Dendrimers, mesoporous silicas and chitosan-based nanosorbents for the removal of heavy-metal ions: A review

Accepted Manuscript Title: Dendrimers, mesoporous silicas and chitosan-based nanosorbents for the removal of heavy-metal ions: A Review Author: E. Vunain AK Mishra BB Mamba PII: DOI: Reference:

S0141-8130(16)30130-1 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.02.005 BIOMAC 5814

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

18-12-2015 28-1-2016 1-2-2016

Please cite this article as: E.Vunain, AK Mishra, BB Mamba, Dendrimers, mesoporous silicas and chitosan-based nanosorbents for the removal of heavymetal ions: A Review, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.02.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dendrimers, mesoporous silicas and chitosan-based nanosorbents for the removal of heavy-metal ions: A Review E. Vunain*, AK Mishra*, BB Mamba Nanotechnology and Water sustainability Research Unit, College of Science, Engineering and Technology, University of South Africa, Florida Campus, Johannesburg, South Africa *

Corresponding authors

Email: [email protected]; [email protected]

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Abstract The application of nanomaterials as nanosorbents in solving environmental problems such as the removal of heavy metals from wastewater has received a lot of attention due to their unique physical and chemical properties. These properties make them more superior and useful in various fields than traditional adsorbents. The present mini-review focuses on the use of nanomaterials such as dendrimers, mesoporous silicas and chitosan nanosorbents in the treatment of wastewater contaminated with toxic heavy-metal ions. Recent advances in the fabrication of these nanoscale materials and processes for the removal of heavy-metal ions from drinking water and wastewater are highlighted, and in some cases their advantages and limitations are given. These next-generation adsorbents have been found to perform very well in environmental remediation and control of heavy-metal ions in wastewater. The main objective of this review is to provide up-to-date information on the research and development in this particular field and to give an account of the applications, advantages and limitations of these particular nanosorbents in the treatment of aqueous solutions contaminated with heavy-metal ions. Keywords: Dendrimers; mesoporous silicas; chitosan; heavy-metal; remediation

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INTRODUCTION The term “nano” is derived from the Greek word for “dwarf”. A nanometer (nm),

from Greek “nanos” for “dwarf” is equal to one billionth of a meter or 10-9 of a meter. Nanotechnology is the art and science of manipulating matter (with at least one dimension size from 1 to 100 nanometers) at the atomic, molecular or supramolecular scale. At the nanoscale level, materials are characterized by different physical, chemical and biological properties than their normal size equivalent [1]. At nanoscale level, the surface area of particles increases with decreasing particles. Nanoscale particles (i. e., particles within the nanometer size-range) exhibit different mechanical, thermal, optical, electrical and magnetic properties from the properties exhibited by macroscopic particles [2–5]. At the nanoscale level, as the particle size decreases, the ratio of surface area to volume increases so much so that the surface properties become the dominant factor. For example at nanoscale level, materials such as polymers and ceramics, metals, metal oxides, carbon derivatives (carbon nanotubes and fullerenes) show a high ratio of surface area to particle size. This large surface

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area provides various unique properties that have widespread applications in a range of industries. Nanotechnology has been useful in the manufacture of products and devices with dimensions in the nanosize range. Nanotechnology is sometimes referred to as a general purpose technology because it has a significant impact on almost all industries and all areas of society [6–12]. Nanotechnology focuses on the fabrication, characterisation and manipulation of substances at sizes in the nanoscale range of approximately between 1 and 100 nm. When materials have one or more dimensions less than 100 nm, the general rules of chemistry and physics no longer apply. As a result, these materials start to show unique and sometimes surprising properties. The rate of reactivity and their ability to conduct electricity increases drastically. For example, reduced to the nanoscale size, solids such as silver show increased anti-microbial properties, gold turns into liquid at room temperature, and inert materials like platinum and gold become catalysts, etc. Generally, materials with smaller particle size in combination with an increased surface area tend to exhibit unique and novel properties, thus creating a vast potential for a wide range of applications. Most importantly, the potential benefits of nanotechnology have already been identified by many researchers in areas such as health, agriculture, energy and water [9,10,13–16]. Nanotechnology offers leapfrogging opportunities in providing new technologies for the treatment of water and wastewater, and once fully utilised, is bound to make a major contribution to achieving the ultimate goal of clean water supply. One of the promising and well-developed environmental applications of nanotechnology has been in water and wastewater treatment where different nanomaterials are being utilised to help purify water through various mechanisms such as adsorption and sequestration of heavy-metal ions and other pollutants, removal and inactivation of pathogens, and finally the transformation of toxic materials into less toxic compounds [16–23].

1.1

Nanomaterials Nanomaterials, also referred to as nanoparticles, are defined as materials within the

size range of 1-100 nm. At the nano-range size, materials often possess novel size-dependent properties quite different from their bulk properties, with many of these properties such as high surface area, high reactivity, fast dissolution and strong sorption being used in water and wastewater treatment [18,24–31]. Nanomaterials have gradually developed an important role in solving water and wastewater problems because of their physical and chemical properties 3

such as enhanced active sites, and abundant functional groups on their surfaces, high activity for adsorption and photocatalysis (high surface to volume ratio), and antimicrobial properties for disinfection. Furthermore, magnetism or other unique optical and electrical properties also make these materials excellent candidates for wastewater treatment [16,32,33]. In addition to water treatment, current research focuses on the development of nanomaterials in other fields such as cosmetics and personal care items, electronics, pharmaceuticals, transport, construction, medicine, agriculture, energy, and sensors, just to name a few [34–37]. Other applications of nanomaterials make use of their discontinuous properties such as superparamagnetism, quantum confinement effect, etc. Nanomaterials have been produced in different shapes and sizes, integrated with a wide range of components and functionalised with a wide range of active components for various applications. For instance, nanomaterials have been incorporated in nanostructured catalytic membranes for application in water treatment. Generally, nanomaterials can be classified as carbon and non-carbon materials as shown in Figure 1 [38]. ]. Carbon-based materials include carbon nanotubes and graphene while the non-carbon materials are inorganic nanomaterials based on metal, metal oxides and quantum dots. However, nanomaterials made of a combination of different materials are currently being developed [39]. Nanomaterials find application in water treatment, which is to reduce concentrations of toxic components (e.g., metal ions, organic and inorganic compounds, radionuclides, as well as bacteria and viruses) to microliter per liter (µL/L) levels [40].

1.2

Nanomaterials as adsorbents For an excellent adsorbent to remove a large amount of pollutant in a short period of

time, it should possess a high surface area, fast adsorption rates and short adsorption equilibrium times. Nanomaterials have been used as nanosorbents due to their high adsorption capabilities because of their large surface areas, i.e. surface area per unit mass and specific functionalities. In addition, nanosorbents can be transported effectively in pore media because they are smaller than the relevant pore spaces and as such they tend to be highly mobile in such media. In their application in water treatment, these sorbents can be employed in situ, within the contaminated zone where treatment is required. Their ability to remove pollutants from subsurface and other environments that are very difficult to access in situ and doing so rapidly and effectively within reasonable costs is the ultimate goal [40–47]. Nanosorbents employed as separation media in water purification to remove inorganic and 4

organic pollutants from contaminated water are nanomaterials, including nanoparticles. The potential of nanomaterials as adsorbents in water treatment cannot be overemphasized. The literature review indicates that much research has been published with the main goal of investigating the removal of pollutants (either in gas or liquid medium) from water and wastewater using nanomaterials as sorbent materials [47–58].

1.3

Objective of this review

The purpose of this paper is to discuss the application of different nanosorbents for the removal of heavy-metal ions from water and wastewater. The primary focus of this review is on nanomaterials such as dendrimers, mesoporous silica materials and chitosan nanomaterials which are currently used in environmental remediation and particularly in the removal of heavy-metal ions from water and wastewater. Different parameters related to adsorption isotherms and kinetics are reported. It is our hope that this review will provide a clearer view and will inspire ideas for the development of next-generation nanomaterials for the removal of heavy-metal ions from water and wastewater. We also trust that the information contained in this review will assist in speeding up the commercialisation of these nanomaterials as alternatives to other adsorbents already in the market. Finally, in this paper some future perspectives are offered to stimulate more exciting developments in this area of water treatment.

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WATER POLLUTION

Water is required for the maintenance of all life forms. Safe drinking water or water that is free of pathogens and toxic chemicals is essential for human health. Clean water also serves as an important feedstock to industries such as pharmaceuticals, food, electronics, etc. It should be noted that accessibility to clean and affordable water is considered one of the most important humanitarian goals and still remains a challenge. The world’s rapid economic development in recent years has led not only to rising incomes, but also to accelerated urbanisation and an uncontrolled population growth [59]. As a consequence, our world is facing a variety of environmental challenges such as water pollution [60–64], excessive generation of solid waste [65,66], climate change/global warming [67], leading to serious environmental degradation in the long term. These environmental problems have a negative impact on economic and social progress. Thus, to effectively address these problems in order to achieve environmental sustainability, pioneering approaches in tackling global water 5

pollution are required [68]. Access to safe drinking water is very important to protect public health and has become a basic necessity of all properly functioning societies. Furthermore, according to estimates from the World Health Organization, 1.1 billion people worldwide still lack access to improved drinking water sources, while about 2.4 billion people do not have access to basic sanitation [69]. Indeed, the health and safety of people, especially the vulnerable groups such as children, the elderly and the poor are closely linked to the accessibility of adequate, safe and affordable water supplies [70,71]. This situation stands to worsen as most freshwater sources have become contaminated by pollutants from natural and industrial sources. Despite their presence at low concentrations, environmental pollutants pose serious treats to freshwater sources, public health and all living organisms [68,72,73].

2.1

Heavy-metal pollution Heavy metals occur as natural constituents of the earth’s crust and are therefore found

naturally in rocks and soils. Although, there is no clear definition of a heavy metal, they are generally regarded as any dense metal or metalloid of environmental concern having atomic weights of between 63.5 and 200.6 Dalton and specific gravity greater than 5 g/mL [74,75]. Toxic heavy metals of concern in water and wastewater include cadmium, lead, copper, chromium, mercury, uranium, nickel, cobalt, zinc, arsenic, selenium, thallium, and antimony [76]. These heavy metals are released to the environment as a consequence of both natural and anthropogenic (human) activities such as mining, electroplating, metal finishing, painting and printing processes, phosphate fertiliser manufacturing, discharge of domestic waste, burning of fossil fuels such as coal, petrol and kerosene, etc., and automobile exhausts (for lead) [77–81]. Heavy metals have been used by humans for decades and when they leach into groundwater, moving along pathways and eventually being deposited in aquifers or are washed away and enter surface waters from runoff, the result is water pollution [79]. In addition, once these heavy metals are released, they tend to bioaccumulate in higher trophic levels of the food chain [82]. Due to accelerated industrial activities and technological development, there has been an increase in the release of these heavy metals into the environment, thus posing a significant threat to the environment and public health. Heavy metals cannot be destroyed or degraded and moreover, mineralisation (degradation) is a very slow process under natural conditions. Immobilisation and concentration of metals onto suitable sorbents seem to be a viable option for their removal from water and wastewater. In this way, the sorbed metals can be removed and reused as raw materials [82]. 6

2.2

Hazardous effects of heavy metals

Heavy metal-induced toxicity has been studied extensively and water contaminated with heavy metals therefore poses serious health risks. Heavy-metal ions are non-biodegradable pollutants in water; they have the ability to persist in natural ecosystems for long periods and to accumulate in food chains and living tissues, and prolonged exposure to high levels of heavy metals causes various diseases and disorders [83–85]. Furthermore, the indiscriminate disposal of domestic waste (containing heavy metals) and industrial waste into aquatic ecosystems continues to threaten the inhabiting aquatic organisms [86]. Toxic heavy metals have been prioritised as the major inorganic contaminant in surface water and groundwater as well as in the environment due to their mobility in aquatic ecosystems and toxicity to higher life forms [87]. Water is the main source of entry of these noxious elements into the human body; beyond the permissible limits, these elements cause direct toxicity to humans. For example, arsenic contamination in drinking water poses health risks for more than 150 million people all over the world [88]. Cadmium is a toxic heavy metal of environmental and occupational concern. Its toxicity may produce bone defects (osteomalacia and osteoporosis) in both humans and animals [89]. It has been identified as a human carcinogen and teratogenic substance severely impacting kidneys, liver, lungs and reproductive organs [90,91]. Lead poisoning is associated with gastrointestinal disorders, constipation, abdominal pains, and central nervous system effects [92]. High concentrations of mercury cause impairment of pulmonary function and kidney, chest pain and dyspnoea [93]. Chromium exists in the environment both as Cr(III) and Cr(VI) forms. However, the Cr(VI) is far more toxic than Cr(III) and exposure to Cr(VI) causes skin irritation, lung cancers, as well as kidney and liver damage [94,95]. Zinc toxicity is not a common problem but can occur due to excessive ingestion. However, it may cause severe damage to various systems in the human body [96]. Mercury and its compounds accumulate in the brain and the central nervous system (CNS) and have neurological and psychiatric effects [97,98]. It is clear that the list of toxic heavy metals is endless. Considering the health risks associated with the exposure to these toxic heavy metals, even at low concentrations, and unless properly treated, their ingestion at levels that exceed regulatory standards causes serious health disorders. Thus, the need for their removal from water and wastewater is currently a major regulatory and academic concern. Considering the health effects associated with these heavy metals, a number of organizations, including the World Health Organization (WHO) and the United

7

States Environmental Protection Agency (USEPA) have defined maximum contamination levels for some metals (Table 1). Faced with more stringent regulations, these metal-ion pollutants are of primary concern in water pollution and are fast becoming one of the most serious environmental problems. It is thus important to prevent heavy-metal pollution. Research is focused on the development of suitable technologies either to prevent heavy-metal pollution or to reduce it to a very low level in order to protect humans and the environment.

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CONVENTIONAL PROCESSES FOR THE TREATMENT OF HEAVY

METALS Many methods are being used for the removal of heavy metals from water, which include the conventional processes such as chemical precipitation, ion exchange, flotation, electrochemical deposition, and adsorption [102]. In industry, chemical precipitation is the most widely used treatment process for heavy-metal removal from inorganic effluents [102–104]. The conceptual mechanism of heavy-metal removal by chemical precipitation is represented in Eq. (1) [102].

M 2  2(OH )   M (OH ) 2 

(1)

Where: M2+ and OH− represent the dissolved metal ions and the precipitant, respectively. M(OH)2 is the insoluble metal hydroxide. In this process, the metal is precipitated from solution in the form of hydroxide [77]. After a pH adjustment to the basic conditions of pH = (9-11), the dissolved metal ions are converted to the insoluble solid phase via a chemical reaction with a precipitant such as lime [80, 105]. The fact that the pH adjustment within the basic range greatly improves the removal of heavy metal using this process is noteworthy. Lime and limestone are commonly employed as precipitant agents due to their availability and low cost [106,107]. Lime precipitation is a simple process, inexpensive in terms of equipment, and convenient and safe to operate. One of its main limitations, however, is that it requires a large amount of chemicals to reduce metals to an acceptable level for discharge. Other drawbacks include slow metal precipitation, poor settling, sludge production that further requires further treatment, aggregation of metal precipitates and the long-term environmental impacts of sludge disposal

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[107]. The conventional chemical precipitation processes include hydroxide precipitation and sulphide precipitation. Ion exchange is another method used successfully in the industry for the removal of heavy metals from effluents due to its many advantages such as high removal efficiency, high treatment capacity and fast kinetics [108]. Ion exchange is the process through which ions in solution are transferred to a solid matrix which in turn releases ions of a different type but of the same charge [109]. Commonly used matrices for ion exchange are synthetic organic ionexchange resins and different types of ion-exchange materials are available commercially. Many researchers have demonstrated that zeolites exhibit good cation-exchange capabilities for heavy-metal ions under different experimental conditions [110–113]. In addition, clinoptilolite, a natural zeolite, has received much attention due to its selectivity for heavy metals. The main advantages of ion-exchange processes are selectivity, convenient recovery of the metals and less sludge volume produced. The limitation of this process is that it cannot handle highly concentrated solutions as the matrix is easily fouled by organics and other solids in the wastewater [102]. Overall, most of these conventional processes suffer from some drawbacks ranging from incomplete removal of pollutants or high-energy cost requirements to the production of toxic sludge [114,115].

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SORPTION ON NEW ADSORBENTS FOR THE TREATMENT OF HEAVY

METALS Adsorption is one of the most promising processes over other processes for the removal of heavy metals because it is rapid and convenient. It produces non-toxic by-products; it also has low initial cost and it is rather simple in terms of design and operation of the treatment unit [116–119]. Most recently, adsorption has become one of the alternative treatment technologies for the treatment of water and wastewater containing heavy-metal ions. Adsorption currently is the most effective method for the removal of trace concentrations of heavy metals in water. Sorption is transfer of ions from solution phase to the solid phase. Sorption describes a group of processes which include adsorption and precipitation reactions and is a mass-transfer process by which a substance is transferred from the liquid phase to the surface of a sorbent, and becomes bound by physical and/or chemical interactions [120]. In general, the three main steps involved in the sorption of a pollutant by the solid sorbent are: (i) the transport of the pollutant from the bulk solution to the sorbent surface; (ii) adsorption on the particle surface; and (iii) transport of the pollutant within the sorbent material. Cost9

effectiveness and technical applicability are the necessary key factors that play a major role in the selection of the most suitable adsorbents for the treatment and removal of heavy-metal ions from water and wastewater. Many adsorbents have been developed for the removal of heavy metals from water and wastewater [121–131]. The prerequisites for ideal adsorbents are: extremely high adsorption capacity, minimum contamination of the water (e.g., minimum ion exchange), zero desorption under environmental conditions and economics of operation. The main purpose of this paper is to provide a review of the use of dendrimers, mesoporous materials, and chitosan nanosorbents as an option in the removal of heavy-metal ions from water and wastewater, so as to improve water quality for social and economic benefits. The authors trust that this paper will inspire further research on the use of these nanosorbents for the removal of heavy metals from water and wastewater.

5.

DENDRIMERS AS NANOSORBENTS

Dendrimers are highly branched polymers with controlled composition and nanoscale dimensions. They form a unique class of synthetic polymers which play an important role in emerging nanotechnology [132,133]. The word dendrimer is derived from the Greek word “dendri”- (tree branch-like) and “meros” (part of) and describes the architecture of this emerging class of polymeric macromolecules which possess 3-D features. Dendrimers consist of three covalently bound components, mainly (a) an interior core; (b) interior branched cells (known as generations) composed of repeating units; and (c) the exterior branched cells (known as surface groups) which are attached to the outermost interior generations [134] (Figure 2). Dendrimers have hyperbranched structures and contain a large number of voids between the branches where other molecules can be trapped. As such, they have become one of the research hotspot areas for host-guest chemistry. The field of dendrimer chemistry has fascinated and challenged researchers for quite some time now, but its commercial applications still remain rather limited. However, numerous applications of dendrimers have been proposed and the application of dendrimers in these areas has been largely due to their unique structural features such as nanoscopic size, spheroidal surface, highly branched architecture, open interior cavities, particle-like topography with numerous end-groups, and exciting properties such as high solubility, high reactivity, and low viscosity [135]. These properties in combination with their high functionalities make them widely applicable in a 10

range of fields such as water treatment [136,137], separating agents [138], drug-delivery systems [139,140], gene therapy [141], catalysts [142–144], electronic applications and chemical sensors [145–147] etc. Their size, shape and topography as well as functionality can easily be controlled at the molecular level and as such they could be employed in this range of applications at the nanomaterial level. The 3-D shapes and the presence of multiple functional groups in dendrimers make them very useful for attracting ions and molecules. The chemistry and the nature of the interactions depend on the presence of functional groups, porosity of the dendrite and the nature of the incoming species. The synthesis and applications of dendrimers and hyperbranched dendrimers have been reviewed [146–149]. The cornerstone in dendrimer chemistry is the synthesis of monodisperse dendrimers which relies on complete substitution of the functional groups. It has been reported that functionalising the dendrimers with photoactive groups expands the possibilities for their application in different fields such as biology, chemistry, physics and medicine [148]. Among the dendritic sorbents for water treatment, polyamidoamine (PAMAM) dendrimers are more promising for the removal of heavy metals from wastewater because they contain numerous cavities and are easy to functionalise with a range of chemical groups that can selectively chelate metal ions from solutions [150–152]. PAMAM dendrimers developed in 1985 by Tomalia et al. [134], are biocompatible, possess terminal modifiable amine-functional

groups,

and

are

non-immunogenic

and

water-soluble.

Indeed,

polyamidoamine (PAMAM) stand as the most studied dendrimer. It is amongst the least toxic and is made from inexpensive, readily available materials [153]. Due to the presence of functional nitrogen groups such as amines and amides and the high density of nitrogen ligands, these PAMAM dendrimers have a strong binding affinity for toxic heavy-metal ions in aqueous solutions. Furthermore, their excellent solubility in water, as well as their reactive amine or ester groups on the periphery, allows for the preparation of polychelatogens with selective complexation of heavy-metal ions [152]. As the complexation of ions with the dendrimer is pH-dependent, the release of metal ions from dendrimers can be achieved by the protonation of amine functional groups at low pH [154]. The adsorbed ions can be readily recovered in a concentrated form for reuse or disposal as the need may be. Thus, these characteristic features make PAMAM dendrimers promising as reusable agents for metal ion separation/recovery from solution. Furthermore, tailoring physical and chemical properties of dendrimers to suit particular applications has considerably enhanced the importance of this novel nanomaterial in various fields. Controlled modification of the interior core, the type 11

and number of repetitive branch units and the terminal surface groups can have a major influence on the structure and chemical properties of dendrimers.

5.1

Dendrimers as sorbents for the removal of heavy metals from aqueous solutions

Dendrimers are a unique class of macromolecules with highly branched, 3-dimensional architecture with high functionality. Dendrimers have a high loading capacity for the encapsulation of pollutants (e.g., heavy metals, polycyclic aromatic hydrocarbons and dyes). Dendrimers and their derivatives are substances with diverse analytical, biomedical and environmental applications [152,155–163], due to their nanocavities (void spaces), easy functionalisation and manipulation of their terminal groups [156,164,165]. Diallo and coworkers [166], first reported the use of dendrimers for the removal of metal ions from water. They used polyamidoamine dendrimers terminated with primary amine as a new class of chelating agents, which were able to bind Cu(II) ions. Later on, Rether and co-workers [152], used PAMAM dendrimers modified with N-benzoylthiourea for selective and recovery of heavy metals such as Co2+, Cu2+, Hg2+, Ni2+, Pb2+ and Zn2+ from wastewater. Around that same period a breakthrough was made by Kovvali and co-workers [167] when they succeeded in developing an immobilised liquid membrane by immersing porous polymer film in pure dendrimer to produce a liquid membrane for the selective separation of CO2 from other gases. Since then, much research has been focused on the use of dendrimers for the removal of heavy metals from water. Researchers have exploited the unique properties of dendrimers for the removal of heavy-metal ions from water and wastewater. More recently, Barakat and co-workers [168] successfully removed Cu(II), Cr(III), and Ni(II) ions from a synthetic water solution by employing generation 4 polyamidoamine (PAMAM) dendrimers with ethylenediamine (EDA) cores (G4-OH) immobilised on titania (TiO2). In this synthesis process, G4-OH dendrimers were immobilised on titania in a slurry process as represented in Figure 3. The scanning electron microscopy (SEM) results of dendrimer/titania composites are shown in Figure 4(a, b). Seen on Fig. 4 (b) is a porous surface with regions not covered by titania and large aggregates of nanoparticles (Fig. 4 (b)) [168]. The pore formation was achieved via removal of the embedded dendrimer by the use of the activated oxygen gas treatment and the titania framework was very stable after removal of the template framework and the pores were replicated from the template dendrimer [168].

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By employing the metal chelating composite as sorbent materials, metal-ion removal was achieved over a wide range of metal concentrations (up to 800 mg/L) within 1 hour at equilibrium time. A maximum metal-ion removal was achieved at pH 7 for both Cu(II) and Cr(III) ions and at pH 9 for Ni(II) ions. As can be seen in the results of this study, Cu(II) and Cr(III) were more easily removed than Ni(II) due to its stability in a basic solution. Furthermore, the equilibrium adsorption data were analysed using adsorption isotherms. The Freundlich isotherm model provided the best fit for the experimental data for the simultaneous sorption of the three metal ions investigated, with linear plots indicating the high affinity of dendrimers towards all three metal ions. The fitting of metal adsorption data to the Freundlich isotherm could well explain the simultaneous removal of heavy metals and this finding was consistent with previous work carried out by Miretzky et al. [169]. Chih-Ming and Hsing-Lung [170] describe the synthesis of dendrimer-conjugated magnetic nanoparticles (Gn-MNPs) as reusable adsorbents for the removal of Zn(II) in water. In this study, the unique adsorption properties of dendrimers and magnetic nanoparticles (MNPs) are combined for the effective removal and recovery of Zn(II) in water. Studies on the effect of pH indicated that Zn(II) adsorption with Gn-MNPs is a function of pH. Zinc ions (Zn(II)) were readily desorbed at pH < 3. The regeneration of Gn-MNPs could be achieved by using very small amounts of dilute HCl solution (0.1 M) and Zn(II) was recovered in the concentrated form. The maximum adsorption capacity of Gn-MNPs as determined by the Langmuir isotherms was 24.3 mg/g at pH 7 and at a temperature of 25 °C. This study revealed that Gn-MNPs underwent ten consecutive adsorption-desorption processes with an average recovery greater than 90%. Results obtained also showed that a synergistic effect between the complexation reaction and the electrostatic interaction may be involved in the Zn(II) adsorption with Gn-MNPs [170]. Thus, the study clearly demonstrates the potential application of Gn-MNPs for the removal and recycling of metal ions from water. The existence of Cr(III), Ni(II), Cd(II), Zn(II) and Cu(II) ions in electroplating wastewater is of great concern due to their potential effects on human health. Therefore, a study of Patel and co-workers [171] demonstrates the adsorptive removal of Cu(II), Ni(II) and Zn(II) ions from aqueous solution using a low-cost hydroxyl terminated S-triazine based dendrimer adsorbent. The authors reported the sorption behaviour of different generations of triazine-based dendrimers (G1, G2 and G3) for the removal of Cu(II), Ni(II) and Zn(II) ions from aqueous solution by EDTA titration. Preparation of the material was carried out from N,N’-bis (4,6-dichloro-1,3,5-triazin-2-yl) hexane-1,6-diamine as core using temperature13

controlled nucleophilic substitution of triazine trichloride [172]. After synthesis, the adsorption capabilities of dendrimers G1, G2 and G3 terminated with 8, 32 and 128 hydroxyl groups, respectively, were evaluated for removal of Cu(II), Ni(II) and Zn(II) ions from aqueous solution as a function of pH, time, and generation number. The sorption of metal ions increases with increased pH and generation number with a maximum sorption for Cu(II) and minimum sorption for Zn(II) ions. Fourier transform infrared (FTIR) spectroscopic analysis of the G3 dendrimer and metal-containing dendrimer showed the presence of hydroxyl groups acting as active binding sites for the metal uptake [172]. In addition, the presence of metal ions in the final metal-containing dendrimer was revealed by thermogravimetric analysis (TGA). Li et al. [173], prepared a novel adsorbent (PAMAM-CD) by using an insoluble crosslinked

copolymer

containing

β-cyclodextrin

( -CD)

structural

units

and

polyamidoamine (PAMAM, generation 2) for the adsorption of Cu(II) and Pb(II) ions and organic compounds (2, 4-dichlorophenol, 2,4,6-trichlorophenol, and Ponceau 4R) from water. The characterisation was carried out by Fourier transform infrared (FTIR) spectroscopy, Xray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), elemental analysis (EA), Brunauer-Emmett-Teller (BET) pore size and surface area analysis, and thermal analysis (thermogravimetric and differential scanning calorimetric measurement, TG/DSC) techniques. Results of the sorption experiments showed that the PAMAM-CD copolymer exhibits high sorption capabilities and high removal efficiencies towards Cu(II) and Pb(II) ions and organic compounds (2,4-dichlorophenol, 2,4,6-trichlorophenol, and Ponceau 4R). Findings of this study offered new opportunities for the fabrication of cyclodextrin-based or polyamidoamine-based materials with enhanced properties that could be useful in applications such as sorption, biomedical sensing, flocculation and therapeutics [173]. Qu et al. [174] synthesised silica-based dendrimer-like highly branched polymer adsorbents (SG-DETA and SG-DETA2) by a combination of homogeneous and heterogeneous methods. Analytical techniques such as FTIR, BET, and SEM showed that the dendrimer-like highly branched polymer was successfully grafted onto the silica surface and the adsorbent was evaluated for the removal of Au(III) ions from water. The maximum adsorption capacities of SG-DETA and SG-DETA2 for Au(III) were 2.09 and 2.12 mmol/g, respectively, at a temperature of 35 °C. Adsorption thermodynamic results and kinetics of SG-DETA and SG-DETA2 showed that these adsorbents had good sorption selectivity for 14

Au(III) with the Langmuir isotherm equation giving the best interpretation of the experimental data. Moreover, the adsorption kinetics of SG-DETA and SG-DETA2 was best described by the pseudo-second-order rate equation. These findings offer yet another opportunity for the use of dendrimers for the sorption of heavy metals from water. In the same manner, Niu and co-workers [163] synthesised silica-gel supported salicylaldehyde modified PAMAM dendrimers for the removal of Hg(II) ions from aqueous solutions. In this study, a series of silica-gel supported salicylaldehyde modified PAMAM (SiO2-GO-SA~SiO2-G2.0-SA) were synthesised by the divergent method [163] as shown in Figure 5. The structure of the supported dendrimer was characterised by FTIR, SEM, XRD, TGA and pore structure analysis. The adsorption capacity was found to depend on parameters such as solution pH, the generation number of salicylaldehyde modified PAMAM, initial concentration, contact time and temperature. Results showed an optimal pH of 6, with an increase in adsorption capacity as the generation number increases. In this study, the density functional theory (DET) method was employed to investigate the coordination geometry and the chelating mechanism between Hg(II) and the salicylaldehyde modified dendrimer. The results indicated that imino nitrogen atoms played a vital role in the coordination. The adsorption kinetics followed the pseudo-second-order model with film diffusion process as the rate determining step. Experimental results fitted the Langmuir isotherm model well, thus implying that the adsorption of Hg(II) proceeded via a monolayer adsorption [163]. The maximum adsorption capacities of SiO2-GO-SA, SiO2-G1.0-SA, and SiO2-G2.0-SA were 0.91, 1.52 and 1.81 mmol/g, respectively. Moreover, based on the thermodynamic values of negative ∆G and positive ∆H, this suggested that the adsorption process was spontaneous and endothermic in nature. These findings also illustrate that dendrimers supported on silica gel could be promising adsorbents for the removal of Hg(II) from aqueous solutions. Diallo et al. [175] employed the use of dendrimers to remove Cu(II) ions from aqueous solutions. They investigated the uptake of Cu(II) by PAMAM dendrimers with an EDA core in aqueous solutions. To probe the structures of Cu(II) complexes with G3.0~G5.0 EDA dendrimers with –NH2 terminal groups in aqueous solutions at neutral pH (7.0), extended X-ray absorption fine structure (EXAFS) spectroscopy was used. The EXAFS results suggest that the Cu(II) binding with the dendrimer involved both the tertiary amine and the terminal groups and that the extent of binding was greatly influenced by the protonation of the functional groups [175].

15

Membrane filtration has received much attention for the treatment of inorganic effluents since it is capable of removing suspended solids, organic compounds and also inorganic contaminants such as heavy metals [102]. Membrane filtration processes such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) can be used for the removal of heavy metals from wastewater depending on the size of the particle that can be retained [102]. Some studies have shown that the use of membranes for the removal of heavy-metal ions from wastewater can reduce the operating costs and overall energy consumption compared to other separation or purification methods [176].A noticeable improvement in the results was observed in UF membranes upon the addition of chelating polymers (dendrimers) that form complexes with metal ions, where the complexes are retained by the UF membrane. This method is known as polymer-assisted UF or polymer-enhanced UF [176]. Rether and Schuster [177] studied the complexation of Co(II), Cu(II), Hg(II), Ni(II), Pb(II) and Zn(II) by using the polymer-supported ultrafiltration (PSU) method. Polyamidoamine (PAMAM) was modified by using a two-step process with benzoylthiourea groups to provide a water-soluble chelating ion exchange dendrimer. 1H NMR, 13C NMR spectroscopy and elemental analysis were used to characterise the benzoylthiourea modified dendrimer and the molecular weight of the dendritic polymers determined by MALDI-TOF experiments was 7.5 kDa. The interaction of dendritic ligands with different heavy-metal ions was determined by measuring metal-ion retention which was highly pH dependent. Results showed that all metal ions under investigation could be retained at pH 9, with Cu(II) and Hg(II) ions forming more stable complexes with the benzoylthiourea modified derivative [177]. However, the bound metal ions could be recovered by decreasing the pH of the solution. Water-soluble polymeric ligands have demonstrated the capability to remove trace metals from aqueous solutions and industrial wastewater through membrane processes [102]. In combination with the ultrafiltration technique, polyethyleneimine (PEI) [178], carboxyl methyl cellulose (CMC) [179] and diethylaminoethyl cellulose [180], have been used as effective water-soluble metalbinding polymers for the selective removal of heavy metals from water. Ferella and coworkers [181] used a surfactant-enhanced ultrafiltration process for the removal of lead and arsenic from water by using cationic (dodecylamine) and anionic (dodecylbenzenesulfonic acid) surfactants. They achieved a removal efficiency of > 99% for lead and 19% for arsenate ions in both systems. More recently, modified UF blend membranes based on sulfonated polyetherimide (SPEI) [182], polycarbonate [183], and cellulose acetate (CA) with polyether ketone [184] have been employed for the removal of heavy metals from water. 16

Furthermore, Algarra et al. [156] successfully removed Cd(II), Hg(II) and Pb(II) from aqueous solutions using an engineered cellulose based membrane, produced by embedding a diaminobutane based poly(propyleneimine) dendrimer functionalised with 16 thiol groups in a swollen cellulosic support. The thiolated dendrimer (DAB-3-(SH)16) was synthesised by the treatment of the third diaminobutane based poly(propyleneimine) dendrimer with DAB-3(NH2)16

with

excess

of

3-mercaptopropanyl-N-hydroxysuccinimide

ester

in

dichloromethane solution at room temperature [156]. The results revealed that the dendrimer inclusion improves the elastic behaviour of the membrane (increase in Young’s modulus of around 20%), while a significant reduction in the permeation of toxic heavy metals (Cd2+, Hg2+ and Pb2+) was also obtained [156], which indicates the possible application of dendrimer-modified membranes in electrochemical devices for water remediation. Han et al. [176] synthesised a composite membrane composed of hyperbranched poly(amidoamine) (HYPAM) and polysulfone (PSf) for the removal of heavy-metal ions from contaminated media. High-density branched polymers containing a number of cavities and functional groups were incorporated in a UF membrane. In this approach, the dendritic chelating agent HYPAM was incorporated into PSf via the phase invasion process to form the HYPAM/PSf membrane. To improve the compatibility of HYPAM with the hydrophobic PSf, HYPAM was modified with palmitoyl chloride (a long-chain fatty acid chloride) during synthesis. The composite was characterised and evaluated for its efficiency in removing Cd(II) ions from wastewater. Results showed that the composite membranes had a much higher removal capacity towards Cd(II) than the HYPAM-free membrane [176]. The composite membrane also showed the possibility of being reused with high recovery efficiencies in the range of 85 – 87%. This study provides useful insight into the use of HYPAM/PSf composite membranes as functional and efficient materials for the removal of heavy metals from wastewater with the possibility of membrane reuse. Table 2 provides additional examples for the utilisation of dendritic-based sorbents for the remediation of heavy metals from water and wastewater.

6

SILICA-BASED NANOSORBENTS AND THEIR SYNTHESIS Silica (SiO2) is an inorganic solid which is made up of three-dimensional network

structures (the silicon atom is covalently bonded to four oxygen atoms) joined by a common atom (bridge), which gives a structure of porous materials having a large surface area. All 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the 17

net chemical formula: SiO2) Silica materials possess excellent physical and chemical properties such as water stability (non-swelling), thermal stability (up to 1 500 °C), good mechanical strength and it is non-toxic. Furthermore, the presence of silanol groups (Si–OH) on the surface offers silica materials a better solid support for the immobilisation of a wide range of inorganic and organic groups [189]. The synthesis of silica materials such as amorphous silica, fumed silica, silica gels, and mesoporous silica has been reviewed [190– 196]. Literature data clearly indicate that well-prepared mesoporous materials show excellent metal adsorption capabilities compared to their amorphous counterparts. This can be attributed to their higher surface areas, good accessibility to active centres, which increase selectivity, and higher mass transport rates inside the porous structure [197]. Examples of mesoporous materials are Mobil Crystalline Material (MCM-41, MCM-48), and Santa Barbara Amorphous type material (SBA-1, SBA-15, and SBA-16). Mesoporous materials possess large surface areas, large pore volumes, large pore sizes and a large amount of surface silanol groups. Mesoporous silicas are prepared by the hydrolysis of alkoxysilane precursors (tetramethoxy- or tetraethoxysilane) in the presence of a suitable surfactant (template) and catalyst to produce a condensed polymerised network of siloxane (Si-O-Si) linkages. The sol-gel chemistry leads to the synthesis of ordered mesoporous silica and recently, surface functionalisation of ordered mesoporous silica has gained a lot of interest as a support material due to its large surface area, fast adsorption kinetics and controllable pore size and pore arrangement [198]. The maximum adsorption capacity of these materials and their adsorbent regeneration capability are very important factors when it comes to determining the reusability of the prepared materials [199]. Mesoporous silica can be further modified by immobilisation of various functional groups to form organic-inorganic hybrid materials [200,201]. These hybrid materials have been reported to exhibit improved sorption properties towards heavy-metal ions after multiple uses, superior to those achieved with pure silica materials [202,203]. The general strategy involves shifting the complexation equilibrium in the opposite direction from metal uptake [197]. A simple metal decomplexation such as washing with acid can be used. The acid wash would liberate the metal ion from the amine (in the case of amine-functionalisation) and recycle the amine functionality for next uptake studies. Despite the high costs of some hybrid mesoporous materials compared to their amorphous homologues, most of these hybrid materials have been shown to be less expensive due to their regeneration capabilities [199]. Functionalised hexagonal MCM-41 and cubic MCM-48 have been reported to show improved selectivity for 18

the removal of heavy metals from wastewater [198, 204–206]. Two approaches are typically employed for the incorporation of the organic moieties in the mesoporous silicas: the grafting method, which refers to post-modification of pre-synthesised mesoporous silica; and the cocondensation (direct synthesis) method [207–213] (Figure 6). Direct co-condensation or one-pot synthesis essentially consists of the co-polymerisation of a silica precursor (usually tetraethyl orthosilicate, TEOS) and an organosilane precursor in the presence of a template [207, 208]. The main advantages of the co-condensation (direct synthesis) method include one-step synthesis, control of the loading and distribution of organic moieties, i.e., organic moieties are homogeneously distributed. However, the main limitation of this synthesis method is that the degree of mesoscopic order of the material decreases with increasing loading of the organic moiety, leading to totally disordered materials. In addition, another disadvantage of this synthesis method is the fact that care must be taken to preserve the organic moiety during the surfactant removal step [197]. On the other hand, the post-modification (grafting) method involves surface modification by covalently linking organosilane species with surface silanol groups of the mesoporous material. In this synthesis procedure, the silica is reacted with an appropriate organosilane in a suitable solvent at reflux. Despite the effective inclusion of a high concentration of covalently bound organic moiety, the limitations of this method include difficulty in controlling the loading and position of the organic moiety, low loading of attached compounds, two steps being involved and hence the process is more time-consuming [214], etc.

6.1

Mesoporous silica sorbents for the removal of heavy metals from aqueous solutions

Heidari and co-workers [215] removed Ni(II), Cd(II) and Pb(II) from a ternary aqueous solution by using amino-functionalised mesoporous and nano-mesoporous silica. In this study, mesoporous MCM-41, nanoparticle MCM-41, amino-functionalised MCM-41 (NH2MCM-41) and nano-NH2-MCM-41 were investigated as sorbents synthesised by using suitable preparation methods for the removal of these heavy metals. The effects of adsorbent dosages, pH, ion concentration and contact time were studied. Results on the effects of adsorbent dosage and pH show that there was an increase in the adsorption of Ni(II), Cd(II) and Pb(II) ions on the surface of adsorbents with increasing solution pH. The NH2-MCM-41 showed the best uptake for Ni(II), Cd(II) and Pb(II) ions. Experimental data were analysed using the Langmuir and Freundlich models. The Langmuir model best described the adsorption behaviour of Ni(II), Cd(II) and Pb(II) ions onto NH2-MCM-41 adsorbent with 19

maximum adsorption values of 12.36, 18.25 and 57.74 mg/g, for Ni(II), Cd(II) and Pb(II) ions, respectively. Modified SBA-15 using an amino group (3-aminopropyltrimethoxy silane) has shown exceptional removal capability for Cd(II), Co(II), Cu(II), Zn(II), Ni(II), Al(III) and Cr(III) ions [216]. Amino-functionalised SBA-15 has shown a high affinity for different metal ions, such as Pb(II), Cu(II), Zn(II), Ni(II), U(VI) and Cr(III) [217-224]. Indeed, the functionalisation of mesoporous silica with aminopropyl group for selective adsorption of heavy-metal ions has been studied [202, 203, 221, 225–230]. Thiol-functionalised mesoporous silicas have shown exceptional selectivity for adsorbing Hg2+ [231–235], Cd2+, Zn2+ and Cd2+ [233, 236, 237], and precious (noble) metals [238, 238–240] from wastewater. The imidazole-derived SBA-15 exhibited high adsorption capacity for Cr(IV) [241, 242]. Very recently, Dindar and co-workers [243] functionalised SBA-15 mesoporous materials for the decontamination of water solutions containing Cr(VI), As(V) and Hg(II) ions. Furthermore, Bao and co-workers [244] applied an amine-functionalised MCM-41 modified UF membrane to remove Cr(VI) and Cu(II) from wastewater. In this study, they employed a thin-film composite UF membrane grafted with amine-functionalised MCM-41 for the removal of the heavy metals from wastewater. The porous NH2-MCM-41 nanoparticles formed a uniform hydrophilic and adsorptive layer on the thin film which endowed the composite membrane with excellent affinity for the heavy metals and improved anti-fouling properties [244]. Results showed an adsorption capacity of 2.8 mg/g and 3.7 mg/g for Cr(VI) and Cu(II) on the membrane, respectively. The continuous UF experiments showed that the functionalised membrane can be used as an effective filter medium to purify wastewater containing trace amounts of heavy metals. Generally, these studies have demonstrated the great interest in preparing hybrid mesoporous materials that are able to complex toxic heavy metals and indeed have opened the door to a new era in analytical strategies for the detection and removal of toxic heavy metals in wastewater

7

CHITOSAN AS NANOSORBENTS

Chitosan, a biopolymer, has attracted the attention of researchers in recent years due to its low cost compared to activated carbon and the presence of amino and hydroxyl groups in its structure which can serve as active sites for the removal of pollutants from wastewater [245– 252]. Chitosan is a natural polysaccharide obtained from partial or full deacetylation of chitin [253, 254]. Chitin, the source material for chitosan, is the most naturally abundant 20

polysaccharide after cellulose. Chitin is an important component of the cell wall of fungi, as well as of the exoskeleton of crustacean water animals and insects. Chitosan is the Ndeacetylated derivative of chitin, a linear and semicrystalline polysaccharide composed of glucosamine and N-acetyl glucosamine units linked by β-(1→4) glycosidic bonds [255, 256] (Figure 7). Chitosan has unique characteristics such as high reactivity, excellent chelation behaviour, chemical stability, and high affinity towards pollutants [257–264], and thus presents an attractive alternative to other biomaterials. Natural chitosan has been chemically modified by various physical or chemical methods or hybrids in order to enhance the adsorption capacity for various types of pollutants and all have been reported and documented in detail in numerous papers [246, 261, 264–272]. Chitosan is modified by utilising the reactivity of the primary amino groups and the primary and secondary hydroxyl groups. The modified chitosan or chitosan derivatives have shown many advantages over flaked or powdered chitosan such as higher internal surface area, and cross-linking of beads making them insoluble in low pH solutions, thus proving their suitability over a broad pH range [259]. Chitosan and its derivatives have a wide range of applications which depend on their physical, chemical and biological properties. 7.1

Chitosan and its derivatives as sorbents for the removal of heavy metals from aqueous

solutions The high adsorption capacity registered by chitosan and its derivatives for the removal of heavy-metal ions from water and wastewater can be attributed to: (1) the presence of multifunctional groups; (2) high hydrophilicity due to a large number of hydroxyl groups on the glucose units; (3) high chemical reactivity of these groups; and (4) the flexible structure of the polymer chain [259, 269]. Copello and co-workers [273] used immobilised chitosan as biosorbent for the removal of Cd(II), Cr(III) and Cr(VI) from aqueous solution. In this study, a layer-by-layer silicate-chitosan composite was prepared and characterised as biosorbent. Its stability and capability in the removal of Cd(II), Cr(III), and Cr(VI) were evaluated with the biosorbent demonstrating good adsorption capacity for these heavy-metal ions. For Cd(II) and Cr(III) metal species, the highest adsorption was obtained at pH = 7 while maximum adsorption of Cr(VI) occurred at pH = 4. The Langmuir and Freundlich isotherms were used to model the equilibrium data with 80% of the adsorbed metal recovered by HNO3 incubation [273]. This 21

study suggests the potential applicability of this cost-effective biosorbent in the removal and recovery of heavy metals from water and wastewater and could also serve as an incentive for researchers to conduct an evaluation of a scale-up procedure. Biosorbents obtained from two natural biomacromolecules, namely chitosan, a derivative of chitin, and sporopollenin, a biopolymer with excellent mechanical properties and great resistance to chemical and biological attack, have been used in the removal of heavy metals from wastewater [274]. The chitosan/sporopollenin microcapsules were prepared via cross-linking and characterised by employing SEM, FTIR and TGA. The sorption performance of chitosan/sporopollenin microcapsules was evaluated for the removal of Cu(II), Cd(II), Cr(III), Ni(II) and Zn(II) ions from wastewater at different pH values, metal concentrations, temperature, amount of adsorbent and sorption time. Results revealed that the adsorption pattern followed the Langmuir isotherm model and the sorption capacity of the chitosan/sporopollenin microcapsules was 1.34 mmol/g, 0.77 mmol/g, 0.99 mmol/g, 0.58 mmol/g, and 0.71 mmol/g for Cd(II), Cr(III), Ni(II) and Zn(II) ions, respectively. However, pure chitosan showed a higher sorption capacity for Cu(II): 1.46 mmol/g, Cr(III): 1.16 mmol/g and Ni(II): 0.81 mmol/g but with lower sorption capacity for Cd(II): 0.15 mmol/g and Zn(II): 0.25 mmol/g. It is likely that the sporopollenin grains entrapped within the polymeric matrices enhanced the sorption of Cd(II) and Zn(II) ions significantly as well as the thermal stability of the microcapsules. The authors then concluded that the chitosan/sporopollenin microcapsules can be used in the sorption of Cd(II) and Zn(II) ions from wastewater [274]. Thiourea-modified magnetic ion-imprinted chitosan/TiO2 composites have been used for the simultaneous removal of cadmium and 2,4-dichlorophenol from wastewater [275]. In this study, a novel type of adsorbent called thiourea-modified magnetic ion-imprinted chitosan/TiO2 (MICT) was developed for effective cadmium adsorption and 2,4-DCP degradation through the combination of ion-imprinted technology and photodegradation technology. The equilibrium data were fitted to the Langmuir isotherm model, and the maximum adsorption capacity for cadmium was found to be 256.41 mg/g and the 2,4-DCP degradation efficiency was up to 98% at an initial 2,4-DCP concentration of 10 mg/L [275]. The adsorbent showed better adsorption and degradation capacity compared to results previously reported [275]. The prepared sorbent was reusable after regeneration through desorption, and the adsorption and degradation were barely affected by the number of cycles. Results revealed that the MICT composite is an ideal platform for the simultaneous removal 22

of heavy metals and organic pollutants in wastewater which could go a long way in facilitating the development processes for the treatment of co-contaminants in aquatic environments. Lakhdhar and co-workers [276] have used electrospun chitosan/PEO nanofibres for the removal of Cu(II) ions from aqueous solutions. In this study, adsorption experiments were carried out using electrospun chitosan (CS)/polyethylene oxide (PEO) nanofibre to investigate the adsorption of Cu(II) from aqueous solutions. By using 75 mg of nanofibre, an optimal adsorption of 94.7% was obtained in 200 min at pH 5.5 and at a temperature of 55.7 °C with an initial copper concentration of 100 mg/L. A number of papers have been published in the literature, giving an account of the capabilities of chitosan and its derivatives as adsorbents for the removal of Cu(II), Ag(I), Co(II), Zn(II), Cd(II), etc. [262, 277–280]. Wu and co-workers [262] studied the adsorption of heavy metals such as Pb(II), Cu(II), Zn(II), Ni(II), Cd(II), Hg(II) and Cr(VI) from sulphate and nitrate solutions on pristine chitosan and its derivatives prepared from cuttlefish wastes. Sorption capabilities of pristine and modified chitosan for adsorption of heavy metals from aqueous media were determined by Langmuir equation. Results revealed that crosslinking or substitution of chitosan led to a significant decrease in the adsorption ability. The magnitude of adsorption capacity of heavy metals on pristine chitosan was used to discuss the mechanism of adsorption, i.e., how many amino groups would coordinate with one heavy-metal ion. For detailed information, the reader is referred to a good number of papers that have been published on the adsorption of heavy metals from water and wastewater [83, 246, 268, 281– 285]. Noteworthy is the fact that in all these adsorption studies, the binding mechanism of metal ions to chitosan is not yet fully understood, although various processes such as adsorption, ion-exchange and chelation have been discussed as possible mechanisms for metal complexation by chitosan [285]. However, in most instances, the type of interaction depends on the metal ion, its chemistry and the pH of the solution [285]. Studies have shown that the chitosan-metal cation complex formation might have likely occurred mainly through the amino groups functioning as ligands.

8

SUMMARY AND PERSPECTIVES This review paper provides a current perspective of the potential of dendrimers,

mesoporous silicas and chitosan nanomaterials as remediation agents to remove heavy-metal 23

ions from water and wastewater. The goal of this review is to highlight the possible application of these materials in removal of heavy-metal ions from aqueous media. Nanomaterials possess some physicochemical properties that make them useful as separation media in wastewater treatment (particularly the removal of heavy metals). These properties include (a) very high surface area to volume ratio compared to larger particles; (b) the ability for easy functionalisation with different organic moieties to enhance their affinity for a given compound; and (c) high selectivity and recyclable ligands for target toxic elements and organic and inorganic solutes/anions in aqueous solutions. The mechanism and kinetics of adsorption or chelation of these metal ions depend on a number of factors such as the chemical nature of nanomaterials, and various physicochemical properties such as temperature, solution pH, adsorbent dosage, and initial metal concentration of the system. It should be noted that when evaluating the adsorption capacity of these nanosorbents these factors need to be taken into account, apart from availability and costs. Some of the major challenges that need to be addressed for full commercialisation of these nanomaterials for the removal of heavy metals are scalability of production, as well as their adsorption capacities. One may argue that not all new research on these materials has turned into a commercial success story; however, worldwide there is a huge need for an effective approach to wastewater remediation and next-generation adsorbents have been found to perform very well in environmental remediation and control of heavy-metal ions in wastewater. The prospects look good as the scientific community continues to make advances in understanding the mechanisms of adsorption and in exploring various materials that have the potential to be applied as next-generation adsorbents. Researchers, academics, practitioners and policy-makers are concerned about water contaminated with heavy metals which poses serious health risks and are committed to finding effective solutions concerning the use of these nanomaterials in a wide range of applications.

Acknowledgments Dr. Ephraim Vunain acknowledges the Postdoctoral Research Fellowship awarded by the University of South Africa (UNISA) in terms of its Postdoctoral Fellowship Programme.

24

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49

Figure 1: Nanomaterials classification according to their physical and chemical properties.

Figure 2: Structure of dendrimer

50

Figure 3: Schematic diagram showing the immobilisation of G4-OH dendrimer on titania to obtain a dendrimer/titania composites for metal-ion complexation (Reprint from ref. 168 with permission from Elsevier).

Figure 4: SEM images of dendrimer/titania composite sample (Reprint from ref. 168 with permission from Elsevier).

51

Figure 5: Ideal synthesis routes of silica-gel supported salicylaldehyde modified PAMAM dendrimers [163].

52

Figure 6. (a) Post-modification; and (b) direct synthesis of mesoporous silica with organic moieties.

Figure 7: Structure of chitosan

53

Table 1. Maximum contaminant limits (MCL) of some heavy metals in drinking water Type

of MCLs (mg/L)

contaminants

EEA [99]

USEPA [100]

WHO [100,101]

Cadmium

0.005

0.005

0.005

Mercury

0.001

0.002

0.001

Lead

0.1

0.015

0.005

Copper

1.3

1.0

selenium

0.05

0.01

0.01

0.01

5.0

5.0

0.1

0.05

0.04

–

Arsenic

0.01

Zinc Chromium

0.05

Nickel

NOTE: Maximum Contaminant Level (MCL) is the highest level of a contaminant that is allowed in drinking water. These MCLs are enforceable standards. EEA = European Environment Agency USEPA = U.S. Environmental Protection Agency WHO = World Health Organization.

54

Table 2: Other applications of dendrimers for removal of heavy metals No

Dendrimer type

1

PAMAM-SBA-15 and EDTA-PAMAM-SBA-15 hybrid Cr(III), Adsorption materials Pb(II) and Zn(II)

Jiang et al. [149]

2

Silica-gel supported hyperbranched polyamidoamine Pb(II) dendrimers

Adsorption

Niu et [163]

3

Polymer assisted ultrafiltration membrane/alginate

Adsorption

Fatin-Rough et al. [185]

Adsorption

Ilaiyaraja et al. [186] Zhang et al. [187]

4 5

Targeted Heavy metal

Pb(II), Cu(II), Zn(II) Ni(II) styrene U(VI)

Polyamidoamine dendron functionalized divinybenzene (PAMAM3-SDB) resin Polyamidoamine dendronized hollow fiber membranes

Cu(II), Pb(II), Cd(II) Au(III)

6

Silica-gel based dendrimers

7

Polyamidoamine dendrimer grafted onto thin-film Pb(II), composite (TFC) nanofiltration (NF) membrane Cu(II), Ni(II), Cd(II), Zn(II) As (V) Silicon quantum dots with dendrimer Cr(VI)

8

55

Type of removal

Reference

al.

and

complexation and Adsorption

Zhang et al. [174] Wen-Ping et al. [157]

and Campos al. [188]

et