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A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems
Journal Pre-proof A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems Botao Liu, Ki-Hyun Kim, Vanish Kumar, Sumin Kim
PII:
S0304-3894(19)31769-8
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121815
Reference:
HAZMAT 121815
To appear in:
Journal of Hazardous Materials
Received Date:
30 August 2019
Revised Date:
2 December 2019
Accepted Date:
2 December 2019
Please cite this article as: Liu B, Kim K-Hyun, Kumar V, Kim S, A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121815
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A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems
Botao Liua, Ki-Hyun Kima, Vanish Kumarb*, Sumin Kimc* a Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 04763, Republic of Korea b National Agri-Food Biotechnology Institute (NABI), S.A.S. Nagar, Punjab 140306, India c Department of Architecture and Architectural Engineering, Yonsei University, Seoul 03722, Republic of Korea
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Correspondence: [email protected]; [email protected]; [email protected]
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Hightlights The presence of arsenic in groundwater is regarded as a significant human health threat. Information on various functional adsorbents is assessed for removal potential for As. The removal potential for As is assessed on the basis of key performance metrics. -The regeneration of sorbents and their disposal after the use are also discussed.
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Graphical abstract
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Abstract
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The presence of arsenic in the water system has been a universal problem over the past several decades. Inorganic arsenic ions mainly occur in two oxidation states, As(V) and As(III), in the natural environment. These
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two oxidation states of arsenic ions are ubiquitous in natural waters and pose significant health hazards to humans when present at or above the allowable limits. Therefore, treatment of arsenic ions has become more stringent
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based on various techniques (e.g., membrane filtration, adsorption, and ion exchange). This paper aims to review the current knowledge on various functional adsorbents through comparison of removal potential for As on the
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basis of key performance metrics, especially the partition coefficient (PC). As a whole, novel materials exhibited far better removal performance for As(V) and As(III) than conventional materials. Of the materials reviewed, the advanced sorbent like ZrO(OH)2/CNTs showcased superior performances such as partition coefficient values of 584.6 (As(V) and 143.8 mol kg-1 M-1 (As(III) with excellent regenerability (>90% of desorption efficiency after three sorption cycles). The results of this review are expected to help researchers to establish a powerful 2
strategy for abatement of arsenic ions in wastewater.
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Keywords: adsorption; arsenic; advanced materials; partition coefficient; nanoparticles
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Table of contents 1. Introduction
1.1. Overview of arsenic contamination 1.2. Classification of various sorbents and framework of this review 2. Brief introduction to removal techniques for arsenic 3. Adsorptive removal of arsenic by conventional materials
3. 2. Zeolites 3. 3. Waste-derived sorbents
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4. Adsorptive removal of arsenic by advanced materials
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3.1. Activated carbon
4.2. Carbon nanotubes
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4.3. Metal organic frameworks
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4.1. Graphene oxide
4.4. Miscellaneous forms
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5. Effects of various factors on adsorption for arsenic removal 6. Performance evaluation of various materials for abatement of arsenic
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7. Regeneration of sorbents and their disposal after the use
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8. Conclusions and perspectives
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1. Introduction 1.1. Overview of arsenic contamination Water contamination is ubiquitous and becoming one of the most challenging problems in the past several decades due to anthropogenic activities. The continuous increase in water contamination poses tremendous threats to the environment and human health. Heavy metal contamination is commonly seen in domestic, industrial, and agricultural wastewaters. In general,
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many heavy metals, such as cadmium, mercury, lead, chromium, and arsenic, are known as toxic components that exert a negative impact on human health and the environment [1]. Of these heavy metals, arsenic is a widely distributed metalloid, originating from various sources and present in
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various forms (e.g., minerals, petroleum refining, and coal fly ash). It is usually found in combined forms with sulfur and other metals [2]. The presence of arsenic in groundwater used for drinking
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is recognized as a significant human health threat in many regions of the world, e.g., China, Chile,
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and India [3]. Further, exposure to arsenic through drinking water was reported to increase the risk of mortality (e.g., through lung, bladder, kidney, stomach, and skin cancer) [4].
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Arsenic may occur in oxidation states of ± 3, 5, and 0 and is found mainly in two forms in the natural environment, namely, AsO33- (III) and AsO43- (V) [5, 6]. In general, pH and redox conditions
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play a dominant role in the oxidation state of arsenic. Accordingly, As(V) is thought to be the most
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stable form of arsenic due to the pH-dependent deprotonated oxyanion effect in aerobic environments. Meanwhile, in moderately reducing anaerobic conditions (e.g., anoxic groundwater conditions), As(III) is more commonly present, and it is far more toxic and mobile than As(V). Hence, As(III) tends to be more difficult to remove by the common water treatment techniques (e.g., adsorption and co-precipitation) due to the shortage of electrostatic force [7]. However, many other factors such as affinity and size exclusion is also important in deciding final adsorption capacity for 5
As(III) removal. Further, the effectiveness of sorbent applied for different phases of contaminated waters (e.g., domestic sewage, industrial, and agricultural wastewater) may be highly variable. For example, several nanoscale zero-valent iron (nZVI) types and iron-biochar (Fe-BC) were used to remove arsenic from model hydraulic fracturing wastewater [8-11]. In these As-containing wastewater treatment studies, removal efficiency values were observed in the range from 40-90%. Apparently, a series of metals/metalloids (e.g., KCl, NaBr, As, and Cu) resulted in either synergistic
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or competitive effects on fracturing wastewater. Thus, some of these materials may be employed as potential sorbents for the in-situ prevention of As migration from fracturing wastewater treatment. For the evaluation of sorbent performance in this paper, we basically relied on the datasets derived
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for the model drinking water which contained single or binary pollutants (i.e., As). Conventional
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arsenic abatement techniques have generally relied on a stepwise approach, such as pretreatment of As(III) and subsequent removal of As(V). As a whole, removal technologies for both As(III) and
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As(V) have received a great deal of attention from many governments and researchers.
1.2. Classification of various sorbents and framework of this review
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In this review, an overview of the seriousness of arsenic contamination was discussed along with a short introduction to various arsenic removal methods. Subsequently, a comparative
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analysis was made to evaluate adsorptive removal performance of arsenic between many advanced sorbents (relative to conventional sorbents). In general, the latter group consists of activated carbon (AC), zeolites, and industrial/agricultural solid waste. AC is formed by pyrolysis of various carbon-containing materials (e.g., wood, husk, coal, and coconut shell), so it exhibits an amorphous property [12]. It is the most commonly used adsorbent for wastewater treatment due 6
to its low cost, good reproducibility, and easy availability. Zeolites are microporous aluminosilicates with different ratios of silicon, aluminum, oxygen, and metals. More than 200 natural and synthetic zeolitic frameworks have been reported as of December 2018 [13]. Sorbents derived from industrial/agricultural solid wastes (such as fly ash, waste slurry, and lignin) have also been reported to have good adsorption capacity for arsenic from wastewater [14]. However, these solid waste materials have scantily been employed for application in other fields.
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Several advanced materials (e.g., metal organic frameworks (MOFs), graphene oxide (GO), and carbon nanotubes (CNT)) have been proposed and developed as sorbents for treatment of As. MOFs are a class of porous materials that are usually formed by binding inorganic metal ions and
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organic ligands. The surface area and porosity of MOFs can be changed or enhanced through
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alteration of metals and/or organic ligands. The feasibility of MOFs has been demonstrated in various fields due to their facile structural fabrication and/or functional modulation [15- 22].
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Another advanced material employed for As removal is GO, also known as single-layer graphite
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oxide, which is generally synthesized by reacting graphite with strong oxidizers [23]. The abundant functionality of GO material is thus derived from the lattice structure and various
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oxygen-containing groups that are enriched on its surface. Then, the rich and effective active sites can be provided by functional groups of the GO surface [24]. As another carbon nanostructure,
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CNTs are well-known for tubular nano-scale graphite crystals with mono or multi-layer graphite sheets crimped around the central axis at a certain helix angle [25]. Their excellent mechanical, electrical, optical, and thermal properties provide them with high potential in various fields of application, such as drug delivery [26], tissue engineering [27], and electrical conduction [28]. The discussion of other advanced materials (e.g., metal oxides, chitosan-based materials, and 7
miscellaneous polymers) will be discussed in detail in each of the following subsections. In this research, an up-to-date knowledge on sorptive removal of As has been reviewed to describe the factors controlling its adsorption (e.g., temperature, pH value, and concentration/ion strength) with respect to the extension of their practical applicability based on more meaningful evaluation on performance between various novel functional and conventional adsorbents (e.g., based on partition coefficient (PC) concept) [29-31]. In light of the limitations involved in the use
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of maximum adsorption capacities, partition coefficient (PC) as the equilibrium ratio of analyte concentration in and on the sorbent to solution concentration was employed to assess the performance of diverse sorbents for As species with the least biases.
Furthermore, knowing that
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an excellent regenerability is also advantageous in lowering the management cost, various aspects
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of sorbent regeneration were described to assess the practical utility of diverse sorbent materials employed for removing As. Finally, the future outlook and conclusions were presented to help
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researchers explore and consummate arsenic removal technologies. As a whole, this review is
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expected to offer valuable insights into adsorptive removal capabilities of As between novel
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functional and conventional adsorbents.
2. Brief introduction to removal techniques for arsenic
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A myriad of techniques has been proposed for treatment of arsenic, including photocatalysis
[32-34], ion exchange [35-37], chemical precipitation [38-40], filtration membranes [41-43], electrochemical treatment [44-46], and adsorption [47-49]. Most of these methods suffer from drawbacks in various respects. Since As(III) is very difficult to remove from the water phase, photocatalysis is generally a preferred pretreatment option used to oxidize As(III) into As(V) in 8
wastewater. Then, the actual removal treatment can be employed through combination with other technologies [50]. Ion exchange approach refers to a physical or chemical process in which anions originated from a typically synthesized ion exchange resin held electrostatically onto the solid phase surface [51]. Then, these anions are exchanged with other ions (e.g., arsenic) having similar charge in a solution. However, widespread use of this method is often restricted due to the high cost of treatment (relative to other methods) [52]. In the case of chemical precipitation, chemicals
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are used to remove As from contaminated water in which As is converted into less soluble As compounds. For example, immobilization of As with metal cations (e.g., Fe2+) led to formation of low-soluble As compounds (e.g., FeAsO4∙2H2O) [53-55]. However, use of this technique is also
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limited since it requires a large amount of precipitants; the generated arsenic-containing waste
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residue may not be fully utilized and thus must be treated carefully, while its long-term accumulation may easily cause secondary pollution [53]. Membrane technology is a purely
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physical process driven by a concentration or pressure difference. Because this method mainly
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utilizes the selective permeability of a membrane, it can be categorized into microfiltration, ultrafiltration, and nanofiltration (based on pore size). Nevertheless, the use of membrane
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technology is confined to some specified conditions (e.g., equipment, membrane quality, and operations requirements) while being used mainly for water purification [56].
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Adsorption of arsenic (adsorbate) can be expressed as adhesion from a solution to the surface of a solid sorbent. Therefore, the performance of adsorptive removal for arsenic is dependent on the affinity between arsenic and adsorbents. Compared to the other methods mentioned above, adsorption is one of the most frequently used water treatment techniques in terms of its costeffectiveness, simple design, and easy operation. However, desorption of arsenic may be difficult 9
once there is a strong affinity between the arsenic and sorbents. That is, the arsenic-containing sorbents may not meet the requirements of environment-friendliness if the sorbents were not disposed well after being exhausted. Therefore, the regeneration and disposal of sorbents after the use are highly essential issue to ensure their cyclic utilization while reducing the potential harmfulness of exhausted sorbents, as discussed in Section 7. In addition, adsorption isotherms provide a quantitative measurement to meaningfully assess the adsorption capacity of arsenic for
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various sorbent materials. It is necessary to substantiate the correlation for the adsorption isotherm equations derivable between various adsorbents and arsenic. The use of proper equation fittings offers reasonable assumptions to describe the sorption mechanism and the related affinities (between
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the sorbents and arsenic). In Supporting Information, detailed descriptions are described for
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Freundlich isotherm, Langmuir isotherm, Redlich-Peterson isotherm, and other isotherms as well as kinetic studies. In some review papers, adsorptive removal of As by various sorbents has been
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attempted [2, 57, 58]. However, these papers did not comprehensively compare the differences in
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adsorptive removal between conventional and advanced materials. Moreover, due to the variations in experimental conditions of different studies, it is very difficult to discriminate the actual
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effectiveness of sorbent for removing arsenic if comparison is made based on the maximum adsorption capacity or removal efficiency. It was already noticed that these two values can be highly
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biased by the arbitration of experimental conditions (e.g., adsorbent dosage and initial concentrations).
3. Adsorptive removal of arsenic by conventional materials A variety of adsorption materials has been employed for the removal of arsenic. As discussed
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above, the majority of available materials cover both conventional and advanced materials. Although our main emphasis lies in advanced materials, our discussion also includes conventional materials, such as activated carbon, zeolite, and waste-derived sorbents in light of their common usage.
3.1. Activated carbon
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AC materials are effective for treatment of various contaminants in wastewater [59]. The large hydrophobic internal surface contributes to its significant affinity for adsorbates. However, virgin AC materials do not easily satisfy the treatment needs for diverse forms of targets. Thus,
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modification of AC has been attracting more attention as an effective means to produce
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unprecedented affinities toward various targets including heavy metal ions. Although many reviewers have discussed adsorptive removal of heavy metal ions by AC [59,
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60], its potential for removal of arsenic in wastewater has been investigated rather scantily. For
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instance, nano-scaled AC was synthesized through the modification of iron and manganese oxides to study As(V) removal in aqueous solution [61]. Adsorptive removal of As(V) by AC impregnated
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with iron/manganese (Fe-Mn-AC) exhibited a far higher capacity (19.35 mg g-1) than other modified (e.g., iron, and iron/cobalt) or unmodified AC. The very low removal efficiency (e.g., less than 20%)
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of the virgin AC material promoted the necessity for their modification. Table 1 shows that removal performances of most modified ACs were not directly reflected by their enhancement in surface area. In other words, modified AC with functional groups may play an important role in sequestration of As regardless of surface area. Essentially, the Fe-Mn-AC displaced the -OH of As(V) molecules, finally forming mono- or bi-dentate complexes on the NFM-impregnated carbon 11
surface to promote removal of As(V). (N stands for Norit initial carbon, while F and M stand for iron and manganese impregnation, respectively.) In another case study, the modification of AC by Fe was also verified to be efficient for the treatment of arsenic [62]. In their study, the presence of iron with AC increased surface area and adsorption capacity (for As(V)) of AC by 3.5 times and 36.5%, respectively. Furthermore, iron-modified AC by different sources (e.g., Fe3O4-decorated AC [63], Fe(OH)3-modified AC [64], and FeCl3- & FeSO4-decorated AC composite [65]) also showed
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elevated As adsorption capacities. In this respect, the removal capability of As was closely related with many variables for Fe (e.g., loaded amount, dispersibility, and the surface availability) within AC. The removal performances of these materials are summarized in Table 1. Nevertheless, the
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performance of As (mainly for As(V)) by these iron-modified AC was not sufficiently satisfactory
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in terms of removal efficiency or adsorption capacity.
Magnetized palm shell waste-based AC was fabricated with lanthanum impregnation (MPSAC-
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La) (Figure 1a) and tested for sorptive removal of As(V) [66]. The MPSAC-La(0.36) (weight ratio
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of La/Fe equal to 0.36) displayed a 15.5 times increment in As (V) adsorption capacity (with an adsorption capacity = 227.6 mg g-1) (Figure 1b) compared to that of virgin PSAC. This enhanced
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As(V) removal capacity was attributed to the precipitation mechanism of sorbents against such a target [67], since La3+ ions easily connect with As(V) to form LaAsO4 precipitate. The well-matched
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H-type Langmuir isotherm model of adsorption data (R2 > 0.92) (e.g., relative to Freundlich model (R2 > 0.77)) by MPSAC-La indicated a strong affinity between adsorbent and adsorbate. In addition, the adsorption process of As(V) was determined to follow a pseudo-first-order kinetic model (R2 = 0.994) (Figure 1c) slightly better than a pseudo-second-order (R2 = 0.991), illustrating that the physisorption should play as an important role in its removal. Finally, a moderately good 12
recyclability was seen for MPSAC-La as it maintained 75% of the adsorption capacity at the third cycle. A type of zirconium-based nanoparticle (NP)-doped AC fiber (ACF) was synthesized and employed for As(V) removal in aqueous phase [68]. During the preparation of NP-doped ACF, hydrogen sulfate ions (HSO4-) were introduced into NP-doped ACF to increase more functional groups. The virgin ACF consisted of long and smooth AC fibers (Figure 2a1) to show nano-sized
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Zr-based NP-containing agglomerates on the ACF surface (Figure 2a2-4). Essentially, the decreased surface area of modified ACF (e.g., from 1,721 to 1,409 m2 g-1) did not reduce the removal capacity for As(V). More specifically, a maximum capacity of 21.7 mg g-1 was obtained for As(V)
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by NP-doped ACF based on the Langmuir model (Figure 2b). HSO4- played an important role in
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the adsorption process as ion exchange between hydrogen sulfate of NP-doped ACF and As(V) can exert direct control on adsorption capability during the adsorption process. In addition, the
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adsorption process was well-matched with the pseudo-second order model, which suggested that
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chemisorption was driven by NP-doped ACF for As(V) removal [68]. Furthermore, arsenate solutions were treated with NP-doped ACF at an initial concentration of 106 µg L-1 through a fixed
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bed adsorption process. Finally, a very low equilibrium concentration was acquired to meet the criteria of the US EPA standard, i.e., 10 ppb (µg L-1). This finding further demonstrated the
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usefulness of Zr-based NP-doped ACF as a promising sorbent for abatement of As(V)-contaminated effluent.
As a whole, unmodified AC materials showed limited adsorption capacity for As and other heavy metals. The low or moderate adsorption capability of unmodified AC made it difficult to meet the US EPA standard (10 ppb) for drinking water despite the low price and easy availability. 13
3.2. Zeolites Over the past 60 years, zeolites have attracted much interest from both academic and industrial fields. Zeolites occur naturally although they are commonly synthesized industrially. In general, as synthetic zeolites showcase enhanced advantages over their pristine form of analogues, it is desirable to design and produce more functional zeolites. Both natural and synthetic zeolites have
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been studied by many researchers as a platform for efficient removal of arsenic [69, 70]. Previously, the adsorption performance on diverse arsenic species (e.g., arsenite (As(III)), arsenate (As(V)), dimethylarisinic acid (DMA), and phenylarsonic acid (PHA)) was compared between natural
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zeolites (Mexico (ZMA, ZME, ZMS, and ZH) and Hungary (ZH)) and synthetic mordenite (ZS-M)
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[69]. However, because of the low sorption efficiency (< 50%) under low concentration (5 mg L-1) and long contact time (2-18 days), the above tested zeolite sorbents were not recommendable for
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such applications. Thus, more efforts were directed toward the modification of zeolite materials to
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help enhance their removal potential. It was reported that Fe-exchanged natural zeolite (Fe-eZ) had an enhanced removal capacity of 100 µg g-1 for As(III) when exposed to its initial concentration of
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20 mg L-1 in the water phase [70]. In another report, natural zeolite was modified by treatment with either 0.1 M NaCl + 0.1 M FeCl3 or 0.1 M NaCl + 0.01 M FeCl3 to attain adsorptive removal
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efficiencies of 9.2 (92% of removal efficiency) or 8.4 µg g-1 (84% of removal efficiency) for As(V) with an initial concentration of 0.1 mg L-1 [71]. Thus, Fe modified zeolite can be considered as an efficient sorbent despite of its low adsorption capacity for As(V) removal from drinking water. However, because of the low As uptake by pristine zeolite materials, more attention was paid to modification of zeolites for treatment of As. 14
Recently, various types of natural zeolites were modified with Fe, Zr, and FeZr and used for adsorptive removal of As(V) from aqueous solutions [72]. Among these modified forms, Zr-ZM (mordenite-rich tuff) and FeZr-ZCH (chabazite-rich tuff) were reported to have the highest uptake values of As(V), with removal capacities of 0.072 and 0.068 mg g-1, respectively [72] (Table 1). All the experimental results fitted well with the Freundlich isotherm model. The highest KF value of FeZr-ZCH among the tested zeolites at 0.36 (mg g-1) (L mg-1)1/n verified the noticeable enhancement
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in affinity between this modified zeolite and As(V). The highest pseudo-second-order rate constant of FeZr-ZCH for As(V) adsorption was consistent with the potential occurrence of numerous binding sites in favor of a faster adsorption process for As(V) removal. It should also be noted that
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formation of an inner sphere complex through the ion exchange reaction may contribute to the
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enhanced adsorption of As(V) [72]. As such, Fe-, Zr-, and FeZr-modified zeolites may have high potential for reduction of As(V) from aqueous medium.
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Adsorptive treatment of As(V) in its single or binary components (e.g., As(V)-Se(VI)) was also
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investigated using nZVI and nZVI-functionalized zeolite (Z-nZVI) (Figure 3a) [73]. In both component systems (i.e., single and binary) for the two sorbents, equilibrium was acquired within
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90 min (Figure 3b). Interestingly, the presence of Se(VI) showed a slight influence on adsorption of As(V) in aqueous media. Both nZVI and Z-nZVI showed significant preferential adsorption
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behavior for As(V) removal. Similar preferential results were acquired in an adsorption study of multiple components of As(V) and P(V) by ferrihydrite Neupane, Donahoe and Arai [74]. Furthermore, the adsorption results for the two components were well-matched with the pseudosecond-order model, indicating chemical adsorption via formation of bidentate complexes [75]. In another study, copper exchanged zeolite (CEZ) was synthesized by ion exchange technology using 15
copper salts and zeolite-A; the prepared material was then employed for the treatment of As(III) and As(V) [76]. CEZ showcased highest adsorption capacities of 1.27 and 1.48 mg g-1 for As(III) and As(V) at the initial loading concentration of 2 mg L-1 when estimated from Langmuir model (R2 = 0.999 and 0.904 for As(III) and As(V)). Further, as seen from the better fitness of Langmuir isotherm model (e.g., relative to Freundlich (R2 = 0.797 and 0.794 for As(III) and As(V)), the monolayer uniform adsorption appears to be prevalent for CEZ surface. The results of the performance test of
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CEZ showcased very low concentrations of arsenic remaining in leachate (such as 0.011 mg L-1 for As(III) and < detection limit for As(V)). As these concentration values are far lower than the USEPA standards of 5 mg L-1, CEZ was found to be a highly efficient media for removing arsenic. The
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results of comparative analysis on the above discussed zeolites are also summarized in Table 1. In
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zeolites are far superior in that respect.
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general, natural zeolites showcased limited efficacy for the removal of arsenic, while the synthetic
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3.3. Waste-derived sorbents
As discussed above, both AC and zeolites showed enhanced adsorption capacity for As removal
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through various modifications. Despite their frequent use, industrial/agricultural solid waste products (i.e., waste-derived sorbents) are also used as adsorbents for metals. They are generally
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cheaper and available locally in large quantities [77] and can be used for reduction of heavy metal ions. Chitosan, waste slurry, and lignin were studied for the treatment of effluent containing metal ions [78] . Likewise, the use of sorbents made of fly ash, blast furnace slag and sludge, and black liquor lignin was also reported for the treatment of heavy metals contaminated in wastewaters [77]. In another review, removal performances of heavy metals from aqueous phase were compared 16
between low-price adsorbents (e.g., sorbents derived from chitosan, fly ash, coal, and oxides) and AC [78]. Nonetheless, it was difficult to assess and identify the best sorbent for specific heavy metals as many of the previous studies were carried out in the presence of multi-component metals or under different operating conditions. The effects of competing ions in multi-component systems occur through complicated processes. Hence, it is difficult to assess removal efficiency and related mechanisms based on simplified experimental conditions.
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A sorbent derived from vegetable oil solid waste was manufactured and employed for As removal [79]. After pretreatment of Fe2+/H2O2 (Fenton reagent) at a 1:17 molar ratio for this sorbent, it showed maximum removal efficiency values for As(III) and As(V); the corresponding adsorption
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capacities with the initial concentration (i.e., 150 µg L-1) were 81% and 75%, respectively. Hence,
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pretreatment with the Fenton reagent was determined to be an efficient technique for enhanced reduction of both As species. In another study, three types of adsorbents were developed using
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industrial wastes (i.e., hydrated cement (HC), brick powder (BP), and marble powder (MP)) for
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removal of binary (arsenic and fluoride) ions from potable water [80]. Among the aforementioned adsorbents, HC exhibited the highest removal efficiency (> 90%) for As(III) at both high and low
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initial concentrations, as summarized in Table 1. The experimental data for the three sorbents were suitably described by the Langmuir isotherm model. Thus, HC exhibited the highest potential for
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treating As(III)-contaminated drinking water with highly selective adsorption of As. Solid sorbent materials derived from waste cement powder and concrete sludge were fabricated
and used to evaluate As(V) removal performance [81]. A maximum adsorption capacity of 175 mg g-1 was acquired for As(V) at a considerably high initial concentration of 700 mg L-1. This high adsorption capacity could be ascribed to the precipitation of calcium arsenic. It should be noted that 17
the aforementioned waste cement materials were rich in calcium. The removal results fit well with the Langmuir equation. Thus, sorbents made of waste concrete were efficient for sequestration of As(V). These kinds of recycled waste sorbents may be a good option for As treatment since they only cost about $100~200 (US dollars) per ton in Japan [81]. Not long before, the enhanced adsorption of As was achieved by alum sludge modified by calcination [82]. Specifically, these authors extracted aluminum oxide from alum sludge by calcination. Then, alum-based adsorbent
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(ABA300, calcination at 300 ℃) exhibited enhanced adsorption capacity (62.9 mg g-1) for As(V) relative to ABA105 (6.07 mg g-1) and ABA500 (10.4 mg g-1). As such, the high efficacy of the alum sludge-derived sorbent for As removal was supported by the dramatic improvement in adsorption
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capacity. Recently, oxidation and adsorption of As were simultaneously conducted onto a composite
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fabricated by alum sludge and graphitic carbon nitride (g-C3N4) [83]. The produced composite displayed both adsorption (40 mg g-1) (24 h-dark state) and oxidation potential (60 mg g-1) (24 h-
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dark plus 24 h-light) to treat As(III). Further, it was demonstrated that both of outer-sphere
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complexation and physisorption stimulated the adsorption of As(III) onto the composite surface. The oxidation of As(III) to As(V) further promoted the adsorption through inner sphere
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complexation and chemisorption in the light condition. Finally, the removal of As proceeded in a more stable manner by the above composite through the combination of adsorption and photo-
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oxidation processes.
Furthermore, sorbents made of solid waste from the leather industry were employed for removal
of As(V) and Cr(VI) contamination from water, with adsorption capacities of 26 and 133 mg g-1, respectively [84]. Interestingly, bio-sorbents made from diverse agricultural wastes (e.g., sugarcane bagasse, rice husk, and wheat straw) were also tested for sequestration of As from aquatic media 18
[85]. Likewise, many other sorbents derived from agricultural waste, such as mango leaf powder (174.9 mg g-1 for As(III)) [86] and orange waste (143.3 mg g-1 for As(III)) [87], also showed excellent removal adsorption capacities for As. As a whole, the innovative recycling of industrial/agricultural waste was demonstrated as an efficient option for sequestration of As from wastewater.
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4. Adsorptive removal of arsenic by advanced materials
As for conventional materials developed for removal of As, advanced novel materials were also explored to treat heavy metals in aqueous systems over the past several decades. In this section, the
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potential of advanced materials for remediation of As from wastewater will be discussed in detail
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4.1. Graphene oxide
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based on some key examples (e.g., GO, CNTs, MOFs, and functionalized porous materials).
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Graphene is considered a 2-dimensional (2-D) structural material formed by sp2-bonded carbon. Graphene generally has carbon as the basic structural element including graphite, charcoal, carbon
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nanotubes, and fullerenes [88]. GO is an oxidized form of graphene with oxygen-containing functional groups, such as epoxide, carbonyl, carboxyl, and hydroxyl groups. The use of graphene-
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based hybrid materials has been widely extended for water treatment [89]. Therefore, many previous reports have indicated that iron-based magnetic GO materials (e.g., magnetite-GO (M-GO) and magnetite-reduced GO (M-rGO)) considerably enhanced removal potentials for heavy metal ions. In this respect, rGO materials were usually fabricated by the reduction of GO through thermal, chemical, or electrical treatments. It is well-known that rGO with some defects on the surface or in 19
the internal structures are also found to maintain oxygen functional groups. Further, different chemical compositions of rGO with various characteristics could be acquired by using different reducing agents or changing the dosage of the agents [90]. It was also noteworthy that the polar oxygen units may exert both positive (e.g., enhancing aqueous dispersion) and negative influences (e.g., decreasing sheet conductivity). Thus, it is very essential to compare the performance of both GO and rGO in terms of As treatment.
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Modification of GO with a humic acid coating and iron was an efficient option for enhancing As removal from water [91]. The use of M-rGO prepared through the addition of a magnetite (particle size of 10 nm) was reported to yield a binding capacity of 13.10 mg g-1 for As(III) and 5.83 mg g-1
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for As(V) [92]. As mentioned earlier, iron-based modifications are very effective for upgrading the
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potential for adsorptivity. Essentially, iron content in a sorbent modified with iron is crucial to determining the actual adsorption capacity of a metal like As. Specifically, the surface complexation
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or electrostatic interaction promoted the adsorption for arsenic. M-GO and M-rGO composites were
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fabricated (Figure 4a) and compared for remediation of As(III) and As(V) from wastewater [93]. M-GO exhibited a higher removal capability for both targets due to the existence of more functional
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groups on M-GO than on M-rGO (Figure 4b and c). The adsorption capacity for As(III) was 85 for M-GO and 57 mg g-1 for M-rGO, while the capacity for As(V) was 38 for M-GO and 12 mg g-1 for
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M-rGO. The presence of more oxygen-containing groups in M-GO contributed to the formation of M-GO with higher Fe3O4 composition to show better adsorption of As by M-GO than by M-rGO. Moreover, The adsorbed As(V) was preferentially connected with two adjacent structural Fe3+ cations (bidentate binuclear-bridging complex), while As(III) adsorption was accompanied by both mono- and bidentate complexes [94, 95]. As mentioned earlier in many studies, As(III) was 20
converted into As(V) before adsorption due to additional difficulties in sorptive treatment of As(III) in comparison to that of As(V). As discussed in Section 1.1., As(III) tends to be more difficult to remove due to the shortage of electrostatic force compared to that of As(V). However, due to the high adsorption capacity of the M-GO/M-rGO composite for As(III), it is not necessary to include the oxidation pretreatment process (e.g., As(III) to As(V)) for removal of As(III). Likewise, many other magnetically modified GO materials showed efficient removal capacities
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for treatment of As-containing effluent. GO modified by iron-manganese binary oxide (FeMnOx/RGO) was explored for removal of As from aqueous solutions [54], [96]. The adsorption capacities of FeMnOx/RGO for As(III) and As(V) were 10.2 mg g-1 and 11.5 mg g-1 (with a 1 mg Linitial concentration), respectively. In contrast, bare RGO showed almost no adsorption of As.
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1
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Initial concentration strongly influenced adsorption capacity, although the enhanced capacity was often accompanied by a slightly decreased removal efficiency (Table 2). Furthermore, the presence
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of PO43- dramatically decreased As adsorption due to its competing effect. More importantly,
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excellent reusability of FeMnOx/RGO (e.g., maintaining > 80% removal efficiency) for both As was observed with an initial concentration of 100 ppb. This result confirmed that FeMnOx/RGO presents
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good performance in real water treatment systems. The rGO-Fe3O4-TiO2 based nanocomposite has great potential for direct removal of As(III) (Benjwal, Kumar, Chamoli and Kar [34]. rGO-Fe3O4-
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TiO2 exhibited a high monolayer adsorption capacity of 147 mg g-1 for As(III) based on the Langmuir adsorption isotherm. In contrast, very low capacity values (e.g., 0.38 mg g-1 for As(III) and 0.27 mg g-1 for As(V)) were seen for Fe3O4-GO (Table 2). As described above, most of the magnetic-based modified GO materials showed exceptional removal performance for As. Similarly, other modified GO materials were also efficient at removing 21
arsenic. A zeolite-reduced GO (ZrGO) composite based on fly ash was tested for removal of As from wastewater (Figure 5a) [97]. Surprisingly, a very low final concentration of 0.008 mg L-1 was attained by ZrGO with an initial As(V) concentration of 0.1 mg L-1 (Figure 5b). As such, the final concentration may meet the permissible limits of the World Health Organization (WHO) (10 ppb). The Redlich Peterson isotherm model fit best with the above noted experimental data, which illustrated the hybrid mechanism and not ideal monolayer adsorption of ZrGO for the adsorption
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process [97]. As a whole, GO-based sorbents showed noticeably enhanced removal efficiency for As compared to that of the conventional materials. The noticeable enhancement in adsorption potential of GO-based materials prompted researchers to further explore new functional materials
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for real-world applications. As such, effective removal of As can be pursued further through
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4.2. Carbon nanotubes
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synthesis of diverse composites of GO, especially with many cost-free materials (e.g., fly ash).
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CNTs consisting of single-wall and multi-wall nanotubes generally have high binding energies through van der Waals forces. As CNTs can showcase dramatic variations in size, shape, purity, and
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structure, they can exert complicated but direct control on various systems. As such, these materials are well-known as highly promising prospects for various environmental applications [98, 99].
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A hydrated nanocomposite consisting of zirconium oxide-coated CNTs (ZrO(OH)2/CNTs) was
fabricated using a filtration-steam hydrolysis technique and employed for adsorptive treatment of As in water [100]. Maximum adsorption capacities for As(III) and As(V) by this nanocomposite were 78.2 and 124.6 mg g-1, respectively, based on the Langmuir model. Noticeably, ZrO(OH)2/CNTs exhibited a far more rapid adsorption rate for both As(III) and As(V) ions than 22
ZrO(OH)2 nanoparticles. The observed differences could be ascribed to uniform dispersion of the former (ZrO(OH)2/CNTs) to provide more adsorption sites. Regeneration is a useful tool of performance metrics for evaluating the economic value of an adsorbent. Removal efficiency of ZrO(OH)2/CNTs material was more than 97% after six cycles, which further validated its effectiveness in removing As from water systems. The initial concentration of 375 µg mL-1 for As(V) and As(III) was reduced to an approximately 10 µg mL-1 final concentration (USEPA standard).
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Zr-based modified CNTs showed significant potential as highly effective adsorbents for remediation of As. Meanwhile, Fe-based materials, such as goethite [101] and natural iron oxide [102], have also gained attention as advanced adsorbents to reduce As from wastewater. GO-CNT
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aerogel was coated with α-FeOOH (iron oxyhydroxide) to form α-FeOOH@GCA through a self-
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assembly technique by in-situ Fe2+ reduction. It was then employed for treating three different Ascontaining species (i.e., As(V), dimethylarsinic acid: DMA, and p-arsanilic acid: p-ASA) (Fig. 6a)
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[103]. α-FeOOH@GCA showed noticeably enhanced maximum removal capacities for the
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aforementioned As species, at 56.4, 24.4, and 102.1 mg g-1, respectively, which were far superior to those of α-FeOOH nanoparticles. The variations in adsorption capacities for different As species by
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α-FeOOH@GCA can be explained at least partially by ligand exchange, which finally promoted formation of inner sphere complexes with different structures of As species. Differences in
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structures and properties of the aforementioned As species were suspected to cause variation in the adsorption mechanism onto α-FeOOH@GCA and, hence, led to the various adsorption capacities. Furthermore, their adsorption results fit well with both the Langmuir and Freundlich isotherm models (Figure 6b). The dynamic affinities between the sorbent and As species were supported by the data of different adsorption removal preferences for As-containing species as well as the 23
predicted results based on the two models. Thus, this study offers new opportunities for modifying natural iron-based minerals with GO-CNT carriers to promote immobilization of As species from wastewater. Meanwhile, a series of other iron-based CNTs has been investigated for remediation of As by many researchers. In this regard, a multi-step, one-pot technique was used to fabricate magnetic CNT composites (MI/CNTs) by employing as-prepared CNTs (p-CNTs) and potassium hydroxide
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(KOH) [104]. MI/CNTs exhibited a larger surface area of 662 m2 g-1, which is ~5 times larger than that of pristine CNT material. They showed moderately low maximum removal capacities for As(V) (9.74 mg g-1) and As(III) (8.13 mg g-1) when estimated using the Langmuir isotherm model. Later,
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Chen’s group also used a one-pot green method to synthesize another magnetic iron oxide/CNTs
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(MIO-CNTs) composite with a surface area of 209.8 m2 g-1 by using p-CNTs as a template through a catalytic chemical vapor deposition technique [105, 106]. Despite the lower surface area, the MIO-
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CNT composite showed better As removal efficiency than the MI-CNT composite. The former
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composite exhibited maximum removal capacities of 47.41 and 24.05 mg g-1 for As(V) and As(III), respectively. It was verified that the oxygen-containing groups of the CNT composite played the
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dominant role for As removal, which formed hydroxylated surface complexes due to the reaction between Fe-OH and As-OH [105]. Therefore, the acquired complexes provided enhanced
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availability of adsorption sites. Furthermore, other modified CNT-based materials, such as CNT/CuO nanocomposite [107], TiO2-coated CNTs [108], and Ce-Fe oxide-decorated CNTs [49], were also reported to present promising removal performances for As from aqueous systems. The adsorption capacities of these CNT-based materials are listed in Table 2, and the associated discussion of CNT materials is presented in Section 4. 24
4.3. Metal organic frameworks MOFs are a new type of micro- or meso-porous material connected by a metal ion and an organic linker through formation of coordination bonds [109]. The facile tenability in their structures and/or inner porosities makes it possible for them to attain desirable porous morphology with a specific surface area and high thermal/water stability [110]. Likewise, a series of MOFs can be developed
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using varying ligands (e.g., 1,3,5-benzenetricarboxylate and imidazolate) and metal clusters (e.g., Zn2+ and Cu2+). As such, various forms of MOFs have been developed and used widely in diverse applications including but not limited to drug delivery [111], catalysis [112], adsorption [113],
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conductivity [114], and gas storage [115].
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As a well-known and commonly used MOF material, ZIF-8 was employed in treatment of As ions from water systems [116]. A nano-sized ZIF-8 fabricated through a facile technique at room
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temperature was used for adsorptive removal of As(III) and As(V) [116] (Figure 7a). On the basis
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of the Langmuir model, ZIF-8 exhibited maximum adsorption capacities of 49.5 and 60.0 mg g-1 for As(III) and As(V), respectively. Depending on the type of co-existing anions, there were dynamic
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effects on As adsorption. Specifically, SO42- and NO3- had slight or no influence on As removal, while PO43- and CO32- adversely affected adsorptive removal of As. Moreover, according to x-ray
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photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses, adsorption of arsenic species was mainly ascribable to electrostatic interactions and formation of complexes with -OH and -NH2 groups. It was noteworthy that a very low final As(V) concentration of 2.8 µg L-1 remained after treatment with ZIF-8 (initial As(V) of 100 µg L-1). As such, ZIF-8 showcased excellent effectiveness in 25
practical drinking water treatment. Study of the synthesis of ZIF-8 and its subsequent applications for removing As(V) have been explored by other researchers. For instance, Wu, Zhou, Zhang, Wu, Li, Qiao, Guan and Li [117] synthesized hierarchical ZIF-8 with the assistance of cetyltrimethylammonium bromide (CTAB) and the amino acid L-histidine (His) as co-templates for removal of As(V). The variation in solvent types and ratios of Zn2+ and ligands may dramatically influence the surface area and As removal performance of ZIF-8. Accordingly, ZIF-8 showed a
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maximum capacity of 90.9 mg g-1 for As(V), which may be explained by the tailored porous structure and the enhanced porosity of the hierarchical ZIF-8 particle. The regeneration experiment demonstrated that this ZIF-8 particle could maintain its adsorption capacity after three cycles to
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support its promising potential for treatment of As and other heavy metal ions.
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In addition, other ZIF-8 or ZIF-based materials were studied to explore their removal capacities for abatement of As. As part of such efforts, water-stable Fe3O4@ZIF-8 composite was fabricated
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for removal of As(III) from water phase to yield a maximum capacity of 100 mg g-1 (Huo, Xu, Yang,
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Cui, Yuan and Fu [118]. From a comparison of three different morphologies of ZIF (i.e., cubic, leafshaped, and dodecahedral ZIFs), it was found that neither particle morphology nor surface area of
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ZIFs played the dominant role in As(III) removal (Liu, Jian, Liu, Yao and Zhang [119] (Table 2). Rather, the hydroxyl substation on ZIFs was primarily responsible for As(III) removal. The leaf-
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shaped 2D ZIF-L framework was also explored by Nasir, Nordin, Goh and Ismail [120] as an As adsorbent. This ZIF-L exhibited a maximum uptake of 43.7 mg g-1 for As(III) but displayed its maximum adsorption capacity in basic conditions (pH 10). In contrast, for most of other the adsorbents used for As removal (Tables 1 and 2), the maximum capacities were usually gained at near neutral pH, consistent with the variation in zeta potential (e.g., from positive to negative values). 26
At pH 9 to 10, the electrostatic attraction of the positively charged ZIF-L surface for As(III) reached a maximum level to showcase enhancement in adsorption uptake of As. Other MOF materials have been also widely studied for removal of As. For instance, a highly water-stable indium-based MOF (AUBM-1) was efficiently employed for As removal from aqueous phase [121]. The fabricated MOF with a pts (tetragonal symmetry) topology exhibited high chemical stability at different pH. The highest As(V) uptake capacity of 103.1 mg g-1 was obtained at pH 7.6
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by matching the experimental data with the Langmuir model (monolayer adsorption) [121]. The kinetic studies indicated that the adsorption process of AUBM-1 fit well with the pseudo-secondorder model. Moreover, its suitability as a regenerable As(V) sorbent was also supported by a
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decrease of ~20% in the As(V) adsorption capacity after three regeneration cycles. Likewise, Li,
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Yang, Sui and Yin [122] successfully developed MOF-808 nanoparticles using a household microwave oven in less than 5 min. MOF-808 displayed an adsorption capacity of 24.9 mg g-1 for
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As(V) at its initial loading concentration of 5 ppm. This MOF maintained > 80% removal efficiency
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for As even after five cycles of adsorption-desorption, while maintaining the integrity of its microcrystalline form as an efficient As(V) removal adsorbent. Overall, MOF materials exhibited
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dramatically enhanced performance for As adsorption compared to that of traditional adsorbents. (For more details, refer to the comparisons among various adsorbents in terms of PC values as
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discussed in Section 4).
4.4. Miscellaneous forms In addition to GO, CNTs, and MOFs, there are many other porous materials (e.g., metal oxides, chitosan-based materials, and polymers) that have been developed as efficient sorbents for removing 27
As from water. More specifically, a nanostructured Fe-Cu binary oxide was introduced for removal of As(V) and As(III) from water systems (Zhang, Ren, Zhang and Chen [123]. During synthesis of Fe-Cu binary oxide, the optimal Cu:Fe molar ratios were determined to acquire the maximum affinity between the adsorbent and As. Interestingly, As sorption increased with an increase in Cu:Fe molar ratio to 1:2 but decreased sharply with further increase in molar ratio. As verified by PXRD and SEM characterizations, the Fe-Cu binary oxide was ferrihydrite-like with poor crystallinity, a
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high BET surface area of 282 m2 g-1, and pore volume of 0.31 cm3 g-1. It exhibited maximum adsorption capabilities for As(V) and As(III) of 82.7 and 122.3 mg g-1, respectively. The similar amorphous 2-line ferrihydrite also showed excellent removal performance for As (Table 2) [124].
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Thus, it was highly speculated that As(V) were adsorbed by formation of inner-sphere surface
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complexes between As and sorbent, while both inner- and outer-sphere surface complexes contributed to adsorption of As(III). These two case studies focused on adsorption of As species
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without converting As(III) to As(V). However, ferrihydrite sorbent at a high Fe/As molar ratio (e.g.,
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50) also acted as a catalyst to oxidize As from As(III) to As(V), which plays a pivotal role in cycling
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arsenic in the natural environment (Zhao, Jia, Xu and Zhao [125].
As discussed earlier, despite large surface areas, the virgin organic or inorganic materials did not
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guarantee good adsorption performance for As ion removal. Hence, modifications of their pristine structure could provide more functional groups on their surface to offer more affinity for As. For instance, the coating technique is a useful tool to modify biopolymer and may enhance the interaction between As and sorbent. In this regard, Boddu, Abburi, Talbott, Smith and Haasch [126] fabricated a chitosan-coated biosorbent (CCB) by coating ceramic alumina with chitosan. The 28
monolayer adsorption capacities for As(III) and As(V) by this sorbent, when estimated based on the Langmuir isotherm, were 56.5 and 96.5 mg g-1, respectively. It can be assumed that biosorption based on bio-materials (e.g., chitosan) may receive more attention in the near future due to the material abundance in nature and biocompatibility. As an example of an advanced biosorbent, chitosan zero-valent iron nanoparticles (CINs) were prepared and tested for removal of As [127]. Surprisingly, very low final concentrations of < 5 µg L-1 for As
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were obtained with the initial concentration of 2 mg L-1 within 180 min. It should be noted that CIN contributed to a considerable reduction in final As concentrations, as listed in Tables 1 and 2. High adsorption capacities of 94 (for As(III)) and 119 mg g-1 (for As(V)) were found based on the
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Langmuir isotherm model. Anions, such as sulfate, phosphate, and silicate, slightly affected the
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adsorption capacities of CIN for both As. Moreover, CIN was also capable of efficiently adsorbing As even after five consecutive cycles, which supported its high potential as a sorbent for As
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treatment in real applications. Further, As concentration of < 14 µg in the leachate were obtained in
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the pH 2.9-6.3 based on an extended toxicity characteristic leaching procedure (TCLP) test [128]. As a whole, biomaterials showed significantly improved performance for As adsorption
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compared to those of conventional materials. However, final adsorption capacity can be sensitively influenced by experimental conditions (e.g., solution pH, initial loading concentration, and
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temperature). The important parameters that can affect performance for As removal are discussed in Section 5.
5. Effects of various factors on adsorption for arsenic removal 29
The adsorptive treatment of target pollutants (e.g., As) in wastewater can be controlled by the combined effects of various parameters such as solution pH, initial concentration, adsorbent dosage, contact time, competing ions, and temperature. More specifically, solution pH plays a significant role in the effluent treatment process as it can directly influence metal speciation schemes in water systems [129]. Meanwhile, it also affects the degree of protonation of functional groups on the surface of the adsorbent and the chemical nature of As. Therefore, its effects can be manifested by
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dramatic change in adsorption efficiency with variation in pH. In most of the adsorption processes, the adsorption capacity for As may increase with pH to neutral and then will follow a downward trend. Tables 1 and 2 show that the maximum adsorption capacities for both As(III) and As(V) were
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acquired at pH 7. In a solution with a higher pH value, As and OH- may form a hydroxide micro-
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precipitate, which may hinder adsorption of As [130].
Temperature is another pivotal experimental condition that is often used to determine the nature
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of the adsorption process in terms of thermodynamics. Thermodynamic parameters can be calculated using the following equations:
𝛥𝑆 𝑅
−
(1) and
𝛥𝐻
(2),
𝑅𝑇
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ln 𝐾 =
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Δ𝐺 = −𝑅𝑇 ln 𝐾
where ΔG is the standard Gibbs free energy (kJ mol-1), R is the gas standard (8.314 J mol-1), T is the
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temperature (K), K is the Langmuir isotherm constant, ΔS is the entropy change (J mol-1 K-1), and ΔH is the enthalpy change (kJ mol-1). In this study, the occurrences of positive and negative ΔH suggested that the adsorption reaction should be endothermic and exothermic, respectively. Furthermore, positive ΔG indicated that the adsorption process needed external energy to convert reactants to products [131], while negative ΔG values implied that the adsorption reaction was 30
spontaneous [64]. However, most of the studies have illustrated slight or limited effects of temperature on adsorption of As [132, 133]. For instance, a modified AC exhibited 526 µg g-1 at 25 ℃ and 588 µg g-1 at 35 ℃ with the initial As(V) concentration of 400 µg L-1 [64]. In groundwater, different ion sources may compete with As for acquisition of adsorption sites, which generally could negatively influence the adsorption capacity of the adsorbent. The effects of numerous co-existing ions (e.g., sulfate, nitrate, chloride, carbonate, bicarbonate, silicate, and
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phosphate) were investigated for their competing roles in adsorption of As. Many studies have reported that carbonate, silicate, and phosphate exhibited more distinctive effects on the adsorption of different As species, regardless of adsorbent type [65, 134, 135]. For instance, the presence of
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phosphate led to a decrease in As(III) ion removal efficiency of modified AC from 90% to 18%
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[135]. This was due to the fact that these anions can be strongly adsorbed onto the surfaces of sorbents through inner-sphere complexation. Therefore, this can lead to a dramatic reduction in
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As(V) adsorption through formation of surface complexes with the OH-group. However, phosphate
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and silicate only showed marginally negative effects on As adsorption behavior in the case study, as discussed (e.g., CIN nanoparticle) in Section 4.4 [127]. This divergence may be explained by the
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various affinities between As and different sorbents. Furthermore, the initial As concentration, or ion strength, was also an important parameter that could affect adsorption, as a higher initial
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concentration may provide a stronger driving force to yield higher adsorption capacities. However, evaluation based on the highest As adsorption capacities may be misleading because such a metric is sensitively affected by initial loading concentration (e.g., use of an unrealistically high initial concentration to induce high capacity values). In real-world conditions, As concentration was prevalent at low (ppb) level. As shown in Table 2, ZIF-8 showed an increased adsorption capacity 31
from 0.486 to 60.03 mg g-1 with an increase in initial As(V) concentration from 0.1 to 100 mg L-1 [116]. This type of improvement in adsorption capacity through control of the driving force parameters (especially initial loading concentration) was also commonly seen in the removal of other contaminants, such as Pb2+ [136], organic dyes [137], and pesticides [138]. There were also some other experimental conditions (e.g., contact time and adsorbent dosage) that could influence the final adsorption capacity, but these parameters were not decisive or could be easily modulated
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based on requirements. As a whole, evaluation of adsorbent performance based on maximum adsorption capacity is not an objective option. The more objective option for such evaluation is
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discussed in the next section.
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6. Performance evaluation of various materials for abatement of arsenic In this section, the PC (mol kg-1 M-1) value was employed as a key metric to assess the
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performance of sorbents with the least bias, so that the best sorbent could be determined more
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objectively for remediation of As from wastewater [139]. The PC was calculated by dividing maximum adsorption capacity (mg g-1 or mol g-1) by final concentration of As (mg L-1 or mol L-1)
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in the solution. Herein, it should be noted that after adsorption of As on sorbents, leaching of the As (with time) into solution can affect the PC values. However, due to the lack of time dependent
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leaching testing in majority of cited articles, we considered the specific adsorption capacity and final As concentration measured for PC calculations. Thus, evaluation of As ion removal was more meaningful if the data are assessed based on PC concept rather than that based on maximum capacity [139]. This is because the former presents the less biased results by reducing the effects of high initial feeding concentrations, while the latter is not free from such effect. It is noteworthy that CIN 32
exhibited the best adsorption performances (PC: ≥ 798 mol kg-1 M-1) for both As(V) and As(III). In fact, this adsorbent showed very high removal efficiency (≥ 99.75%) even at a low initial concentration of 2 mg L-1. This enhanced performance could be ascribed to the synergistic combination between strong affinity (As and iron) and size effect of the CIN particle. This type of nano-sized particle could be efficiently modified to increase its adsorption removal performance for abatement of organic pollutants and inorganic metal ions, demonstrating advantages over other
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sorbents. It should also be noted that Eu-MGO/Au@MWCNT showed the highest values for both As(III) (320 mg g-1) and As(V) (298 mg g-1) in terms of adsorption capacities. However, it was difficult to obtain their related PC values due to the lack of data regarding remaining As
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concentration or removal efficiency. In the case of MPSAC-La (0.36), the low removal efficiency
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at a high initial loading of As(V) resulted in a moderate PC value (1.98 mol kg-1 M-1) for As(V), although it showed a high adsorption capacity (227.6 mg g-1) for As(V) [66]. This case study on
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MPSAC-La (0.36) should remind researchers that sorbents with a high adsorption capacity do not
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necessarily perform well for removal of As from wastewater. According to evaluation of sorbent performances, the CIN particle appeared to have great potential as a sorbent for abatement of both
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As(V) and As(III). Therefore, synthesis of nano-sized particles and their application for metal remediation can be touted as a highly effective option for treatment of As and other toxic heavy
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metal ions. Furthermore, the evaluation of various sorbents in terms of regeneration study is discussed subsequently in the following section.
7. Regeneration of sorbents and their disposal after the use Regeneration and disposal of exhausted sorbents have been discussed intensely in recent years 33
so as to ensure their cyclic utilization while reducing the potential harmfulness of exhausted sorbents [140, 141]. Generally, the unreasonable disposal of the exhausted sorbents loaded with As may cause severe environment problems. Thus, it is necessary to safely dispose the exhausted sorbents, such as landfill, mixing with livestock waste, and incorporating within construction materials [142]. In this regard, stabilization/solidification (S/S) is considered as a suitable wasted sorbent treatment technology. As such, such option has been utilized very broadly for almost all toxic spent waste
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management alternatives because it may convert toxic waste into a less toxic or less mobile form of waste by chemical, physical, or thermal processes [143, 144]. Further, because inorganic wastes are more compatible with the used cementitious binders, the S/S process is used more frequently to
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convert inorganic wastes into valuable products. More specifically, monolithic solids with the
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advantageous properties (e.g., enhanced structural integrity, long-term stability, and minimal leaching) may be fabricated by encapsulating or incorporating the waste sorbents into the binder
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system [142]. Cement and hydrated lime (Ca(OH)2) has been extensively studied for stabilizing As
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containing materials [145]. For example, the calcium leaching may influence the leaching of As in all S/S system so that As concentrations can dramatically decrease with the increase of Ca leachate
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concentration [144]. Not long before, As bearing spent adsorbents (e.g., thermally treated (TTL), acid-base treated laterite (ABTL), and aluminum oxide/hydroxide nanoparticles (AHNP)) were well
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stabilized in the form of clay bricks [146]. Before stabilization, TTL, ABLT, and AHNP exhibited adsorption capacities of 6.43, 9.25, and 48.5 µg g-1, respectively. The subsequent characterizations for the bricks showed satisfactory results of density (2.3 g cm-3), shrinkage (10.2%), water absorption (11~14%), and comprehensive strength (35~150 kgf cm-2). Herein, it should be noted that converting one waste material into another beneficial feedstock is highly recommendable for 34
sustainable development. Thus, Table 3 lists a series of studies that focused on the S/S of Ascontaining solid wastes into useful construction materials. The effectiveness of a cement-free S/S technique to treat soils contaminated with As and Pb was realized using clay minerals as the green compatible binding material [147]. In this research, Ca3(AsO4)2∙4H2O and Pb3(NO3)(OH)5 precipitates were first produced by reacting those soils with Ca(OH)2, respectively. Subsequently, these precipitates were mixed separately with metakaolin (MK), lime (L), limestone, and
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phosphogypsum to generate the clay-based S/S products. Finally, the immobilization efficiency of As and Pb in S/S treated soil samples improved significantly by 96.2% (As) and 98.8% (Pb) in M6L (molar ratio of MK to L, 1:6) samples to support its efficacy. As a whole, As containing wastes
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have been widely stabilized into bricks, cements, and ceramics as shown in Table 3. However, it
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was noteworthy most of the mentioned studies were not sorbents related wastes. Therefore, the immobilization of spent sorbents into various construction materials still need to be studied further.
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Elution is of great importance for the reutilization of exhausted sorbents and the recovery of the
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adsorbed heavy metal ions (e.g., As). A sorbent with good regenerability implies that it can considerably lower the operating and maintenance price for the whole adsorption system [148].
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Generally, an effective regeneration process is based on the principle that the desorbed sorbents can recover its initial adsorption performance perfectly [149]. In this respect, the adsorption
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performance of various sorbents will be evaluated in terms of regenerability. Nearly 90% of the listed sorbents in Tables 1, 2, and 4 did not address any data for regeneration test, especially traditional materials. Thus, it can be speculated that most of traditional sorbents possess low desorption efficiency for As. In Table 3, NaOH was widely used as the desorbent for studying the regenerability of As adsorption-desorption. It should be noted that in the alkaline pH condition, the 35
adsorption capacity may dramatically decrease due to the competition between As with the hydroxyl ions [148]. In Table 4, only 6 sorbents possessed >80% desorption efficiency after 3 cycles of regeneration study. In this regard, FeMnOx/RGO, ZrO(OH)2/CNTs, and Fe-Cu binary oxide showed ideal regeneration performance because all these three sorbents had desorption efficiency of more than 90% for treating As(III) and/or As(V) after three sorption cycles. Especially, ZrO(OH)2/CNTs exhibited a high desorption efficiency of 97% after six cycles for As(III). The findings of the high
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removal capability in low concentration level (with high PC value) imply that ZrO(OH)2/CNTs is one of the best sorbents for treating arsenic, especially for As(V) (PC: 584.6 mol kg-1 M-1). Thus, further research is needed to find more advanced materials with high adsorption performance that
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are straightforwardly regeneratable over several adsorption-desorption cycles without noticeable
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loss of sorption capacity. Further, in Section 6, although CIN showcased the highest PC value, the low desorption efficiency may limit its use in water treatment. To sum up, some hybridized materials
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(like ZrO(OH)2/CNTs) should be the better choice in terms of performance (e.g., PC value) and
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regenerability.
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8. Conclusions and perspectives
The presence of As in ground and/or drinking water is one of the most important environmental
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problems around the world. In this review, a wide range of purification techniques, such as ion exchange, photocatalysis, and membrane technology, were briefly discussed for treatment of As in such systems. Among such options, adsorption technology is considered a highly attractive option due to its simple design, low price, and easy operation. Further, the potential use of numerous functional materials was explored along with conventional 36
materials. The adsorption performances of these materials were first assessed in terms of adsorption capacities. The results were assessed further in terms of PC and regenerability for a better evaluation of their performances. It was evident from the listed articles and compilation of data (as summarized in Tables 1 and 2) that many of the advanced functional materials (e.g., GO, CNTs, MOFs, and other miscellaneous forms) are much better options for efficient treatment of both As(V) and As(III) in comparison to conventional adsorbents, as evaluated in terms of enhanced recyclability and high
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PC values (e.g., ZrO(OH)2/CNTs with the PC values of 584.6 (As(V)) and 143.8 mol kg-1 M-1 (As(III))). Therefore, advanced materials were found to hold considerably high potential as an alternative to various conventional materials for effective treatment or remediation of As from
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drinking water or wastewater. Adsorptive removal of low-level As by novel materials may provide
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a great motivation for researchers to further synthesize more advanced functional materials for
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various applications.
Declarations
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Biotechnology Advances requires that all authors sign a declaration of conflicting interests. If you have
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nothing to declare in any of these categories then this should be stated.
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There is no competing interests.
A grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1E1A1A01940995).
Acknowledgments We would like to acknowledge support by the R&D Center for Green Patrol Technologies through 37
the R&D for Global Top Environmental Technologies funded by the Ministry of Environment (MOE Grant No: 2018001850001) and by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (Grant No: 2016R1E1A1A01940995). VK would like to acknowledge support from the Department of Science and Technology, New Delhi, India, in the form of an INSPIRE Faculty Award and from the Science and Engineering Research Board (SERB), Government of India, under the Early Career Research (ECR) award (File No. ECR/2018/000748). References [1] M. Jaishankar, T. Tseten, N. Anbalagan, B.B. Mathew, K.N. Beeregowda, Toxicity, mechanism and health effects of some heavy metals, Interdisciplinary toxicology, 7 (2014) 60-72. Journal of environmental management, 166 (2016) 387-406.
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47
f
Target
pH
Initial concentration [As] (mg L-1)
Isotherm
Removal efficiency (%)
18
4.0
0.16
980.40
71
11.05
1.27
973.4
68.0
10.87
1.13
670.3
96.0
19.35
16.13
649.4
49
2.45
1.20
863.00
45.6
2.28
1.05
388.00
Activated carbon
Adsorption capacity (mg g-1)
pr
Materials
e-
S. no.
oo
Table 1. Comparison of adsorption performance of conventional adsorbents for As(V)/As(III) removal. Partition coefficient (mol kg-1.M-1)
Surface area (m2 g-1)
AC
As(V)
3
30
F
2
Fe-AC
As(V)
3
30.0
F
3
Fe-Cu-AC
As(V)
3
30.0
F
4
Fe-Mn-AC
As(V)
5
CF-MP
As(V)
7
4.0
6
CW-MP
As(V)
7
4.0
7
Fe3O4-AC(MN)
As(V)
8
100.0
L
20
28
0.35
572.00
8
H2SO4-AC(MY)
As(V)
8
100.0
F
25
61
0.81
349.00
9
30.0
na l
Jo ur
Fe-AC
3
Pr
1
L
References
[61]
[62]
[63]
As(V)
6
1.0
L
51.3
2.565
As(III)
8
1.0
L
39.2
1.960
0.847
[150]
10
F400-M
As(V)
7
0.3
L
[64]
11
NP-doped ACF
As(V)
3
120.0
F
25
21.74
0.24
[68]
12
MPSAC-La(0.36)
As(V)
6
350.0
L
67.1
227.6
1.98
[66]
*The symbols F, L, and R-P
48
f 14
MPSAC
As(V)
6
15
KOH-AC (C)
As(V)
7.5
75
L
16
Ca alginate beads (G)
As(V)
7.5
75
L
17
G/AC composite beads (GC)
As(V)
7.5
75
L
18
AC-3
As(V)
7
20
L
19
Oxidized AC (ACO-3)
As(V)
7
20
L
20
AC-9
As(V)
7
20
21
ACO-9
As(V)
7
20
22
AC
As(V)
6
23
Iron oxide/AC
As(V)
24
F
13.8
were used to
141.8
indicate 1621
42.4
32.9
66.7
733.6
27
[151]
8
0.53
770
43.5
14
1.24
74
Peterson isotherms,
25
6.67
43
L
86.5
28
10.37
26
250
L
36
17.86
0.11
1022.6
6
250
L
40
20.24
0.13
678.3
As(III)
9
20.0
F
60
0.1
0.0125
As(V)
9
20.0
F
55
0.05
0.0056
Fe-eZ
[152]
Redlich-
25
81.25
na l
Freundlich, Langmuir, and
L
Jo ur
Zeolites
350.0
oo
6
pr
As(V)
e-
PSAC
Pr
13
respectively. [65]
[70]
25
GC
As(V)
7
0.1
15
1.50E-03
0.018
34.1
26
Na-GC
As(V)
7
0.1
17
1.70E-03
0.020
33.2
27
Fe1-GC
As(V)
7
0.1
92
9.20E-03
1.15
79.3
28
Fe2-GC
As(V)
7
0.1
84
8.40E-03
0.53
36.9
[71]
49
f oo
Fe-ZCH
As(V)
6.3
0.1
F
0.038
30
Fe-ZM
As(V)
6.3
0.1
F
0.033
31
Fe-ZC
As(V)
6.3
0.1
F
0.024
32
Zr-ZM
As(V)
6.3
0.1
F
33
Zr-ZC
As(V)
6.3
0.1
F
34
Zr-ZCH
As(V)
6.3
0.1
F
35
FeZr-ZCH
As(V)
6.3
0.1
F
36
FeZr-ZM
As(V)
6.3
0.1
37
FeZr-ZC
As(V)
6.3
0.1
Fe-Mn loaded zeolite (MFM)
As(III)
7
300
58
318
38
As(V)
7
300
41
-
96
19.39
pr
29
0.072
0.056
na l
Pr
0.068
F
0.062
F
0.062
39
Zeolite
As(V)
2.5
20
40
Magnetite nanoparticle coated zeolite (MNCZ)
As(V)
2.5
20
F
As(III)
7
2
L
1.37
41
CEZ
As(V)
7
2
L
1.48
As(III)
2
0.15
Jo ur
[72]
e-
0.058
339.4
2.54
340
[153]
24.2
126
[154]
[71]
Waste-derived sorbents 42
FMSWVOI
81
[79]
50
f 5
0.15
75
HC
As(III)
7
1
HC
As(III)
7
0.1
BP
As(III)
8
1
BP
As(III)
8
0.1
MP
As(III)
7
1
MP
As(III)
7
0.1
46
Waste cement (untreated)
As(V)
700
47
Waste cement (heat)
As(V)
700
As(V)
700
167
As(V)
700
175
L
97
L
Leather waste
As(V)
1
51
ABA300
As(V)
6
Jo ur
50
S. no
Alum sludge composite
Materials
As(V)
7
Target
As(V)/As(III) removal.
95
Pr
95.3
28.3 34.7 [81]
26 F
[84]
62.9
As(III)
52
[80]
e-
L
na l
49
adsorbents for
88
96.4
45
sludge
of novel functional
pr
98.8
44
Concrete (untreated) Concrete sludge (heat)
adsorption performance
1.92
43
48
Table 2. Comparison of
oo
As(V)
181.3
[82]
40 1000
[83] 15 Initial
pH
concentration [As]
Isotherm
Removal
Adsorption
Partition
efficiency
Capacity
Coefficient
(%)
(mg L-1)
51
(mg
g-1)
(mol
kg-1.M-1)
Surface area (m2 g-1)
References
4
rGO–Fe3O4–TiO2
Fe3O4-GO 5
macroscopic composite
Carbon nanotube Eu-MGO/ Au@MWCNT
7
ZrO(OH)2
8
ZrO(OH)2/CNTs
9
α-FeOOH/@GCA
f [93]
57
As(V) As(III)
7
1
As(V)
7
1
As(III)
7
7
As(V)
7
7
As(III)
7
As(III)
7
As(III)
7
As(III) As(V)
12 10.2 11
411
47.1
147.1
0.86
0.38
0.43
0.20
0.25
0.12
1.06
0.27
0.48
0.20
As(V)
0.27
0.08
As(III)
320
As(V)
298
[34]
[155]
[156]
As(III)
0.375
L
2.1
38.18
As(V)
0.375
L
6.7
209.38
As(III)
7
0.375
L
95.7
2.3
143.75
As(V)
7
0.375
L
96.5
7.6
584.62
As(V)
200
L,F
35.0
56
0.43
As (DMA)
200
L,F
15.0
24
0.14
As (p-ASA)
200
L,F
60.0
102
1.28
52
[96]
49.01
As(V)
Jo ur
6
As(III)
pr
FeMnOx/RGO
38
e-
3
M- rGO
85
As(III) As(V)
Pr
2
M-GO
na l
1
oo
Graphene oxide
87.80 [100] 126
150.1
[103]
MI-CNT-G
13
CNT/CuO
14
TiO2-CNT
Ce-Fe/CNT 15 Ce-Fe/CNT-A Metal organic frameworks
17
AUBM-1
11
As(V)
11
As(V)
10
As(III)
10
As(V)
5
1
As(III)
7
1
As(V)
7
0.9
As(III)
7
As(V) As(III) As(V)
ZIF-8
f
As(III)
As(III)
14.5 8.13 9.74 47.4 24.1
L
2.40
L
2.27 1.9
0.9
2.2
20
48
31.0
2.98
20
50
28.7
2.87
20
38.5
16.8
1.37
20
39.5
16.2
1.34
As(V)
7.6
180
L
As(III)
7
100
F
49.5
As(V)
7
100
F
60.0
As(III)
7
0.1
27
0.135
1.85
As(V)
7
0.1
97.2
0.486
173.57
Jo ur
16
200
oo
12
As (p-ASA)
8
pr
MI/CNTs
200
e-
11
As (DMA)
25.7
Pr
α-FeOOH
200
na l
10
As(V)
[104]
209.8
[105]
480
[107]
196
[108]
216.3 [49] 189
[121]
1063.5
18
ZIF-8
As(V)
19
Fe3O4@ZIF-8
As(III)
8
40
100
1,133
Cubic ZIFs
As(III)
8.5
80
122.6
958.4
Leaf-shaped ZIFs
As(III)
8.5
80
108.1
12.7
20
40
662.1
90.9 L
53
[116]
[117] [118] [119]
8.5
21
MOF-808
As(V)
22
ZIF-L
As(III)
10
100
As(V)
7
60
As(III)
7
60
As(V)
7
40
As(III)
7
40
As(V)
4
As(III)
4
As(V)
7
As(III)
7
As(V)
7
As(III)
7
f
As(III)
80
117.5
5 F
24
Fe-Zr binary oxide
25
CCB
CIN
730.29
40
43.74
0.73
67.02
[122]
50
82.7
2.76
282
F
46.7
122.3
3.82
282
F
25
46.1
1.54
12.5
120
3.43
F
R-P
96.46
R-P
56.5
60
97
119
66.11
60
93
94
22.38
2
99.75
4
800
2
99.75
3.99
798
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na l
26
24.83
F
e-
Fe-Cu binary oxide
Pr
23
1,151.2
99.32
pr
Miscellaneous forms
oo
Dodecahedral ZIFs
Table 3. Stabilization/solidification of As ions from solid wastes into construction materials.
54
[120]
[123]
339
[124]
125.24
[126]
69
[127]
f 1
Iron oxide coated As(III) cement (IOCC)
2
Arsenic sludge
4
Products
Slurry preparation of Monolithic IOCC/cement/Ca(OH)2 matrices
Mechanochemical activation, mixing of Heavy metals Treated sludge modified ZVI and As sludge, and drying Drying, milling, sieving, As and F ions Clay bricks and sintering
Leaching test
Results
Semi-dynamic leach test
References
Single cement, or lime, or both are very suitable to reduce As leachability
[144]
Toxicity characteristic leaching Decrease As leaching concentrations from 72.5 to procedure (TCLP) tests [157] 0.62 mg L-1 (EPA1992)
e-
TTL/ABTL/ AHNP As-Fe hydroxide sludge
3
Methods
oo
Pollutants
pr
Solid wastes
As(III) As (V)
and
Mixing cement with sludge
Ca-As compounds
5
As-Fe sludge As
Crushing, mixing
and Clay-sludge bricks
6
Ascontaining mine tailings
As
Slurry preparation of mine Cements tailings and cement
7
Tannery sludge
Heavy metals
Mixing, drying
8
Calamine wastes
9
Fly ash
10
Contaminate d soil
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seieving,
grinding,
and
Oxides and Mixing, pressed under high heavy metals pressure, drying Mixing and sealed for 28 As days Mixing, stirring, and As and Pb transferred into moulds, and drying
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California Waste Extraction Maximum values in leachate are below the [146] Test (WET) permissibles of USEPA [158]
Pr
S. no.
Clay bricks Bricks Compacted samples Clay mineral pastes
USEPA 1992, acidic, alkaline, 6% (safely maximum) of sludge by weight could be [159] and neutral condition mixed with clay The addition of 7.5% cement remarkably decreased TCLP tests (Korean standard, the leachability [160] US EPA 1312, US EPA 1311) TCLP tests (USEPA 1311) and Netherlands tank leaching test (NEN 7345) Dutch standard (NEN-7375) and TCLP (USEPA 1311) American Nuclear Society (ANS) 16.1 test TCLP (US EPA, 1992)
Meet the criteria for bricks in construction and the leached concentration is far below USEPA limits [161] Dramatically reduce the consumption of finite [162] resources and harmful environmental impact 98.3% release reduction upon addition of 25% fly [163] ash The leachability of As and Pb in treated soil samples reduced by 96.2% and 98.7%, respectively
[147]
Table 4. Comparative regeneration studies for benzene by various sorbents. S. no. 1
2 3
Sorbents
Targets
MPSAC-La(0.36) MNCZ FeMnOx/RGO
As(V) As(V) As(III)
Initial concentration (mg L-1) 350
Desorption conditions
20 0.1
[154] 0.1 M NaOH 82% after 5 cycles 0.1 M NaOH + 0.1 M 95% after 3 cycles, slightly [96]
0.5 M NaOH
55
Regeneration results (Desorption efficiency) 75% after 3 cycles
References [66]
As(V) As(III)
20
5
Fe3O4@ZIF-8
As(III)
-
6
Dodecahedral ZIFs
As(III)
10
7
MOF-808
As(V)
5
8
Fe-Cu binary oxide
9
CIN
As(V)
f
0.5 M NaOH
1
0.1 M NaOH
1000
1 M NaOH
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As(III) As(III) Alum sludge composite As(V)
25
0.5 M Na2SO4
Pr
As(III)
0.4 mM NaOH
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10
As(V)
Hydrolysis, 75 vapor, 90 min 0.2 M NaOH
pr
ZrO(OH)2/CNTs
e-
4
decreased 93% after 5cycles 94% after 3 cycles, but decreased to 78% after 5 cycles ℃ 97% after 6 cycles, but decreased to 50% after 10 cycles 65% after 3 cycles 97% after 2 cycles, but decreased to 88% after 3 cycles 88% after 3 cycles, decreased to 82% after 5 cycles 94% after 4 cycles 90% after 4 cycles 50-60% after 5 cycles 50-60% after 5 cycles 75% after 5 cycles 80% after 5 cycles
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NaClO
56
[100] [118] [119] [122] [123] [127]
[83]
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Figure 1. Adsorptive removal of As(V) by MPSAC-La (0.36) [66]: a) Preparation procedures for MPSAC-La (0.36), b) Kinetics for removal of As(V) by MPSAC-La (0.36), and c) Intra-particle
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ur
na
lP
re
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diffusion modeling of As(V) removal by MPSAC-La (0.36).
57
f oo pr ePr na l
Figure 2. Removal of As(V) by modified ACF through adsorption [68]. Surface morphologies of various ACF: a1) virgin ACF (1,000 magnification); Zr-
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based, NP-doped ACF with different magnifications of a2) 1,000, a3) 5,000, and a4) 30,000. b) Adsorption isotherms of As(V) ions by modified ACF.
58
f oo pr ePr
Figure 3. Z-nZVI adsorbents for treatment As(V) in aqueous medium [73]. a) Schematic of AsO43- and AsO43-/SeO42- removal by Z-nZVI; Sorption kinetics
Jo ur
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for abatement of As(V) in single b1) and binary b2) systems.
59
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Figure 4. Remediation of As ions by M-GO and M-rGO [93]. a) Preparation process for M-GO
Jo
ur
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and M-rGO; Adsorption isotherm model of As ions onto b) M-GO and c) M-rGO.
60
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re
Figure 5. As(V) treatment by ZrGO from an aqueous system [97]. a) Preparation procedure for
Jo
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na
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ZrGO; b) Adsorption isotherms of As(V) ions with three different concentrations.
61
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Figure 6. Application in adsorption of As species by α-FeOOH@GCA [103]. a) Adsorption
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mechanism for As(V) removal by α-FeOOH@GCA; b) Adsorption isotherms of removal of three As-containing species (i.e., DMA (dimethylarsinic acid (DMA), As(V) (sodium arsenate), p-ASA
Jo
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(p-arsanilic acid)) at three different temperatures: 25, 35, and 45 ℃.
62
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Figure 7. Adsorptive removal of As(III) and As(V) ions from aqueous solution by ZIF-8 [116]. a)
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Synthetic process of ZIF-8 particles; b-c) Adsorption isotherms of As(III) and As(V) onto ZIF-8
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under different ionic strengths.
63
PII:
S0304-3894(19)31769-8
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121815
Reference:
HAZMAT 121815
To appear in:
Journal of Hazardous Materials
Received Date:
30 August 2019
Revised Date:
2 December 2019
Accepted Date:
2 December 2019
Please cite this article as: Liu B, Kim K-Hyun, Kumar V, Kim S, A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121815
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A review of functional sorbents for adsorptive removal of arsenic ions in aqueous systems
Botao Liua, Ki-Hyun Kima, Vanish Kumarb*, Sumin Kimc* a Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 04763, Republic of Korea b National Agri-Food Biotechnology Institute (NABI), S.A.S. Nagar, Punjab 140306, India c Department of Architecture and Architectural Engineering, Yonsei University, Seoul 03722, Republic of Korea
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Correspondence: [email protected]; [email protected]; [email protected]
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Hightlights The presence of arsenic in groundwater is regarded as a significant human health threat. Information on various functional adsorbents is assessed for removal potential for As. The removal potential for As is assessed on the basis of key performance metrics. -The regeneration of sorbents and their disposal after the use are also discussed.
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Graphical abstract
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Abstract
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The presence of arsenic in the water system has been a universal problem over the past several decades. Inorganic arsenic ions mainly occur in two oxidation states, As(V) and As(III), in the natural environment. These
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two oxidation states of arsenic ions are ubiquitous in natural waters and pose significant health hazards to humans when present at or above the allowable limits. Therefore, treatment of arsenic ions has become more stringent
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based on various techniques (e.g., membrane filtration, adsorption, and ion exchange). This paper aims to review the current knowledge on various functional adsorbents through comparison of removal potential for As on the
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basis of key performance metrics, especially the partition coefficient (PC). As a whole, novel materials exhibited far better removal performance for As(V) and As(III) than conventional materials. Of the materials reviewed, the advanced sorbent like ZrO(OH)2/CNTs showcased superior performances such as partition coefficient values of 584.6 (As(V) and 143.8 mol kg-1 M-1 (As(III) with excellent regenerability (>90% of desorption efficiency after three sorption cycles). The results of this review are expected to help researchers to establish a powerful 2
strategy for abatement of arsenic ions in wastewater.
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Keywords: adsorption; arsenic; advanced materials; partition coefficient; nanoparticles
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Table of contents 1. Introduction
1.1. Overview of arsenic contamination 1.2. Classification of various sorbents and framework of this review 2. Brief introduction to removal techniques for arsenic 3. Adsorptive removal of arsenic by conventional materials
3. 2. Zeolites 3. 3. Waste-derived sorbents
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4. Adsorptive removal of arsenic by advanced materials
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3.1. Activated carbon
4.2. Carbon nanotubes
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4.3. Metal organic frameworks
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4.1. Graphene oxide
4.4. Miscellaneous forms
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5. Effects of various factors on adsorption for arsenic removal 6. Performance evaluation of various materials for abatement of arsenic
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7. Regeneration of sorbents and their disposal after the use
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8. Conclusions and perspectives
4
1. Introduction 1.1. Overview of arsenic contamination Water contamination is ubiquitous and becoming one of the most challenging problems in the past several decades due to anthropogenic activities. The continuous increase in water contamination poses tremendous threats to the environment and human health. Heavy metal contamination is commonly seen in domestic, industrial, and agricultural wastewaters. In general,
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many heavy metals, such as cadmium, mercury, lead, chromium, and arsenic, are known as toxic components that exert a negative impact on human health and the environment [1]. Of these heavy metals, arsenic is a widely distributed metalloid, originating from various sources and present in
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various forms (e.g., minerals, petroleum refining, and coal fly ash). It is usually found in combined forms with sulfur and other metals [2]. The presence of arsenic in groundwater used for drinking
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is recognized as a significant human health threat in many regions of the world, e.g., China, Chile,
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and India [3]. Further, exposure to arsenic through drinking water was reported to increase the risk of mortality (e.g., through lung, bladder, kidney, stomach, and skin cancer) [4].
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Arsenic may occur in oxidation states of ± 3, 5, and 0 and is found mainly in two forms in the natural environment, namely, AsO33- (III) and AsO43- (V) [5, 6]. In general, pH and redox conditions
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play a dominant role in the oxidation state of arsenic. Accordingly, As(V) is thought to be the most
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stable form of arsenic due to the pH-dependent deprotonated oxyanion effect in aerobic environments. Meanwhile, in moderately reducing anaerobic conditions (e.g., anoxic groundwater conditions), As(III) is more commonly present, and it is far more toxic and mobile than As(V). Hence, As(III) tends to be more difficult to remove by the common water treatment techniques (e.g., adsorption and co-precipitation) due to the shortage of electrostatic force [7]. However, many other factors such as affinity and size exclusion is also important in deciding final adsorption capacity for 5
As(III) removal. Further, the effectiveness of sorbent applied for different phases of contaminated waters (e.g., domestic sewage, industrial, and agricultural wastewater) may be highly variable. For example, several nanoscale zero-valent iron (nZVI) types and iron-biochar (Fe-BC) were used to remove arsenic from model hydraulic fracturing wastewater [8-11]. In these As-containing wastewater treatment studies, removal efficiency values were observed in the range from 40-90%. Apparently, a series of metals/metalloids (e.g., KCl, NaBr, As, and Cu) resulted in either synergistic
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or competitive effects on fracturing wastewater. Thus, some of these materials may be employed as potential sorbents for the in-situ prevention of As migration from fracturing wastewater treatment. For the evaluation of sorbent performance in this paper, we basically relied on the datasets derived
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for the model drinking water which contained single or binary pollutants (i.e., As). Conventional
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arsenic abatement techniques have generally relied on a stepwise approach, such as pretreatment of As(III) and subsequent removal of As(V). As a whole, removal technologies for both As(III) and
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As(V) have received a great deal of attention from many governments and researchers.
1.2. Classification of various sorbents and framework of this review
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In this review, an overview of the seriousness of arsenic contamination was discussed along with a short introduction to various arsenic removal methods. Subsequently, a comparative
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analysis was made to evaluate adsorptive removal performance of arsenic between many advanced sorbents (relative to conventional sorbents). In general, the latter group consists of activated carbon (AC), zeolites, and industrial/agricultural solid waste. AC is formed by pyrolysis of various carbon-containing materials (e.g., wood, husk, coal, and coconut shell), so it exhibits an amorphous property [12]. It is the most commonly used adsorbent for wastewater treatment due 6
to its low cost, good reproducibility, and easy availability. Zeolites are microporous aluminosilicates with different ratios of silicon, aluminum, oxygen, and metals. More than 200 natural and synthetic zeolitic frameworks have been reported as of December 2018 [13]. Sorbents derived from industrial/agricultural solid wastes (such as fly ash, waste slurry, and lignin) have also been reported to have good adsorption capacity for arsenic from wastewater [14]. However, these solid waste materials have scantily been employed for application in other fields.
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Several advanced materials (e.g., metal organic frameworks (MOFs), graphene oxide (GO), and carbon nanotubes (CNT)) have been proposed and developed as sorbents for treatment of As. MOFs are a class of porous materials that are usually formed by binding inorganic metal ions and
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organic ligands. The surface area and porosity of MOFs can be changed or enhanced through
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alteration of metals and/or organic ligands. The feasibility of MOFs has been demonstrated in various fields due to their facile structural fabrication and/or functional modulation [15- 22].
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Another advanced material employed for As removal is GO, also known as single-layer graphite
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oxide, which is generally synthesized by reacting graphite with strong oxidizers [23]. The abundant functionality of GO material is thus derived from the lattice structure and various
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oxygen-containing groups that are enriched on its surface. Then, the rich and effective active sites can be provided by functional groups of the GO surface [24]. As another carbon nanostructure,
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CNTs are well-known for tubular nano-scale graphite crystals with mono or multi-layer graphite sheets crimped around the central axis at a certain helix angle [25]. Their excellent mechanical, electrical, optical, and thermal properties provide them with high potential in various fields of application, such as drug delivery [26], tissue engineering [27], and electrical conduction [28]. The discussion of other advanced materials (e.g., metal oxides, chitosan-based materials, and 7
miscellaneous polymers) will be discussed in detail in each of the following subsections. In this research, an up-to-date knowledge on sorptive removal of As has been reviewed to describe the factors controlling its adsorption (e.g., temperature, pH value, and concentration/ion strength) with respect to the extension of their practical applicability based on more meaningful evaluation on performance between various novel functional and conventional adsorbents (e.g., based on partition coefficient (PC) concept) [29-31]. In light of the limitations involved in the use
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of maximum adsorption capacities, partition coefficient (PC) as the equilibrium ratio of analyte concentration in and on the sorbent to solution concentration was employed to assess the performance of diverse sorbents for As species with the least biases.
Furthermore, knowing that
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an excellent regenerability is also advantageous in lowering the management cost, various aspects
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of sorbent regeneration were described to assess the practical utility of diverse sorbent materials employed for removing As. Finally, the future outlook and conclusions were presented to help
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researchers explore and consummate arsenic removal technologies. As a whole, this review is
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expected to offer valuable insights into adsorptive removal capabilities of As between novel
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functional and conventional adsorbents.
2. Brief introduction to removal techniques for arsenic
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A myriad of techniques has been proposed for treatment of arsenic, including photocatalysis
[32-34], ion exchange [35-37], chemical precipitation [38-40], filtration membranes [41-43], electrochemical treatment [44-46], and adsorption [47-49]. Most of these methods suffer from drawbacks in various respects. Since As(III) is very difficult to remove from the water phase, photocatalysis is generally a preferred pretreatment option used to oxidize As(III) into As(V) in 8
wastewater. Then, the actual removal treatment can be employed through combination with other technologies [50]. Ion exchange approach refers to a physical or chemical process in which anions originated from a typically synthesized ion exchange resin held electrostatically onto the solid phase surface [51]. Then, these anions are exchanged with other ions (e.g., arsenic) having similar charge in a solution. However, widespread use of this method is often restricted due to the high cost of treatment (relative to other methods) [52]. In the case of chemical precipitation, chemicals
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are used to remove As from contaminated water in which As is converted into less soluble As compounds. For example, immobilization of As with metal cations (e.g., Fe2+) led to formation of low-soluble As compounds (e.g., FeAsO4∙2H2O) [53-55]. However, use of this technique is also
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limited since it requires a large amount of precipitants; the generated arsenic-containing waste
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residue may not be fully utilized and thus must be treated carefully, while its long-term accumulation may easily cause secondary pollution [53]. Membrane technology is a purely
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physical process driven by a concentration or pressure difference. Because this method mainly
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utilizes the selective permeability of a membrane, it can be categorized into microfiltration, ultrafiltration, and nanofiltration (based on pore size). Nevertheless, the use of membrane
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technology is confined to some specified conditions (e.g., equipment, membrane quality, and operations requirements) while being used mainly for water purification [56].
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Adsorption of arsenic (adsorbate) can be expressed as adhesion from a solution to the surface of a solid sorbent. Therefore, the performance of adsorptive removal for arsenic is dependent on the affinity between arsenic and adsorbents. Compared to the other methods mentioned above, adsorption is one of the most frequently used water treatment techniques in terms of its costeffectiveness, simple design, and easy operation. However, desorption of arsenic may be difficult 9
once there is a strong affinity between the arsenic and sorbents. That is, the arsenic-containing sorbents may not meet the requirements of environment-friendliness if the sorbents were not disposed well after being exhausted. Therefore, the regeneration and disposal of sorbents after the use are highly essential issue to ensure their cyclic utilization while reducing the potential harmfulness of exhausted sorbents, as discussed in Section 7. In addition, adsorption isotherms provide a quantitative measurement to meaningfully assess the adsorption capacity of arsenic for
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various sorbent materials. It is necessary to substantiate the correlation for the adsorption isotherm equations derivable between various adsorbents and arsenic. The use of proper equation fittings offers reasonable assumptions to describe the sorption mechanism and the related affinities (between
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the sorbents and arsenic). In Supporting Information, detailed descriptions are described for
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Freundlich isotherm, Langmuir isotherm, Redlich-Peterson isotherm, and other isotherms as well as kinetic studies. In some review papers, adsorptive removal of As by various sorbents has been
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attempted [2, 57, 58]. However, these papers did not comprehensively compare the differences in
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adsorptive removal between conventional and advanced materials. Moreover, due to the variations in experimental conditions of different studies, it is very difficult to discriminate the actual
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effectiveness of sorbent for removing arsenic if comparison is made based on the maximum adsorption capacity or removal efficiency. It was already noticed that these two values can be highly
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biased by the arbitration of experimental conditions (e.g., adsorbent dosage and initial concentrations).
3. Adsorptive removal of arsenic by conventional materials A variety of adsorption materials has been employed for the removal of arsenic. As discussed
10
above, the majority of available materials cover both conventional and advanced materials. Although our main emphasis lies in advanced materials, our discussion also includes conventional materials, such as activated carbon, zeolite, and waste-derived sorbents in light of their common usage.
3.1. Activated carbon
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AC materials are effective for treatment of various contaminants in wastewater [59]. The large hydrophobic internal surface contributes to its significant affinity for adsorbates. However, virgin AC materials do not easily satisfy the treatment needs for diverse forms of targets. Thus,
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modification of AC has been attracting more attention as an effective means to produce
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unprecedented affinities toward various targets including heavy metal ions. Although many reviewers have discussed adsorptive removal of heavy metal ions by AC [59,
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60], its potential for removal of arsenic in wastewater has been investigated rather scantily. For
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instance, nano-scaled AC was synthesized through the modification of iron and manganese oxides to study As(V) removal in aqueous solution [61]. Adsorptive removal of As(V) by AC impregnated
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with iron/manganese (Fe-Mn-AC) exhibited a far higher capacity (19.35 mg g-1) than other modified (e.g., iron, and iron/cobalt) or unmodified AC. The very low removal efficiency (e.g., less than 20%)
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of the virgin AC material promoted the necessity for their modification. Table 1 shows that removal performances of most modified ACs were not directly reflected by their enhancement in surface area. In other words, modified AC with functional groups may play an important role in sequestration of As regardless of surface area. Essentially, the Fe-Mn-AC displaced the -OH of As(V) molecules, finally forming mono- or bi-dentate complexes on the NFM-impregnated carbon 11
surface to promote removal of As(V). (N stands for Norit initial carbon, while F and M stand for iron and manganese impregnation, respectively.) In another case study, the modification of AC by Fe was also verified to be efficient for the treatment of arsenic [62]. In their study, the presence of iron with AC increased surface area and adsorption capacity (for As(V)) of AC by 3.5 times and 36.5%, respectively. Furthermore, iron-modified AC by different sources (e.g., Fe3O4-decorated AC [63], Fe(OH)3-modified AC [64], and FeCl3- & FeSO4-decorated AC composite [65]) also showed
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elevated As adsorption capacities. In this respect, the removal capability of As was closely related with many variables for Fe (e.g., loaded amount, dispersibility, and the surface availability) within AC. The removal performances of these materials are summarized in Table 1. Nevertheless, the
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performance of As (mainly for As(V)) by these iron-modified AC was not sufficiently satisfactory
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in terms of removal efficiency or adsorption capacity.
Magnetized palm shell waste-based AC was fabricated with lanthanum impregnation (MPSAC-
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La) (Figure 1a) and tested for sorptive removal of As(V) [66]. The MPSAC-La(0.36) (weight ratio
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of La/Fe equal to 0.36) displayed a 15.5 times increment in As (V) adsorption capacity (with an adsorption capacity = 227.6 mg g-1) (Figure 1b) compared to that of virgin PSAC. This enhanced
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As(V) removal capacity was attributed to the precipitation mechanism of sorbents against such a target [67], since La3+ ions easily connect with As(V) to form LaAsO4 precipitate. The well-matched
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H-type Langmuir isotherm model of adsorption data (R2 > 0.92) (e.g., relative to Freundlich model (R2 > 0.77)) by MPSAC-La indicated a strong affinity between adsorbent and adsorbate. In addition, the adsorption process of As(V) was determined to follow a pseudo-first-order kinetic model (R2 = 0.994) (Figure 1c) slightly better than a pseudo-second-order (R2 = 0.991), illustrating that the physisorption should play as an important role in its removal. Finally, a moderately good 12
recyclability was seen for MPSAC-La as it maintained 75% of the adsorption capacity at the third cycle. A type of zirconium-based nanoparticle (NP)-doped AC fiber (ACF) was synthesized and employed for As(V) removal in aqueous phase [68]. During the preparation of NP-doped ACF, hydrogen sulfate ions (HSO4-) were introduced into NP-doped ACF to increase more functional groups. The virgin ACF consisted of long and smooth AC fibers (Figure 2a1) to show nano-sized
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Zr-based NP-containing agglomerates on the ACF surface (Figure 2a2-4). Essentially, the decreased surface area of modified ACF (e.g., from 1,721 to 1,409 m2 g-1) did not reduce the removal capacity for As(V). More specifically, a maximum capacity of 21.7 mg g-1 was obtained for As(V)
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by NP-doped ACF based on the Langmuir model (Figure 2b). HSO4- played an important role in
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the adsorption process as ion exchange between hydrogen sulfate of NP-doped ACF and As(V) can exert direct control on adsorption capability during the adsorption process. In addition, the
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adsorption process was well-matched with the pseudo-second order model, which suggested that
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chemisorption was driven by NP-doped ACF for As(V) removal [68]. Furthermore, arsenate solutions were treated with NP-doped ACF at an initial concentration of 106 µg L-1 through a fixed
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bed adsorption process. Finally, a very low equilibrium concentration was acquired to meet the criteria of the US EPA standard, i.e., 10 ppb (µg L-1). This finding further demonstrated the
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usefulness of Zr-based NP-doped ACF as a promising sorbent for abatement of As(V)-contaminated effluent.
As a whole, unmodified AC materials showed limited adsorption capacity for As and other heavy metals. The low or moderate adsorption capability of unmodified AC made it difficult to meet the US EPA standard (10 ppb) for drinking water despite the low price and easy availability. 13
3.2. Zeolites Over the past 60 years, zeolites have attracted much interest from both academic and industrial fields. Zeolites occur naturally although they are commonly synthesized industrially. In general, as synthetic zeolites showcase enhanced advantages over their pristine form of analogues, it is desirable to design and produce more functional zeolites. Both natural and synthetic zeolites have
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been studied by many researchers as a platform for efficient removal of arsenic [69, 70]. Previously, the adsorption performance on diverse arsenic species (e.g., arsenite (As(III)), arsenate (As(V)), dimethylarisinic acid (DMA), and phenylarsonic acid (PHA)) was compared between natural
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zeolites (Mexico (ZMA, ZME, ZMS, and ZH) and Hungary (ZH)) and synthetic mordenite (ZS-M)
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[69]. However, because of the low sorption efficiency (< 50%) under low concentration (5 mg L-1) and long contact time (2-18 days), the above tested zeolite sorbents were not recommendable for
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such applications. Thus, more efforts were directed toward the modification of zeolite materials to
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help enhance their removal potential. It was reported that Fe-exchanged natural zeolite (Fe-eZ) had an enhanced removal capacity of 100 µg g-1 for As(III) when exposed to its initial concentration of
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20 mg L-1 in the water phase [70]. In another report, natural zeolite was modified by treatment with either 0.1 M NaCl + 0.1 M FeCl3 or 0.1 M NaCl + 0.01 M FeCl3 to attain adsorptive removal
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efficiencies of 9.2 (92% of removal efficiency) or 8.4 µg g-1 (84% of removal efficiency) for As(V) with an initial concentration of 0.1 mg L-1 [71]. Thus, Fe modified zeolite can be considered as an efficient sorbent despite of its low adsorption capacity for As(V) removal from drinking water. However, because of the low As uptake by pristine zeolite materials, more attention was paid to modification of zeolites for treatment of As. 14
Recently, various types of natural zeolites were modified with Fe, Zr, and FeZr and used for adsorptive removal of As(V) from aqueous solutions [72]. Among these modified forms, Zr-ZM (mordenite-rich tuff) and FeZr-ZCH (chabazite-rich tuff) were reported to have the highest uptake values of As(V), with removal capacities of 0.072 and 0.068 mg g-1, respectively [72] (Table 1). All the experimental results fitted well with the Freundlich isotherm model. The highest KF value of FeZr-ZCH among the tested zeolites at 0.36 (mg g-1) (L mg-1)1/n verified the noticeable enhancement
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in affinity between this modified zeolite and As(V). The highest pseudo-second-order rate constant of FeZr-ZCH for As(V) adsorption was consistent with the potential occurrence of numerous binding sites in favor of a faster adsorption process for As(V) removal. It should also be noted that
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formation of an inner sphere complex through the ion exchange reaction may contribute to the
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enhanced adsorption of As(V) [72]. As such, Fe-, Zr-, and FeZr-modified zeolites may have high potential for reduction of As(V) from aqueous medium.
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Adsorptive treatment of As(V) in its single or binary components (e.g., As(V)-Se(VI)) was also
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investigated using nZVI and nZVI-functionalized zeolite (Z-nZVI) (Figure 3a) [73]. In both component systems (i.e., single and binary) for the two sorbents, equilibrium was acquired within
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90 min (Figure 3b). Interestingly, the presence of Se(VI) showed a slight influence on adsorption of As(V) in aqueous media. Both nZVI and Z-nZVI showed significant preferential adsorption
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behavior for As(V) removal. Similar preferential results were acquired in an adsorption study of multiple components of As(V) and P(V) by ferrihydrite Neupane, Donahoe and Arai [74]. Furthermore, the adsorption results for the two components were well-matched with the pseudosecond-order model, indicating chemical adsorption via formation of bidentate complexes [75]. In another study, copper exchanged zeolite (CEZ) was synthesized by ion exchange technology using 15
copper salts and zeolite-A; the prepared material was then employed for the treatment of As(III) and As(V) [76]. CEZ showcased highest adsorption capacities of 1.27 and 1.48 mg g-1 for As(III) and As(V) at the initial loading concentration of 2 mg L-1 when estimated from Langmuir model (R2 = 0.999 and 0.904 for As(III) and As(V)). Further, as seen from the better fitness of Langmuir isotherm model (e.g., relative to Freundlich (R2 = 0.797 and 0.794 for As(III) and As(V)), the monolayer uniform adsorption appears to be prevalent for CEZ surface. The results of the performance test of
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CEZ showcased very low concentrations of arsenic remaining in leachate (such as 0.011 mg L-1 for As(III) and < detection limit for As(V)). As these concentration values are far lower than the USEPA standards of 5 mg L-1, CEZ was found to be a highly efficient media for removing arsenic. The
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results of comparative analysis on the above discussed zeolites are also summarized in Table 1. In
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zeolites are far superior in that respect.
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general, natural zeolites showcased limited efficacy for the removal of arsenic, while the synthetic
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3.3. Waste-derived sorbents
As discussed above, both AC and zeolites showed enhanced adsorption capacity for As removal
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through various modifications. Despite their frequent use, industrial/agricultural solid waste products (i.e., waste-derived sorbents) are also used as adsorbents for metals. They are generally
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cheaper and available locally in large quantities [77] and can be used for reduction of heavy metal ions. Chitosan, waste slurry, and lignin were studied for the treatment of effluent containing metal ions [78] . Likewise, the use of sorbents made of fly ash, blast furnace slag and sludge, and black liquor lignin was also reported for the treatment of heavy metals contaminated in wastewaters [77]. In another review, removal performances of heavy metals from aqueous phase were compared 16
between low-price adsorbents (e.g., sorbents derived from chitosan, fly ash, coal, and oxides) and AC [78]. Nonetheless, it was difficult to assess and identify the best sorbent for specific heavy metals as many of the previous studies were carried out in the presence of multi-component metals or under different operating conditions. The effects of competing ions in multi-component systems occur through complicated processes. Hence, it is difficult to assess removal efficiency and related mechanisms based on simplified experimental conditions.
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A sorbent derived from vegetable oil solid waste was manufactured and employed for As removal [79]. After pretreatment of Fe2+/H2O2 (Fenton reagent) at a 1:17 molar ratio for this sorbent, it showed maximum removal efficiency values for As(III) and As(V); the corresponding adsorption
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capacities with the initial concentration (i.e., 150 µg L-1) were 81% and 75%, respectively. Hence,
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pretreatment with the Fenton reagent was determined to be an efficient technique for enhanced reduction of both As species. In another study, three types of adsorbents were developed using
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industrial wastes (i.e., hydrated cement (HC), brick powder (BP), and marble powder (MP)) for
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removal of binary (arsenic and fluoride) ions from potable water [80]. Among the aforementioned adsorbents, HC exhibited the highest removal efficiency (> 90%) for As(III) at both high and low
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initial concentrations, as summarized in Table 1. The experimental data for the three sorbents were suitably described by the Langmuir isotherm model. Thus, HC exhibited the highest potential for
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treating As(III)-contaminated drinking water with highly selective adsorption of As. Solid sorbent materials derived from waste cement powder and concrete sludge were fabricated
and used to evaluate As(V) removal performance [81]. A maximum adsorption capacity of 175 mg g-1 was acquired for As(V) at a considerably high initial concentration of 700 mg L-1. This high adsorption capacity could be ascribed to the precipitation of calcium arsenic. It should be noted that 17
the aforementioned waste cement materials were rich in calcium. The removal results fit well with the Langmuir equation. Thus, sorbents made of waste concrete were efficient for sequestration of As(V). These kinds of recycled waste sorbents may be a good option for As treatment since they only cost about $100~200 (US dollars) per ton in Japan [81]. Not long before, the enhanced adsorption of As was achieved by alum sludge modified by calcination [82]. Specifically, these authors extracted aluminum oxide from alum sludge by calcination. Then, alum-based adsorbent
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(ABA300, calcination at 300 ℃) exhibited enhanced adsorption capacity (62.9 mg g-1) for As(V) relative to ABA105 (6.07 mg g-1) and ABA500 (10.4 mg g-1). As such, the high efficacy of the alum sludge-derived sorbent for As removal was supported by the dramatic improvement in adsorption
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capacity. Recently, oxidation and adsorption of As were simultaneously conducted onto a composite
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fabricated by alum sludge and graphitic carbon nitride (g-C3N4) [83]. The produced composite displayed both adsorption (40 mg g-1) (24 h-dark state) and oxidation potential (60 mg g-1) (24 h-
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dark plus 24 h-light) to treat As(III). Further, it was demonstrated that both of outer-sphere
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complexation and physisorption stimulated the adsorption of As(III) onto the composite surface. The oxidation of As(III) to As(V) further promoted the adsorption through inner sphere
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complexation and chemisorption in the light condition. Finally, the removal of As proceeded in a more stable manner by the above composite through the combination of adsorption and photo-
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oxidation processes.
Furthermore, sorbents made of solid waste from the leather industry were employed for removal
of As(V) and Cr(VI) contamination from water, with adsorption capacities of 26 and 133 mg g-1, respectively [84]. Interestingly, bio-sorbents made from diverse agricultural wastes (e.g., sugarcane bagasse, rice husk, and wheat straw) were also tested for sequestration of As from aquatic media 18
[85]. Likewise, many other sorbents derived from agricultural waste, such as mango leaf powder (174.9 mg g-1 for As(III)) [86] and orange waste (143.3 mg g-1 for As(III)) [87], also showed excellent removal adsorption capacities for As. As a whole, the innovative recycling of industrial/agricultural waste was demonstrated as an efficient option for sequestration of As from wastewater.
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4. Adsorptive removal of arsenic by advanced materials
As for conventional materials developed for removal of As, advanced novel materials were also explored to treat heavy metals in aqueous systems over the past several decades. In this section, the
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potential of advanced materials for remediation of As from wastewater will be discussed in detail
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4.1. Graphene oxide
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based on some key examples (e.g., GO, CNTs, MOFs, and functionalized porous materials).
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Graphene is considered a 2-dimensional (2-D) structural material formed by sp2-bonded carbon. Graphene generally has carbon as the basic structural element including graphite, charcoal, carbon
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nanotubes, and fullerenes [88]. GO is an oxidized form of graphene with oxygen-containing functional groups, such as epoxide, carbonyl, carboxyl, and hydroxyl groups. The use of graphene-
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based hybrid materials has been widely extended for water treatment [89]. Therefore, many previous reports have indicated that iron-based magnetic GO materials (e.g., magnetite-GO (M-GO) and magnetite-reduced GO (M-rGO)) considerably enhanced removal potentials for heavy metal ions. In this respect, rGO materials were usually fabricated by the reduction of GO through thermal, chemical, or electrical treatments. It is well-known that rGO with some defects on the surface or in 19
the internal structures are also found to maintain oxygen functional groups. Further, different chemical compositions of rGO with various characteristics could be acquired by using different reducing agents or changing the dosage of the agents [90]. It was also noteworthy that the polar oxygen units may exert both positive (e.g., enhancing aqueous dispersion) and negative influences (e.g., decreasing sheet conductivity). Thus, it is very essential to compare the performance of both GO and rGO in terms of As treatment.
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Modification of GO with a humic acid coating and iron was an efficient option for enhancing As removal from water [91]. The use of M-rGO prepared through the addition of a magnetite (particle size of 10 nm) was reported to yield a binding capacity of 13.10 mg g-1 for As(III) and 5.83 mg g-1
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for As(V) [92]. As mentioned earlier, iron-based modifications are very effective for upgrading the
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potential for adsorptivity. Essentially, iron content in a sorbent modified with iron is crucial to determining the actual adsorption capacity of a metal like As. Specifically, the surface complexation
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or electrostatic interaction promoted the adsorption for arsenic. M-GO and M-rGO composites were
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fabricated (Figure 4a) and compared for remediation of As(III) and As(V) from wastewater [93]. M-GO exhibited a higher removal capability for both targets due to the existence of more functional
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groups on M-GO than on M-rGO (Figure 4b and c). The adsorption capacity for As(III) was 85 for M-GO and 57 mg g-1 for M-rGO, while the capacity for As(V) was 38 for M-GO and 12 mg g-1 for
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M-rGO. The presence of more oxygen-containing groups in M-GO contributed to the formation of M-GO with higher Fe3O4 composition to show better adsorption of As by M-GO than by M-rGO. Moreover, The adsorbed As(V) was preferentially connected with two adjacent structural Fe3+ cations (bidentate binuclear-bridging complex), while As(III) adsorption was accompanied by both mono- and bidentate complexes [94, 95]. As mentioned earlier in many studies, As(III) was 20
converted into As(V) before adsorption due to additional difficulties in sorptive treatment of As(III) in comparison to that of As(V). As discussed in Section 1.1., As(III) tends to be more difficult to remove due to the shortage of electrostatic force compared to that of As(V). However, due to the high adsorption capacity of the M-GO/M-rGO composite for As(III), it is not necessary to include the oxidation pretreatment process (e.g., As(III) to As(V)) for removal of As(III). Likewise, many other magnetically modified GO materials showed efficient removal capacities
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for treatment of As-containing effluent. GO modified by iron-manganese binary oxide (FeMnOx/RGO) was explored for removal of As from aqueous solutions [54], [96]. The adsorption capacities of FeMnOx/RGO for As(III) and As(V) were 10.2 mg g-1 and 11.5 mg g-1 (with a 1 mg Linitial concentration), respectively. In contrast, bare RGO showed almost no adsorption of As.
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1
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Initial concentration strongly influenced adsorption capacity, although the enhanced capacity was often accompanied by a slightly decreased removal efficiency (Table 2). Furthermore, the presence
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of PO43- dramatically decreased As adsorption due to its competing effect. More importantly,
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excellent reusability of FeMnOx/RGO (e.g., maintaining > 80% removal efficiency) for both As was observed with an initial concentration of 100 ppb. This result confirmed that FeMnOx/RGO presents
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good performance in real water treatment systems. The rGO-Fe3O4-TiO2 based nanocomposite has great potential for direct removal of As(III) (Benjwal, Kumar, Chamoli and Kar [34]. rGO-Fe3O4-
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TiO2 exhibited a high monolayer adsorption capacity of 147 mg g-1 for As(III) based on the Langmuir adsorption isotherm. In contrast, very low capacity values (e.g., 0.38 mg g-1 for As(III) and 0.27 mg g-1 for As(V)) were seen for Fe3O4-GO (Table 2). As described above, most of the magnetic-based modified GO materials showed exceptional removal performance for As. Similarly, other modified GO materials were also efficient at removing 21
arsenic. A zeolite-reduced GO (ZrGO) composite based on fly ash was tested for removal of As from wastewater (Figure 5a) [97]. Surprisingly, a very low final concentration of 0.008 mg L-1 was attained by ZrGO with an initial As(V) concentration of 0.1 mg L-1 (Figure 5b). As such, the final concentration may meet the permissible limits of the World Health Organization (WHO) (10 ppb). The Redlich Peterson isotherm model fit best with the above noted experimental data, which illustrated the hybrid mechanism and not ideal monolayer adsorption of ZrGO for the adsorption
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process [97]. As a whole, GO-based sorbents showed noticeably enhanced removal efficiency for As compared to that of the conventional materials. The noticeable enhancement in adsorption potential of GO-based materials prompted researchers to further explore new functional materials
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for real-world applications. As such, effective removal of As can be pursued further through
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4.2. Carbon nanotubes
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synthesis of diverse composites of GO, especially with many cost-free materials (e.g., fly ash).
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CNTs consisting of single-wall and multi-wall nanotubes generally have high binding energies through van der Waals forces. As CNTs can showcase dramatic variations in size, shape, purity, and
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structure, they can exert complicated but direct control on various systems. As such, these materials are well-known as highly promising prospects for various environmental applications [98, 99].
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A hydrated nanocomposite consisting of zirconium oxide-coated CNTs (ZrO(OH)2/CNTs) was
fabricated using a filtration-steam hydrolysis technique and employed for adsorptive treatment of As in water [100]. Maximum adsorption capacities for As(III) and As(V) by this nanocomposite were 78.2 and 124.6 mg g-1, respectively, based on the Langmuir model. Noticeably, ZrO(OH)2/CNTs exhibited a far more rapid adsorption rate for both As(III) and As(V) ions than 22
ZrO(OH)2 nanoparticles. The observed differences could be ascribed to uniform dispersion of the former (ZrO(OH)2/CNTs) to provide more adsorption sites. Regeneration is a useful tool of performance metrics for evaluating the economic value of an adsorbent. Removal efficiency of ZrO(OH)2/CNTs material was more than 97% after six cycles, which further validated its effectiveness in removing As from water systems. The initial concentration of 375 µg mL-1 for As(V) and As(III) was reduced to an approximately 10 µg mL-1 final concentration (USEPA standard).
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Zr-based modified CNTs showed significant potential as highly effective adsorbents for remediation of As. Meanwhile, Fe-based materials, such as goethite [101] and natural iron oxide [102], have also gained attention as advanced adsorbents to reduce As from wastewater. GO-CNT
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aerogel was coated with α-FeOOH (iron oxyhydroxide) to form α-FeOOH@GCA through a self-
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assembly technique by in-situ Fe2+ reduction. It was then employed for treating three different Ascontaining species (i.e., As(V), dimethylarsinic acid: DMA, and p-arsanilic acid: p-ASA) (Fig. 6a)
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[103]. α-FeOOH@GCA showed noticeably enhanced maximum removal capacities for the
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aforementioned As species, at 56.4, 24.4, and 102.1 mg g-1, respectively, which were far superior to those of α-FeOOH nanoparticles. The variations in adsorption capacities for different As species by
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α-FeOOH@GCA can be explained at least partially by ligand exchange, which finally promoted formation of inner sphere complexes with different structures of As species. Differences in
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structures and properties of the aforementioned As species were suspected to cause variation in the adsorption mechanism onto α-FeOOH@GCA and, hence, led to the various adsorption capacities. Furthermore, their adsorption results fit well with both the Langmuir and Freundlich isotherm models (Figure 6b). The dynamic affinities between the sorbent and As species were supported by the data of different adsorption removal preferences for As-containing species as well as the 23
predicted results based on the two models. Thus, this study offers new opportunities for modifying natural iron-based minerals with GO-CNT carriers to promote immobilization of As species from wastewater. Meanwhile, a series of other iron-based CNTs has been investigated for remediation of As by many researchers. In this regard, a multi-step, one-pot technique was used to fabricate magnetic CNT composites (MI/CNTs) by employing as-prepared CNTs (p-CNTs) and potassium hydroxide
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(KOH) [104]. MI/CNTs exhibited a larger surface area of 662 m2 g-1, which is ~5 times larger than that of pristine CNT material. They showed moderately low maximum removal capacities for As(V) (9.74 mg g-1) and As(III) (8.13 mg g-1) when estimated using the Langmuir isotherm model. Later,
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Chen’s group also used a one-pot green method to synthesize another magnetic iron oxide/CNTs
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(MIO-CNTs) composite with a surface area of 209.8 m2 g-1 by using p-CNTs as a template through a catalytic chemical vapor deposition technique [105, 106]. Despite the lower surface area, the MIO-
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CNT composite showed better As removal efficiency than the MI-CNT composite. The former
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composite exhibited maximum removal capacities of 47.41 and 24.05 mg g-1 for As(V) and As(III), respectively. It was verified that the oxygen-containing groups of the CNT composite played the
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dominant role for As removal, which formed hydroxylated surface complexes due to the reaction between Fe-OH and As-OH [105]. Therefore, the acquired complexes provided enhanced
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availability of adsorption sites. Furthermore, other modified CNT-based materials, such as CNT/CuO nanocomposite [107], TiO2-coated CNTs [108], and Ce-Fe oxide-decorated CNTs [49], were also reported to present promising removal performances for As from aqueous systems. The adsorption capacities of these CNT-based materials are listed in Table 2, and the associated discussion of CNT materials is presented in Section 4. 24
4.3. Metal organic frameworks MOFs are a new type of micro- or meso-porous material connected by a metal ion and an organic linker through formation of coordination bonds [109]. The facile tenability in their structures and/or inner porosities makes it possible for them to attain desirable porous morphology with a specific surface area and high thermal/water stability [110]. Likewise, a series of MOFs can be developed
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using varying ligands (e.g., 1,3,5-benzenetricarboxylate and imidazolate) and metal clusters (e.g., Zn2+ and Cu2+). As such, various forms of MOFs have been developed and used widely in diverse applications including but not limited to drug delivery [111], catalysis [112], adsorption [113],
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conductivity [114], and gas storage [115].
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As a well-known and commonly used MOF material, ZIF-8 was employed in treatment of As ions from water systems [116]. A nano-sized ZIF-8 fabricated through a facile technique at room
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temperature was used for adsorptive removal of As(III) and As(V) [116] (Figure 7a). On the basis
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of the Langmuir model, ZIF-8 exhibited maximum adsorption capacities of 49.5 and 60.0 mg g-1 for As(III) and As(V), respectively. Depending on the type of co-existing anions, there were dynamic
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effects on As adsorption. Specifically, SO42- and NO3- had slight or no influence on As removal, while PO43- and CO32- adversely affected adsorptive removal of As. Moreover, according to x-ray
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photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses, adsorption of arsenic species was mainly ascribable to electrostatic interactions and formation of complexes with -OH and -NH2 groups. It was noteworthy that a very low final As(V) concentration of 2.8 µg L-1 remained after treatment with ZIF-8 (initial As(V) of 100 µg L-1). As such, ZIF-8 showcased excellent effectiveness in 25
practical drinking water treatment. Study of the synthesis of ZIF-8 and its subsequent applications for removing As(V) have been explored by other researchers. For instance, Wu, Zhou, Zhang, Wu, Li, Qiao, Guan and Li [117] synthesized hierarchical ZIF-8 with the assistance of cetyltrimethylammonium bromide (CTAB) and the amino acid L-histidine (His) as co-templates for removal of As(V). The variation in solvent types and ratios of Zn2+ and ligands may dramatically influence the surface area and As removal performance of ZIF-8. Accordingly, ZIF-8 showed a
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maximum capacity of 90.9 mg g-1 for As(V), which may be explained by the tailored porous structure and the enhanced porosity of the hierarchical ZIF-8 particle. The regeneration experiment demonstrated that this ZIF-8 particle could maintain its adsorption capacity after three cycles to
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support its promising potential for treatment of As and other heavy metal ions.
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In addition, other ZIF-8 or ZIF-based materials were studied to explore their removal capacities for abatement of As. As part of such efforts, water-stable Fe3O4@ZIF-8 composite was fabricated
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for removal of As(III) from water phase to yield a maximum capacity of 100 mg g-1 (Huo, Xu, Yang,
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Cui, Yuan and Fu [118]. From a comparison of three different morphologies of ZIF (i.e., cubic, leafshaped, and dodecahedral ZIFs), it was found that neither particle morphology nor surface area of
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ZIFs played the dominant role in As(III) removal (Liu, Jian, Liu, Yao and Zhang [119] (Table 2). Rather, the hydroxyl substation on ZIFs was primarily responsible for As(III) removal. The leaf-
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shaped 2D ZIF-L framework was also explored by Nasir, Nordin, Goh and Ismail [120] as an As adsorbent. This ZIF-L exhibited a maximum uptake of 43.7 mg g-1 for As(III) but displayed its maximum adsorption capacity in basic conditions (pH 10). In contrast, for most of other the adsorbents used for As removal (Tables 1 and 2), the maximum capacities were usually gained at near neutral pH, consistent with the variation in zeta potential (e.g., from positive to negative values). 26
At pH 9 to 10, the electrostatic attraction of the positively charged ZIF-L surface for As(III) reached a maximum level to showcase enhancement in adsorption uptake of As. Other MOF materials have been also widely studied for removal of As. For instance, a highly water-stable indium-based MOF (AUBM-1) was efficiently employed for As removal from aqueous phase [121]. The fabricated MOF with a pts (tetragonal symmetry) topology exhibited high chemical stability at different pH. The highest As(V) uptake capacity of 103.1 mg g-1 was obtained at pH 7.6
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by matching the experimental data with the Langmuir model (monolayer adsorption) [121]. The kinetic studies indicated that the adsorption process of AUBM-1 fit well with the pseudo-secondorder model. Moreover, its suitability as a regenerable As(V) sorbent was also supported by a
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decrease of ~20% in the As(V) adsorption capacity after three regeneration cycles. Likewise, Li,
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Yang, Sui and Yin [122] successfully developed MOF-808 nanoparticles using a household microwave oven in less than 5 min. MOF-808 displayed an adsorption capacity of 24.9 mg g-1 for
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As(V) at its initial loading concentration of 5 ppm. This MOF maintained > 80% removal efficiency
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for As even after five cycles of adsorption-desorption, while maintaining the integrity of its microcrystalline form as an efficient As(V) removal adsorbent. Overall, MOF materials exhibited
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dramatically enhanced performance for As adsorption compared to that of traditional adsorbents. (For more details, refer to the comparisons among various adsorbents in terms of PC values as
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discussed in Section 4).
4.4. Miscellaneous forms In addition to GO, CNTs, and MOFs, there are many other porous materials (e.g., metal oxides, chitosan-based materials, and polymers) that have been developed as efficient sorbents for removing 27
As from water. More specifically, a nanostructured Fe-Cu binary oxide was introduced for removal of As(V) and As(III) from water systems (Zhang, Ren, Zhang and Chen [123]. During synthesis of Fe-Cu binary oxide, the optimal Cu:Fe molar ratios were determined to acquire the maximum affinity between the adsorbent and As. Interestingly, As sorption increased with an increase in Cu:Fe molar ratio to 1:2 but decreased sharply with further increase in molar ratio. As verified by PXRD and SEM characterizations, the Fe-Cu binary oxide was ferrihydrite-like with poor crystallinity, a
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high BET surface area of 282 m2 g-1, and pore volume of 0.31 cm3 g-1. It exhibited maximum adsorption capabilities for As(V) and As(III) of 82.7 and 122.3 mg g-1, respectively. The similar amorphous 2-line ferrihydrite also showed excellent removal performance for As (Table 2) [124].
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Thus, it was highly speculated that As(V) were adsorbed by formation of inner-sphere surface
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complexes between As and sorbent, while both inner- and outer-sphere surface complexes contributed to adsorption of As(III). These two case studies focused on adsorption of As species
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without converting As(III) to As(V). However, ferrihydrite sorbent at a high Fe/As molar ratio (e.g.,
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50) also acted as a catalyst to oxidize As from As(III) to As(V), which plays a pivotal role in cycling
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arsenic in the natural environment (Zhao, Jia, Xu and Zhao [125].
As discussed earlier, despite large surface areas, the virgin organic or inorganic materials did not
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guarantee good adsorption performance for As ion removal. Hence, modifications of their pristine structure could provide more functional groups on their surface to offer more affinity for As. For instance, the coating technique is a useful tool to modify biopolymer and may enhance the interaction between As and sorbent. In this regard, Boddu, Abburi, Talbott, Smith and Haasch [126] fabricated a chitosan-coated biosorbent (CCB) by coating ceramic alumina with chitosan. The 28
monolayer adsorption capacities for As(III) and As(V) by this sorbent, when estimated based on the Langmuir isotherm, were 56.5 and 96.5 mg g-1, respectively. It can be assumed that biosorption based on bio-materials (e.g., chitosan) may receive more attention in the near future due to the material abundance in nature and biocompatibility. As an example of an advanced biosorbent, chitosan zero-valent iron nanoparticles (CINs) were prepared and tested for removal of As [127]. Surprisingly, very low final concentrations of < 5 µg L-1 for As
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were obtained with the initial concentration of 2 mg L-1 within 180 min. It should be noted that CIN contributed to a considerable reduction in final As concentrations, as listed in Tables 1 and 2. High adsorption capacities of 94 (for As(III)) and 119 mg g-1 (for As(V)) were found based on the
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Langmuir isotherm model. Anions, such as sulfate, phosphate, and silicate, slightly affected the
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adsorption capacities of CIN for both As. Moreover, CIN was also capable of efficiently adsorbing As even after five consecutive cycles, which supported its high potential as a sorbent for As
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treatment in real applications. Further, As concentration of < 14 µg in the leachate were obtained in
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the pH 2.9-6.3 based on an extended toxicity characteristic leaching procedure (TCLP) test [128]. As a whole, biomaterials showed significantly improved performance for As adsorption
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compared to those of conventional materials. However, final adsorption capacity can be sensitively influenced by experimental conditions (e.g., solution pH, initial loading concentration, and
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temperature). The important parameters that can affect performance for As removal are discussed in Section 5.
5. Effects of various factors on adsorption for arsenic removal 29
The adsorptive treatment of target pollutants (e.g., As) in wastewater can be controlled by the combined effects of various parameters such as solution pH, initial concentration, adsorbent dosage, contact time, competing ions, and temperature. More specifically, solution pH plays a significant role in the effluent treatment process as it can directly influence metal speciation schemes in water systems [129]. Meanwhile, it also affects the degree of protonation of functional groups on the surface of the adsorbent and the chemical nature of As. Therefore, its effects can be manifested by
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dramatic change in adsorption efficiency with variation in pH. In most of the adsorption processes, the adsorption capacity for As may increase with pH to neutral and then will follow a downward trend. Tables 1 and 2 show that the maximum adsorption capacities for both As(III) and As(V) were
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acquired at pH 7. In a solution with a higher pH value, As and OH- may form a hydroxide micro-
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precipitate, which may hinder adsorption of As [130].
Temperature is another pivotal experimental condition that is often used to determine the nature
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of the adsorption process in terms of thermodynamics. Thermodynamic parameters can be calculated using the following equations:
𝛥𝑆 𝑅
−
(1) and
𝛥𝐻
(2),
𝑅𝑇
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ln 𝐾 =
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Δ𝐺 = −𝑅𝑇 ln 𝐾
where ΔG is the standard Gibbs free energy (kJ mol-1), R is the gas standard (8.314 J mol-1), T is the
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temperature (K), K is the Langmuir isotherm constant, ΔS is the entropy change (J mol-1 K-1), and ΔH is the enthalpy change (kJ mol-1). In this study, the occurrences of positive and negative ΔH suggested that the adsorption reaction should be endothermic and exothermic, respectively. Furthermore, positive ΔG indicated that the adsorption process needed external energy to convert reactants to products [131], while negative ΔG values implied that the adsorption reaction was 30
spontaneous [64]. However, most of the studies have illustrated slight or limited effects of temperature on adsorption of As [132, 133]. For instance, a modified AC exhibited 526 µg g-1 at 25 ℃ and 588 µg g-1 at 35 ℃ with the initial As(V) concentration of 400 µg L-1 [64]. In groundwater, different ion sources may compete with As for acquisition of adsorption sites, which generally could negatively influence the adsorption capacity of the adsorbent. The effects of numerous co-existing ions (e.g., sulfate, nitrate, chloride, carbonate, bicarbonate, silicate, and
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phosphate) were investigated for their competing roles in adsorption of As. Many studies have reported that carbonate, silicate, and phosphate exhibited more distinctive effects on the adsorption of different As species, regardless of adsorbent type [65, 134, 135]. For instance, the presence of
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phosphate led to a decrease in As(III) ion removal efficiency of modified AC from 90% to 18%
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[135]. This was due to the fact that these anions can be strongly adsorbed onto the surfaces of sorbents through inner-sphere complexation. Therefore, this can lead to a dramatic reduction in
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As(V) adsorption through formation of surface complexes with the OH-group. However, phosphate
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and silicate only showed marginally negative effects on As adsorption behavior in the case study, as discussed (e.g., CIN nanoparticle) in Section 4.4 [127]. This divergence may be explained by the
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various affinities between As and different sorbents. Furthermore, the initial As concentration, or ion strength, was also an important parameter that could affect adsorption, as a higher initial
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concentration may provide a stronger driving force to yield higher adsorption capacities. However, evaluation based on the highest As adsorption capacities may be misleading because such a metric is sensitively affected by initial loading concentration (e.g., use of an unrealistically high initial concentration to induce high capacity values). In real-world conditions, As concentration was prevalent at low (ppb) level. As shown in Table 2, ZIF-8 showed an increased adsorption capacity 31
from 0.486 to 60.03 mg g-1 with an increase in initial As(V) concentration from 0.1 to 100 mg L-1 [116]. This type of improvement in adsorption capacity through control of the driving force parameters (especially initial loading concentration) was also commonly seen in the removal of other contaminants, such as Pb2+ [136], organic dyes [137], and pesticides [138]. There were also some other experimental conditions (e.g., contact time and adsorbent dosage) that could influence the final adsorption capacity, but these parameters were not decisive or could be easily modulated
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based on requirements. As a whole, evaluation of adsorbent performance based on maximum adsorption capacity is not an objective option. The more objective option for such evaluation is
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discussed in the next section.
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6. Performance evaluation of various materials for abatement of arsenic In this section, the PC (mol kg-1 M-1) value was employed as a key metric to assess the
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performance of sorbents with the least bias, so that the best sorbent could be determined more
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objectively for remediation of As from wastewater [139]. The PC was calculated by dividing maximum adsorption capacity (mg g-1 or mol g-1) by final concentration of As (mg L-1 or mol L-1)
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in the solution. Herein, it should be noted that after adsorption of As on sorbents, leaching of the As (with time) into solution can affect the PC values. However, due to the lack of time dependent
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leaching testing in majority of cited articles, we considered the specific adsorption capacity and final As concentration measured for PC calculations. Thus, evaluation of As ion removal was more meaningful if the data are assessed based on PC concept rather than that based on maximum capacity [139]. This is because the former presents the less biased results by reducing the effects of high initial feeding concentrations, while the latter is not free from such effect. It is noteworthy that CIN 32
exhibited the best adsorption performances (PC: ≥ 798 mol kg-1 M-1) for both As(V) and As(III). In fact, this adsorbent showed very high removal efficiency (≥ 99.75%) even at a low initial concentration of 2 mg L-1. This enhanced performance could be ascribed to the synergistic combination between strong affinity (As and iron) and size effect of the CIN particle. This type of nano-sized particle could be efficiently modified to increase its adsorption removal performance for abatement of organic pollutants and inorganic metal ions, demonstrating advantages over other
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sorbents. It should also be noted that Eu-MGO/Au@MWCNT showed the highest values for both As(III) (320 mg g-1) and As(V) (298 mg g-1) in terms of adsorption capacities. However, it was difficult to obtain their related PC values due to the lack of data regarding remaining As
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concentration or removal efficiency. In the case of MPSAC-La (0.36), the low removal efficiency
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at a high initial loading of As(V) resulted in a moderate PC value (1.98 mol kg-1 M-1) for As(V), although it showed a high adsorption capacity (227.6 mg g-1) for As(V) [66]. This case study on
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MPSAC-La (0.36) should remind researchers that sorbents with a high adsorption capacity do not
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necessarily perform well for removal of As from wastewater. According to evaluation of sorbent performances, the CIN particle appeared to have great potential as a sorbent for abatement of both
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As(V) and As(III). Therefore, synthesis of nano-sized particles and their application for metal remediation can be touted as a highly effective option for treatment of As and other toxic heavy
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metal ions. Furthermore, the evaluation of various sorbents in terms of regeneration study is discussed subsequently in the following section.
7. Regeneration of sorbents and their disposal after the use Regeneration and disposal of exhausted sorbents have been discussed intensely in recent years 33
so as to ensure their cyclic utilization while reducing the potential harmfulness of exhausted sorbents [140, 141]. Generally, the unreasonable disposal of the exhausted sorbents loaded with As may cause severe environment problems. Thus, it is necessary to safely dispose the exhausted sorbents, such as landfill, mixing with livestock waste, and incorporating within construction materials [142]. In this regard, stabilization/solidification (S/S) is considered as a suitable wasted sorbent treatment technology. As such, such option has been utilized very broadly for almost all toxic spent waste
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management alternatives because it may convert toxic waste into a less toxic or less mobile form of waste by chemical, physical, or thermal processes [143, 144]. Further, because inorganic wastes are more compatible with the used cementitious binders, the S/S process is used more frequently to
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convert inorganic wastes into valuable products. More specifically, monolithic solids with the
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advantageous properties (e.g., enhanced structural integrity, long-term stability, and minimal leaching) may be fabricated by encapsulating or incorporating the waste sorbents into the binder
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system [142]. Cement and hydrated lime (Ca(OH)2) has been extensively studied for stabilizing As
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containing materials [145]. For example, the calcium leaching may influence the leaching of As in all S/S system so that As concentrations can dramatically decrease with the increase of Ca leachate
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concentration [144]. Not long before, As bearing spent adsorbents (e.g., thermally treated (TTL), acid-base treated laterite (ABTL), and aluminum oxide/hydroxide nanoparticles (AHNP)) were well
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stabilized in the form of clay bricks [146]. Before stabilization, TTL, ABLT, and AHNP exhibited adsorption capacities of 6.43, 9.25, and 48.5 µg g-1, respectively. The subsequent characterizations for the bricks showed satisfactory results of density (2.3 g cm-3), shrinkage (10.2%), water absorption (11~14%), and comprehensive strength (35~150 kgf cm-2). Herein, it should be noted that converting one waste material into another beneficial feedstock is highly recommendable for 34
sustainable development. Thus, Table 3 lists a series of studies that focused on the S/S of Ascontaining solid wastes into useful construction materials. The effectiveness of a cement-free S/S technique to treat soils contaminated with As and Pb was realized using clay minerals as the green compatible binding material [147]. In this research, Ca3(AsO4)2∙4H2O and Pb3(NO3)(OH)5 precipitates were first produced by reacting those soils with Ca(OH)2, respectively. Subsequently, these precipitates were mixed separately with metakaolin (MK), lime (L), limestone, and
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phosphogypsum to generate the clay-based S/S products. Finally, the immobilization efficiency of As and Pb in S/S treated soil samples improved significantly by 96.2% (As) and 98.8% (Pb) in M6L (molar ratio of MK to L, 1:6) samples to support its efficacy. As a whole, As containing wastes
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have been widely stabilized into bricks, cements, and ceramics as shown in Table 3. However, it
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was noteworthy most of the mentioned studies were not sorbents related wastes. Therefore, the immobilization of spent sorbents into various construction materials still need to be studied further.
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Elution is of great importance for the reutilization of exhausted sorbents and the recovery of the
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adsorbed heavy metal ions (e.g., As). A sorbent with good regenerability implies that it can considerably lower the operating and maintenance price for the whole adsorption system [148].
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Generally, an effective regeneration process is based on the principle that the desorbed sorbents can recover its initial adsorption performance perfectly [149]. In this respect, the adsorption
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performance of various sorbents will be evaluated in terms of regenerability. Nearly 90% of the listed sorbents in Tables 1, 2, and 4 did not address any data for regeneration test, especially traditional materials. Thus, it can be speculated that most of traditional sorbents possess low desorption efficiency for As. In Table 3, NaOH was widely used as the desorbent for studying the regenerability of As adsorption-desorption. It should be noted that in the alkaline pH condition, the 35
adsorption capacity may dramatically decrease due to the competition between As with the hydroxyl ions [148]. In Table 4, only 6 sorbents possessed >80% desorption efficiency after 3 cycles of regeneration study. In this regard, FeMnOx/RGO, ZrO(OH)2/CNTs, and Fe-Cu binary oxide showed ideal regeneration performance because all these three sorbents had desorption efficiency of more than 90% for treating As(III) and/or As(V) after three sorption cycles. Especially, ZrO(OH)2/CNTs exhibited a high desorption efficiency of 97% after six cycles for As(III). The findings of the high
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removal capability in low concentration level (with high PC value) imply that ZrO(OH)2/CNTs is one of the best sorbents for treating arsenic, especially for As(V) (PC: 584.6 mol kg-1 M-1). Thus, further research is needed to find more advanced materials with high adsorption performance that
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are straightforwardly regeneratable over several adsorption-desorption cycles without noticeable
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loss of sorption capacity. Further, in Section 6, although CIN showcased the highest PC value, the low desorption efficiency may limit its use in water treatment. To sum up, some hybridized materials
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(like ZrO(OH)2/CNTs) should be the better choice in terms of performance (e.g., PC value) and
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regenerability.
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8. Conclusions and perspectives
The presence of As in ground and/or drinking water is one of the most important environmental
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problems around the world. In this review, a wide range of purification techniques, such as ion exchange, photocatalysis, and membrane technology, were briefly discussed for treatment of As in such systems. Among such options, adsorption technology is considered a highly attractive option due to its simple design, low price, and easy operation. Further, the potential use of numerous functional materials was explored along with conventional 36
materials. The adsorption performances of these materials were first assessed in terms of adsorption capacities. The results were assessed further in terms of PC and regenerability for a better evaluation of their performances. It was evident from the listed articles and compilation of data (as summarized in Tables 1 and 2) that many of the advanced functional materials (e.g., GO, CNTs, MOFs, and other miscellaneous forms) are much better options for efficient treatment of both As(V) and As(III) in comparison to conventional adsorbents, as evaluated in terms of enhanced recyclability and high
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PC values (e.g., ZrO(OH)2/CNTs with the PC values of 584.6 (As(V)) and 143.8 mol kg-1 M-1 (As(III))). Therefore, advanced materials were found to hold considerably high potential as an alternative to various conventional materials for effective treatment or remediation of As from
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drinking water or wastewater. Adsorptive removal of low-level As by novel materials may provide
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a great motivation for researchers to further synthesize more advanced functional materials for
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various applications.
Declarations
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Biotechnology Advances requires that all authors sign a declaration of conflicting interests. If you have
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nothing to declare in any of these categories then this should be stated.
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There is no competing interests.
A grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1E1A1A01940995).
Acknowledgments We would like to acknowledge support by the R&D Center for Green Patrol Technologies through 37
the R&D for Global Top Environmental Technologies funded by the Ministry of Environment (MOE Grant No: 2018001850001) and by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (Grant No: 2016R1E1A1A01940995). VK would like to acknowledge support from the Department of Science and Technology, New Delhi, India, in the form of an INSPIRE Faculty Award and from the Science and Engineering Research Board (SERB), Government of India, under the Early Career Research (ECR) award (File No. ECR/2018/000748). References [1] M. Jaishankar, T. Tseten, N. Anbalagan, B.B. Mathew, K.N. Beeregowda, Toxicity, mechanism and health effects of some heavy metals, Interdisciplinary toxicology, 7 (2014) 60-72. Journal of environmental management, 166 (2016) 387-406.
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47
f
Target
pH
Initial concentration [As] (mg L-1)
Isotherm
Removal efficiency (%)
18
4.0
0.16
980.40
71
11.05
1.27
973.4
68.0
10.87
1.13
670.3
96.0
19.35
16.13
649.4
49
2.45
1.20
863.00
45.6
2.28
1.05
388.00
Activated carbon
Adsorption capacity (mg g-1)
pr
Materials
e-
S. no.
oo
Table 1. Comparison of adsorption performance of conventional adsorbents for As(V)/As(III) removal. Partition coefficient (mol kg-1.M-1)
Surface area (m2 g-1)
AC
As(V)
3
30
F
2
Fe-AC
As(V)
3
30.0
F
3
Fe-Cu-AC
As(V)
3
30.0
F
4
Fe-Mn-AC
As(V)
5
CF-MP
As(V)
7
4.0
6
CW-MP
As(V)
7
4.0
7
Fe3O4-AC(MN)
As(V)
8
100.0
L
20
28
0.35
572.00
8
H2SO4-AC(MY)
As(V)
8
100.0
F
25
61
0.81
349.00
9
30.0
na l
Jo ur
Fe-AC
3
Pr
1
L
References
[61]
[62]
[63]
As(V)
6
1.0
L
51.3
2.565
As(III)
8
1.0
L
39.2
1.960
0.847
[150]
10
F400-M
As(V)
7
0.3
L
[64]
11
NP-doped ACF
As(V)
3
120.0
F
25
21.74
0.24
[68]
12
MPSAC-La(0.36)
As(V)
6
350.0
L
67.1
227.6
1.98
[66]
*The symbols F, L, and R-P
48
f 14
MPSAC
As(V)
6
15
KOH-AC (C)
As(V)
7.5
75
L
16
Ca alginate beads (G)
As(V)
7.5
75
L
17
G/AC composite beads (GC)
As(V)
7.5
75
L
18
AC-3
As(V)
7
20
L
19
Oxidized AC (ACO-3)
As(V)
7
20
L
20
AC-9
As(V)
7
20
21
ACO-9
As(V)
7
20
22
AC
As(V)
6
23
Iron oxide/AC
As(V)
24
F
13.8
were used to
141.8
indicate 1621
42.4
32.9
66.7
733.6
27
[151]
8
0.53
770
43.5
14
1.24
74
Peterson isotherms,
25
6.67
43
L
86.5
28
10.37
26
250
L
36
17.86
0.11
1022.6
6
250
L
40
20.24
0.13
678.3
As(III)
9
20.0
F
60
0.1
0.0125
As(V)
9
20.0
F
55
0.05
0.0056
Fe-eZ
[152]
Redlich-
25
81.25
na l
Freundlich, Langmuir, and
L
Jo ur
Zeolites
350.0
oo
6
pr
As(V)
e-
PSAC
Pr
13
respectively. [65]
[70]
25
GC
As(V)
7
0.1
15
1.50E-03
0.018
34.1
26
Na-GC
As(V)
7
0.1
17
1.70E-03
0.020
33.2
27
Fe1-GC
As(V)
7
0.1
92
9.20E-03
1.15
79.3
28
Fe2-GC
As(V)
7
0.1
84
8.40E-03
0.53
36.9
[71]
49
f oo
Fe-ZCH
As(V)
6.3
0.1
F
0.038
30
Fe-ZM
As(V)
6.3
0.1
F
0.033
31
Fe-ZC
As(V)
6.3
0.1
F
0.024
32
Zr-ZM
As(V)
6.3
0.1
F
33
Zr-ZC
As(V)
6.3
0.1
F
34
Zr-ZCH
As(V)
6.3
0.1
F
35
FeZr-ZCH
As(V)
6.3
0.1
F
36
FeZr-ZM
As(V)
6.3
0.1
37
FeZr-ZC
As(V)
6.3
0.1
Fe-Mn loaded zeolite (MFM)
As(III)
7
300
58
318
38
As(V)
7
300
41
-
96
19.39
pr
29
0.072
0.056
na l
Pr
0.068
F
0.062
F
0.062
39
Zeolite
As(V)
2.5
20
40
Magnetite nanoparticle coated zeolite (MNCZ)
As(V)
2.5
20
F
As(III)
7
2
L
1.37
41
CEZ
As(V)
7
2
L
1.48
As(III)
2
0.15
Jo ur
[72]
e-
0.058
339.4
2.54
340
[153]
24.2
126
[154]
[71]
Waste-derived sorbents 42
FMSWVOI
81
[79]
50
f 5
0.15
75
HC
As(III)
7
1
HC
As(III)
7
0.1
BP
As(III)
8
1
BP
As(III)
8
0.1
MP
As(III)
7
1
MP
As(III)
7
0.1
46
Waste cement (untreated)
As(V)
700
47
Waste cement (heat)
As(V)
700
As(V)
700
167
As(V)
700
175
L
97
L
Leather waste
As(V)
1
51
ABA300
As(V)
6
Jo ur
50
S. no
Alum sludge composite
Materials
As(V)
7
Target
As(V)/As(III) removal.
95
Pr
95.3
28.3 34.7 [81]
26 F
[84]
62.9
As(III)
52
[80]
e-
L
na l
49
adsorbents for
88
96.4
45
sludge
of novel functional
pr
98.8
44
Concrete (untreated) Concrete sludge (heat)
adsorption performance
1.92
43
48
Table 2. Comparison of
oo
As(V)
181.3
[82]
40 1000
[83] 15 Initial
pH
concentration [As]
Isotherm
Removal
Adsorption
Partition
efficiency
Capacity
Coefficient
(%)
(mg L-1)
51
(mg
g-1)
(mol
kg-1.M-1)
Surface area (m2 g-1)
References
4
rGO–Fe3O4–TiO2
Fe3O4-GO 5
macroscopic composite
Carbon nanotube Eu-MGO/ Au@MWCNT
7
ZrO(OH)2
8
ZrO(OH)2/CNTs
9
α-FeOOH/@GCA
f [93]
57
As(V) As(III)
7
1
As(V)
7
1
As(III)
7
7
As(V)
7
7
As(III)
7
As(III)
7
As(III)
7
As(III) As(V)
12 10.2 11
411
47.1
147.1
0.86
0.38
0.43
0.20
0.25
0.12
1.06
0.27
0.48
0.20
As(V)
0.27
0.08
As(III)
320
As(V)
298
[34]
[155]
[156]
As(III)
0.375
L
2.1
38.18
As(V)
0.375
L
6.7
209.38
As(III)
7
0.375
L
95.7
2.3
143.75
As(V)
7
0.375
L
96.5
7.6
584.62
As(V)
200
L,F
35.0
56
0.43
As (DMA)
200
L,F
15.0
24
0.14
As (p-ASA)
200
L,F
60.0
102
1.28
52
[96]
49.01
As(V)
Jo ur
6
As(III)
pr
FeMnOx/RGO
38
e-
3
M- rGO
85
As(III) As(V)
Pr
2
M-GO
na l
1
oo
Graphene oxide
87.80 [100] 126
150.1
[103]
MI-CNT-G
13
CNT/CuO
14
TiO2-CNT
Ce-Fe/CNT 15 Ce-Fe/CNT-A Metal organic frameworks
17
AUBM-1
11
As(V)
11
As(V)
10
As(III)
10
As(V)
5
1
As(III)
7
1
As(V)
7
0.9
As(III)
7
As(V) As(III) As(V)
ZIF-8
f
As(III)
As(III)
14.5 8.13 9.74 47.4 24.1
L
2.40
L
2.27 1.9
0.9
2.2
20
48
31.0
2.98
20
50
28.7
2.87
20
38.5
16.8
1.37
20
39.5
16.2
1.34
As(V)
7.6
180
L
As(III)
7
100
F
49.5
As(V)
7
100
F
60.0
As(III)
7
0.1
27
0.135
1.85
As(V)
7
0.1
97.2
0.486
173.57
Jo ur
16
200
oo
12
As (p-ASA)
8
pr
MI/CNTs
200
e-
11
As (DMA)
25.7
Pr
α-FeOOH
200
na l
10
As(V)
[104]
209.8
[105]
480
[107]
196
[108]
216.3 [49] 189
[121]
1063.5
18
ZIF-8
As(V)
19
Fe3O4@ZIF-8
As(III)
8
40
100
1,133
Cubic ZIFs
As(III)
8.5
80
122.6
958.4
Leaf-shaped ZIFs
As(III)
8.5
80
108.1
12.7
20
40
662.1
90.9 L
53
[116]
[117] [118] [119]
8.5
21
MOF-808
As(V)
22
ZIF-L
As(III)
10
100
As(V)
7
60
As(III)
7
60
As(V)
7
40
As(III)
7
40
As(V)
4
As(III)
4
As(V)
7
As(III)
7
As(V)
7
As(III)
7
f
As(III)
80
117.5
5 F
24
Fe-Zr binary oxide
25
CCB
CIN
730.29
40
43.74
0.73
67.02
[122]
50
82.7
2.76
282
F
46.7
122.3
3.82
282
F
25
46.1
1.54
12.5
120
3.43
F
R-P
96.46
R-P
56.5
60
97
119
66.11
60
93
94
22.38
2
99.75
4
800
2
99.75
3.99
798
Jo ur
na l
26
24.83
F
e-
Fe-Cu binary oxide
Pr
23
1,151.2
99.32
pr
Miscellaneous forms
oo
Dodecahedral ZIFs
Table 3. Stabilization/solidification of As ions from solid wastes into construction materials.
54
[120]
[123]
339
[124]
125.24
[126]
69
[127]
f 1
Iron oxide coated As(III) cement (IOCC)
2
Arsenic sludge
4
Products
Slurry preparation of Monolithic IOCC/cement/Ca(OH)2 matrices
Mechanochemical activation, mixing of Heavy metals Treated sludge modified ZVI and As sludge, and drying Drying, milling, sieving, As and F ions Clay bricks and sintering
Leaching test
Results
Semi-dynamic leach test
References
Single cement, or lime, or both are very suitable to reduce As leachability
[144]
Toxicity characteristic leaching Decrease As leaching concentrations from 72.5 to procedure (TCLP) tests [157] 0.62 mg L-1 (EPA1992)
e-
TTL/ABTL/ AHNP As-Fe hydroxide sludge
3
Methods
oo
Pollutants
pr
Solid wastes
As(III) As (V)
and
Mixing cement with sludge
Ca-As compounds
5
As-Fe sludge As
Crushing, mixing
and Clay-sludge bricks
6
Ascontaining mine tailings
As
Slurry preparation of mine Cements tailings and cement
7
Tannery sludge
Heavy metals
Mixing, drying
8
Calamine wastes
9
Fly ash
10
Contaminate d soil
na l
seieving,
grinding,
and
Oxides and Mixing, pressed under high heavy metals pressure, drying Mixing and sealed for 28 As days Mixing, stirring, and As and Pb transferred into moulds, and drying
Jo ur
California Waste Extraction Maximum values in leachate are below the [146] Test (WET) permissibles of USEPA [158]
Pr
S. no.
Clay bricks Bricks Compacted samples Clay mineral pastes
USEPA 1992, acidic, alkaline, 6% (safely maximum) of sludge by weight could be [159] and neutral condition mixed with clay The addition of 7.5% cement remarkably decreased TCLP tests (Korean standard, the leachability [160] US EPA 1312, US EPA 1311) TCLP tests (USEPA 1311) and Netherlands tank leaching test (NEN 7345) Dutch standard (NEN-7375) and TCLP (USEPA 1311) American Nuclear Society (ANS) 16.1 test TCLP (US EPA, 1992)
Meet the criteria for bricks in construction and the leached concentration is far below USEPA limits [161] Dramatically reduce the consumption of finite [162] resources and harmful environmental impact 98.3% release reduction upon addition of 25% fly [163] ash The leachability of As and Pb in treated soil samples reduced by 96.2% and 98.7%, respectively
[147]
Table 4. Comparative regeneration studies for benzene by various sorbents. S. no. 1
2 3
Sorbents
Targets
MPSAC-La(0.36) MNCZ FeMnOx/RGO
As(V) As(V) As(III)
Initial concentration (mg L-1) 350
Desorption conditions
20 0.1
[154] 0.1 M NaOH 82% after 5 cycles 0.1 M NaOH + 0.1 M 95% after 3 cycles, slightly [96]
0.5 M NaOH
55
Regeneration results (Desorption efficiency) 75% after 3 cycles
References [66]
As(V) As(III)
20
5
Fe3O4@ZIF-8
As(III)
-
6
Dodecahedral ZIFs
As(III)
10
7
MOF-808
As(V)
5
8
Fe-Cu binary oxide
9
CIN
As(V)
f
0.5 M NaOH
1
0.1 M NaOH
1000
1 M NaOH
na l
As(III) As(III) Alum sludge composite As(V)
25
0.5 M Na2SO4
Pr
As(III)
0.4 mM NaOH
Jo ur
10
As(V)
Hydrolysis, 75 vapor, 90 min 0.2 M NaOH
pr
ZrO(OH)2/CNTs
e-
4
decreased 93% after 5cycles 94% after 3 cycles, but decreased to 78% after 5 cycles ℃ 97% after 6 cycles, but decreased to 50% after 10 cycles 65% after 3 cycles 97% after 2 cycles, but decreased to 88% after 3 cycles 88% after 3 cycles, decreased to 82% after 5 cycles 94% after 4 cycles 90% after 4 cycles 50-60% after 5 cycles 50-60% after 5 cycles 75% after 5 cycles 80% after 5 cycles
oo
NaClO
56
[100] [118] [119] [122] [123] [127]
[83]
ro of
Figure 1. Adsorptive removal of As(V) by MPSAC-La (0.36) [66]: a) Preparation procedures for MPSAC-La (0.36), b) Kinetics for removal of As(V) by MPSAC-La (0.36), and c) Intra-particle
Jo
ur
na
lP
re
-p
diffusion modeling of As(V) removal by MPSAC-La (0.36).
57
f oo pr ePr na l
Figure 2. Removal of As(V) by modified ACF through adsorption [68]. Surface morphologies of various ACF: a1) virgin ACF (1,000 magnification); Zr-
Jo ur
based, NP-doped ACF with different magnifications of a2) 1,000, a3) 5,000, and a4) 30,000. b) Adsorption isotherms of As(V) ions by modified ACF.
58
f oo pr ePr
Figure 3. Z-nZVI adsorbents for treatment As(V) in aqueous medium [73]. a) Schematic of AsO43- and AsO43-/SeO42- removal by Z-nZVI; Sorption kinetics
Jo ur
na l
for abatement of As(V) in single b1) and binary b2) systems.
59
ro of -p re lP
Figure 4. Remediation of As ions by M-GO and M-rGO [93]. a) Preparation process for M-GO
Jo
ur
na
and M-rGO; Adsorption isotherm model of As ions onto b) M-GO and c) M-rGO.
60
ro of -p
re
Figure 5. As(V) treatment by ZrGO from an aqueous system [97]. a) Preparation procedure for
Jo
ur
na
lP
ZrGO; b) Adsorption isotherms of As(V) ions with three different concentrations.
61
ro of -p re
Figure 6. Application in adsorption of As species by α-FeOOH@GCA [103]. a) Adsorption
lP
mechanism for As(V) removal by α-FeOOH@GCA; b) Adsorption isotherms of removal of three As-containing species (i.e., DMA (dimethylarsinic acid (DMA), As(V) (sodium arsenate), p-ASA
Jo
ur
na
(p-arsanilic acid)) at three different temperatures: 25, 35, and 45 ℃.
62
ro of -p re
Figure 7. Adsorptive removal of As(III) and As(V) ions from aqueous solution by ZIF-8 [116]. a)
lP
Synthetic process of ZIF-8 particles; b-c) Adsorption isotherms of As(III) and As(V) onto ZIF-8
Jo
ur
na
under different ionic strengths.
63