By Lee D. Wilson, PhD
This article presents an overview of chitosan results from the literature that deal mostly with the adsorptive uptake of inorganic arsenic species. Biopolymers such as chitosan and its modified forms represent a suitable alternative to conventional adsorbents because of their abundance, low cost and synthetic versatility. Chitosan-adsorbent materials are anticipated to play an ever increasingly important role in adsorption technology for applications related to contaminant remediation or chemical separations. The future demand for improved adsorbent materials with good adsorption properties and molecular selectivity is anticipated due to the uncontrolled release of contaminants into the environment.
The occurrence of arsenic-based waterborne contaminants represents an emerging concern to global water and food security, as evidenced by food production, human health and sustainable development.1,2 The widespread occurrence of elevated levels of arsenic in water supplies is referred to as hot spots and are well known.3-6 For example, certain regions of China, India and Bangladesh have been referred to as cancer belts, in part, because of the elevated levels of arsenic among other heavy metal contaminants due to the geochemistry of certain regions. In Canada,4 a number of hot spots have been identified along with the US and South America.3 Aside from natural geological deposits, industrial sources of arsenic-based compounds include agrochemicals (fertilizers, pesticides), petrochemicals and pharmaceuticals, production of pigments and dyes and mining activities. The occurrence of arsenic contaminants is especially relevant for mine tailings and geological formations containing arsenopyrite mineral phases.7
Arsenic is a unique chemical species in terms of its chemical diversity as evidenced by its two common oxidation states [As (III) and As (V)], chemical reactivity and protolytic behavior.8,9 Arsenate and arsenous anions (see Table 1) are the typical inorganic species of As (V/III) in aquatic environments, depending on the environmental conditions. Arsenic acid (H3AsO4) displays similar chemical structure and acid dissociation properties relative to phosphoric acid (i.e. H3PO4), as seen in Table 1. The occurrence of such different chemical forms is an important consideration for understanding the fate and transport in the environment.
In addition to the inorganic species of arsenic, there exist a wide range of organoarsenicals (e.g., methylated arsenic, phenylarsonic acids, arseno-sugars, etc.).11-13 Organoarsenicals have been the subject of recent studies because of the potential impact of these compounds on human and ecosystem health.9,14,15 The widespread application of organoarsenicals as animal feed additives, wood preservatives, insecticides and pharmaceutical compounds has raised concerns regarding their fate, occurrence and distribution in the environment.16-18 Among the various organoarsenicals, Roxarsone (3-nitro-4-hydroxyphenylarsonic acid) was used as a feed additive for poultry production due to its antimicrobial properties and growth enhancement effects. Since 2012, Roxarsone was banned by the US Department of Agriculture due to questions regarding the potential toxicity of its decomposition byproducts, including uncertainty regarding their fate and distribution in the environment.19
In view of the similarity of the chemical behavior of arsenate and phosphate species (see Table 1), conventional water treatment offers some level of removal through the use of lime-based approaches through precipitation of a mineralized calcium species.10 Advanced water treatment methods that employ inorganic or organic coagulants and flocculants have shown some success, as described in a recent review by Renault and coworkers.20 Other methods of arsenic removal include electrocoagulation, dissolved air floatation, ion exchange, solvent extraction, catalytic oxidation, bioremediation and adsorption methods.21-23 Adsorption-based processes occur when a chemical species (adsorbate) is transferred either from solution or in the gas phase onto the surface of a solid adsorbent material due to physical or chemical interactions as depicted in Figure 2.
Figure 2. Adsorptive removal strategy of waterborne contaminants, such as arsenic species (sphere) onto the surface of an adsorbent material (rectangle) n an aqueous environment. Note that water is omitted for the sake of clarity.
Mohan and Pittman23 carried out an extensive review on the use of adsorbent materials that covered the literature up to 2007. In view of the low infrastructure requirements, adsorption methods are among the least expensive and facile technologies for the efficient uptake of chemical contaminants in complex matrices, such as water and wastewater.24 Among the range of adsorbent materials, granular activated carbon (GAC), polymer resins, activated alumina, granular iron (hydr)oxide, zeolites and ion exchange resins are some examples of industrial adsorbent materials. In view of relative availability, renewability, efficacy and cost of biomaterials, there is merit in the consideration of such materials for their application in water remediation. In particular, chitosan-based adsorbents have been shown to display favorable uptake of metals and arsenic-based contaminants.17,25-27 This article will provide an overview of some recent studies concerning the use of chitosan-based materials for arsenic removal from water.
Metal oxides, polymers, carbonaceous materials and their composites were used as adsorbent materials for the removal of arsenic from drinking water.9,22,23 Qu22 has examined a range of adsorbents in conjunction with catalytic oxidation-reduction processes for heavy metal and arsenic removal. Metal oxide (e.g., Ti, Fe, Cu, Mn, etc.) supported materials for arsenic removal are potential sorbents of interest, as shown by their variable uptake values in Table 2.22,28-31 The use of composite adsorbents containing metal oxyhydroxides enables the conversion of toxic As (III) to a less toxic form [As (V)]. The use of adsorbents with a dual-functioning behavior (adsorption + oxidation) offer advantages when compared with conventional oxidants (chlorine, peroxide, etc.) in water containing natural organic matter. The use of dual function adsorbents avoids potential side reactions during adsorption processes. In particular, US EPA has endorsed the use of iron (hydr)oxide based materials for the removal of arsenic.32 Aside from the efficient uptake of arsenic species in variable oxidation states, these materials are advantageious in terms of their relative abundance and overall lower cost.
According to Table 2, the range of uptake values varies considerably depending on the nature of the single-component metal oxide and its formulation as a composite material, as evidenced by the variable uptake value (1.7 to 251 mg/g). The overall uptake efficiency of contaminants generally depends on the surface chemistry and textural properties of the materials and the adsorption conditions (e.g., adsorbate concentration, adsorbent dosage, pH, ionic strength). These physicochemical properties can be modified in the case of composite materials by varying the composition of the respective components, especially in the case of supported nanomaterials.
Figure 3. Generalized molecular structure of chitosan, where R represents -H or -COCH3 (acetyl) groups depending on the level of deacetylation of the polymer chain.
Chitosan is an aminopolysaccharide biopolymer derived from the partial deacetylation of chitin. Figure 3 illustrates the generalized molecular structure of chitosan. Chitosan and its derivatives are relatively low cost and renewable materials that are suitable for the efficient removal of arsenic under variable conditions.12,13,33 In part, the utility of chitosan and its modified forms, or as a support material for composites by further modification of the various hydroxyl or amine groups, make this polysaccharide attractive as a modular sorbent material.34 Chitosan has been widely studied because of its synthetic versatility and for its efficacy in the removal of metal ion species via complex formation with the abundant hydroxyl and amine function groups.27,35,36 The favorable uptake of metal cation species is largely attributed to the availability of the amine or acetyl functional group on the polymer backbone of partially deacetylated chitosan.
Pontoni and Fabbricino37 have recently reviewed the use of chitosan-based materials for the removal of arsenic species. Table 3 lists some selected examples of modified chitosan adsorbents for the adsorptive uptake of inorganic As (III/V) species.38-47 In general, native chitosan displays relatively low uptake toward As (III/V) species, whereas, chemical or structurally modified chitosan reveal greater uptake. It should be noted that the pKa of chitosan is approximately 5.5, depending on the degree of polymerization and acetylation.25 At pH values that exceed the pKa of chitosan, there should be no pronounced electrostatic contributions to adsorption from ion exchange, particularly when the amine group is in a neutral charged state above pH 6. In cases where pH is > 7 and when adsorption is significant, other factors such as van der Waals and H-bonding interactions may predominate along with structural contributions that relate to surface and hydration effects. The results illustrate that chemical and structural modification of chitosan produce materials with variable uptake capacity, as evidenced by variable uptake values that range from 1.84 to 228 mg/g for As(V) species.
In the case of composite materials containing chitosan and metal (hydr)oxides, such materials have variable surface chemistry in accordance with their relative composition. Kwon et al. prepared a series of iron (hydr)oxides as adsorbents supported onto activated carbon (AC) for the uptake of a model organo arsenical (Roxarsone). Greater uptake of Roxarsone was observed as the iron oxide content of the AC composites decreased. This is in contrast with the uptake of inorganic arsenic species, since arsenate forms stable complexes with iron (hydr)oxide materials. The dual adsorbent nature of composites toward both inorganic and organic species depends on the relative composition of the adsorbent and the nature of the contaminant species. Roxarsone is relatively insoluble at ambient pH conditions, whereas, inorganic arsenate species has a greater solubility. The different hydration behavior of such contaminants results in significant differences in their adsorption behavior with organic and mineral surfaces. Composite materials involve a range of weak (e.g., H-bonding and van der Waals) interactions to stronger coordination (e.g., metal-ligand complexation) that involve both inner and outer sphere. In a recent review by Da Sacco and Masotti,17 the adsorptive interactions between chitosan and As2O3 were examined. The authors attribute H-bonding and ion exchange as the predominant interactions for chitosan and As (III/V) species at various pH conditions (see Figure 1 in17). Depending on the ionization state of the adsorbent-adsorbate system, the adsorption may vary in strength from physisorption to chemisorption in nature. Ionic bonding between oppositely charged groups or metal-ligand coordination is representative of strong adsorptive interactions, while weaker non-covalent interactions are lesser in magnitude. However, the structure and textural properties of the sorbent material require further consideration since nanomaterials, molecular imprinted polymers and microporous materials display relatively high uptake. This is understood in terms of their greater surface area and number of available adsorption sites. The formation of preorganized structures that are zeolite-like in nature may lead to cooperative binding interactions and structural considerations, which are especially relevant in advanced materials with unique structure and morphology.
Further evidence of surface area effects, structure and its influence on the uptake capacity toward arsenic species is illustrated by nano-sized metal oxides (NMOs).48-53 Examples of NMOs include ferric, manganese, aluminum, titanium, magnesium and cerium oxides. TiO2-based arsenic removal methods have generally been focused on the photocatalytic oxidation (PCO) of arsenite and organic arsenic species to less toxic byproducts, such as arsenate in conjunction with adsorptive removal.22 As mentioned above, the use of biopolymer supports such as chitosan serve several purposes and are anticipated to play an increasingly important role in advanced water treatment processes. In the case of NMOs, chitosan serves to stabilize the NMOs to prevent aggregation and loss of nanomaterials. The latter is especially relevant in flow systems due to their ability to pass through micron-level filters. High surface-to-volume ratios in the case of NMOs44-47 provide abundant surface sites for the uptake of arsenic and other contaminants from aqueous systems, as evidenced by the results presented in Tables 2 and 3.
The anticipated use of chitosan or its modified materials described in Tables 2 and 3 are anticipated to be similar to deployment methods used for commercially activated carbon sorbents in conventional water treatment systems. Thus, the use of chitosan and its modified materials in various structural forms, such as powders, beads and granules, are amenable to the use of filter-cartridge modules in conventional treatment systems. If one compares the results for the uptake in Tables 2 and 3, it is evident that iron (hydr)oxide materials and its composites offer potential materials for removal of arsenic species at a relatively low cost. Detailed CAPEX estimates are not provided herein for the use of chitosan and its modified materials because one must consider the OPEX (i.e. operational and regeneration costs) as part of the overall process, although the cost of raw materials (USD/kg) are variable (chitosan, $15-30; iron oxide $0.75-1.00; activated carbon $0.5-2.5).54
Among the materials listed, iron oxide materials are considered relatively efficient, safe and economical water treatment technolology according to US EPA. The costs of regeneration, however, are more difficult to estimate and depend significantly on the quality of the source water supplies. In the case of chitosan-supported NMOs, stabilization of the nanomaterials may be achieved while enhancing surface area and uptake efficiency when compared with monolithic mineral oxides. By comparison, the regeneration costs of purely organic chitosan adsorbents are anticipated to be lower for such materials that adsorb arsenic species by physical interactions. Regeneration of chitosan can be achieved using simple backwashing with alkaline solution.
Notwithstanding the differences in materials outlined above, the potentially lower regeneration and operational costs of chitosan and its modified materials warrant further evaluation. The focus of this review deals with adsorbent materials for the removal of arsenic species in view of the technological simplicity of adsorption and its relatively lower operational costs relative to membrane based methods. The general utility of reverse osmosis has been examined as a potential technological solution elsewhere.55-57 A detailed overview of RO is beyond the scope of this article; however, there are concerns with membrane fouling and operational costs relative to simple adsorption technology described herein. In this manner, further consideration of chitosan and its composite materials should be given since they represent an emerging class of materials in view of their synthetic versatility and structural diversity. According to the examples described in Tables 2 and 3, the physicochemical properties (e.g., surface area and surface chemistry) of chitosan adsorbent materials offer opportunities for the future development of technological solutions to address challenging water treatment issues under different environmental conditions.
Conclusion and future outlook
The level of arsenic species dissolved in ground and surface water depends on various physicochemical factors, including the available primary and secondary sources. The solution chemistry of the environment (pH, redox potential, ions present, ionic strength, organic matter content, etc.) plays a role in terms of the efficacy of the use of adsorbent materials. Surface chemical interactions at the solid and fluid interface will affect the relative uptake of contaminant species, redox process, dissolution/precipitation, ion exchange and adsorption/desorption processes. Chitosan and its composite materials represent a promising class of adsorbent materials for the remediation of waterborne contaminants such as arsenic. Further advances in the development of nanomaterials and composites that contain chitosan are anticipated to result in materials with improved adsorption properties, increasing the use of industrial-scale adsorption technology for POU remediation of mine tailings, petroleum byproducts and chemical separations. The unintended release of arsenic and other contaminants into the environment due to anthropogenic activities are anticipated to become increasingly important issues and the demand for such adsorbent technology will continue well into the future.
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About the author
Lee D. Wilson completed a PhD in physical chemistry from the University of Saskatchewan (1998) and was an NSERC (The Natural Sciences and Engineering Research Council of Canada) Visiting Fellow at the National Research Council of Canada at the Steacie Institute for Molecular Sciences. Currently an Associate Professor of chemistry at the University of Saskatchewan and on the Advisory Board of NanoStruck Technologies, Wilson has led the development of a nano-chitosan copolymer powder, a derivative of crustacean shells being used exclusively by NanoStruck Technologies for its water remediation technologies. Email: firstname.lastname@example.org; tel. +001-306-966-2961; fax. +001-306-966-4730.
About the company
NanoStruck Technologies Inc. (www.nanostruck.com) is a Canadian company with a suite of technologies that remove molecular-sized particles. Through propriety additions to conventional technologies and insertion of disruptive organic copolymers, the company’s patented technologies provide environmentally safe solutions for water purification and precious-metal recovery. NanoStruck owns the exclusive rights by license to manufacture, market, distribute and sell the nano-biotechnology worldwide, formulated at the University of Saskatchewan.