoparticles [235-238]. For example, to enhance the sorption performance, biochar was manufactured in nano-composites form using metal oxide nanoparticles such as Cuo, CoFe2O4, MgO, MnO, and TiO2; therefore, the environmental and 29
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biological risk of these metal oxides should be considered [42, 88]. Thus, it is essential to evaluate the stability and toxicity of newly manufactured nanocomposites. An evaluation of
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metal ion leaching from nanocomposites using the U.S. EPA Toxicity Character Leaching
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Protocol (TCLP) will provide an indication of their behavior [239]. To pinpoint any adverse
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effects of nanocomposite fertilizers on health, it will be necessary to initiate studies on various issues including the fate and transport of the nutrient loaded nanocomposites in the soil, the
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ecosystem, the uptake and accumulation in plants, and the evolution of the toxicity of
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nanomaterials. Risk assessment studies should be conducted before any agricultural application, and the effects should be monitored continuously throughout the application.
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Although biochar has shown a wide variety of benefits, such as improved soil fertility,
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improved plant growth, and increased crop yields, possible negative consequences should also be
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considered. For example, the micronutrients mentioned above (essential elements) are required in trace quantities for normal plant growth, but when present at higher concentrations, these elements are toxic to living organisms. Before these nutrients are applied as fertilizers via postadsorbents, the materials should be evaluated to assess whether or not toxic compounds, such as heavy metals, would be released into soils, thereby violating current environmental protocols. Biochars have been engineered from various organic feedstock materials, which may have contained the potential toxic elements (PTEs) Pb, Cd, Cu, and As, in addition to carbon, nutrients, and minerals. These materials also contain polycyclic aromatic hydrocarbons (PAHs) and highly volatile organic compounds (VOC), which are notable because of their adverse effects on plant growth and on microbial communities [240, 241]. However, after converting feedstock into biochar, contents of PTEs in the resulting biochar were reported to be low or high 30
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with respect to their feedstocks [242, 243]. Although small quantities of bioavailable PTEs are expected in soils, the subsequent negative effects on crops are undesirable, and lead to health
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problems in animals and humans via the food chain [244, 245]. Furthermore, the practice of
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pyrolysis may alter the oxidation state of some PTEs, specifically Cr(III) or Cr(VI), which could
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significantly increase or decrease toxicity. Considering the importance of potential toxicities of biochar depends on pyrolysis temperature Lyu et al. [191] investigated the toxic effects of
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biochars produced at temperatures ranging from 200-750 oC. They denoted that biochars
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manufactured at pyrolysis temperature >400 oC are less toxic and are more appropriate for soil usages. There are no legislative standards limiting PTE concentrations in biochar that is to be
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applied to soils. Instead, biochar containing materials are to be certified according to the
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International Biochar Initiative (IBI) guidelines [246], and the European Biochar Certificate
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guidelines [247]. These certificate systems were established by researchers as a preliminary assessment of the toxic characteristics of biochar materials. According to the IBI, As, Cd, Cr, Co, Cu, Pb, Hg, Mo, Ni, and Zn are the PTEs of concern in biochar, and Co and Mo [246] are also priority pollutants as per the U.S. EPA [248, 249]. The European law on EC fertilizers suggests that heavy metals should not be added purposely to ammonium nitrate fertilizers, and the concentration of copper should be below 10 mg/Kg [250]. Thus it is essential to investigate the concentrations of PTEs in biomass-based materials that are manufactured from new feedstock materials according to the extant legislation or regional regulations. These strategies will ensure the development of environmentally-friendly post-sorbent material enriched for nutrients, and the application of these materials as fertilizers will improve the nutrient efficiency of crops and play a positive role in reaching the 2050 food and environmental security goals [146, 251, 252]. 31
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The industrialization of post-sorbent materials that are manufactured by sorbing nutrients onto various sorbents as fertilizers have been limited by the following key issues, (i) release of toxic
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metal ions from sorbent and/or composite sorbents causes antagonistic effects on plant growth,
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(ii) uncontrolled release of sorbed metal ions, (iii) cost, (iv) high volumes are required due to the
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lower amounts of sorbed nutrients compared with commercial fertilizers, and (v) uneven distribution of sorbed metal ions results either high nutrient availability or low nutrient
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availability.
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2.2. Carbon-encapsulated metal nanoparticles (CEMNPs) Before providing a detailed account of the applications for post-sorbents in
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manufacturing carbon-encapsulated metal nanoparticles, it is relevant to introduce three major
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problems encountered in three divergent scientific fields (stable carbon-nanomaterial fabrication,
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biomass gasification, and wastewater treatment), all of which were solved by post-sorption technology. First, the preparation of stable metal nanoparticles is difficult. Pure metal nanoparticles (MNPs), such as non-precious 3d-block transition metals (Fe, Cu, Co and Ni) and their alloys (CoFe, CoNiFe, etc.), have charmed researchers for years due to their optical, catalytic, magnetic, mechanical, photo-thermal, and electrical properties [253]. These interesting characteristics have led to stimulating applications in diverse areas such as nanosensing, nanocatalysis, water treatment, drug delivery, spectroscopy, magnetic resonance imaging, and plasmonics [253-257]. However, the high surface-to-volume ratio of pure metal nanoparticles is also associated with instability via oxidation in air, dissolution in acid, and aggregation due to high surface energy, leading to the deterioration of their magnetic properties among other things [258]. This restricts hands-on applications and creates difficulties in assessing the properties of 32
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these materials [259-262]. Thus, inert materials either organics or inorganic, have often been used to coat pure metal nanoparticles through in-situ and ex-situ techniques to shield them from
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oxidation and avert changes in their properties. Encapsulation of highly dispersed metal/metal
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oxide nanoparticles inside of carbon materials is a useful method; the carbon layer protects the
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naked nanoparticles from oxidation, agglomeration, harsh chemical environments, and maintains high magnetization [263]. Compared with other types of coatings, carbon provides the unique
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advantages of high chemical and thermal stability, high dielectric permittivity, magnetic
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permeability, better conductivity, and biocompatibility [264-268]. Carbon-encapsulated metal nanoparticles (CEMNPs) are a novel type of nanostructured carbon materials that have a unique
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core-shell construction of metal cores surrounded by thin carbon coat (shells) (Fig. 8), and the
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crystallinity of this coating is graphitic in nature and alike to CNTs [269, 270].
Fig. 8. Effect of air exposure and encapsulation of metal nanoparticles (MNP)
CEMNPs have attracted much attention due to their unique physical and chemical properties, especially for application in catalysts, drug delivery, water treatment, hyperthermia and 33
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information technologies such as magnetic data storage, xerography, electronics, and biomedicine [271, 272]. There are several methods available for fabricating CEMNPs; these
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include high-temperature annealing [271], chemical vapor deposition [273], arc discharging, and
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modified arc discharging [274]. However, in situ generation of nanoparticles using metal ion
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loaded biomass through carbothermal reduction is of particular interest due to the renewability of biomass, and also due to its economic and eco-friendly nature.
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Preparation of stable carbon-nanocomposites for the treatment of contaminated water is
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another problem, which was also solved using post-sorption technology. The general manufacturing process for composites involves synthesis of carbon and nanoparticles
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individually, and then further mixing of these two distinct phases to obtain the composite
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materials. This approach has limitations including: (i) the challenges of carbon manufacturing,
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which requires high temperature, high voltage, and is difficult to control, (ii) the poor stability and high cost of composites, and (iii) the undesirable impregnation of particles or adherence to surfaces and pores. Additionally, when composites are used in water treatment and come into contact with acidic solutions, there is a possibility of nanoparticle leaching. When composites are manufactured through the pyrolysis of biomass loaded with metal ions (post-sorbent), the nanoparticles that are harvested are held tightly inside the carbon (during the in situ generation of nanoparticles which are covered by the carbon). Thus, there is less chance of leaching, agglomerating and oxidizing. The process of post-sorbent pyrolysis is further advantageous because the manufactured composite is eco-friendly and the process is scalable and economically viable. The economic benefit of this approach is due to the use of low-cost of the biomass used as the carbon precursor. The process is facile in that it occurs in a few steps: (i) loading of metal 34
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ions (ii) drying (iii) pyrolysis, and (iv) washing. Materials prepared in this way exhibit a high surface area of >1000 mg/g [275].
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The third problem that was solved using the post-sorbent pyrolysis technique was in
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biomass gasification. Biomass is inexpensive and is an abundant renewable source of carbon that
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has recently been widely used as renewable source of energy and fuel production through a thermochemical conversion process. Production of synthesis gas (also termed as syngas) through
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gasification/pyrolysis is a promising technique to convert biomass into value-added chemicals,
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hydrogen, gas fuels, liquids, and char. Moreover, the generation of syngas compounds (H2, CO, CO2 and N2) through the gasification process also produces undesirable by-products such as tar
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(comprising of benzene, naphthalene, phenol, toluene, and xylene) and inorganics (S, K, P etc.).
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Tar, comprised of benzene and polycyclic aromatic hydrocarbon, is of major concern because it
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is an environmental pollutant and damages process equipment, engines and turbines [276, 277]. For effective use of syngas, these impurities, particularly tar, must be removed or treated. Catalytic reforming/cracking of the tars and hydrocarbons is an efficient technique that provides additional syngas, enhanced H2 content, high heating values, and higher biomass carbon utilization [278]. Transition metal-based catalysts, chiefly nickel-based catalysts are effective in tar reduction and syngas reforming during biomass pyrolysis/gasification [279-281]. Among the different catalyst integration strategies, insertion of the catalyst metal precursor into the biomass (metal-impregnated biomass) through adsorption prior to thermo-chemical conversion was effective, and had a significant effect on the yields of tar, gas and char [282-284]. Adsorption plays an important role in the preparation of metal-impregnated biomass because the chemical functional groups present in biomass act as adsorption sites during the impregnation step, and 35
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mineral cations present in biomass are also replaced by catalyst metal precursors through ion exchange [282, 284]. High yield and good dispersion of the metal precursor are accomplished
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using this approach.
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Biomass is an abundant carbon source that can be used as a precursor for manufacturing
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carbon-encapsulated metal nanoparticles. Biomass and derivative materials have been used widely in water treatment for the removal of metal ions from aqueous solutions, because they are
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abundantly available, low cost, and environmentally-friendly [120, 285-290]. Biomass materials
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contain numerous oxygenated functional groups, such as carboxyl, carbonyl, amine, hydroxyl, and ether groups [291]. These functional groups act as ligands for metal ion complexation in
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aqueous media. Robalds et al. [292] classified the biosorption mechanism and their review and
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references therein are recommended for further reading on this subject. Due to the ability of
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biomass to complex metals, it can function as a natural biotemplate to generate dispersed metal ions. In this respect, biomass has been used as both a template and a carbon source for harvesting nanoparticles. Recently, there have been several attempts to fabricate carbonaceous metal nanoparticles using the abundant, economical and environmentally friendly metal-loaded biomass (post-sorbent) as a precursor material. This process involves the adsorption of transition metal ions (Cu, Ni, and Co, etc.) onto biomass, and then conversion to carbonaceous metal nanoparticles using catalytic pyrolysis/gasification techniques. Catalytic biomass gasification is an effective technique used to convert metal-loaded biomass into carbon, char, syngas, and nanoparticles. Harvesting carbonaceous metal nanoparticles involves the following processes: (i) sorption of metal ions onto biomass, (ii) grinding and drying, (iii) pyrolysis of metal-laden biomass, and (iv) in situ 36
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generation of metal nanoparticles. In the first step, the target metal cations are adsorbed onto the numerous oxygenated groups (hydroxyl, carboxyl, amine etc.) of biomass while in aqueous
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solutions, and at the same time metal ions are dispersed inside the biomass matrix. In the second
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step, the biomass is thermally decomposed, and the major reactions that occur during thermo-
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chemical conversion are summarized in the following equations [293]: (1) (2) (3) (4)
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(5)
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(6)
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CxHyOz represents the major types of biomass molecules, including aldehydes, ketones, alcohols, carboxylic acid, and cresols. Adsorbed metal ions in the biomass matrix are reduced by the reducing components in the reaction (ex. CO, CH4, H2, CxHyOz and the bio-char). The carbothermal reduction of metal oxides is a well-known technique [294]. In the absence of oxygen, the abundant reductants molecules reduce the high-valency (Mn+) metal ions dispersed in the biomass into metal nanoparticles (Mo, M2+, MO, MFe2O4). The carbothermal reduction of metal ions is described in the following equations: (7) (8) The advantages of this process are: (i) the metal nanoparticles and the carbon can be harvested simultaneously, (ii) the metal compounds have a catalytic effect, and (iii) it is a one step process. 37
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Catalytic biomass gasification was principally used to obtain synthesis gas, bio-oil, bio-char, and H2 [295]. For further reading on carbon-based nanoparticles, including carbothermal synthesis,
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catalytic biomass gasification, biomass pyrolysis, and carbon supported catalysts see other
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reviews on the subject [157, 296-303]. A schematic of carbon-nanoparticle harvested through
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carbothermal-reduction is shown in Fig. 9.
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Fig. 9. Synthesis of carbon-nanoparticles through carbothermal synthesis approach
In situ generation of nanoparticles through biomass pyrolysis/gasification depends upon biomass composition, metal ion impregnation and pyrolysis conditions. Few of the significant factors, such as effects of metal ions, metal salts, and impregnation conditions (pH, atmosphere, and vacuum) have been reported [278, 282, 304-306]. pH is one of the key parameters that influences metal ion speciation in aqueous solutions and influences the functional groups present on biomass [307, 308]. Biomass consists of surface chemical groups such as carboxyl, carbonyl, hydroxyl, amine, amide, phosphonate, etc. [291], which groups acts as sorption sites. The pH influences the charges of these sorption sites. As the pH decreases, the biomass functional groups 38
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are protonated due to the high concentration of protons. This situation is not favorable for the sorption of positively charged metal ions due to the electrostatic repulsion. In the other hands, as
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the pH increases, a number of negatively charged sites increases, facilitating sorption of cations.
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Furthermore, the pH also influences the complexation process which involves the formation of
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coordination bonds between metal ions and functional groups containing a lone pair of electrons [287]. This complexation process is highly pH dependent; change in pH causes change in
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sorption efficiency of biomass [287].
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Besides, the pH also affects the solution chemistry of metal ions. In aqueous solution,
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metal ions (M2+) undergo solvation, hydrolysis and polymerization [309-311] as shown below: (9)
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(10)
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(11)
Most of the metal cations exist as free ions (M2+) in the pH range 2-5, as the pH is increased (>2) the positively charged metal cations quickly approach the negatively charged adsorbent surface. With further increase in solution pH, reactions between metal ions with these OH- ions results in the precipitation of metal hydroxides. This phenomenon also impacts the biomass sorption performance. When the influence of pH on Ni loading onto beechwood was studied, the findings revealed that metal loading increased with increasing pH of the solution [278]. Because nickel hydroxide formation occurs at pH>7 and wood chips have a pHZPC (6.7), optimum pH conditions of 6-7 were chosen to attain high dispersion of nickel cations into the wood biomass matrix. Nickel-loaded biomass pyrolysis resulted in well dispersed Ni0 nanoparticles of 2-4 nm in the 39
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carbon matrix. The findings also suggested that both the initial nickel loading and final pyrolysis temperature influenced the crystallite size of Nio, and these in situ generated NPs enhanced both
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tar conversion and H2 production. To evaluate the effect of precursor metal salts, another study
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describes the loading of Fe and Ni nitrates onto oak sawdust biomass and evaluation of the
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catalytic effect of the metals [304]. Higher H2 yields were observed for Ni-laden biomass samples than for Fe-loaded biomass, whereas the latter reduces the production of gases. Similar
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findings were also reported for nickel-loaded biomass, which led to high levels of hydrogen
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production when compared to iron-loaded biomass, despite the higher char yield for iron-loaded biomass [306]. The effect of atmospheric pressure and vacuum impregnation on nickel
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adsorption onto beech wood has been explored [282]. Furthermore, the catalytic effect of NPs
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has been evaluated by inserting Ni0 NPs into biomass prior to pyrolysis. One interesting finding
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was that under atmospheric impregnation Ni2+ adsorption onto biomass involved outer-sphere complexation, whereas under vacuum impregnation it involved inner-sphere complexation. The improved Ni insertion was achieved under vacuum impregnation due to this inner-sphere complexation, and higher catalytic efficiency was observed for in situ generated Ni0 NPs as compared to Ni0 NPs that were inserted into biomass before pyrolysis. The effect of boron addition was examined in a study of nickel impregnated rice husk (RH) biomass modified with sodium borohydride [284]. The results revealed that the addition of NaBH4 encouraged the generation of Nio NPs by deoxidizing NiO into metallic Nio NPs, and also enhanced tar conversion during pyrolysis. Nanda et al. [312] used hydrothermal gasification to study the influence of temperature conditions, biomass-to-water ratios, and residence time on nickelimpregnated pinewood and wheat straw biomasses. Nickel impregnation onto biomass was 40
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carried out at pH 7-8 for quick sorption. Compared with non-catalytic gasification higher yields of gas, hydrogen with superior carbon gasification efficiency was observed for nickel-
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impregnated biomasses.
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Recently, composite materials comprising two unique constituents have gained particular
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attention in areas such as catalysis, water treatment, and biological applications. Among all composites developed to date, carbon composites made up of metal nanoparticles or magnetic
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particles have been used widely in water treatment due to their high surface area, which is an
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advantage because it facilitates recovery of the composite after water treatment [41, 42, 81, 92]. CEMNPs have shown high sorption capacity and faster removal rates for inorganics compared
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with traditional activated carbon [269, 313]. Nitric acid treated CEMNPs have shown high
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sorption capacity i.e. 91.0 mg/g for Cd(II) compared with CNTs (20.37 mg/g) and AC (9.91
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mg/g) [314]. Considering the high sorption performance of CEMNPs several attempts were made to treat various pollutants. In a study porous carbon sheets containing carbon-encapsulated Fe/Fe3C (Fe/Fe3C@PCS) was fabricated by using gelatin biomass and Fe(NO3)3. 9H2O as precursors [315]. The material successfully removed uranium from aqueous medium via surface adsorption and led to in situ reduction of U(VI) into U(IV). Comparison was made between the sorption efficiencies of activated carbon and Fe/Fe3C@PCS for U(VI), activated carbon (AC) has limited sorption performance which was effective at low initial concentrations due to the limited active sites of AC. Whereas even at high concentration up to 140 mg/L Fe/Fe3C@PCS was effective for U(VI) removal (~100%). In another approach, agar was used as the biomass precursor, along with Fe(NO3)3. 9H2O and Al(NO)3. 9H2O as metal precursors, to manufacture porous carbon sheets containing carbon stabilized Fe/Fe3C nanoparticles [275]. The material 41
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exhibited a high surface area of 1023 m2/g and successfully adsorbed the organic pollutants methylene blue, methyl orange, and crystal violet with sorption capacities of 1616, 1062, and
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1728 mg/g, respectively. These materials have the added advantage that they can be easily
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removed from aqueous solutions due to their magnetic properties, unlike other materials. Cu NP
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(average particle size of 21.2 nm) anchored magnetic carbon (Cu and Fe3O4-mc) was manufactured by fast pyrolysis of Fe/Cu adsorbed fir sawdust biomass and the material was used
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as a catalyst for the catalytic reduction of 4-nitrophenol [316]. When the performance of Cu and
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Fe3O4-mc was compared to that of magnetic carbon, the Cu and Fe 3O4-mc showed more effective catalytic reduction of 4-nitrophenol, likely due to the presence of Cu nanoparticles,
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which have exceptional catalytic properties. This catalyst retained 95% of its reduction efficiency
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after five cycles, suggesting that it is effectively recyclable. Release of the heavy metal Cu
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throughout the five cycles was measured to determine the risk of toxicity to aquatic environments, and was found to be < 1.0 mg/L, which is below surface water limits. In another study, compressed carbon black was used to manufacture C-Feo particles using adsorption and impregnation procedures, and the resulting materials were tested for Cr(VI) remediation [317]. The results revealed that the BET surface area of C-Feo particles that were prepared through adsorption was higher (130 m2/g) than that of the starting carbon material (80 m2/g). The presence of C-Feo reduced the Cr(VI) concentration from 10 to ~1ppm. Most of the studies described above focus on metal-loaded biomass, but dye loaded adsorbents have also been used to manufacture nanoparticles by adding the appropriate metal ion precursor to the post-sorbent. Tian et al. [98] used an interesting strategy to prepare a multifunctional nanocomposite by combining AgNO3 and methyl violet-loaded palygorskite in a 42
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hydrothermal process. The dye adsorbed onto the palygorskite acted as carbon source, whereas the Ag(I) promoted carbonization of the dye molecule. This composite exhibited good sorption
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efficiency for methylene blue (99.2%), methyl violate (86.9%), chlortetracycline hydrochloride
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(68.7%), and tetracycline (46.2%). It also exhibited good catalytic properties by reducing 4-
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nitrophenol to 4-aminophenol with good reusability (8 cycles). A summary of adsorbents and metal ion precursors used in the fabrication of carbon-nanoparticles is given in Table 3.
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Biosorption, the sorption of metal ions from various biomaterial wastes, has been one of
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the more highly studied topics due to its effective removal of various metal ions from aqueous solutions. However, researchers are still looking for alternative strategies for the safe disposal of
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spent or metal-loaded biosorbents. Post-sorbed metal ions on biomass could be utilized as
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feedstock for harvesting nanoparticles in a carbon matrix through modern pyrolysis processes.
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Several factors that influence the sorption ability of biomass include pH, temperature, vacuum/atmosphere, biomass characteristics, and type of metal salts (ex. nitrate, chlorides, etc.). One of the key factors that influences the harvesting of nanoparticles is the pH during impregnation [318]. pH is always controlled, because the optimum sorption of metal ions occurs when metal ions are well dispersed and in high amounts on the biomass. This ideal method will provide an opportunity to manufacture value-added products such as (i) syngas, (ii) nanoparticles, (iii) biochar, (iv) reusable catalysts, and (v) resource recovery. The cost of raw materials influences the overall cost of any technology, and for in situ generation of nanoparticles, the cost of metal ion precursors is significant. Due to its good catalytic activity, nickel is one of the most widely used precursors for harvesting nanoparticles. Electroplating wastewater is rich in nickel ions and can be used for loading nickel ions into a biomass matrix 43
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[284, 319]. In addition to metal-containing wastewaters, seawater, which is rich in metal ions, can also be useful; because it is an inexpensive resource, the overall cost of carbon-nanoparticles
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synthesis would be reduced. For instance, in one study, seawater-rich MgCl2 was used as a
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precursor along with sawdust biomass for manufacturing mesoporous carbon stabilized MgO
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nanoparticles, and the resulting material was successfully used for CO2 capture [320]. Although CEMNPs have a wide range of applications in diverse fields, there are several
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limitations. Ni-based catalysts are often used to prepare CEMNPs for their enhanced catalytic
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activity and reduced cost. However, Ni-based catalysts are less stable and further prone to coke development [284, 321]. Uniform distribution of metal ions in the biomass is required.
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Otherwise, carbon formation without metal nanoparticles is possible and in the case of water
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treatment it's hard to separate the fine carbon powder. Fabrication of these materials requires
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high technology instruments and high temperatures. For producing CEMNPs both precursor biomass and metal ions should be obtained at low cost (i.e. in the form of waste); otherwise, the overall fabrication cost of CEMNPs will be higher. In recent years toxicity of emerging materials is of big debate. In the case of CEMNPs although nanoparticles are covered by carbon shell which protects from direct contact with biological fluids and environment toxicity of CEMNPs is off under scrutiny [272]. The results revealed cytotoxic response of these particles depends on their physicochemical characteristics specifically surface characteristics [272, 322-324].
44
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Fe/Al
Post adsorbent characterization and applications
800 oC /N2
Fe/Fe3C
(size Surfac Application e area m2/g
References
Efficiency
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NPs nm)
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Temperature /Atmosphere
1023
Dye removal
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MB (1615.9)
[275]
MO (1062.4)
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Agar
Metal precursor
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Adsorbent
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Table 3. Sorption based carbon-metal nanoparticles and their characteristics
CV (1728.3) mg/g
[Ni(NO3)2. 6 H2O]
700 oC
Gelatin
Fe
800 oC/ N2 (5 o C/min)
Fe/Fe3C
Beech wood chips
[Ni(NO3)2. 6 H2O]
700 oC/N2 (5 o C/min)
Ni0
H2 rich syngas production
[318]
Sawdust
[MgCl2. 6H2O]
773-973 K/N2
MgO
CO2 capture
[320]
Fir sawdust
Cu(NO3)2, Fe(NO3)2
773 K/N2
Cu&Fe3O4
186
4-nitrphenol reduction
Sawdust
FeCl3
773 K/N2
Fe3O4
296
Acid (Benzyl
AC
CE
Beech wood
Ni0
[282]
Aromatic tar conversion 95.9
Uranium removal
acetate) 45
100% mg/L
95%
catalyst 93%, reusable
(140 [315]
[316] [325]
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[Ni(NO3)2. 6 H2O]
750 oC/N2
Ni0
-
Tar conversion
-
[284]
Beechwood chips
[Ni(NO3)2. 6 H2O]
700 oC/N2 (5 Ni0 o C/min)
-
H2 production/Tar conversion
[278]
Pinewood & wheat
[Ni(NO3)2. 6 H2O]
700 oC/argon (10 oC/min)
Ni0
-
H2 production
-
[312]
Beech woodchips
[Ni(NO3)2. 6 H2O]
850 oC/N2
Ni
465
H2 production
-
[326]
Rice husk char
Fe(NO3)3·9H2O & 600 oC Ni(NO3)2·6H2O
Ni-Fe
-
Tar removal
92.3%
[327]
Rice husk
[Ni(NO3)2. 6 H2O]
Ni0
135
Tar conversion
-
[328]
Pinewood
[Ni(NO3)2. 6 H2O]
Ni (<100)
-
Gas and H2 yields
-
[312]
Wheat straw
[Ni(NO3)2. 6 H2O]
Ni (<100)
-
Gas and H2 yields
-
Compressed carbon black
Fe(NO3)3·9H2O
800 oC/Ar
C-Fe0 (~20 130 to 150 nm)
Cr(VI) remediation
-
[317]
Palygorskite
AgNO3
180 o C/Autoclave
Ag
Sorption/catalysis
-
[98]
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SC MA
PT ED
CE
AC
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straw
~700 oC/N2
PT
Rice husk
205
46
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2.3. Feed additives Trace elements, also known as trace minerals (dietary minerals), are essential inorganic
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feed additives that provide the key nutrients required for various metabolic functions in living
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organisms. These are required in tiny quantities for proper growth and, development, and play a
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vital role in innumerable biological processes and molecules, including nerve transmission, hormones, energy production, blood sugar levels, metabolism, digestion, and cholesterol [329].
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Of the 90 elements that occurring naturally, approximately 26 are considered to be essential for
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mammalian life, including B, Co, Cu, F, Fe, I, Li, Mn, Mo, Ni, Se, Si, V, and Zn, and possibly As, Cr, and Sn [330-333]. Although all of these elements are involved in biological functions, the
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trace elements required for good health are copper, cobalt, iron, iodine, manganese,
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molybdenum, selenium, and zinc [334-338]. Deficiencies of these minor trace elements can adversely effect animal and human health and can lead to chronic illnesses such as anemia,
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diabetes, Alzheimer’s disease, aging, hypertension, fertility issues, and reduced immune system function [339-342]. As an example, Zn is one of the essential micronutrients required for normal growth and development [343, 344]; however, it is estimated that 17% of the global population is at risk of Zn deficiency due to low dietary Zn intake [345]. Certain trace mineral insufficiencies in livestock can be classified as either a primary or secondary shortcoming in nature. A primary deficiency occurs as a result of the consumption of a diet that is low in one or more trace minerals, while a secondary deficiency results when there is reduced nutrient absorption or either enhanced metabolism or secretion of a specific nutrient [346, 347]. When an animal suffers from mineral deficiencies due to lack of trace elements (such as iodine and selenium), the meat or milk produced from those animals also contains inadequate 47
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levels of these trace elements. Consumption of products from deficient animals results in a similar deficiency in humans [348]. As an example of successful reversal of this effect, in the
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United Kingdom, cases of iodine(I) deficiency in humans have decreased after iodine
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supplementation was given to dairy cows, presumably because iodine progresses into the cow’s
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milk, which is then ingested by humans [349]. Animals with mineral insufficiency might not exhibit any outward symptoms; however, they may have reduced performance potential and less
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ideal health conditions. It is also possible to ingest excessive levels of trace minerals naturally or
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through diet supplementation, and this too has a negative effect on health in that it can lead to acute toxicity problems and possibly induction of deficiencies in other related trace minerals
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[333, 336, 350]. Scientific studies have carefully defined the nutritional needs of animals that
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feed formulations with appropriate levels of micronutrients are available to support the efficiency
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of animal husbandry and food production. Ideally, eatable animal foodstuffs (meat, eggs, dairy products, etc.) should contain optimal quantities of trace minerals to ensure human health. To handle micronutrients as precisely as possible, premix and feed producers must rely on sources of trace minerals that have ideal physical and technological properties. For instance, copper and zinc are critical trace minerals that improve production and reproduction in farm animals [351]. However, the form of the food, organic or inorganic, makes a difference in the availability of these minerals to the animal, because the inorganic form of trace minerals are not adequately absorbed and retained [351]. In contrast, the organic form is absorbed well and is maintained in tissues, improving performance, immunity, health and reproduction [352]. Biosorption technology, which enables sorption of a broad range of metal ions, is one of the best solutions for manufacturing fine-tuned trace mineral supplements. Trace minerals can be 48
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loaded onto the inexpensive, edible, and abundant biomass, and the resulting feed can be supplied to animal husbandry. Several researchers have focused on developing feed supplements
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utilizing biomass-based sorbents. In particular, Prof. Katarzyna Chojnacka’s group in Poland has
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been one of the most successful teams, making valuable progress in this research area and
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bringing this technology from the laboratory to the pilot scale [353-355]. This research group studied a broad range of trace metal-enriched biosorbents as feed additives and found promising
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results. This group published an excellent review on manufacturing agrochemicals, which
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includes discussion of the application of metal-enriched biosorbents to feed supplements [356]. Seaweed biomass, macroalgae, soybean meal, and spent mushroom substrates have often
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been used to adsorb metal ions, and were studied for their subsequent influence on animal
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performance (meat, milk and eggs). Taking into consideration the importance of Zn(II) and
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Cu(II) for animal growth, macroalga Enteromorpha sp. biosorbent was enriched with these two trace elements through biosorption technology, and studied for effectiveness as pig feed [357]. Inorganic salts were used for the control group and compared to experimental groups who were feed with biomass-enriched metal ions. A mineral composition analysis of the meat from each pig revealed that experimental groups had a significantly higher level of Mn (49%) than the control groups. In another study, microalgal biomass Spirulina maxima was utilized as a feed supplement by enriching it with the metal ions Cu(II) and Fe(II), and fed to swine and laying hens [358, 359]. In the experiments on laying hens, it was observed that the trace minerals Fe, Zn, and Mn were present at higher levels in the egg albumin of the experimental groups as compared to the control groups [359]. A pilot plant arrangement was also used to manufacture Zn(II), Cu(II), Mn(II) and Fe(II)- enriched soybean meal using sorption technology and the post49
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sorbent material was enriched with metal ions and used for zootechnical tests [355]. Some of the adsorbents were used as feed additives, and the nutritional content of the feed and their
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applications are summarized in Table 4.
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A wide variety of sorbents can be useful for manufacturing nutrient enriched feed; however, biomass substrate has often been chosen for this purpose. Zeolites should also be
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considered as substrate for trace element enrichment and subsequent application as a feed
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supplement. No study has reported the application of nutrient-loaded zeolite as feed supplements. However, application of zeolites as feed additive has been reported and its use has been accepted
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by various regulatory agencies. For example, both the U.S. FDA and the EU have approved
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clinoptilolite as a feed additive [230]. An in vitro study evaluating the effect of zeolite on the
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gastrointestinal tracts of swine found no signs of toxicity [360]. The release of extremely low concentrations of Pb and other toxic elements was observed; however, the concentration of Pb was far less than that allowed by European Regulations (E566). Prasai et al. [361] studied the effects of feed supplementation containing biochar, zeolite, and bentonite on layer chickens and the findings revealed that these feed additives are effective in controlling zoonotic pathogens. Several studies have reported that zeolites have a positive influence on disease resistance as well as, immunological and hematological factors [362]. Natural zeolites are microporous crystalline, hydrated aluminosilicates of alkali and alkaline earth cations that have unique physicochemical characteristics including ion-exchange and adsorption-desorption abilities. A wide variety of natural and synthetic zeolites have been used for wastewater treatment, effectively removing metal ions such as copper, zinc, manganese, and boron [82, 363-367]. Further studies should be 50
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carried out to recognize the potential of trace element-enriched zeolite application as feed supplementation.
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Most of the research has focused on enrichment of sorbents with various trace elements
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and using this as a supplement to animal feed. However, vitamins and antibiotics are also
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essential to the performance of livestock and prevention of disease. For example, pharmaceuticals are often used in farm animals for both therapeutic and prophylactic purposes,
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and are distributed through feed or drinking water [368]. Thus, drugs and/or vitamin-loaded feed
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supplements could be developed using sorption-based technology. Several researchers have reported that sorption technology has been successful in the removal/recovery of various drugs
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[369-374]. Thus, biomass used to feed animals could be tailored with drugs/vitamins, and should
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be considered a method for manufacturing feed supplements (Fig. 10). Because Fe, I, Zn, Ca,
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Mg, Cu and Se are often deficient in the human diet, these particular elements are important to consider when manufacturing feed-additives [135].
Fig. 10. Feed-additive applications of post-sorption technology Several challenges still exist for the development of feed supplements manufactured through biosorption technology. Manufacturing biosorbents enriched with the optimal concentration of trace elements is one of the greatest challenges. Before feed can be 51
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manufactured, it is essential to identify the deficiency and sufficiency levels within the diet for each type of animal [350]. Even though most studies have focused on enrichment with a single
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trace element, clearly it is essential to develop techniques to manufacture multi-mineral nutrients
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to meet accurate complex nutrient demands. Additionally, emission of ammonia from manure is
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a significant environmental challenges faced by the poultry industry. Supplementing dietary trace minerals (eg. zinc) may be one solution, because it is possible to reduce both nitrogen excretion
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and emission. In addition, mineral supplementation inhibits microbial uricase activity, which
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then decreases the production of ammonia [375]. Feed supplementation with zeolite could be particular useful as it has high selectivity for ammonia [230]. Farming strategies can be
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developed to include custom tailored post-sorbent that is rich in the specific trace elements or a
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combination of elements needed for each particular type of animal. Moreover, it is also essential
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to identify the sorbate (trace mineral)-sorbent (biosorbent) interactions and their metal complexation form. It is conceivable to regulate this supplementation and to determine the metalreleasing performance for each. Analytical determination of metal toxins in animal feed is often challenging due to the complexity of the feed matrices and extremely low concentrations of metal ions. This should be considered an important issue in manufacturing feed-additives. One of the major problems with feed additive use is that, as in humans, elevated levels of particular elements can lead to acute health problems in animals. Furthermore, consumption of the food materials (meat, eggs, milk and dairy products) derived from these animals may severely affect human health. Thus, the safety and toxicity of feed supplements should be thoroughly assessed prior to their application and supplementation should be carefully controlled, with particular care taken to measure any toxic effects of trace elements in that may 52
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be present in high quantities. Some biomaterial feedstocks contain potentially toxic elements, such as Pb, Cd, Hg, and As, that are harmful to animals and subsequently to humans [376]. For
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instance toxic effects of organic arsenical compounds that were widely used in poultry feed were
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well reported [377-379]; inorganic arsenic was found in meat and manure [378, 380]. Yao et al.
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[381] reported the quantitative delivery of roxarsone via chicken diet to rice plant and their subsequent environmental and health issues. Thus, before biosorbent feed is utilized, a complete
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elemental composition analysis should be carried out. Global meat consumption has undergone
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dramatic changes (increased), and it has been estimated that there will be a growing demand for animal-derived products and meat in coming decades [382, 383]. Meat is an excellent carrier of
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trace elements that provides them in an organic form [384, 385]. Thus, enrichment of meat with
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of investigation.
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supplements as a means of transfering essential elements to humans via the food-chain is worthy
Several international regulations address the impact of trace minerals on living organisms, providing stringent regulations on maximum supplementation levels [386]. The European Union, according to the Commission Regulations (EC) 1334/2003, has reduced the recommended trace element levels in animal feed in recent years to address the importance of food safety and the environment [387]. In the EU, animals intended for slaughter are permitted to have maximal levels of cobalt, copper, iron, iodine, manganese, selenium, and zinc in feed at 2, 25-35, 750, 10, 150, 05 and 150 mg/Kg, respectively [383, 388]. Furthermore, the toxic effects of certain metals, such as chromium, vary with oxidation states, so this should be taken into consideration [389, 390]. The use of organic arsenical compounds in broiler chicken industry as feed-additives was well-known; however, considering their toxic effects the US FDA banned 53
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arsanilic, carbarsone and roxarsone in 2013 [391]. Antibiotics used to boost growth of farm animals are now being regulated, and no trace of the parent compound or its metabolites may
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remain in food products, as per the European Union feed additive legislation (Council Directives
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70/524/EEC and 96/51/EC) [392]. Milk, cheese, meat, and eggs are a major food source.
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Because levels of trace minerals in these animals exist in their animal products, careful study and regulation is needed when manufacturing trace mineral laden sorbents (feed-additives), and in
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screening the produced materials to avoid food toxicity and environmental pollution.
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Although it seems there is a large scope available for the development of commercial feed additives through sorption-based technology still several limitations exists. Feed safety is
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most vital norm as the material used for animal feed should not be a threat to either animal or
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human health and also to environment. The most vital limitations in post-sorbent based feed-
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additives is the sorbed element should not exceed the maximum concentration levels if so it will causes damage to the animal and human health. In such case caution is required to dispose the manufactured feed safely otherwise it will contaminate the environment. Another most vital issue is some of the biomass-based materials used as sorbents are complex matrix that contains wide variety of elements and natural products thus complete sorbent analysis is essential. Loading of pharmaceuticals is needed additional care as it is well-known that pharmaceuticals undergo transformation depends on the environment, thus careful residual analysis is required to determine the presence of original drug or its metabolite residues in the animal meat or their excretions. Thus it is essential to carefully analyze the animal products to identify the presence of any harmful pharmaceutical by-products. Further research is required to study the risk assessment of post-sorbent based feed additives. Another significant limitation is cost of the feed 54
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SC
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manufactured through sorption-based technology should not exceed the available feed costs.
55
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SC
Goats
8.2% (Milk)b
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Estimated nutrient after feeding
Mn(II)
5.92±0.77a
Zn(II)
9.06±1.18a
14.6% (Milk)b
Fe(II)
11.9±1.5a
-----
Zn(II)
-
Cu(II) Cu(II)
Soya bean meal
Cu(II)
MA
11.7±1.5a
Spirulina maxima
Soya bean meal
Feeded Animal /Time
Cu(II)
PT ED
Macroalga Enteromorpha sp.
Post-sorption applications
CE
Soya bean meal
Micronutrient Content of microelement in feed
Reference
[353]
29.2% (Milk)b
Pig/3 months
>9.5% (blood)b
[357]
-
Pigs/87 days
18.3% (Meat)b
[358]
11.7±1.5a
Pigs/91 days
10.4% (Loin)b
[393]
-
AC
Sorbent
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Table. 4. Sorption based feed additives (nutrient-loaded adsorbents) and their characteristics
Fe(II)
11.9±1.5a
20.4% (Loin)b
Mn(II)
5.92±0.77a
9.0% (Loin)b
Zn(II)
9.06±1.18a
7.8% (Loin)b
Cu(II)
15.690 ± 0.370a Laying hens
-
56
[394]
Fe(II)
16.337 ± 0.228a
Cr(III)
20.588 ± 0.212a
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14.088 ± 0.403a
Cu(II), Zn(II), Mo(II), Co(II), Cr(III)
Laying hens
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Macroalgae
Zn(II)
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a= mg/g; b=percentage increase compared with control.
57
Cu-56%, Zn- 171%, Cr-723% [395] (Egg white)b
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2.4. Catalysts Catalysis is a key step that can be modified for developing sustainable practice necessary
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to produce chemical compounds [396]. Heterogeneous catalysis is an effective technique that
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involves the fabrication of a stable catalyst on a solid support [397]. The use of metal catalysts,
SC
such as Ni, Ru, and Pd, on a homogenous solid catalyst support enhances the catalytic activity [398-401]. Solid supports have often been required to prepare highly disperse metal catalysts,
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and they have several advantages, including recovery, regeneration, and reuse [402]. Adsorption
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has been used to adhere metal ions onto solid adsorbents, and these post-sorbents containing metal ions that have catalytic oxidation properties may be useful for the treatment of organic
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contaminants. In recent years, there has been much interest in reusable catalysts, which can be
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easily separated and easy to recover. Among these, magnetic and reusable catalysts have gained
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attention. Magnetic adsorbents have been widely used for water treatment, and these may exhibit exceptional sorption capacity for various metal ions. These metal- loaded magnetic sorbents can be efficiently used as catalysts, and therefore several attempts have been made to use metalloaded adsorbents for this purpose. Post-sorbents that had been loaded with metal ions have also been utilized as catalysts for the degradation of various toxic pollutants. Wu et al. [403] manufactured nickel-sorbed porous graphene oxide/sawdust composite (NiO@GNCC), and the post-sorbent was reduced by NaBH4 to create a catalyst this was further used for degradation of phenol in aqueous solutions. A high sorption capacity of 158.98 mg/g for phenol was accomplished using NiO@GNCC; however, reusability and stability of the catalyst was not studied. Biomass, such as fly ash and sawdust, has been used to recover nickel ions from wastewater, and then post-adsorbent was 58
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reused for the degradation of 2-chlorophenol [404]. Prior to use as catalyst, the post-adsorbent was dried, blended, and calcined. Improved surface area was observed for the catalyst compared
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with fly ash which plays a key role in the catalytical activity. Naturally, abundant fir sawdust has
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been used to preload Ni(II)/Fe(III), and then the sorbed biomass was pyrolyzed to obtain Ni-
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NiFe2O4/CNF catalyst [405]. Fast pyrolysis of metal preloaded biomass resulted in magnetic NiFe2O4, Ni(0) nanoparticles, and carbon nanofibers. This manufactured catalyst exhibited good
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performance in the hydrogenation of aromatic nitrobenzene to aniline without detectable by-
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products azobenzene or azoxybenzene. Catalyst also showed good reusable performance; even after 7 reused cycles no significant decrease in aniline yield was observed. High aniline yield
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(~100%) was accomplished with Ni-NiFe2O4/CNF, whereas individual Ni NPs, NiFe2O4, and
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CNF exhibited lower yields such as 53%, 29%, and 14%, respectively. Most of the sorption
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studies focused on the removal of single metal pollutant. However, wastewaters often contain metal mixture of both cationic and anionic metal ions. Considering this an attempt is made to remove cationic (Cu2+) and anionic (CrO42-) pollutants by using layered double hydroxide adsorbent [406]. The adsorbent was useful for treating both metal ions due to the synergetic effect between both Cu and Cr ions during sorption process. Further spent Cu/Cr loaded sorbent has been used as a catalyst for MO degradation. The catalytic activity of sorbent, Cu loaded sorbent, Cr loaded sorbent, and Cu/Cr loaded sorbent were evaluated and found that Cu/Crloaded spent sorbent was effective with the highest degradation (86.5%). In another approach, bacterial biomass was used as precursor to manufacture biomass-Pd catalyst and its catalytic efficiency towards Cr(VI) reduction was found to be 90% of Cr(VI) reduced bioPd D. desulfuricans and bioPdE. Coli after 30 min [407]. Catalysts manufactured through retrieval of metal ions from 59
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aqueous solutions were also used for oxidation of air pollutants, including volatile organic compounds. In one study, Cr(VI) loaded Arthrobacter viscosus supported on NaY zeolite was
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obtained through sorption experiments [408]. The manufactured materials were calcined at 400 o
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C in air flow and the prepared catalyst was used for the oxidation of ethyl acetate, ethanol and
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toluene in the following sequence: ethanol>ethyl acetate>toluene. Similar Cr-loaded sorbents were reported and used as catalysts for the catalytic oxidation of 1,2-dichlorobenzene [409] and
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cyclohexene [410]. These studies suggest that spent sorbents can be used as efficient catalysts for
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degradation of organic compounds and, in addition, most were easy to separate after the catalytic process was complete.
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Although the post-sorbents have shown good catalytic performance, still few limitations
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exist which hinders their practical applications. Most of the sorbent materials are prepared from
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expensive raw materials through complex synthetic routes to enhance the overall catalytic process. The researcher should focus on the fabrication of economic catalysts by utilizing lowcost biomass waste (sorbent) materials and utilizing metal (sorbate) contained in wastewater through facile synthetic routes. In addition to the studies in a laboratory scale, further pilot-scale studies are required to bring this technology to industrial scale. Major limitation that confines the practicality of this approach is the presence of toxic metal ions and nanoparticles in the postsorbent, which may be released into the adjacent environment during catalytic processing and after their usage (storage and landfills). Leaching of toxic metal ions and nanoparticles from the adsorbents and catalysts has been well reported [81, 411]. Leaching of metal ions causes the generation of Mn+ ions and sometimes development of nanoparticles. Among the various metal ions used in catalysis often nickel and copper are considered to be sustainable and green. 60
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However, Egorova et al. [411] discussed that these metal ions are also toxic. Safe storage or disposal of the spent catalysts comprising metals or nanoparticles is of high concern due to the
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stringent environmental legislations. For instance, USEPA classified spent catalysts as hazardous
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waste due to the existence of the considerable amount of heavy metal ions [412]. To overcome
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this limitation, it is feasible to fabricate Ca-based catalysts that are non-toxic and economical by loading Ca onto adsorbents [413].
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2.5. Bioactive compounds
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Along with the applications described thus far, metal-loaded sorbents have also been shown to be useful for other functions, including biological activity. Most of the post-sorbents
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exhibit disinfection properties and other shielding biological effects. These biological effects
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have been attributed to the constituent metal ions, particularly Cu, Zn, and Mn. As a result of these observations, post-sorbents have been evaluated to determine their antimicrobial and
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antioxidant effects, and here we summarize those that have shown interesting biological effects. Knowing that silver is a potent antimicrobial, biochar loaded with Ag+ was evaluated for its antimicrobial ability and found to efficiently inhibit the growth of Escherichia coli, whereas biochar without silver adsorbed promoted the growth of Escherichia coli [414]. In another study, Cu was loaded onto chitosan/nano Al2O3, and exhibited good antimicrobial activity against E. coli as compared to chitosan and chitosan/nano Al2O3 alone [415]. The effect was attributed to the presence of Cu2+, which has antibacterial properties. The adsorbent was also used for removal of As(III) with a sorption potential of 18.32 mg/g at pH 7. Bioactive ceramic hydroxyapatite [Ca10(PO4)6(OH)2] was used to retrieve chlorhexidine digluconate through an adsorption process, and the post-sorbent inhibited Enterococcus faecalis growth in vitro for up to 61
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6 days [416]. Serbian clinoptilolite was used for the adsorption of Zn(II) and exhibited maximum sorption capacity of 16.8 mg/g [417]. In the presence of air, the post-sorbent was thermally
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treated at 540 oC to produce ZnO-clinoptilolite. Both materials, Zn-clinoptilolite and ZnO-
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clinoptilolite, were tested against Acinetobacter junii, and the former exhibited good antibacterial
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activity. Four different algal biomass powders were loaded with zinc, copper and silver through sorption equilibrium and tested for their antimicrobial properties against Escherichia coli [418].
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Algae loaded with Ag had a strong bactericidal effect than Zn-loaded algae and algae alone. The
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microalgae Chlorella vulgaris loaded with polyphenols was evaluated based on antioxidant activity, and biomass enriched with quercetin has highest antioxidant activity [419]. These
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interesting findings suggested that post-sorbents that comprise biologically active metal ions,
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particularly Ag, Cu, and Zn can be employed as antibacterial, antifungal and antioxidant agents.
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Although post-sorbents have shown antimicrobial activities, further research is required to assess the biocompatibility and toxicity of these materials of these post-sorbents in various applications. 2.6. Miscellaneous applications
Other than the above applications focused on post-sorbents, few studies explored the recovery of adsorbate after post-sorption. Considering the cost of critical metals and their resource limitations, several studies have focused on the recovery of metal ions from postsorbents using incineration or pyrolysis techniques [420-424]. Bark species were used in one study for the removal of copper and incinerated in the presence of air and nitrogen [420]. A thermogravimetric and X-ray powder diffraction (XRD) analysis of incinerated ashes indicated that CuO (≈80%) and Cuo (≈14%) were present in the produced solid. In another study, olive solid waste generated from an olive oil mill was used for copper and nickel sorption, and 62
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accomplished a sorption capacity of 3.6 and 1.7 mg/g for copper and nickel, respectively [421]. Metal-loaded biosorbent was burned at 850 oC in an oven and the resulting ashes contained 96%
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of each metal. Desorption is the most often used conventional process for the recovery of sorbate
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from adsorbent, however, this process requires a large amount of desorbing agent. This will
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invariably increase cost of sorption based technology, besides secondary pollution is possible through this approach. Liu et al. [425] made an attempt to understand the technical and economic
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feasibility of biosorption-pyrolysis technology compared with biosorption-desorption process for
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lead recovery. The results demonstrated that biosorption-pyrolysis technology cost $ 0.06 per ton wastewater is more beneficial compared with biosorption-leaching process which costs $0.19 for
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HCl and $ 4.41 for EDTA to treat per ton wastewater. Considering the importance of precious
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metal ions in a wide variety of applications and their resource limitation, our group investigated
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gold and ruthenium recovery after post-sorbent incineration [422, 423]. We successfully recovered 99.75% of Ru and more than 91% of gold through a sorption-incineration approach. In another study, the precious metal platinum was recovered using an amine modified corn husk, followed by calcination of the post-sorbent at 800 oC for 12 h [426]. The results revealed that 54% of the calcined powder was platinum. Dye-loaded biosorbent has been used to manufacture biochar material [54]. Through a pyrolysis technique, dye-laden biosorbent was further converted into biochar, which was then used for methylene blue (MB) removal. Biochar showed good removal performance at lower initial MB concentration when compared to other common biosorbents. In another study methylene blue loaded carboxymethyl cellulose was used for methyl orange (MO) removal, MB loaded CMC exhibited high sorption efficiency of > 100 mg/g for MO and is much higher than 63
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CMC sorption efficiency for MO [427]. Post-sorbents have also been used for fuel cell technology. In a fuel cell, yeast biomass loaded with Pt (PtCl62-) was used as an electro-catalytic
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anode to generate electricity from glucose and ethanol [428]. Metal-loaded adsorbent was also
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used to obtain metal oxide nanoparticles; specifically, clinoptilolite was shown to remove Ni(II)
resulted in nano-sized NiO particles of ~5nm [429].
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ions from aqueous solutions. Thermal treatment of post-sorbent (Ni(II) loaded clinoptilolite)
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Interestingly, only a few post-sorbent applications in energy production have been
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proposed [52, 53, 430]. Energy is one the most vital 3E issues. It is a key factor for the economic development of all countries as industries and individuals alike are highly dependent on energy
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supplies. The amount of available petroleum fuels is diminishing, and there is a lack of
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acceptance of nuclear energy [431-434]. To address these problems and meet the future energy
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demands of growing populations and increasing industrial activities, utilization of alternative energy resources, such as wind, tide, solar, geothermal and biomass, is becoming more important [303]. Among these, bioenergy produced from biomass has become an alternative to fossil fuels with great potential to provide a stable source of power. For instance, in the United States, use of biomass-derived energy increased by 60% between 2002 and 2013 [435]. Considering the importance of energy production from biomass derived materials, few studies have been conducted to evaluate the post-sorbents for power generation. Converting biomass into valuable energy and chemical products through pyrolysis is one of the frontier thermochemical technologies; this process results in gas (pyrolytic gas), liquid (bio-oil), and solid (bio-char) [436-438]. Bio-oil and/or gas obtained through biomass pyrolysis are used for energy production. Several researchers made attempts to utilize the exhausted 64
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biosorbent (post-sorbent) to get valuable resources through pyrolysis, combustion and gasification process, while simultaneously reducing the volume of waste. However, biomass
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pyrolysis depends on various parameters such as biomass pretreatment, heating rate, reaction
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atmosphere, temperature and biomass source [439]. Exhausted or spent sorbent consists of metal
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ions which influence the biomass pyrolysis process. Thus it is essential to evaluate the sorbate effect on pyrolysis process. As an example, pine cone shell biomass was used for Pb(II) and
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Cu(II) removal from aqueous solution in a packed bed column [52]. Effects of Pb(II) and Cu(II)
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on thermal decomposition of the biosorbent were studied, revealing that these metals did not influence the major degradation pathways. Thermal decomposition of the metal-loaded
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biosorbent resulted in chars, with 95% and 99% recovery of copper and lead, respectively.
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Combustion of olive tree pruning biomass loaded with Pb(II) was studied in a thermobalance, a
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laboratory flow reactor and an atmospheric oven [53]. The total concentration of Pb(II) remains in the ashes suggesting that this does not influence the usage of biomass as fuel. However, high amount of Pb(II) >35% present in ash should need further treatment or applications should be explored considering the toxic nature of lead. Pine cone shells were used in one study to sorb copper from aqueous solution and achieved a sorption capacity of 6.81 mg/g before the postsorbent was thermally decomposed [430]. Both sorbent and post-sorbent exhibited similar pyrolysis and combustion behaviors, with major compounds being CO, CO2, CH4, C2H4, CH3OH, and SO2. These studies suggest the feasibility of application of metal loaded sorbents for energy production.
3. Conclusions and future perspectives Adsorption is one of the key techniques that have been used in separation science to 65
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retrieve various pollutants and analytes from the aqueous phase, and therefore, has attracted substantial interest for scientific and commercial applications over the past several decades. Due
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to the accessibility of widespread, naturally abundant adsorbent materials, as well as the
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emergence of new generation materials, much research has gone into exploring the use of these
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materials in water treatment applications. High sorption efficiencies, rapid removal rates, and selectivity are the chief properties to consider when designing adsorbents, whereas less attention
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has been paid to post-sorbent management or possible further utilization of post-sorbents. In
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recent years, there has been increasing interest in the development of sustainable technologies to minimize the cost of materials/techniques and to utilize resources more efficiently. Recent
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reports on sorption-based technology have shown that post-sorbents with multiple functionalities
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provide an opportunity for diverse applications. Post-sorption has been explored as a useful
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method for the manufacture of value-added key products such fertilizers, catalysts, nanoparticles, antimicrobial agents, and feed-additives. This sustainable materials production pathway is economically and ecologically appropriate. This review summarizes the relevant work on postsorption applications, strategies, challenges, reported applications, and issues surrounding toxicity. Major challenges involved in the efficient development of these materials for the above applications include the stability of the sorbent, controlling the release rate of an element from the sorbent, and toxicity issues. The major conclusions drawn are as follows: (i) Adsorption is often used to retrieve various pollutants from aqueous solutions using diverse adsorbent materials, and these could potentially be applied to the production of new value-added products. (ii) Natural, low-cost materials loaded with macronutrients, such as N and P, can be utilized as 66
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fertilizer with controlled slow-release rates for soil applications. (iii) Prior to the application of post-sorbent as fertilizer or soil conditioner it is essential to
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determine the levels of PTEs present.
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(iv) Feed-additives, manufactured using various types of biomass, including microbial biomass,
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can supply the essential trace elements required for animal husbandry.
(v) Carbonaceous metal nanoparticles can be synthesized using biomass laden with metal ions,
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and successfully used as catalysts and adsorbents.
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(vi) Dye-loaded adsorbents could be used to manufacture value-added biochar materials. The dye-loaded sorbent may be further mixed with metal additives to manufacture carbonaceous
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nanoparticles.
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subsequent adverse effects.
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(vii) Regulations should be followed closely when manufacturing feed additives to avoid any
(viii) Several other applications, such as energy and biological applications (antimicrobial and antioxidant), should be explored more fully.
Post-sorbent based functional materials will play a major role in the development of value-added green materials production in the near future. Adsorption technology can be used to engineer tailored post-sorbents that have a wide variety of functional elements/materials, as outlined in this review. More versatile, custom, and facile manufacturing strategies are expected to emerge in the future for the continuous advancement of post-sorbent applications. Though several challenges remain, the rapid development of post-sorbent applications in various fields foreshadows a bright future for sustainable adsorption technology. Since exploration of post67
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sorbent applications as value-added products has just begun, there is room for further studies in this area to reveal the opportunities that may exist. Designing and manufacturing green
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adsorbents using non-toxic and economical precursors will provide an opportunity for successful
Acknowledgements word
was
financially supported
by
the
Korean
Government
through
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This
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utilization of post-sorbents loaded with nutrients and elements as feed-additives and fertilizers.
NRF
(2014R1A2A1A09007378) grants. One of the authors D.H.K. Reddy acknowledges the BK21
List of abbreviations
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plus Postdoctoral Fellowship for financial support.
Biochar
CEMNs
Carbon encapsulated metal nanoparticles
CEMNPs CEN CMC EC
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CEC
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BC
Cation exchange capacity Carbon-encapsulated metal nanoparticles European Standardization Committee Carboxymethyl cellulose Commission Regulations
EU
European Union
FAO
Food and Agriculture Organization
FDA
U S Food and Drug Administration
Fe/Fe3C@PCS
Porous carbon sheets containing carbon-encapsulated Fe/Fe3C
IBI
International Biochar Initiative
Mag-PCMA
Magnetic permanently confined micelle arrays
MB
Methylene blue 68
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Metal nanoparticles
MO
Methyl orange
MWCNTs
Multiwalled carbon nanotubes
NiO@GNCC
Nickel sorbed porous graphene oxide/sawdust composite
NiFe2O4/CNF
Ni–NiFe2O4/carbon nanofiber
NPs
Nanoparticles
NUE
Nutrient use efficiency
PAHs
Polycyclic aromatic hydrocarbons
PSA–GO
Poly(sodium acrylate)–graphene oxide
PTEs
Potential toxic elements
SOC
Soil organic carbon
SWCNTs
Single walled carbon nanotubes
TCLP
Toxicity characteristic leaching procedure
USEPA
US Environmental Protection Agency
VOC
Volatile organic compounds
XRD
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WHO
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MNPs
World Health Organization X-ray powder diffraction
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Graphical Abstract
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Highlights Review of the valorisation of post-sorption materials.
Strategies required for tuning post-sorption materials are discussed.
Advantages and toxicity issues were highlighted.
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S0001-8686(16)30218-4 doi: 10.1016/j.cis.2016.12.002 CIS 1703
To appear in:
Advances in Colloid and Interface Science
Please cite this article as: Harikishore Kumar Reddy D, Vijayaraghavan K, Kim Jeong Ae, Yun Yeoung-Sang, Valorisation of post-sorption materials: Opportunities, strategies, and challenges, Advances in Colloid and Interface Science (2016), doi: 10.1016/j.cis.2016.12.002
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ACCEPTED MANUSCRIPT Valorisation of post-sorption materials: Opportunities, strategies, and challenges D. Harikishore Kumar Reddya, K. Vijayaraghavanc, Jeong Ae Kima, Yeoung-Sang Yuna,b* Division of Semiconductor and Chemical Engineering, Chonbuk National University,
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a.
Department of Bioprocess Engineering, Chonbuk National University, Jeonbuk 561-756,
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b.
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Jeonbuk 561-756, Republic of Korea.
Republic of Korea.
Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai
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C.
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600036, India
*
Corresponding author:
Prof. Yeoung-Sang Yun E-mail address: [email protected] (Y.-S. Yun) Tel.: +82 63 270 2308; Fax: +82 63 270 2306
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ACCEPTED MANUSCRIPT Content 1. Introduction
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1.1. Adsorption
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1.2. Post-sorbents
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1.3. Objective of this review 2. Post-sorption applications
2.2. Carbon- metal nanoparticles
2.4. Catalysts
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2.5. Bioactive compounds
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2.3. Feed additives
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2.1. Fertilizers & Soil conditioners
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2.6. Miscellaneous applications
3. Conclusions and future perspectives
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4. References
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ACCEPTED MANUSCRIPT ABSTRACT Adsorption is a facile, economic, eco-friendly and low-energy requiring technology
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that aims to separate diverse compounds (ions and molecules) from one phase to another
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phase using a wide variety of adsorbent materials. To date, this technology has been used
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most often for removal/recovery of pollutants from aqueous solutions; however, emerging post-sorption technologies are now enabling the manufacture of value-added key adsorption products that can subsequently be used for (i) fertilizers, (ii) catalysis, (iii) carbonaceous
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metal nanoparticle synthesis, (iv) feed additives, and (v) biologically active compounds.
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These new strategies ensure the sustainable valorisation of post-sorption materials as an economically viable alternative to the engineering of other green chemical products because
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of the ecological affability, biocompatibility, and widespread accessibility of post-sorption
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materials. Fertilizers and feed additives manufactured using sorption technology contain elements such as N, P, Cu, Mn, and Zn, which improve soil fertility and provide essential
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nutrients to animals and humans. This green and effective approach to managing postsorption materials is an important step in reaching the global goals of sustainability and
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healthy human nutrition. Post-sorbents have also been utilized for the harvesting of metal nanoparticles via modern catalytic pyrolysis techniques. The resulting materials exhibited a high surface area (>1000 m2/g) and are further used as catalysts and adsorbents. Together with the above possibilities, energy production from post-sorbents is under exploration. Many of the vital 3E (energy, environment, and economy) problems can be addressed using postsorption materials. In this review, we summarize a new generation of applications of postadsorbents as value-added green chemical products. At the end of each section, scientific challenges, further opportunities, and issues related to toxicity are discussed. We believe this critical evaluation not only delivers essential contextual information to researchers in the field but also stimulates new ideas and applications to further advance post-sorbent applications. 3
ACCEPTED MANUSCRIPT Keywords: Post-adsorption; post-sorbent; exhausted sorbent; spent sorbent; adsorption; biosorption; fertilizers; feed-additives; catalysis; carbothermal synthesis; nanoparticles; water
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treatment; aqueous medium
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1. Introduction
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Development of sustainable materials and technologies is a currently a popular research area in many fields of science. The three major factors that drive this area of research are: (i) environment, (ii) economy, and (iii) depletion of natural reserves. Due to the
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rapid population growth worldwide, there is a high demand for various commercial goods and products. A large number of these products contain metals (Fe, Cd, Cu, Pb, etc.) and
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materials (Nano, plastic, etc.) that contribute to significant environmental pollution problems when they are discarded or damaged [1]. One notable example is the vast quantities of “e-
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waste” generated globally from discarded electric and electronic equipment, which will build
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up even more rapidly in the near future [2]. This waste contains various toxic metal ions, such
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as Al, Fe, Pb, Cd, Co, Cu, La, Ni, and Hg along with traces of rare earth and precious elements such as Au, Ag, Sn, Se, Te, Pt, Pd, Ta, Co, and In [2-6], which, when released into
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the environment, cause severe health problems and environmental issues either directly or indirectly. Additionally, nanomaterials/nanoparticles used in various commercial products have become a serious concern [7]. Nanomaterials are one of the key ingredients in products such as sunscreen, cosmetics, photovoltaics, liquid magnets, automotive coatings, and more [8-10]. There is an ongoing debate about the safety and toxicity of nanomaterials because their fate and behavior in air, water, and soil environments have not been entirely examined [11-20]. Iron (Fe) is one of the technologically important key elements with different valence states (0, II, III, IV, V, and VI) and its polymorphs such as Fe2O3 and FeO(OH) have been widely used around the globe for various industrial applications [21]. For instance, iron is commonly used in steel production by combining with other alloying elements [21, 22]. 4
ACCEPTED MANUSCRIPT Magnetic iron-based materials are of particular and emerging interest because of their facile separation property, and these materials have been used in various fields such as water
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treatment [23, 24], food analysis [25], photocatalysis [26], electrochemical biosensing [27],
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magnetic hyperthermia [28], drug delivery [29], sensors [30]. Nonetheless, there is still
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continuing discussion about the safety and toxicity of iron oxide nanoparticles [31-33]. Hightech materials are also of economic concern because the manufacturing of most of these products requires very costly raw materials. Furthermore, the cost of these materials is
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increasing annually due to high demand; thus, it is important to find sustainable ways to reuse
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or recover these materials from products after their use. Additionally, because most of these materials are manufactured using naturally occurring resources, reserves of these resources
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are limited, and are rapidly being depleted [34]. For example, natural reserves of metal ions
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widely used in technology manufacturing have measurably decreased [35]. Although there are still untapped reserves, such as sea water, that could be a source of various critical metal
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ions [36], environmental, technical and economic issues limit the practicality of mining from these sources [37-40]. The need for sustainable technologies and materials is more essential
issues.
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than ever in order to overcome these environmental, economic, and resource availability
Adsorption is one of the most successful techniques in the field of separation science. Diverse solid materials (adsorbents) have been used to remove or recover a wide range of contaminants, including metal ions, dyes, organics, and pharmaceuticals (adsorbates) (Fig. 1). Charcoal and activated carbon are the most popular and widely used adsorbents for removing various contaminants and remain at the forefront for commercial applications. These materials can be disposed of easily, or sent to a landfill after they are no longer useful. However, although carbon-based materials show a high efficiency of binding organic pollutants, they have a limited tendency to bind inorganics. To overcome this limitation and 5
ACCEPTED MANUSCRIPT to improve their performance, researchers have focused on the development of new and diverse adsorbent materials, such as zeolites, biosorbents, polymers, metal oxides,
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nanomaterials, chemically modified adsorbents, and others [41-48]. Most of the research into
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these adsorbent materials has focused on improved performance and increased removal rates,
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whereas less emphasis has been placed on the storage, disposal, and reuse of these adsorbents once they are spent. Some of the newer-generation adsorbent materials used in water treatment have been examined for possible toxic effects on living organisms [17, 49-51]. Also
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of concern is the high cost of manufacturing some of the adsorbents due to the use of
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expensive raw materials. Thus, in recent years, researchers have focused on the use of postsorbents, which are sorbents with bound materials, for various applications to meet
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environmental, energy, agriculture, and dietary needs [52-55].
Fig. 1. Numbers of publications focused on adsorption of metal ions, dyes and pharmaceuticals (data from Scopus® search date: 14-06-2016)
6
ACCEPTED MANUSCRIPT 1.1.
Adsorption
Adsorption is a naturally occurring phase transfer process and is widely used for the
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removal of adsorbates (molecules or ions) from fluid phase (gases or liquids) using adsorbent
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materials (typically solid). Using adsorption to remove notorious toxic pollutants from the
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liquid (aqueous) phase has been one of the most widely employed techniques to obtain clean water for drinking and for industrial purposes. Although the term “adsorption” was not coined until 1881, the process itself has been used for many years. Egyptians and Sumerians used
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charcoal for the reduction of copper, zinc, and tin ores in the manufacturing of bronze around
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the year 3750 B.C. [56] and in 1550 B.C., the Egyptians applied charcoal for medicinal purposes [57]. In the year 1881, the term adsorption was introduced into the literature by H. Kayser, at the suggestion of E. du Bois-Reymond [58, 59]. Prior to this, both absorption and
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the adsorption phenomenon were referred to as “absorption” [60]. The tremendous interest in adsorption has led to numerous publications including research reports, reviews, monographs
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on historical evolution, and information about mechanisms and applications of adsorption many of which are referred herein [56, 61-66]. Although adsorption can be used for a variety
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of applications, it has historically been used primarily for wastewater treatment to address both strict environmental regulations and the growing level of contaminants (heavy metals, dyes, pharmaceuticals, micro pollutants, biological and plastics) in water. Progress in designing and developing novel adsorbents with multi-functional properties has made adsorption a key technology in wastewater treatment (Fig. 2). The goals for development of these materials have focused on achieving a high sorption capacity, a rapid sorption rate, and high affinity. Therefore, the new generation of adsorbents now includes nanomaterials, composites, metal-organic frames works, and layered-double hydroxides. Review articles published in the past year have described the removal of organics, metal ions and dyes, and the overall number of reviews on adsorption is indicative of the importance of this field [677
ACCEPTED MANUSCRIPT 87]. Individual reviews discuss the performance of particular adsorbent materials such as aerogels [76], biochar [68], biochar-based nano-composites [88], carbon nanotubes [71],
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chitosan [69], coal-based adsorbents [89], composite fibers [77], engineered nanomaterials
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[90], hydrogel [91], layered double hydroxides [67], magnetic chitosan composites [92],
palygorskite
[74],
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metal-organic frameworks [75, 85, 86], nanoadsorbents [70], nanocomposites [78, 79, 83], polymer-functionalized
nanocomposites
[83],
polypyrrole-based
adsorbents [73], and spinel ferrites [81]. These reviews primarily describe the efficiency and
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performance of adsorbent materials in water treatment, and none of these discuss post-sorbent
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applications. Biomaterials (plant, microbial and agricultural wastes), which when used for adsorption are known as biosorbents, are also widely used to address contaminants problems
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because they are abundant in nature, low-cost and easily available. The process of biosorption
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is a very popular technique and involves the use of solid biomass for the removal/recovery of various toxic pollutants through various physico-chemical mechanisms. Although there are
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slight differences between adsorption and biosorption, we will use the term adsorption to refer to both processes throughout this review because the intention is to focus on post-
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sorbent applications of both.
Fig. 2. The development of adsorbents
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ACCEPTED MANUSCRIPT 1.2.
Post-sorbents Two separate terms are used to describe sorbents after they have been used: post sorbents
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and spent (exhausted) sorbents. Post-sorbent is a term used to describe a solid adsorbent
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concentrated with adsorbate ions or molecules that have been removed from dilute aqueous
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phase through the adsorption process. Spent or exhausted sorbent is the solid waste generated after adsorbate has been regenerated or recovered from adsorbent. The major drawback of adsorption is that this is a nondestructive technology; the adsorbate does not degrade or
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disappear when the adsorbate is transferred from aqueous to solid phase. Thus, post-sorbent
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(adsorbate-laden sorbent) management is of utmost importance for the sustainability of adsorption technology. Current options for managing post-sorbents are limited to: (i) recovery
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of the sorbate, (ii) disposal in a landfill, and (iii) safe storage or disposal. Landfill disposal of
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post-sorbent (laden with toxic contaminants) leads to secondary pollution problems, such as soil, underground water or surface water contamination through natural leaching or
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desorption [54]. Recovery of the sorbate (contaminant) from the sorbent is possible; however, this process has several disadvantages. Regeneration of adsorbents is dependent on factors
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such as the type of adsorbent, the adsorbate itself, adsorbate-adsorbent interactions, the treatment process, and the cost of the regeneration process. Most of the techniques used for sorbent regeneration, including the use of thermal energy [93], microwave irradiation [94], ultra-sonication [95], dielectric barrier discharge plasma [96], and others, have notable limitations. For example, spent activated carbon can be regenerated through thermal treatment, which reduces the active surface area and decreases the adsorption efficiency in subsequent adsorption cycles. When a harsh chemical compound, such as an acid or alkali, is used in the regeneration process there are both economic and environmental consequences. For example, regeneration of a fixed-bed granular activated carbon operation comprises 75% of the total operation and maintenance cost [97]. Although adsorbents are recycled, 9
ACCEPTED MANUSCRIPT inadequate or incomplete removal of the toxic adsorbate can occur. Additionally, stable adsorbent-adsorbate interactions limit the effectiveness of the recovery process when
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conventional solvents (acids, bases and organic solvents) are used [98].
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The two methods used to manage both post-sorbents and spent (exhausted) sorbent wastes
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are landfill disposal and incineration [99]. The factors that influence these methods include: (i) cost of the adsorbent, (ii) pollutant type, (iii) operating cost, (iv) cost of the combustion and incineration plant, and (v) fees for disposal. Although landfills have typically been used
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for the disposal of sorbents, this method has a subsequent pollution risk when toxic
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compounds leach from adsorbents into the soil. With this in mind, strict environmental regulations have been put in place. Both Spanish and U.S. EPA waste disposal guidelines,
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prevent the disposal of metal- laden ashes in landfills [100]. Some of the newly developed
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adsorbents, such as nanoparticles, metal oxides, and composites, are themselves composed of toxic materials and metal ions. These have often been used as high-efficiency adsorbents for
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treating a variety of contaminants because of their high sorption capacities and rapid rates of adsorbent removal. The safety of these materials is a topic of debate because most cause
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problems when they enter air, soil, and water environments. Researchers have paid more attention to technology development and less attention to safe treatment and effective management of post-sorbents and spent adsorbents. Adsorbent cost is another important issue because most of the emerging adsorbents are in the form of composites and hybrid materials that comprise two or more distinct cost-effective materials. For example, nanomaterials such as graphite/graphene, single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWCNTs) have been used to fabricate composite adsorbents [101-104], and the cost of these materials is in the range of 0.10 to 1000$ per gram depending on their quality, grade and functionalization [90, 105]. The cost of some of the most commonly used adsorbents are listed in Table 1 and are estimated 10
ACCEPTED MANUSCRIPT primarily from the cost of raw materials alone. A true estimation of the overall cost of an adsorbent includes the cost of raw materials, energy, labor, and processing. Additionally, safe
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management (storage or disposal) costs should be taken into consideration as well. Adeleye et
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al. [90] thoroughly describe the costs and environmental issues regarding engineered
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nanomaterials used in water treatment processes. Adsorbent cost is one of the key factors that influences real-world industrial applications. As most of the emerging adsorbents are
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useful application of these materials (Fig. 3).
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expensive to produce and some are potentially toxic themselves, reutilization is essential for
Fig. 3. Schematic of pollutant removal using adsorbents and post-treatment options
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ACCEPTED MANUSCRIPT Table 1. Estimated costs of adsorbents Cost ($/kg)
Reference
Mag-PCMA
70a, ~4b
[90, 106]
Biochar
0.08-13.48
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Adsorbent
Activated carbon
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Granular activated carbon MWCNTs
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Graphene
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Modified Ficus carica fibers
PSA–GO
1-22
[108-110]
3.30-6.71
[111, 112]
6000
[111]
990
[111]
0.2
[113]
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Cost ($/t)
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Polyampholyte hydrogel
[107]
[114]
6.93 × 103
[115]
8.08 x 103
[115, 116]
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Amino siloxane oligomer-linked graphene oxide
6.3×103
*a=Laboratory scale, *b=factory scale
1.3.
Objective of this review A considerable amount of progress has been made in adsorption research with the aim of
removing toxic pollutants from wastewater using a wide variety of adsorbent materials [80, 102, 117-119]. For an adsorption method to be sustainable, regeneration of the post-sorbent is a particularly important step. However, most of the existing technologies require harsh chemicals, energy, and processing steps, and may limit the lifetime of the adsorbent itself. 12
ACCEPTED MANUSCRIPT Thus it is essential to identify alternative ways to utilize post-sorbent materials as a valueadded product. Numerous reviews have been published describing adsorption [68, 81, 92,
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120-124], but most focus on the performance of materials, sorption isotherm and kinetics, and
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mechanisms. Safety management and reuse or disposal problems associated with post-
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adsorbents or spent adsorbents have seldom been addressed. In fact, to date few studies are addressing the management of spent regenerated solutions, spent adsorbents and exhausted sorbent applications [54, 125, 126]. Feng et al. [54] discusses the disposal problems
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associated with spent adsorbents and their management. Gómez-Pastora et al. [125] reviews
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the management of spent magnetic adsorbents through regeneration and reuse. Dodson et al. [127] discusses the value-added applications of metal-loaded sorbents. It is necessary to find
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sustainable pathways for effective utilization of pollutant-loaded adsorbents as value-added
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products to resolve the economic, toxic and management issues associated with these materials. In this review, we propose alternative strategies for the effective utilization of post-
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sorbents based on their emerging applications (Fig. 4).
Fig. 4. Past and present status of post-sorbents
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ACCEPTED MANUSCRIPT 2. Post-sorbent applications 2.1. Fertilizers and soil conditioners
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World food resources depend primarily on agriculture, and it is predicted that global
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agricultural production must increase by 70% by the year 2050 [128] to meet the demands of
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a rapidly growing world population (9.7 billion by the year 2050) [129-132]. To confront this global food demand, modern farming techniques that maximize agricultural production have been employed; however, these lead to depletion of carbon and nutrients in the soil. Crops
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cultivated in these overused soils become less productive [133] and they cannot provide the
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essential elements required for a healthy human diet, and therefore lead to cases of microand macro malnutrition [131, 134, 135]. Dimpka et al. [136] comprehensively discusses the
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importance of micronutrients in effective agronomic production. Billions of people around
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the globe suffer from an insufficient intake of micronutrients due to mineral-deficient food crops [137]. Micro- and macro- malnutrition lead to diverse health effects, including mental
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retardation a weakened immune system and, an overall decrease in health [138]. A conference held in 2014 by the FAO/WHO (Rome, Italy) emphasized that dietary deficiencies in
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essential micronutrients affects more than two billion individuals around the world, with women and children particularly at risk [139, 140]. At the conference, a Rome Declaration on Nutrition was put formulated, with one goal being “the elimination of malnutrition in all its forms”. A reliable and appropriately balanced supply of micro and macronutrients is essential to enhance plant growth and to overcome malnutrition problems [141]. Fertilizers have been widely used to enhance crop yields. Fertilizers are typically comprised of six macronutrients, including calcium (Ca), magnesium (Mg), phosphorus (P), nitrogen (N), potassium (P), and sulfur (S), and eight micronutrients including boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), zinc (Zn) [142]. The nutrients (micro or macro) are applied in either organic or inorganic form to 14
ACCEPTED MANUSCRIPT soils that are deficient. The use of fertilizer in agriculture is likely to increase considerably in the future [143]. The IFA (International Fertilizer Association) [144] website reports that the
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global use of nutrient fertilizer (nitrogen, phosphate, and potash) is estimated at 186 million
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tons, and will reach around 200 million tons by the year 2018.
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Several studies demonstrate that excessive usage of fertilizers leads to the release of high concentrations of compounds such as nitrogen and phosphorus into the environment [143, 145-147]. The enrichment of these nutrients in particular ecosystems can cause
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eutrophication, or the excessive richness of nutrients in the water, likely due to land runoff.
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This is detrimental to the environment as it results in a lack of oxygen that promotes plant formation while increasing animal mortality [34, 146, 148]. The raw resources required for
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the production of N and P fertilizers are non-renewable. Furthermore, the limited natural
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reserves of these materials are being exhausted due to their widespread use as fertilizers [149151]. Thus effective utilization of fertilizers applied to the crops is essential.
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A limited supply of nutrients leads to low crop production, soil degradation, and poor human nutrition; in contrast, high concentrations lead to environmental pollution [147]. The
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nutrient use efficiency (NUE) of crops is poor under realistic conditions. According to the IFA, in the initial year of nutrient fertilizer application, the utilization rate of both N and K is about 50-60%, while only 10-25% for P [152]. The remaining unused nutrients are destined to degrade, release or leach, all of which may cause surface and groundwater pollution. The major disadvantages of using conventional fertilizers are the low efficiency and excessive waste, thereby resulting in numerous severe environmental and health complications. Thus, for the sustainable management of fertilizers, it is critical to develop slow and controlledrelease technologies (Fig. 5). The European Standardization Committee (CEN) Task Force stipulated that slow-release fertilizers should meet the following conditions at a temperature of 25 oC: (i) no more than 15% release in 24 h, (ii) no more than 75% release in 28 days, and 15
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(iii) at least 75% released by the stated release time [153].
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Fig. 5. Conceptual nutrient use efficiency of slow and rapid release fertilizers
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Various techniques have been explored for the purpose of regulating the quantity of nutrients that discharge into water. Of these, adsorption is one of the more proficient and economical practices and has the following benefits: ease of handling, a wide choice of adsorbents, minimal operation costs, less sludge release, and fewer disposal complications. Adsorption is the only technology for which it is feasible to use nutrient-laden adsorbents as fertilizer and soil conditioner in agricultural systems [154]. Compared to conventional fertilizers, sorption-based fertilizers offer several advantages: (i) soil-deliverability, (ii) controlled nutrient release, (iii) fewer adverse effects, (iv) biocompatibility, and (v) excellent nutrient use efficiency. Manufacturing fertilizers by sorption technology would be more sustainable and economic because, precursor materials used for nutrient loading are 16
ACCEPTED MANUSCRIPT renewable (biomass, zeolites, biochar and manure) and available at low or no cost. However, the successful application of adsorbent as fertilizer relies heavily upon the choice of
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adsorbent materials because these will define the relevant properties, such as nutrient loading
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capacity, cost, nutrient release rate, adaptability to soil conditions, and water
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retention/holding capacity. Several specific adsorbents could be fabricated to load both cationic (NH4+, K+), anionic (NO3-, PO43-), and trace mineral (Cu, Fe, Mn and Zn) nutrients in
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aqueous solutions, which could be successfully applied as fertilizer (Fig. 6).
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Fig. 6. Fertilizer applications of post-sorption technology
One of the most successful adsorbents used for fertilizer applications is biochar, a solid carbonaceous porous product acquired by partial combustion of organic materials using modern pyrolysis techniques [55, 155-157]. The biochar pore structure directly contributes to the soil porosity providing water, nutrient, and mineral storage and accessibility. Biochar exhibits typical pore structure which might be divided into three kinds such as macro, meso, and micropores as shown in Fig. 7. This division is based on pore sizes, large pore sizes > 50 nm are macropores, pore size in the range of 5-20 nm are mesopores, whereas smaller pore size in the range of <2 nm are micropores. The micropores in biochars have high sorption 17
ACCEPTED MANUSCRIPT ability for small molecules [158], thus during soil applications biochars with high micropores might reduce the nutrient loss during volatilization of organic molecules.
Mesopore (2-50 nm)
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Macropore (>50 nm)
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Micropore (<2 nm)
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Fig. 7. Representative image of biochar showing the macro, meso and micropores.
Biochar has been widely used to improve soil fertility and soil carbon sequestration
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in soil amendment fields, which generally improves soil quality and helps to mitigate processes associated with climate change [159-161]. Utilization of biochar has received
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global attention in the past few decades and several reviews on this application have been published [162-168]. The soil organic carbon (SOC) levels that are essential for plant growth have decreased around the world due to intensive agricultural activities. Biochar, derived from carbon-rich biomass, has a high organic C content that could enhance SOC levels. According to published reports, the carbon content of biochar can reach 70% depending on the feedstock material [169]. Soil containing biochar has been shown to have measurable improvements in water-holding capacity, structure, and porosity, cation exchange capacity, electrical conductivity, and leaching of nutrient and agrochemicals [170-174]. Not only can biochar improve the growth of plants through its natural mineral/nutrients composition, it also protects plants from the abiotic stress of toxic elements and organic pollutants by 18
ACCEPTED MANUSCRIPT adsorbing or precipitating toxic pollutants [175]. Biochar can be manufactured from a widerange of materials that are of low to no cost because they are typically discarded as waste.
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These include: agricultural wastes, animal manure, forest waste, and sewage sludge. For all
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of these reasons, several researchers have focused on the application of biochar as a sorbent
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for the recovery/enrichment of nutrients and its subsequent application as fertilizer. Although biochar is one of the strategic key resources that have been used to enhance soil fertility, it does not show the same performance and effectiveness under all conditions. This
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inconsistency is because the physicochemical characteristics of biochar depend upon the
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properties of the feedstock materials (available nutrients, carbon content, and porosity) and the pyrolysis conditions (time and temperature) that are used. The nutrient (both micro and
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macro) availability of biochar-amended soil depends on several physical and chemical
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properties such as pH, surface area, porosity, cation exchange capacity (CEC), and the transfer of nutrients into the amended soil. However, it is possible to prepare custom or
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engineered biochars in such a way that they function effectively under specified conditions. A major drawback of biochar is that it is ineffective at recovering nutrients because
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most nutrients (nitrate and phosphate) are anionic, while the surface of biochar is highly negatively charged. In one study, thirteen different biochars derived from various feedstocks were tested for nitrate or phosphate adsorption, and most exhibited little or no sorption ability [176]. Similar results were observed with nine other biochars prepared from different feedstock materials [177]. These two studies were demonstrated that nutrient sorption ability of biochar significantly differs with biochar and nutrient kind. To improve the anionic nutrient sorption ability of biochar, composites were manufactured by combining inorganics, such as Al, Ca, Mg, Fe, La, Ca-Mg, and Mg-Al, with biochar [178-188]. Efforts were focused on the development of Ca and Mg biochar nanocomposites because they have the attractive properties of biocompatibility, high surface area, high nutrient loading capacity, and 19
ACCEPTED MANUSCRIPT controlled release of nutrient. Mg-enriched tomato tissue has been used as an engineered biochar for phosphate recovery from aqueous solution and then repurposed as a soil P-
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fertilizer [55]. Post-sorption characterization results revealed that both precipitation and
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surface deposition are key mechanisms involved in P recovery. Bioassay results showed that
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the addition of P-loaded biochar nanocomposite improved seed germination rates from 53% to 85% (p < 0.001). To enhance the anionic sorption capacity and ease of separation of biochar from aqueous solutions, magnetic MgO-decorated biochar was manufactured using
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ferrous chloride, and magnesium chloride, and subsequently tested for phosphate recovery
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and reuse [156]. Mg content during the impregnation increased from ~0.44 to 20%, and at an Mg content of 20%, the biochar had a high phosphate sorption capacity. The sorbate-sorbent
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interactions were attributed to protonation, surface electrostatic attraction, surface inner-
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sphere complexation, and precipitation.
Biochar characteristics may significantly vary with pyrolysis temperature which
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further influences the sorption ability of biochar [189-191]. Considering this in a study the effect of temperature during pyrolysis (200-800 oC) on the characteristics and phosphate
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sorption ability on biochar prepared using Undaria Pinnatifida roots as feed-stock were investigated [192]. The results revealed that when pyrolysis temperature increased, the following were observed: (i) decreased yield, (ii) increased aromaticity, and (iii) decreased polarity. Phosphate sorption increased with pyrolysis temperature upto 400 oC, and then decreased as the temperature was further increased. When the fertilizer efficiency of phosphate-loaded biochar was studied using potted indoor lettuces, the pot with supplied biochar grew faster and had a larger leaf area and size in the first 21 days, and larger and wider sizes of leaves after 30 days when compared with control pot containing commercial compost [192]. Biochar as a soil conditioner was also examined in a study on ammonia-laden corn cobs [193]. The biochars were manufactured at two different pyrolysis temperatures, 20
ACCEPTED MANUSCRIPT 400 and 600 oC, and biochar prepared at the lower temperature had a higher sorption capacity than that prepared at the higher temperature.
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Zeolite and clays, which are abundant in nature, have also been used extensively for
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nutrient recovery and subsequently as fertilizers [194-197]. Citing the advantages of air
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permeability, good soil osmosis, and water storage efficiency, a recent study reported the use of Kanuma and Akadam clays, along with zeolites, to manufacture ceramic adsorbent for the recovery of ammonium that was then repurposed as a fertilizer [198]. A maximum nitrate
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sorption capacity of 75.5 mg/g was obtained using an initial concentration of 10,000 mg/L
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NH4+-N was used.
As a demonstration of the value of the high nutrient loading ability of composite
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materials, a 30%, palygorskite nanocomposite was manufactured using carboxymethyl
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chitosan, acrylic acid, and palygorskite and the resulting composite exhibited a high ammonium loading capacity of 237.6 mg/g [199]. The composite exhibited both slow-release
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of nitrogen, with 60% of the nitrogen released into the soil over ten days, and good waterretention properties. In addition to nitrogen, palygorskite composites also contain nutrients
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such as P, K, Ca, Fe, Mg, and Mn, which are useful for plant growth. In another study, six surfactants comprising functionalized and unmodified synthetic silicate were manufactured and used for the recovery and reuse of anionic nitrate [200]. Surfactant-loaded clinoptilolite had a higher sorption capacity, of 125 mg/g, compared with other unmodified and modified silicates. The results also showed that surfactant-modified silicates could be effectively used as fertilizer due to their slow release performance, wherein nitrate continued to be released after 15-20 days of observation. The same group also employed surfactant-modified silicates for phosphate removal and for application as a slow release fertilizer [201]. Phosphate sorption was efficient at pH 7 and a maximum sorption capacity of 93.46 mg/g was perceived for hexadecyltrimethylammonium bromide modified clinoptilolite. The sorption mechanisms 21
ACCEPTED MANUSCRIPT between phosphate and surfactant-modified silicates were identified as electrostatic attraction, Van der Waals interactions, and interlayer adsorption. Nutrient leaching studies in
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soil-column tests revealed phosphate leaching even after 15 days demonstrates the slow-
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release characteristics of surfactant modified silicates.
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In addition to the above materials, biomass such as wheat straw [202, 203], chicken feathers [204], waste biomass [205], seaweed [206], and spent mushrooms [207, 208] have been utilized for nutrient loading and then reused in fertilizer applications. In one study, straw
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cellulose adsorbent was prepared and used for the recovery of both cationic and anionic
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NH4+and H2PO4− nutrients from aqueous solutions [209]. Soil experiments were performed to evaluate the slow-release behavior and water retention capacity of nutrient loaded sorbent,
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and the results were positive, 95.1% N released within 20 days and 60.0% P released within
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30 days, with increasing fertilizer dosage from 0.5 to 2 wt% soil water-retention capacity was increased. In another application, recognizing the importance of Mg in phosphate
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sequestration Wang et al. [210] prepared a hybrid artemia shell–Mg adsorbent by impregnating magnesium nitrate into artemia shells in an in situ process. Phosphate sorption
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experiments revealed a maximum sorption capacity of 32.7 mg/g at pH 6.4-6.9. The post sorbent (shell-Mg-P) has a slow-release behavior; only 65.8% of the phosphate had been released after 30 days. In addition to nitrogen and phosphorous other micronutrients, such as Mn(II) and Zn(II), have also been recovered and repurposed as fertilizer. In one study, spent mushrooms were converted to fertilizer on a pilot plant scale by loading Zn(II) and Mn(II) elements using sorption-based technology [207]. The biomass exhibited a sorption capacity of 15.84 mg/g for Zn(II) and 11.81 mg/g for Mn(II). Another study incorporated, feather protein, extracted from chicken feathers, as the basic macromolecular skeletal material for manufacturing hydrogel sorbent, and used this sorbent for the removal of cationic ammonium and anionic phosphate [204]. The authors propose that the resulting nutrient-laden sorbent 22
ACCEPTED MANUSCRIPT could be utilized further as fertilizer. A summary of the adsorbents that have been used as
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fertilizers, the target nutrients recovered, and their applications are presented in Table 2.
23
ACCEPTED MANUSCRIPT
Nutrient
Sorption capacity (mg/g)
Post-sorption applications
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Adsorbent
RI
PT
Table 2. Sorption based fertilizers (nutrient-loaded adsorbents) and their characteristics
Fertilizer
Reference
Nutrient release
P
116.600±26.590
Seed germination 7555.5 ± 10.5 mg [55] (85%) P/ kg
MgO decorated biochar
Phosphate
121
Ryegrass seedling 37.53% growth
[156]
Porous MgO-biochar nanocomposites
Phosphate
835
-
-
[184]
Nitrate
95
-
-
Proposed as fertilizer
-
[186]
327
-
1.66%
[187]
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81.8
AC
Mg/Al layered double Phosphate hydroxide functionalized biochar
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Engineered biochar
Ca-Mg biochar
Phosphorous
Undaria pinnatifida roots
Phosphate
32.6
Lactuca sativa plant growth
[192]
Corn cob
Ammonia
6.37
Soil conditioner
[193]
Ceramic adsorbent
NH4+–N
75.5
Soil conditioner 24
-
[198]
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NH4+
Soil fertility
60% in 10 days
[199]
125
Soil fertility
75% in 6th irrigation
[200]
93.5
Soil
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Surfactant modified Nitrate silicates (HDTMAB
238
-
[201]
Soil conditioner
69.2% in 10 days
[202]
Proposed as fertilizer
-
[207]
Soil
95.1% N (21 days)
[209]
Soil
60.0% P (30 days)
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Surfactant modified Phosphate silicates (HDTMAB
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loaded clinoptilolite)
Ammonium
7.15 mmol/g
Spent mushroom
Mn(II)
11.8
Zn(II)
15.8
NH4+
68
H2PO4–
38.6
Artemia egg shell
Phosphate
AC
CE
PT ED
Wheat straw
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loaded clinoptilolite)
Straw cellulose
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Palygorskite nanocomposite
32.7
Mung bean seedling growth
69.41 ± 2.2 mg [210] P/kg
Ca impregnated ramie
Phosphate
105
-
-
[211]
P
252
Proposed
-
[212]
Biomass Fe-Mn binary oxide nanoparticles
25
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-
Starch stabilized Mn nanoparticles
313
-
Fe-
-
PT
298
-
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CMC stabilized Fe-Mn nanoparticles
Phosphorous
454.5 mmol/g
Soil conditioner, P release even after [213] P nutrient 1080 hrs
Natural zeolite
Ammonia
6.89± 0.64
Cultivation of A. platensis
35% ammonia
[214]
Surfactant-modified zeolite
Nitrate
91 mmol/Kg
Sweet corn (Zea mays) plant growth
-
[215]
Seaweed growth
-
[216]
Correct soil acidity and
-
[217]
Proposed as fertilizer
20.4 (±6.35)
[218]
Zn(II)
Anaerobically digested Phosphorus sewage sludge and ochre
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PT ED
Manure/straw 1:3
49
225 mmok/Kg
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double Phosphate
6.70
AC
NiAl layered hydroxide
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Surfactant-Modified Zeolite
26
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Very little research has addressed the utilization of macronutrient (nitrogen, phosphorous)- loaded sorbents as fertilizers and soil conditioners, and a few studies have
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evaluated the efficiency of micronutrient-loaded sorbents as fertilizers. However, recent attention
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has turned toward health-related issues associated with malnutrition attributed to micronutrient
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deficiency [219-221]. For instance, a zinc deficiency in the diet of people in developing countries is one of the 10 key issues contributing to health concerns according to the World Health
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Organization (WHO) [222, 223]. This micronutrient deficiency can be overcome through
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supplementation with fertilizers. For example, in the 1970s, in Finland, an extremely low Se consumption of 0.025 mg/day was detected and to overcome this situation, in 1984, sodium
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selenite multi nutrient fertilizers were recommended [224]. Since 1985, most of the fertilizers in
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Finland have included Se, and currently, all crop fertilizers contain 15 mg Se/Kg. Thus, it is
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conceivable to pinpoint a nutrient deficiency in a particular geographic location and address the problem by developing customized, nutrient-loaded sorbent fertilizers that provide the optimal nutrient supplementation. In another example, anemia, which is a significant global health problem affecting 1.62 billion individuals worldwide [225], is frequently caused by an iron deficiency [226, 227]. Anand et al. [228] report that iron deficiency anemia is one of the chief public health complications in developing countries such as India. Insufficiencies of minerals, specifically iron (Fe), zinc (Zn), and calcium (Ca), are a major problem in developing countries where they lead to adverse effects on human health [229]. Again, the possibility of manufacturing custom-tailored post-adsorbents (laden with essential trace elements) that can then be used as multi nutrient fertilizers may be a solution to improving public health. Zeolites are ideal substrates because they carry nutrients and release them gradually, and they have 27
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hydration and dehydration capacities that are helpful for maintaining water balance [197]. Zeolites also possess selective adsorption efficiency towards ammonia (NH 4+), which is useful in
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manufacturing of nitrogen-based fertilizers [230]. The specific adsorbent-nutrient interactions
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and the nutrient release rate are both important properties to consider. The following strategic
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points are important to consider when manufacturing nutrient-loaded sorbent fertilizers: (i) affinity for both cations and anions (NH4+, K+, NO3-, and PO43-), (ii) stability under soil
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ability to function for long periods of time.
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environmental conditions, (iii) slow-release characteristics, (iv) water-holding capacity, and (v)
Although adding post-sorbent (nutrient-loaded sorbents) to soil improves agricultural
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yields and nutritional qualities (laboratory scale), socio-economic conditions will determine
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whether or not the materials can be applied to commercial agriculture. The key issues that
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influence the application of post-sorbent fertilizers are the availability of feedstocks, the technology to manufacture fertilizer, and investment capacity. Further studies should carry out economic analyses of post-sorbent fertilizers to provide the appropriate information about the application of these materials to agricultural systems. A recent cost analysis for manufacturing Zn(II) and Mn(II) loaded mushroom material was carried out [207]. A cost estimation analysis for manufacturing the fertilizer estimated $ 4.76/Kg for the production of 100 kg/day in the pilot plant. Raw materials cost significantly influences the overall post-sorbent fertilizer manufacturing cost; thus low-cost sources should be explored. Nutrients used to load onto adsorbent were of high-cost; however, these metal ions are available in various waters at low or no cost. Seawater and wastewater, which are rich in various trace metal ions, should be considered as sources of micronutrients for sorption technology because this will decrease the 28
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costs of manufacturing nutrient-laden sorbents [36, 231-233]. This strategy will be useful in twoways: one is to remove toxic metal ions from wastewater and second one is to reuse them, thus
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this strategy reduces the manufacturing cost of nutrient-laden sorbents. In one study, poultry
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litter (PL) containing P was used as a source and Fe-Mn binary oxide nanoparticles were used to
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control the leaching of P from PL; the treatment cost was estimated to be $0.0036–0.0056/kg-PL [212]. Composite materials have also been developed as sorbents using several types of metal
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oxide nanoparticles, such as MgO, which is widely used to fabricate biochar composites. For
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preparing MgO composite materials, alternative Mg resources should be explored. In one study, carbon materials with stabilized MgO nanoparticles (~17-18 nm) were manufactured using an
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MgCl2 solution by using average Mg concentration in seawater i.e. 1540 mg/L [234]. Utilization
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of untapped reserves to manufacture nutrient-loaded sorbent fertilizers will invariably reduce the
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overall product costs. A comprehensive assessment of the economic, technical, and environmental strength and weakness of post-nutrient laden fertilizer manufacturing technologies should be carried for a better understanding of the real-world applications of the developed materials.
It is important that post-nutrient laden sorbent fertilizers be examined for toxic characteristics, which would limit their validity and application. For example, to enhance both nutrient loading capacity and controlled release properties, several researchers have employed nanocomposite fertilizers. However, in recent years there has been much debate about the environmental safety and toxicity of nanoparticles [235-238]. For example, to enhance the sorption performance, biochar was manufactured in nano-composites form using metal oxide nanoparticles such as Cuo, CoFe2O4, MgO, MnO, and TiO2; therefore, the environmental and 29
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biological risk of these metal oxides should be considered [42, 88]. Thus, it is essential to evaluate the stability and toxicity of newly manufactured nanocomposites. An evaluation of
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metal ion leaching from nanocomposites using the U.S. EPA Toxicity Character Leaching
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Protocol (TCLP) will provide an indication of their behavior [239]. To pinpoint any adverse
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effects of nanocomposite fertilizers on health, it will be necessary to initiate studies on various issues including the fate and transport of the nutrient loaded nanocomposites in the soil, the
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ecosystem, the uptake and accumulation in plants, and the evolution of the toxicity of
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nanomaterials. Risk assessment studies should be conducted before any agricultural application, and the effects should be monitored continuously throughout the application.
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Although biochar has shown a wide variety of benefits, such as improved soil fertility,
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improved plant growth, and increased crop yields, possible negative consequences should also be
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considered. For example, the micronutrients mentioned above (essential elements) are required in trace quantities for normal plant growth, but when present at higher concentrations, these elements are toxic to living organisms. Before these nutrients are applied as fertilizers via postadsorbents, the materials should be evaluated to assess whether or not toxic compounds, such as heavy metals, would be released into soils, thereby violating current environmental protocols. Biochars have been engineered from various organic feedstock materials, which may have contained the potential toxic elements (PTEs) Pb, Cd, Cu, and As, in addition to carbon, nutrients, and minerals. These materials also contain polycyclic aromatic hydrocarbons (PAHs) and highly volatile organic compounds (VOC), which are notable because of their adverse effects on plant growth and on microbial communities [240, 241]. However, after converting feedstock into biochar, contents of PTEs in the resulting biochar were reported to be low or high 30
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with respect to their feedstocks [242, 243]. Although small quantities of bioavailable PTEs are expected in soils, the subsequent negative effects on crops are undesirable, and lead to health
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problems in animals and humans via the food chain [244, 245]. Furthermore, the practice of
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pyrolysis may alter the oxidation state of some PTEs, specifically Cr(III) or Cr(VI), which could
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significantly increase or decrease toxicity. Considering the importance of potential toxicities of biochar depends on pyrolysis temperature Lyu et al. [191] investigated the toxic effects of
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biochars produced at temperatures ranging from 200-750 oC. They denoted that biochars
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manufactured at pyrolysis temperature >400 oC are less toxic and are more appropriate for soil usages. There are no legislative standards limiting PTE concentrations in biochar that is to be
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applied to soils. Instead, biochar containing materials are to be certified according to the
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International Biochar Initiative (IBI) guidelines [246], and the European Biochar Certificate
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guidelines [247]. These certificate systems were established by researchers as a preliminary assessment of the toxic characteristics of biochar materials. According to the IBI, As, Cd, Cr, Co, Cu, Pb, Hg, Mo, Ni, and Zn are the PTEs of concern in biochar, and Co and Mo [246] are also priority pollutants as per the U.S. EPA [248, 249]. The European law on EC fertilizers suggests that heavy metals should not be added purposely to ammonium nitrate fertilizers, and the concentration of copper should be below 10 mg/Kg [250]. Thus it is essential to investigate the concentrations of PTEs in biomass-based materials that are manufactured from new feedstock materials according to the extant legislation or regional regulations. These strategies will ensure the development of environmentally-friendly post-sorbent material enriched for nutrients, and the application of these materials as fertilizers will improve the nutrient efficiency of crops and play a positive role in reaching the 2050 food and environmental security goals [146, 251, 252]. 31
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The industrialization of post-sorbent materials that are manufactured by sorbing nutrients onto various sorbents as fertilizers have been limited by the following key issues, (i) release of toxic
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metal ions from sorbent and/or composite sorbents causes antagonistic effects on plant growth,
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(ii) uncontrolled release of sorbed metal ions, (iii) cost, (iv) high volumes are required due to the
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lower amounts of sorbed nutrients compared with commercial fertilizers, and (v) uneven distribution of sorbed metal ions results either high nutrient availability or low nutrient
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availability.
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2.2. Carbon-encapsulated metal nanoparticles (CEMNPs) Before providing a detailed account of the applications for post-sorbents in
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manufacturing carbon-encapsulated metal nanoparticles, it is relevant to introduce three major
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problems encountered in three divergent scientific fields (stable carbon-nanomaterial fabrication,
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biomass gasification, and wastewater treatment), all of which were solved by post-sorption technology. First, the preparation of stable metal nanoparticles is difficult. Pure metal nanoparticles (MNPs), such as non-precious 3d-block transition metals (Fe, Cu, Co and Ni) and their alloys (CoFe, CoNiFe, etc.), have charmed researchers for years due to their optical, catalytic, magnetic, mechanical, photo-thermal, and electrical properties [253]. These interesting characteristics have led to stimulating applications in diverse areas such as nanosensing, nanocatalysis, water treatment, drug delivery, spectroscopy, magnetic resonance imaging, and plasmonics [253-257]. However, the high surface-to-volume ratio of pure metal nanoparticles is also associated with instability via oxidation in air, dissolution in acid, and aggregation due to high surface energy, leading to the deterioration of their magnetic properties among other things [258]. This restricts hands-on applications and creates difficulties in assessing the properties of 32
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these materials [259-262]. Thus, inert materials either organics or inorganic, have often been used to coat pure metal nanoparticles through in-situ and ex-situ techniques to shield them from
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oxidation and avert changes in their properties. Encapsulation of highly dispersed metal/metal
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oxide nanoparticles inside of carbon materials is a useful method; the carbon layer protects the
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naked nanoparticles from oxidation, agglomeration, harsh chemical environments, and maintains high magnetization [263]. Compared with other types of coatings, carbon provides the unique
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advantages of high chemical and thermal stability, high dielectric permittivity, magnetic
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permeability, better conductivity, and biocompatibility [264-268]. Carbon-encapsulated metal nanoparticles (CEMNPs) are a novel type of nanostructured carbon materials that have a unique
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core-shell construction of metal cores surrounded by thin carbon coat (shells) (Fig. 8), and the
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crystallinity of this coating is graphitic in nature and alike to CNTs [269, 270].
Fig. 8. Effect of air exposure and encapsulation of metal nanoparticles (MNP)
CEMNPs have attracted much attention due to their unique physical and chemical properties, especially for application in catalysts, drug delivery, water treatment, hyperthermia and 33
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information technologies such as magnetic data storage, xerography, electronics, and biomedicine [271, 272]. There are several methods available for fabricating CEMNPs; these
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include high-temperature annealing [271], chemical vapor deposition [273], arc discharging, and
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modified arc discharging [274]. However, in situ generation of nanoparticles using metal ion
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loaded biomass through carbothermal reduction is of particular interest due to the renewability of biomass, and also due to its economic and eco-friendly nature.
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Preparation of stable carbon-nanocomposites for the treatment of contaminated water is
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another problem, which was also solved using post-sorption technology. The general manufacturing process for composites involves synthesis of carbon and nanoparticles
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individually, and then further mixing of these two distinct phases to obtain the composite
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materials. This approach has limitations including: (i) the challenges of carbon manufacturing,
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which requires high temperature, high voltage, and is difficult to control, (ii) the poor stability and high cost of composites, and (iii) the undesirable impregnation of particles or adherence to surfaces and pores. Additionally, when composites are used in water treatment and come into contact with acidic solutions, there is a possibility of nanoparticle leaching. When composites are manufactured through the pyrolysis of biomass loaded with metal ions (post-sorbent), the nanoparticles that are harvested are held tightly inside the carbon (during the in situ generation of nanoparticles which are covered by the carbon). Thus, there is less chance of leaching, agglomerating and oxidizing. The process of post-sorbent pyrolysis is further advantageous because the manufactured composite is eco-friendly and the process is scalable and economically viable. The economic benefit of this approach is due to the use of low-cost of the biomass used as the carbon precursor. The process is facile in that it occurs in a few steps: (i) loading of metal 34
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ions (ii) drying (iii) pyrolysis, and (iv) washing. Materials prepared in this way exhibit a high surface area of >1000 mg/g [275].
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The third problem that was solved using the post-sorbent pyrolysis technique was in
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biomass gasification. Biomass is inexpensive and is an abundant renewable source of carbon that
SC
has recently been widely used as renewable source of energy and fuel production through a thermochemical conversion process. Production of synthesis gas (also termed as syngas) through
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gasification/pyrolysis is a promising technique to convert biomass into value-added chemicals,
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hydrogen, gas fuels, liquids, and char. Moreover, the generation of syngas compounds (H2, CO, CO2 and N2) through the gasification process also produces undesirable by-products such as tar
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(comprising of benzene, naphthalene, phenol, toluene, and xylene) and inorganics (S, K, P etc.).
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Tar, comprised of benzene and polycyclic aromatic hydrocarbon, is of major concern because it
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is an environmental pollutant and damages process equipment, engines and turbines [276, 277]. For effective use of syngas, these impurities, particularly tar, must be removed or treated. Catalytic reforming/cracking of the tars and hydrocarbons is an efficient technique that provides additional syngas, enhanced H2 content, high heating values, and higher biomass carbon utilization [278]. Transition metal-based catalysts, chiefly nickel-based catalysts are effective in tar reduction and syngas reforming during biomass pyrolysis/gasification [279-281]. Among the different catalyst integration strategies, insertion of the catalyst metal precursor into the biomass (metal-impregnated biomass) through adsorption prior to thermo-chemical conversion was effective, and had a significant effect on the yields of tar, gas and char [282-284]. Adsorption plays an important role in the preparation of metal-impregnated biomass because the chemical functional groups present in biomass act as adsorption sites during the impregnation step, and 35
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mineral cations present in biomass are also replaced by catalyst metal precursors through ion exchange [282, 284]. High yield and good dispersion of the metal precursor are accomplished
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using this approach.
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Biomass is an abundant carbon source that can be used as a precursor for manufacturing
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carbon-encapsulated metal nanoparticles. Biomass and derivative materials have been used widely in water treatment for the removal of metal ions from aqueous solutions, because they are
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abundantly available, low cost, and environmentally-friendly [120, 285-290]. Biomass materials
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contain numerous oxygenated functional groups, such as carboxyl, carbonyl, amine, hydroxyl, and ether groups [291]. These functional groups act as ligands for metal ion complexation in
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aqueous media. Robalds et al. [292] classified the biosorption mechanism and their review and
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references therein are recommended for further reading on this subject. Due to the ability of
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biomass to complex metals, it can function as a natural biotemplate to generate dispersed metal ions. In this respect, biomass has been used as both a template and a carbon source for harvesting nanoparticles. Recently, there have been several attempts to fabricate carbonaceous metal nanoparticles using the abundant, economical and environmentally friendly metal-loaded biomass (post-sorbent) as a precursor material. This process involves the adsorption of transition metal ions (Cu, Ni, and Co, etc.) onto biomass, and then conversion to carbonaceous metal nanoparticles using catalytic pyrolysis/gasification techniques. Catalytic biomass gasification is an effective technique used to convert metal-loaded biomass into carbon, char, syngas, and nanoparticles. Harvesting carbonaceous metal nanoparticles involves the following processes: (i) sorption of metal ions onto biomass, (ii) grinding and drying, (iii) pyrolysis of metal-laden biomass, and (iv) in situ 36
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generation of metal nanoparticles. In the first step, the target metal cations are adsorbed onto the numerous oxygenated groups (hydroxyl, carboxyl, amine etc.) of biomass while in aqueous
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solutions, and at the same time metal ions are dispersed inside the biomass matrix. In the second
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step, the biomass is thermally decomposed, and the major reactions that occur during thermo-
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chemical conversion are summarized in the following equations [293]: (1) (2) (3) (4)
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(5)
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(6)
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CxHyOz represents the major types of biomass molecules, including aldehydes, ketones, alcohols, carboxylic acid, and cresols. Adsorbed metal ions in the biomass matrix are reduced by the reducing components in the reaction (ex. CO, CH4, H2, CxHyOz and the bio-char). The carbothermal reduction of metal oxides is a well-known technique [294]. In the absence of oxygen, the abundant reductants molecules reduce the high-valency (Mn+) metal ions dispersed in the biomass into metal nanoparticles (Mo, M2+, MO, MFe2O4). The carbothermal reduction of metal ions is described in the following equations: (7) (8) The advantages of this process are: (i) the metal nanoparticles and the carbon can be harvested simultaneously, (ii) the metal compounds have a catalytic effect, and (iii) it is a one step process. 37
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Catalytic biomass gasification was principally used to obtain synthesis gas, bio-oil, bio-char, and H2 [295]. For further reading on carbon-based nanoparticles, including carbothermal synthesis,
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catalytic biomass gasification, biomass pyrolysis, and carbon supported catalysts see other
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reviews on the subject [157, 296-303]. A schematic of carbon-nanoparticle harvested through
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carbothermal-reduction is shown in Fig. 9.
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Fig. 9. Synthesis of carbon-nanoparticles through carbothermal synthesis approach
In situ generation of nanoparticles through biomass pyrolysis/gasification depends upon biomass composition, metal ion impregnation and pyrolysis conditions. Few of the significant factors, such as effects of metal ions, metal salts, and impregnation conditions (pH, atmosphere, and vacuum) have been reported [278, 282, 304-306]. pH is one of the key parameters that influences metal ion speciation in aqueous solutions and influences the functional groups present on biomass [307, 308]. Biomass consists of surface chemical groups such as carboxyl, carbonyl, hydroxyl, amine, amide, phosphonate, etc. [291], which groups acts as sorption sites. The pH influences the charges of these sorption sites. As the pH decreases, the biomass functional groups 38
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are protonated due to the high concentration of protons. This situation is not favorable for the sorption of positively charged metal ions due to the electrostatic repulsion. In the other hands, as
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the pH increases, a number of negatively charged sites increases, facilitating sorption of cations.
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Furthermore, the pH also influences the complexation process which involves the formation of
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coordination bonds between metal ions and functional groups containing a lone pair of electrons [287]. This complexation process is highly pH dependent; change in pH causes change in
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sorption efficiency of biomass [287].
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Besides, the pH also affects the solution chemistry of metal ions. In aqueous solution,
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metal ions (M2+) undergo solvation, hydrolysis and polymerization [309-311] as shown below: (9)
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(10)
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(11)
Most of the metal cations exist as free ions (M2+) in the pH range 2-5, as the pH is increased (>2) the positively charged metal cations quickly approach the negatively charged adsorbent surface. With further increase in solution pH, reactions between metal ions with these OH- ions results in the precipitation of metal hydroxides. This phenomenon also impacts the biomass sorption performance. When the influence of pH on Ni loading onto beechwood was studied, the findings revealed that metal loading increased with increasing pH of the solution [278]. Because nickel hydroxide formation occurs at pH>7 and wood chips have a pHZPC (6.7), optimum pH conditions of 6-7 were chosen to attain high dispersion of nickel cations into the wood biomass matrix. Nickel-loaded biomass pyrolysis resulted in well dispersed Ni0 nanoparticles of 2-4 nm in the 39
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carbon matrix. The findings also suggested that both the initial nickel loading and final pyrolysis temperature influenced the crystallite size of Nio, and these in situ generated NPs enhanced both
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tar conversion and H2 production. To evaluate the effect of precursor metal salts, another study
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describes the loading of Fe and Ni nitrates onto oak sawdust biomass and evaluation of the
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catalytic effect of the metals [304]. Higher H2 yields were observed for Ni-laden biomass samples than for Fe-loaded biomass, whereas the latter reduces the production of gases. Similar
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findings were also reported for nickel-loaded biomass, which led to high levels of hydrogen
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production when compared to iron-loaded biomass, despite the higher char yield for iron-loaded biomass [306]. The effect of atmospheric pressure and vacuum impregnation on nickel
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adsorption onto beech wood has been explored [282]. Furthermore, the catalytic effect of NPs
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has been evaluated by inserting Ni0 NPs into biomass prior to pyrolysis. One interesting finding
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was that under atmospheric impregnation Ni2+ adsorption onto biomass involved outer-sphere complexation, whereas under vacuum impregnation it involved inner-sphere complexation. The improved Ni insertion was achieved under vacuum impregnation due to this inner-sphere complexation, and higher catalytic efficiency was observed for in situ generated Ni0 NPs as compared to Ni0 NPs that were inserted into biomass before pyrolysis. The effect of boron addition was examined in a study of nickel impregnated rice husk (RH) biomass modified with sodium borohydride [284]. The results revealed that the addition of NaBH4 encouraged the generation of Nio NPs by deoxidizing NiO into metallic Nio NPs, and also enhanced tar conversion during pyrolysis. Nanda et al. [312] used hydrothermal gasification to study the influence of temperature conditions, biomass-to-water ratios, and residence time on nickelimpregnated pinewood and wheat straw biomasses. Nickel impregnation onto biomass was 40
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carried out at pH 7-8 for quick sorption. Compared with non-catalytic gasification higher yields of gas, hydrogen with superior carbon gasification efficiency was observed for nickel-
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impregnated biomasses.
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Recently, composite materials comprising two unique constituents have gained particular
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attention in areas such as catalysis, water treatment, and biological applications. Among all composites developed to date, carbon composites made up of metal nanoparticles or magnetic
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particles have been used widely in water treatment due to their high surface area, which is an
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advantage because it facilitates recovery of the composite after water treatment [41, 42, 81, 92]. CEMNPs have shown high sorption capacity and faster removal rates for inorganics compared
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with traditional activated carbon [269, 313]. Nitric acid treated CEMNPs have shown high
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sorption capacity i.e. 91.0 mg/g for Cd(II) compared with CNTs (20.37 mg/g) and AC (9.91
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mg/g) [314]. Considering the high sorption performance of CEMNPs several attempts were made to treat various pollutants. In a study porous carbon sheets containing carbon-encapsulated Fe/Fe3C (Fe/Fe3C@PCS) was fabricated by using gelatin biomass and Fe(NO3)3. 9H2O as precursors [315]. The material successfully removed uranium from aqueous medium via surface adsorption and led to in situ reduction of U(VI) into U(IV). Comparison was made between the sorption efficiencies of activated carbon and Fe/Fe3C@PCS for U(VI), activated carbon (AC) has limited sorption performance which was effective at low initial concentrations due to the limited active sites of AC. Whereas even at high concentration up to 140 mg/L Fe/Fe3C@PCS was effective for U(VI) removal (~100%). In another approach, agar was used as the biomass precursor, along with Fe(NO3)3. 9H2O and Al(NO)3. 9H2O as metal precursors, to manufacture porous carbon sheets containing carbon stabilized Fe/Fe3C nanoparticles [275]. The material 41
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exhibited a high surface area of 1023 m2/g and successfully adsorbed the organic pollutants methylene blue, methyl orange, and crystal violet with sorption capacities of 1616, 1062, and
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1728 mg/g, respectively. These materials have the added advantage that they can be easily
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removed from aqueous solutions due to their magnetic properties, unlike other materials. Cu NP
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(average particle size of 21.2 nm) anchored magnetic carbon (Cu and Fe3O4-mc) was manufactured by fast pyrolysis of Fe/Cu adsorbed fir sawdust biomass and the material was used
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as a catalyst for the catalytic reduction of 4-nitrophenol [316]. When the performance of Cu and
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Fe3O4-mc was compared to that of magnetic carbon, the Cu and Fe 3O4-mc showed more effective catalytic reduction of 4-nitrophenol, likely due to the presence of Cu nanoparticles,
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which have exceptional catalytic properties. This catalyst retained 95% of its reduction efficiency
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after five cycles, suggesting that it is effectively recyclable. Release of the heavy metal Cu
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throughout the five cycles was measured to determine the risk of toxicity to aquatic environments, and was found to be < 1.0 mg/L, which is below surface water limits. In another study, compressed carbon black was used to manufacture C-Feo particles using adsorption and impregnation procedures, and the resulting materials were tested for Cr(VI) remediation [317]. The results revealed that the BET surface area of C-Feo particles that were prepared through adsorption was higher (130 m2/g) than that of the starting carbon material (80 m2/g). The presence of C-Feo reduced the Cr(VI) concentration from 10 to ~1ppm. Most of the studies described above focus on metal-loaded biomass, but dye loaded adsorbents have also been used to manufacture nanoparticles by adding the appropriate metal ion precursor to the post-sorbent. Tian et al. [98] used an interesting strategy to prepare a multifunctional nanocomposite by combining AgNO3 and methyl violet-loaded palygorskite in a 42
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hydrothermal process. The dye adsorbed onto the palygorskite acted as carbon source, whereas the Ag(I) promoted carbonization of the dye molecule. This composite exhibited good sorption
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efficiency for methylene blue (99.2%), methyl violate (86.9%), chlortetracycline hydrochloride
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(68.7%), and tetracycline (46.2%). It also exhibited good catalytic properties by reducing 4-
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nitrophenol to 4-aminophenol with good reusability (8 cycles). A summary of adsorbents and metal ion precursors used in the fabrication of carbon-nanoparticles is given in Table 3.
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Biosorption, the sorption of metal ions from various biomaterial wastes, has been one of
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the more highly studied topics due to its effective removal of various metal ions from aqueous solutions. However, researchers are still looking for alternative strategies for the safe disposal of
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spent or metal-loaded biosorbents. Post-sorbed metal ions on biomass could be utilized as
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feedstock for harvesting nanoparticles in a carbon matrix through modern pyrolysis processes.
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Several factors that influence the sorption ability of biomass include pH, temperature, vacuum/atmosphere, biomass characteristics, and type of metal salts (ex. nitrate, chlorides, etc.). One of the key factors that influences the harvesting of nanoparticles is the pH during impregnation [318]. pH is always controlled, because the optimum sorption of metal ions occurs when metal ions are well dispersed and in high amounts on the biomass. This ideal method will provide an opportunity to manufacture value-added products such as (i) syngas, (ii) nanoparticles, (iii) biochar, (iv) reusable catalysts, and (v) resource recovery. The cost of raw materials influences the overall cost of any technology, and for in situ generation of nanoparticles, the cost of metal ion precursors is significant. Due to its good catalytic activity, nickel is one of the most widely used precursors for harvesting nanoparticles. Electroplating wastewater is rich in nickel ions and can be used for loading nickel ions into a biomass matrix 43
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[284, 319]. In addition to metal-containing wastewaters, seawater, which is rich in metal ions, can also be useful; because it is an inexpensive resource, the overall cost of carbon-nanoparticles
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synthesis would be reduced. For instance, in one study, seawater-rich MgCl2 was used as a
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precursor along with sawdust biomass for manufacturing mesoporous carbon stabilized MgO
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nanoparticles, and the resulting material was successfully used for CO2 capture [320]. Although CEMNPs have a wide range of applications in diverse fields, there are several
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limitations. Ni-based catalysts are often used to prepare CEMNPs for their enhanced catalytic
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activity and reduced cost. However, Ni-based catalysts are less stable and further prone to coke development [284, 321]. Uniform distribution of metal ions in the biomass is required.
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Otherwise, carbon formation without metal nanoparticles is possible and in the case of water
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treatment it's hard to separate the fine carbon powder. Fabrication of these materials requires
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high technology instruments and high temperatures. For producing CEMNPs both precursor biomass and metal ions should be obtained at low cost (i.e. in the form of waste); otherwise, the overall fabrication cost of CEMNPs will be higher. In recent years toxicity of emerging materials is of big debate. In the case of CEMNPs although nanoparticles are covered by carbon shell which protects from direct contact with biological fluids and environment toxicity of CEMNPs is off under scrutiny [272]. The results revealed cytotoxic response of these particles depends on their physicochemical characteristics specifically surface characteristics [272, 322-324].
44
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Fe/Al
Post adsorbent characterization and applications
800 oC /N2
Fe/Fe3C
(size Surfac Application e area m2/g
References
Efficiency
SC
NPs nm)
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Temperature /Atmosphere
1023
Dye removal
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MB (1615.9)
[275]
MO (1062.4)
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Agar
Metal precursor
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Adsorbent
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Table 3. Sorption based carbon-metal nanoparticles and their characteristics
CV (1728.3) mg/g
[Ni(NO3)2. 6 H2O]
700 oC
Gelatin
Fe
800 oC/ N2 (5 o C/min)
Fe/Fe3C
Beech wood chips
[Ni(NO3)2. 6 H2O]
700 oC/N2 (5 o C/min)
Ni0
H2 rich syngas production
[318]
Sawdust
[MgCl2. 6H2O]
773-973 K/N2
MgO
CO2 capture
[320]
Fir sawdust
Cu(NO3)2, Fe(NO3)2
773 K/N2
Cu&Fe3O4
186
4-nitrphenol reduction
Sawdust
FeCl3
773 K/N2
Fe3O4
296
Acid (Benzyl
AC
CE
Beech wood
Ni0
[282]
Aromatic tar conversion 95.9
Uranium removal
acetate) 45
100% mg/L
95%
catalyst 93%, reusable
(140 [315]
[316] [325]
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[Ni(NO3)2. 6 H2O]
750 oC/N2
Ni0
-
Tar conversion
-
[284]
Beechwood chips
[Ni(NO3)2. 6 H2O]
700 oC/N2 (5 Ni0 o C/min)
-
H2 production/Tar conversion
[278]
Pinewood & wheat
[Ni(NO3)2. 6 H2O]
700 oC/argon (10 oC/min)
Ni0
-
H2 production
-
[312]
Beech woodchips
[Ni(NO3)2. 6 H2O]
850 oC/N2
Ni
465
H2 production
-
[326]
Rice husk char
Fe(NO3)3·9H2O & 600 oC Ni(NO3)2·6H2O
Ni-Fe
-
Tar removal
92.3%
[327]
Rice husk
[Ni(NO3)2. 6 H2O]
Ni0
135
Tar conversion
-
[328]
Pinewood
[Ni(NO3)2. 6 H2O]
Ni (<100)
-
Gas and H2 yields
-
[312]
Wheat straw
[Ni(NO3)2. 6 H2O]
Ni (<100)
-
Gas and H2 yields
-
Compressed carbon black
Fe(NO3)3·9H2O
800 oC/Ar
C-Fe0 (~20 130 to 150 nm)
Cr(VI) remediation
-
[317]
Palygorskite
AgNO3
180 o C/Autoclave
Ag
Sorption/catalysis
-
[98]
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SC MA
PT ED
CE
AC
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straw
~700 oC/N2
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Rice husk
205
46
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2.3. Feed additives Trace elements, also known as trace minerals (dietary minerals), are essential inorganic
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feed additives that provide the key nutrients required for various metabolic functions in living
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organisms. These are required in tiny quantities for proper growth and, development, and play a
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vital role in innumerable biological processes and molecules, including nerve transmission, hormones, energy production, blood sugar levels, metabolism, digestion, and cholesterol [329].
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Of the 90 elements that occurring naturally, approximately 26 are considered to be essential for
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mammalian life, including B, Co, Cu, F, Fe, I, Li, Mn, Mo, Ni, Se, Si, V, and Zn, and possibly As, Cr, and Sn [330-333]. Although all of these elements are involved in biological functions, the
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trace elements required for good health are copper, cobalt, iron, iodine, manganese,
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molybdenum, selenium, and zinc [334-338]. Deficiencies of these minor trace elements can adversely effect animal and human health and can lead to chronic illnesses such as anemia,
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diabetes, Alzheimer’s disease, aging, hypertension, fertility issues, and reduced immune system function [339-342]. As an example, Zn is one of the essential micronutrients required for normal growth and development [343, 344]; however, it is estimated that 17% of the global population is at risk of Zn deficiency due to low dietary Zn intake [345]. Certain trace mineral insufficiencies in livestock can be classified as either a primary or secondary shortcoming in nature. A primary deficiency occurs as a result of the consumption of a diet that is low in one or more trace minerals, while a secondary deficiency results when there is reduced nutrient absorption or either enhanced metabolism or secretion of a specific nutrient [346, 347]. When an animal suffers from mineral deficiencies due to lack of trace elements (such as iodine and selenium), the meat or milk produced from those animals also contains inadequate 47
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levels of these trace elements. Consumption of products from deficient animals results in a similar deficiency in humans [348]. As an example of successful reversal of this effect, in the
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United Kingdom, cases of iodine(I) deficiency in humans have decreased after iodine
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supplementation was given to dairy cows, presumably because iodine progresses into the cow’s
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milk, which is then ingested by humans [349]. Animals with mineral insufficiency might not exhibit any outward symptoms; however, they may have reduced performance potential and less
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ideal health conditions. It is also possible to ingest excessive levels of trace minerals naturally or
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through diet supplementation, and this too has a negative effect on health in that it can lead to acute toxicity problems and possibly induction of deficiencies in other related trace minerals
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[333, 336, 350]. Scientific studies have carefully defined the nutritional needs of animals that
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feed formulations with appropriate levels of micronutrients are available to support the efficiency
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of animal husbandry and food production. Ideally, eatable animal foodstuffs (meat, eggs, dairy products, etc.) should contain optimal quantities of trace minerals to ensure human health. To handle micronutrients as precisely as possible, premix and feed producers must rely on sources of trace minerals that have ideal physical and technological properties. For instance, copper and zinc are critical trace minerals that improve production and reproduction in farm animals [351]. However, the form of the food, organic or inorganic, makes a difference in the availability of these minerals to the animal, because the inorganic form of trace minerals are not adequately absorbed and retained [351]. In contrast, the organic form is absorbed well and is maintained in tissues, improving performance, immunity, health and reproduction [352]. Biosorption technology, which enables sorption of a broad range of metal ions, is one of the best solutions for manufacturing fine-tuned trace mineral supplements. Trace minerals can be 48
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loaded onto the inexpensive, edible, and abundant biomass, and the resulting feed can be supplied to animal husbandry. Several researchers have focused on developing feed supplements
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utilizing biomass-based sorbents. In particular, Prof. Katarzyna Chojnacka’s group in Poland has
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been one of the most successful teams, making valuable progress in this research area and
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bringing this technology from the laboratory to the pilot scale [353-355]. This research group studied a broad range of trace metal-enriched biosorbents as feed additives and found promising
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results. This group published an excellent review on manufacturing agrochemicals, which
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includes discussion of the application of metal-enriched biosorbents to feed supplements [356]. Seaweed biomass, macroalgae, soybean meal, and spent mushroom substrates have often
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been used to adsorb metal ions, and were studied for their subsequent influence on animal
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performance (meat, milk and eggs). Taking into consideration the importance of Zn(II) and
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Cu(II) for animal growth, macroalga Enteromorpha sp. biosorbent was enriched with these two trace elements through biosorption technology, and studied for effectiveness as pig feed [357]. Inorganic salts were used for the control group and compared to experimental groups who were feed with biomass-enriched metal ions. A mineral composition analysis of the meat from each pig revealed that experimental groups had a significantly higher level of Mn (49%) than the control groups. In another study, microalgal biomass Spirulina maxima was utilized as a feed supplement by enriching it with the metal ions Cu(II) and Fe(II), and fed to swine and laying hens [358, 359]. In the experiments on laying hens, it was observed that the trace minerals Fe, Zn, and Mn were present at higher levels in the egg albumin of the experimental groups as compared to the control groups [359]. A pilot plant arrangement was also used to manufacture Zn(II), Cu(II), Mn(II) and Fe(II)- enriched soybean meal using sorption technology and the post49
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sorbent material was enriched with metal ions and used for zootechnical tests [355]. Some of the adsorbents were used as feed additives, and the nutritional content of the feed and their
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applications are summarized in Table 4.
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A wide variety of sorbents can be useful for manufacturing nutrient enriched feed; however, biomass substrate has often been chosen for this purpose. Zeolites should also be
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considered as substrate for trace element enrichment and subsequent application as a feed
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supplement. No study has reported the application of nutrient-loaded zeolite as feed supplements. However, application of zeolites as feed additive has been reported and its use has been accepted
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by various regulatory agencies. For example, both the U.S. FDA and the EU have approved
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clinoptilolite as a feed additive [230]. An in vitro study evaluating the effect of zeolite on the
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gastrointestinal tracts of swine found no signs of toxicity [360]. The release of extremely low concentrations of Pb and other toxic elements was observed; however, the concentration of Pb was far less than that allowed by European Regulations (E566). Prasai et al. [361] studied the effects of feed supplementation containing biochar, zeolite, and bentonite on layer chickens and the findings revealed that these feed additives are effective in controlling zoonotic pathogens. Several studies have reported that zeolites have a positive influence on disease resistance as well as, immunological and hematological factors [362]. Natural zeolites are microporous crystalline, hydrated aluminosilicates of alkali and alkaline earth cations that have unique physicochemical characteristics including ion-exchange and adsorption-desorption abilities. A wide variety of natural and synthetic zeolites have been used for wastewater treatment, effectively removing metal ions such as copper, zinc, manganese, and boron [82, 363-367]. Further studies should be 50
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carried out to recognize the potential of trace element-enriched zeolite application as feed supplementation.
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Most of the research has focused on enrichment of sorbents with various trace elements
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and using this as a supplement to animal feed. However, vitamins and antibiotics are also
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essential to the performance of livestock and prevention of disease. For example, pharmaceuticals are often used in farm animals for both therapeutic and prophylactic purposes,
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and are distributed through feed or drinking water [368]. Thus, drugs and/or vitamin-loaded feed
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supplements could be developed using sorption-based technology. Several researchers have reported that sorption technology has been successful in the removal/recovery of various drugs
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[369-374]. Thus, biomass used to feed animals could be tailored with drugs/vitamins, and should
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be considered a method for manufacturing feed supplements (Fig. 10). Because Fe, I, Zn, Ca,
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Mg, Cu and Se are often deficient in the human diet, these particular elements are important to consider when manufacturing feed-additives [135].
Fig. 10. Feed-additive applications of post-sorption technology Several challenges still exist for the development of feed supplements manufactured through biosorption technology. Manufacturing biosorbents enriched with the optimal concentration of trace elements is one of the greatest challenges. Before feed can be 51
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manufactured, it is essential to identify the deficiency and sufficiency levels within the diet for each type of animal [350]. Even though most studies have focused on enrichment with a single
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trace element, clearly it is essential to develop techniques to manufacture multi-mineral nutrients
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to meet accurate complex nutrient demands. Additionally, emission of ammonia from manure is
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a significant environmental challenges faced by the poultry industry. Supplementing dietary trace minerals (eg. zinc) may be one solution, because it is possible to reduce both nitrogen excretion
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and emission. In addition, mineral supplementation inhibits microbial uricase activity, which
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then decreases the production of ammonia [375]. Feed supplementation with zeolite could be particular useful as it has high selectivity for ammonia [230]. Farming strategies can be
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developed to include custom tailored post-sorbent that is rich in the specific trace elements or a
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combination of elements needed for each particular type of animal. Moreover, it is also essential
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to identify the sorbate (trace mineral)-sorbent (biosorbent) interactions and their metal complexation form. It is conceivable to regulate this supplementation and to determine the metalreleasing performance for each. Analytical determination of metal toxins in animal feed is often challenging due to the complexity of the feed matrices and extremely low concentrations of metal ions. This should be considered an important issue in manufacturing feed-additives. One of the major problems with feed additive use is that, as in humans, elevated levels of particular elements can lead to acute health problems in animals. Furthermore, consumption of the food materials (meat, eggs, milk and dairy products) derived from these animals may severely affect human health. Thus, the safety and toxicity of feed supplements should be thoroughly assessed prior to their application and supplementation should be carefully controlled, with particular care taken to measure any toxic effects of trace elements in that may 52
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be present in high quantities. Some biomaterial feedstocks contain potentially toxic elements, such as Pb, Cd, Hg, and As, that are harmful to animals and subsequently to humans [376]. For
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instance toxic effects of organic arsenical compounds that were widely used in poultry feed were
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well reported [377-379]; inorganic arsenic was found in meat and manure [378, 380]. Yao et al.
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[381] reported the quantitative delivery of roxarsone via chicken diet to rice plant and their subsequent environmental and health issues. Thus, before biosorbent feed is utilized, a complete
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elemental composition analysis should be carried out. Global meat consumption has undergone
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dramatic changes (increased), and it has been estimated that there will be a growing demand for animal-derived products and meat in coming decades [382, 383]. Meat is an excellent carrier of
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trace elements that provides them in an organic form [384, 385]. Thus, enrichment of meat with
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of investigation.
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supplements as a means of transfering essential elements to humans via the food-chain is worthy
Several international regulations address the impact of trace minerals on living organisms, providing stringent regulations on maximum supplementation levels [386]. The European Union, according to the Commission Regulations (EC) 1334/2003, has reduced the recommended trace element levels in animal feed in recent years to address the importance of food safety and the environment [387]. In the EU, animals intended for slaughter are permitted to have maximal levels of cobalt, copper, iron, iodine, manganese, selenium, and zinc in feed at 2, 25-35, 750, 10, 150, 05 and 150 mg/Kg, respectively [383, 388]. Furthermore, the toxic effects of certain metals, such as chromium, vary with oxidation states, so this should be taken into consideration [389, 390]. The use of organic arsenical compounds in broiler chicken industry as feed-additives was well-known; however, considering their toxic effects the US FDA banned 53
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arsanilic, carbarsone and roxarsone in 2013 [391]. Antibiotics used to boost growth of farm animals are now being regulated, and no trace of the parent compound or its metabolites may
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remain in food products, as per the European Union feed additive legislation (Council Directives
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70/524/EEC and 96/51/EC) [392]. Milk, cheese, meat, and eggs are a major food source.
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Because levels of trace minerals in these animals exist in their animal products, careful study and regulation is needed when manufacturing trace mineral laden sorbents (feed-additives), and in
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screening the produced materials to avoid food toxicity and environmental pollution.
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Although it seems there is a large scope available for the development of commercial feed additives through sorption-based technology still several limitations exists. Feed safety is
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most vital norm as the material used for animal feed should not be a threat to either animal or
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human health and also to environment. The most vital limitations in post-sorbent based feed-
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additives is the sorbed element should not exceed the maximum concentration levels if so it will causes damage to the animal and human health. In such case caution is required to dispose the manufactured feed safely otherwise it will contaminate the environment. Another most vital issue is some of the biomass-based materials used as sorbents are complex matrix that contains wide variety of elements and natural products thus complete sorbent analysis is essential. Loading of pharmaceuticals is needed additional care as it is well-known that pharmaceuticals undergo transformation depends on the environment, thus careful residual analysis is required to determine the presence of original drug or its metabolite residues in the animal meat or their excretions. Thus it is essential to carefully analyze the animal products to identify the presence of any harmful pharmaceutical by-products. Further research is required to study the risk assessment of post-sorbent based feed additives. Another significant limitation is cost of the feed 54
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SC
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manufactured through sorption-based technology should not exceed the available feed costs.
55
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SC
Goats
8.2% (Milk)b
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Estimated nutrient after feeding
Mn(II)
5.92±0.77a
Zn(II)
9.06±1.18a
14.6% (Milk)b
Fe(II)
11.9±1.5a
-----
Zn(II)
-
Cu(II) Cu(II)
Soya bean meal
Cu(II)
MA
11.7±1.5a
Spirulina maxima
Soya bean meal
Feeded Animal /Time
Cu(II)
PT ED
Macroalga Enteromorpha sp.
Post-sorption applications
CE
Soya bean meal
Micronutrient Content of microelement in feed
Reference
[353]
29.2% (Milk)b
Pig/3 months
>9.5% (blood)b
[357]
-
Pigs/87 days
18.3% (Meat)b
[358]
11.7±1.5a
Pigs/91 days
10.4% (Loin)b
[393]
-
AC
Sorbent
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Table. 4. Sorption based feed additives (nutrient-loaded adsorbents) and their characteristics
Fe(II)
11.9±1.5a
20.4% (Loin)b
Mn(II)
5.92±0.77a
9.0% (Loin)b
Zn(II)
9.06±1.18a
7.8% (Loin)b
Cu(II)
15.690 ± 0.370a Laying hens
-
56
[394]
Fe(II)
16.337 ± 0.228a
Cr(III)
20.588 ± 0.212a
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14.088 ± 0.403a
Cu(II), Zn(II), Mo(II), Co(II), Cr(III)
Laying hens
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Macroalgae
Zn(II)
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a= mg/g; b=percentage increase compared with control.
57
Cu-56%, Zn- 171%, Cr-723% [395] (Egg white)b
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2.4. Catalysts Catalysis is a key step that can be modified for developing sustainable practice necessary
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to produce chemical compounds [396]. Heterogeneous catalysis is an effective technique that
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involves the fabrication of a stable catalyst on a solid support [397]. The use of metal catalysts,
SC
such as Ni, Ru, and Pd, on a homogenous solid catalyst support enhances the catalytic activity [398-401]. Solid supports have often been required to prepare highly disperse metal catalysts,
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and they have several advantages, including recovery, regeneration, and reuse [402]. Adsorption
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has been used to adhere metal ions onto solid adsorbents, and these post-sorbents containing metal ions that have catalytic oxidation properties may be useful for the treatment of organic
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contaminants. In recent years, there has been much interest in reusable catalysts, which can be
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easily separated and easy to recover. Among these, magnetic and reusable catalysts have gained
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attention. Magnetic adsorbents have been widely used for water treatment, and these may exhibit exceptional sorption capacity for various metal ions. These metal- loaded magnetic sorbents can be efficiently used as catalysts, and therefore several attempts have been made to use metalloaded adsorbents for this purpose. Post-sorbents that had been loaded with metal ions have also been utilized as catalysts for the degradation of various toxic pollutants. Wu et al. [403] manufactured nickel-sorbed porous graphene oxide/sawdust composite (NiO@GNCC), and the post-sorbent was reduced by NaBH4 to create a catalyst this was further used for degradation of phenol in aqueous solutions. A high sorption capacity of 158.98 mg/g for phenol was accomplished using NiO@GNCC; however, reusability and stability of the catalyst was not studied. Biomass, such as fly ash and sawdust, has been used to recover nickel ions from wastewater, and then post-adsorbent was 58
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reused for the degradation of 2-chlorophenol [404]. Prior to use as catalyst, the post-adsorbent was dried, blended, and calcined. Improved surface area was observed for the catalyst compared
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with fly ash which plays a key role in the catalytical activity. Naturally, abundant fir sawdust has
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been used to preload Ni(II)/Fe(III), and then the sorbed biomass was pyrolyzed to obtain Ni-
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NiFe2O4/CNF catalyst [405]. Fast pyrolysis of metal preloaded biomass resulted in magnetic NiFe2O4, Ni(0) nanoparticles, and carbon nanofibers. This manufactured catalyst exhibited good
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performance in the hydrogenation of aromatic nitrobenzene to aniline without detectable by-
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products azobenzene or azoxybenzene. Catalyst also showed good reusable performance; even after 7 reused cycles no significant decrease in aniline yield was observed. High aniline yield
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(~100%) was accomplished with Ni-NiFe2O4/CNF, whereas individual Ni NPs, NiFe2O4, and
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CNF exhibited lower yields such as 53%, 29%, and 14%, respectively. Most of the sorption
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studies focused on the removal of single metal pollutant. However, wastewaters often contain metal mixture of both cationic and anionic metal ions. Considering this an attempt is made to remove cationic (Cu2+) and anionic (CrO42-) pollutants by using layered double hydroxide adsorbent [406]. The adsorbent was useful for treating both metal ions due to the synergetic effect between both Cu and Cr ions during sorption process. Further spent Cu/Cr loaded sorbent has been used as a catalyst for MO degradation. The catalytic activity of sorbent, Cu loaded sorbent, Cr loaded sorbent, and Cu/Cr loaded sorbent were evaluated and found that Cu/Crloaded spent sorbent was effective with the highest degradation (86.5%). In another approach, bacterial biomass was used as precursor to manufacture biomass-Pd catalyst and its catalytic efficiency towards Cr(VI) reduction was found to be 90% of Cr(VI) reduced bioPd D. desulfuricans and bioPdE. Coli after 30 min [407]. Catalysts manufactured through retrieval of metal ions from 59
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aqueous solutions were also used for oxidation of air pollutants, including volatile organic compounds. In one study, Cr(VI) loaded Arthrobacter viscosus supported on NaY zeolite was
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obtained through sorption experiments [408]. The manufactured materials were calcined at 400 o
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C in air flow and the prepared catalyst was used for the oxidation of ethyl acetate, ethanol and
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toluene in the following sequence: ethanol>ethyl acetate>toluene. Similar Cr-loaded sorbents were reported and used as catalysts for the catalytic oxidation of 1,2-dichlorobenzene [409] and
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cyclohexene [410]. These studies suggest that spent sorbents can be used as efficient catalysts for
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degradation of organic compounds and, in addition, most were easy to separate after the catalytic process was complete.
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Although the post-sorbents have shown good catalytic performance, still few limitations
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exist which hinders their practical applications. Most of the sorbent materials are prepared from
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expensive raw materials through complex synthetic routes to enhance the overall catalytic process. The researcher should focus on the fabrication of economic catalysts by utilizing lowcost biomass waste (sorbent) materials and utilizing metal (sorbate) contained in wastewater through facile synthetic routes. In addition to the studies in a laboratory scale, further pilot-scale studies are required to bring this technology to industrial scale. Major limitation that confines the practicality of this approach is the presence of toxic metal ions and nanoparticles in the postsorbent, which may be released into the adjacent environment during catalytic processing and after their usage (storage and landfills). Leaching of toxic metal ions and nanoparticles from the adsorbents and catalysts has been well reported [81, 411]. Leaching of metal ions causes the generation of Mn+ ions and sometimes development of nanoparticles. Among the various metal ions used in catalysis often nickel and copper are considered to be sustainable and green. 60
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However, Egorova et al. [411] discussed that these metal ions are also toxic. Safe storage or disposal of the spent catalysts comprising metals or nanoparticles is of high concern due to the
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stringent environmental legislations. For instance, USEPA classified spent catalysts as hazardous
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waste due to the existence of the considerable amount of heavy metal ions [412]. To overcome
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this limitation, it is feasible to fabricate Ca-based catalysts that are non-toxic and economical by loading Ca onto adsorbents [413].
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2.5. Bioactive compounds
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Along with the applications described thus far, metal-loaded sorbents have also been shown to be useful for other functions, including biological activity. Most of the post-sorbents
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exhibit disinfection properties and other shielding biological effects. These biological effects
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have been attributed to the constituent metal ions, particularly Cu, Zn, and Mn. As a result of these observations, post-sorbents have been evaluated to determine their antimicrobial and
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antioxidant effects, and here we summarize those that have shown interesting biological effects. Knowing that silver is a potent antimicrobial, biochar loaded with Ag+ was evaluated for its antimicrobial ability and found to efficiently inhibit the growth of Escherichia coli, whereas biochar without silver adsorbed promoted the growth of Escherichia coli [414]. In another study, Cu was loaded onto chitosan/nano Al2O3, and exhibited good antimicrobial activity against E. coli as compared to chitosan and chitosan/nano Al2O3 alone [415]. The effect was attributed to the presence of Cu2+, which has antibacterial properties. The adsorbent was also used for removal of As(III) with a sorption potential of 18.32 mg/g at pH 7. Bioactive ceramic hydroxyapatite [Ca10(PO4)6(OH)2] was used to retrieve chlorhexidine digluconate through an adsorption process, and the post-sorbent inhibited Enterococcus faecalis growth in vitro for up to 61
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6 days [416]. Serbian clinoptilolite was used for the adsorption of Zn(II) and exhibited maximum sorption capacity of 16.8 mg/g [417]. In the presence of air, the post-sorbent was thermally
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treated at 540 oC to produce ZnO-clinoptilolite. Both materials, Zn-clinoptilolite and ZnO-
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clinoptilolite, were tested against Acinetobacter junii, and the former exhibited good antibacterial
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activity. Four different algal biomass powders were loaded with zinc, copper and silver through sorption equilibrium and tested for their antimicrobial properties against Escherichia coli [418].
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Algae loaded with Ag had a strong bactericidal effect than Zn-loaded algae and algae alone. The
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microalgae Chlorella vulgaris loaded with polyphenols was evaluated based on antioxidant activity, and biomass enriched with quercetin has highest antioxidant activity [419]. These
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interesting findings suggested that post-sorbents that comprise biologically active metal ions,
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particularly Ag, Cu, and Zn can be employed as antibacterial, antifungal and antioxidant agents.
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Although post-sorbents have shown antimicrobial activities, further research is required to assess the biocompatibility and toxicity of these materials of these post-sorbents in various applications. 2.6. Miscellaneous applications
Other than the above applications focused on post-sorbents, few studies explored the recovery of adsorbate after post-sorption. Considering the cost of critical metals and their resource limitations, several studies have focused on the recovery of metal ions from postsorbents using incineration or pyrolysis techniques [420-424]. Bark species were used in one study for the removal of copper and incinerated in the presence of air and nitrogen [420]. A thermogravimetric and X-ray powder diffraction (XRD) analysis of incinerated ashes indicated that CuO (≈80%) and Cuo (≈14%) were present in the produced solid. In another study, olive solid waste generated from an olive oil mill was used for copper and nickel sorption, and 62
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accomplished a sorption capacity of 3.6 and 1.7 mg/g for copper and nickel, respectively [421]. Metal-loaded biosorbent was burned at 850 oC in an oven and the resulting ashes contained 96%
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of each metal. Desorption is the most often used conventional process for the recovery of sorbate
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from adsorbent, however, this process requires a large amount of desorbing agent. This will
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invariably increase cost of sorption based technology, besides secondary pollution is possible through this approach. Liu et al. [425] made an attempt to understand the technical and economic
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feasibility of biosorption-pyrolysis technology compared with biosorption-desorption process for
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lead recovery. The results demonstrated that biosorption-pyrolysis technology cost $ 0.06 per ton wastewater is more beneficial compared with biosorption-leaching process which costs $0.19 for
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HCl and $ 4.41 for EDTA to treat per ton wastewater. Considering the importance of precious
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metal ions in a wide variety of applications and their resource limitation, our group investigated
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gold and ruthenium recovery after post-sorbent incineration [422, 423]. We successfully recovered 99.75% of Ru and more than 91% of gold through a sorption-incineration approach. In another study, the precious metal platinum was recovered using an amine modified corn husk, followed by calcination of the post-sorbent at 800 oC for 12 h [426]. The results revealed that 54% of the calcined powder was platinum. Dye-loaded biosorbent has been used to manufacture biochar material [54]. Through a pyrolysis technique, dye-laden biosorbent was further converted into biochar, which was then used for methylene blue (MB) removal. Biochar showed good removal performance at lower initial MB concentration when compared to other common biosorbents. In another study methylene blue loaded carboxymethyl cellulose was used for methyl orange (MO) removal, MB loaded CMC exhibited high sorption efficiency of > 100 mg/g for MO and is much higher than 63
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CMC sorption efficiency for MO [427]. Post-sorbents have also been used for fuel cell technology. In a fuel cell, yeast biomass loaded with Pt (PtCl62-) was used as an electro-catalytic
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anode to generate electricity from glucose and ethanol [428]. Metal-loaded adsorbent was also
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used to obtain metal oxide nanoparticles; specifically, clinoptilolite was shown to remove Ni(II)
resulted in nano-sized NiO particles of ~5nm [429].
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ions from aqueous solutions. Thermal treatment of post-sorbent (Ni(II) loaded clinoptilolite)
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Interestingly, only a few post-sorbent applications in energy production have been
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proposed [52, 53, 430]. Energy is one the most vital 3E issues. It is a key factor for the economic development of all countries as industries and individuals alike are highly dependent on energy
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supplies. The amount of available petroleum fuels is diminishing, and there is a lack of
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acceptance of nuclear energy [431-434]. To address these problems and meet the future energy
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demands of growing populations and increasing industrial activities, utilization of alternative energy resources, such as wind, tide, solar, geothermal and biomass, is becoming more important [303]. Among these, bioenergy produced from biomass has become an alternative to fossil fuels with great potential to provide a stable source of power. For instance, in the United States, use of biomass-derived energy increased by 60% between 2002 and 2013 [435]. Considering the importance of energy production from biomass derived materials, few studies have been conducted to evaluate the post-sorbents for power generation. Converting biomass into valuable energy and chemical products through pyrolysis is one of the frontier thermochemical technologies; this process results in gas (pyrolytic gas), liquid (bio-oil), and solid (bio-char) [436-438]. Bio-oil and/or gas obtained through biomass pyrolysis are used for energy production. Several researchers made attempts to utilize the exhausted 64
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biosorbent (post-sorbent) to get valuable resources through pyrolysis, combustion and gasification process, while simultaneously reducing the volume of waste. However, biomass
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pyrolysis depends on various parameters such as biomass pretreatment, heating rate, reaction
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atmosphere, temperature and biomass source [439]. Exhausted or spent sorbent consists of metal
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ions which influence the biomass pyrolysis process. Thus it is essential to evaluate the sorbate effect on pyrolysis process. As an example, pine cone shell biomass was used for Pb(II) and
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Cu(II) removal from aqueous solution in a packed bed column [52]. Effects of Pb(II) and Cu(II)
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on thermal decomposition of the biosorbent were studied, revealing that these metals did not influence the major degradation pathways. Thermal decomposition of the metal-loaded
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biosorbent resulted in chars, with 95% and 99% recovery of copper and lead, respectively.
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Combustion of olive tree pruning biomass loaded with Pb(II) was studied in a thermobalance, a
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laboratory flow reactor and an atmospheric oven [53]. The total concentration of Pb(II) remains in the ashes suggesting that this does not influence the usage of biomass as fuel. However, high amount of Pb(II) >35% present in ash should need further treatment or applications should be explored considering the toxic nature of lead. Pine cone shells were used in one study to sorb copper from aqueous solution and achieved a sorption capacity of 6.81 mg/g before the postsorbent was thermally decomposed [430]. Both sorbent and post-sorbent exhibited similar pyrolysis and combustion behaviors, with major compounds being CO, CO2, CH4, C2H4, CH3OH, and SO2. These studies suggest the feasibility of application of metal loaded sorbents for energy production.
3. Conclusions and future perspectives Adsorption is one of the key techniques that have been used in separation science to 65
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retrieve various pollutants and analytes from the aqueous phase, and therefore, has attracted substantial interest for scientific and commercial applications over the past several decades. Due
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to the accessibility of widespread, naturally abundant adsorbent materials, as well as the
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emergence of new generation materials, much research has gone into exploring the use of these
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materials in water treatment applications. High sorption efficiencies, rapid removal rates, and selectivity are the chief properties to consider when designing adsorbents, whereas less attention
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has been paid to post-sorbent management or possible further utilization of post-sorbents. In
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recent years, there has been increasing interest in the development of sustainable technologies to minimize the cost of materials/techniques and to utilize resources more efficiently. Recent
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reports on sorption-based technology have shown that post-sorbents with multiple functionalities
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provide an opportunity for diverse applications. Post-sorption has been explored as a useful
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method for the manufacture of value-added key products such fertilizers, catalysts, nanoparticles, antimicrobial agents, and feed-additives. This sustainable materials production pathway is economically and ecologically appropriate. This review summarizes the relevant work on postsorption applications, strategies, challenges, reported applications, and issues surrounding toxicity. Major challenges involved in the efficient development of these materials for the above applications include the stability of the sorbent, controlling the release rate of an element from the sorbent, and toxicity issues. The major conclusions drawn are as follows: (i) Adsorption is often used to retrieve various pollutants from aqueous solutions using diverse adsorbent materials, and these could potentially be applied to the production of new value-added products. (ii) Natural, low-cost materials loaded with macronutrients, such as N and P, can be utilized as 66
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fertilizer with controlled slow-release rates for soil applications. (iii) Prior to the application of post-sorbent as fertilizer or soil conditioner it is essential to
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determine the levels of PTEs present.
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(iv) Feed-additives, manufactured using various types of biomass, including microbial biomass,
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can supply the essential trace elements required for animal husbandry.
(v) Carbonaceous metal nanoparticles can be synthesized using biomass laden with metal ions,
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and successfully used as catalysts and adsorbents.
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(vi) Dye-loaded adsorbents could be used to manufacture value-added biochar materials. The dye-loaded sorbent may be further mixed with metal additives to manufacture carbonaceous
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nanoparticles.
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subsequent adverse effects.
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(vii) Regulations should be followed closely when manufacturing feed additives to avoid any
(viii) Several other applications, such as energy and biological applications (antimicrobial and antioxidant), should be explored more fully.
Post-sorbent based functional materials will play a major role in the development of value-added green materials production in the near future. Adsorption technology can be used to engineer tailored post-sorbents that have a wide variety of functional elements/materials, as outlined in this review. More versatile, custom, and facile manufacturing strategies are expected to emerge in the future for the continuous advancement of post-sorbent applications. Though several challenges remain, the rapid development of post-sorbent applications in various fields foreshadows a bright future for sustainable adsorption technology. Since exploration of post67
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sorbent applications as value-added products has just begun, there is room for further studies in this area to reveal the opportunities that may exist. Designing and manufacturing green
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adsorbents using non-toxic and economical precursors will provide an opportunity for successful
Acknowledgements word
was
financially supported
by
the
Korean
Government
through
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This
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utilization of post-sorbents loaded with nutrients and elements as feed-additives and fertilizers.
NRF
(2014R1A2A1A09007378) grants. One of the authors D.H.K. Reddy acknowledges the BK21
List of abbreviations
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plus Postdoctoral Fellowship for financial support.
Biochar
CEMNs
Carbon encapsulated metal nanoparticles
CEMNPs CEN CMC EC
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CEC
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BC
Cation exchange capacity Carbon-encapsulated metal nanoparticles European Standardization Committee Carboxymethyl cellulose Commission Regulations
EU
European Union
FAO
Food and Agriculture Organization
FDA
U S Food and Drug Administration
Fe/Fe3C@PCS
Porous carbon sheets containing carbon-encapsulated Fe/Fe3C
IBI
International Biochar Initiative
Mag-PCMA
Magnetic permanently confined micelle arrays
MB
Methylene blue 68
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Metal nanoparticles
MO
Methyl orange
MWCNTs
Multiwalled carbon nanotubes
NiO@GNCC
Nickel sorbed porous graphene oxide/sawdust composite
NiFe2O4/CNF
Ni–NiFe2O4/carbon nanofiber
NPs
Nanoparticles
NUE
Nutrient use efficiency
PAHs
Polycyclic aromatic hydrocarbons
PSA–GO
Poly(sodium acrylate)–graphene oxide
PTEs
Potential toxic elements
SOC
Soil organic carbon
SWCNTs
Single walled carbon nanotubes
TCLP
Toxicity characteristic leaching procedure
USEPA
US Environmental Protection Agency
VOC
Volatile organic compounds
XRD
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WHO
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MNPs
World Health Organization X-ray powder diffraction
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Graphical Abstract
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Highlights Review of the valorisation of post-sorption materials.
Strategies required for tuning post-sorption materials are discussed.
Advantages and toxicity issues were highlighted.
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