6
Introduction Heavy metal contamination due to natural and anthropo- genic sources is now of a global environmental concern. Accumulation of such metals in living beings often results in several disease conditions (USEPA, 2004). Major sources of these which contaminate the environment are smelting, roasting and plating of minerals, fertilizer impurities (Cd), refining, pulp production, electroplating processes (Cr), primary copper, lead smelters, and manufacture of chemi- cals (As), dental amalgam fillings, household products, fluorescent light bulbs, broken thermometers and indus- trial settings (Hg), mining activities, algaecides, chromated copper arsenate, pressure treated lumber, copper pipes (Cu), and plating wastes (Ni). e presence of these metals in the environment and their potential for harmful effects therefore calls for the attention of scientists in the investiga- tion of mechanisms of the speciation and mobility of metal contaminants. Distribution and forms of heavy metals in environmental compartments Distribution, mobility, and toxicity of metals are strongly related to the different forms in which they exist (Ure and Davidson, 1995). It is likely that water with a high total metal concentration can be, in fact, less toxic than water with a lower total metal concentration. For example, many workers (Nielsen and Andersen, 1970; Bryan, 1971; LaGrega et al., 1994) concluded that ionic copper is far more toxic toward aquatic organisms than organically bound copper since ionic copper is more soluble and hence bioavailable to the organisms. e organically bound copper being more stable has very low solubility and thus lower toxicity to aquatic life. Even in the case of treatment plants, knowledge of the chemical forms of dissolved metals is important for efficient operations of the treatment plant. e efficiency of the treat- ment operation depends on whether the metal is an ionic, complexed, colloidal, or particulate form (Florence and Batley, 1977). Lead in water samples is found in general as elemental lead, lead oxides, lead hydroxides and lead oxy- anion complexes (Smith et al., 1995). Under most conditions Pb 2+ and lead–hydroxy complexes are the most stable forms of lead (Lee and Saunders, 2003). Low solubility compounds are formed by complexation with Cl , CO 3 2− , SO 4 2− , PO 4 3− , and organic ligands (humic, fulvic, and amino acids) (Bodek et al., 1988). Chromium (Cr) does not occur naturally in elemental form, but only in compounds. Chromium is mined as a primary ore product in the form of the mineral chromite, FeCr 2 O 4 . Major sources of Cr contamination include releases from electroplating processes and the disposal of chro- mium containing wastes. Cr (VI) is the form of chromium (Accepted 25 August 2009) ISSN 0738-8551 print/ISSN 1549-7801 online © 2009 Informa UK Ltd DOI: 10.1080/07388550903284462 http://www.informahealthcare.com/bty REVIEW ARTICLE Biological methods for speciation of heavy metals: different approaches Neha Singh, and Ranu Gadi Indira Gandhi Institute of Technology, GGS Indraprastha University, Delhi, India Abstract Heavy metals are found in their different forms in the environment. The distribution, mobility, and toxicity of metals are strongly related to these different forms. This necessitates the exploration of different methods for the remediation and speciation of heavy metals. Some direct and indirect physico-chemical methods such as filtra- tion, chemical precipitation, ion-exchange, electro deposition, and membrane systems have been used for the last four decades. However, it is only in last few years that reliable biological methods have also been used. The biological methods include the use of microorganisms (fungi, algae, bacteria), plants (live or dead) and biopoly- mers. The use of these methods for the speciation of heavy metals is reviewed here. Keywords: Biopolymers; bioremediation; metal toxicity; microorganisms; plants Critical Reviews in Biotechnology, 2009; 29(4): 307–312 Address for Correspondence: Indira Gandhi Institute of Technology, GGS Indraprastha University, Delhi-110006, India. E-mail: [email protected] Critical Reviews in Biotechnology Downloaded from informahealthcare.com by Serials Unit - Library on 09/10/13 For personal use only.

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Page 1: Biological methods for speciation of heavy metals: different approaches

Introduction

Heavy metal contamination due to natural and anthropo-genic sources is now of a global environmental concern. Accumulation of such metals in living beings often results in several disease conditions (USEPA, 2004). Major sources of these which contaminate the environment are smelting, roasting and plating of minerals, fertilizer impurities (Cd), refining, pulp production, electroplating processes (Cr), primary copper, lead smelters, and manufacture of chemi-cals (As), dental amalgam fillings, household products, fluorescent light bulbs, broken thermometers and indus-trial settings (Hg), mining activities, algaecides, chromated copper arsenate, pressure treated lumber, copper pipes (Cu), and plating wastes (Ni). The presence of these metals in the environment and their potential for harmful effects therefore calls for the attention of scientists in the investiga-tion of mechanisms of the speciation and mobility of metal contaminants.

Distribution and forms of heavy metals in environmental compartmentsDistribution, mobility, and toxicity of metals are strongly related to the different forms in which they exist (Ure and Davidson, 1995). It is likely that water with a high total metal concentration can be, in fact, less toxic than water with a

lower total metal concentration. For example, many workers (Nielsen and Andersen, 1970; Bryan, 1971; LaGrega et al., 1994) concluded that ionic copper is far more toxic toward aquatic organisms than organically bound copper since ionic copper is more soluble and hence bioavailable to the organisms. The organically bound copper being more stable has very low solubility and thus lower toxicity to aquatic life. Even in the case of treatment plants, knowledge of the chemical forms of dissolved metals is important for efficient operations of the treatment plant. The efficiency of the treat-ment operation depends on whether the metal is an ionic, complexed, colloidal, or particulate form (Florence and Batley, 1977). Lead in water samples is found in general as elemental lead, lead oxides, lead hydroxides and lead oxy-anion complexes (Smith et al., 1995). Under most conditions Pb2+ and lead–hydroxy complexes are the most stable forms of lead (Lee and Saunders, 2003). Low solubility compounds are formed by complexation with Cl−, CO

32−, SO

42−, PO

43−, and

organic ligands (humic, fulvic, and amino acids) (Bodek et al., 1988).

Chromium (Cr) does not occur naturally in elemental form, but only in compounds. Chromium is mined as a primary ore product in the form of the mineral chromite, FeCr

2O

4. Major sources of Cr contamination include releases

from electroplating processes and the disposal of chro-mium containing wastes. Cr (VI) is the form of chromium

(Accepted 25 August 2009)

ISSN 0738-8551 print/ISSN 1549-7801 online © 2009 Informa UK LtdDOI: 10.1080/07388550903284462 http://www.informahealthcare.com/bty

R E V I E W A R T I C L E

Biological methods for speciation of heavy metals: different approaches

Neha Singh, and Ranu Gadi

Indira Gandhi Institute of Technology, GGS Indraprastha University, Delhi, India

AbstractHeavy metals are found in their different forms in the environment. The distribution, mobility, and toxicity of metals are strongly related to these different forms. This necessitates the exploration of different methods for the remediation and speciation of heavy metals. Some direct and indirect physico-chemical methods such as filtra-tion, chemical precipitation, ion-exchange, electro deposition, and membrane systems have been used for the last four decades. However, it is only in last few years that reliable biological methods have also been used. The biological methods include the use of microorganisms (fungi, algae, bacteria), plants (live or dead) and biopoly-mers. The use of these methods for the speciation of heavy metals is reviewed here.

Keywords: Biopolymers; bioremediation; metal toxicity; microorganisms; plants

Critical Reviews in Biotechnology, 2009; 29(4): 307–312Critical Reviews in Biotechnology

2009

1–6

iFirst

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25 August 2009

0738-8551

1549-7801

© 2009 Informa UK Ltd

10.1080/07388550903284462

Address for Correspondence: Indira Gandhi Institute of Technology, GGS Indraprastha University, Delhi-110006, India. E-mail: [email protected]

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Page 2: Biological methods for speciation of heavy metals: different approaches

308 Neha Singh and Ranu Gadi

commonly found at contaminated sites. Chromium can also occur in the +III oxidation state, depending on pH and redox conditions. Cr (VI) is the dominant form of chromium in shallow aquifers where aerobic conditions exist. Cr (VI) can be reduced to Cr (III) by soil organic matter, S2− and Fe2+ ions under anaerobic conditions often encountered in deeper groundwater. Major Cr (VI) species include chro-mate (CrO

42−) and dichromate (Cr

2O

72−) which precipitate

readily in the presence of metal cations (especially Ba2+, Pb2+, and Ag+). Chromate and dichromate also adsorb on soil surfaces, especially iron and aluminum oxides. Cr (III) is the dominant form of chromium at low pH (<4). Cr3+ forms solu-tion complexes with NH

3, OH−, Cl−, CN−, SO

42−, and soluble

organic ligands. Cr (VI) is the more toxic form of chromium and is also more mobile. Cr (III) mobility is decreased by adsorption to clays and oxide minerals below pH 5 and its solubility decreases above pH 5 with the formation of solid Cr (OH)

3 (Evanko and Dzombak, 1997).

In aerobic environment As (V) is dominant, usually in the form of arsenate (AsO

43−) in various protonation states:

H3AsO

4, H

2AsO

4−, H

2AsO

42−, AsO

43−. Arsenate and other ani-

onic forms of arsenic behave as chelates and can precipitate when metal cations are present. Metal arsenate complexes are stable only under specific conditions. As (V) coprecipi-tates with or adsorb onto iron oxyhydroxides under acidic and moderately reducing conditions. Coprecipitates are immobile under these conditions. However, arsenic mobil-ity increases as pH increases. Under reducing conditions As (III) dominates, existing as arsenite (AsO

33−) and in proto-

nated forms: H3AsO

3, H

2AsO

3−, HAsO

32−. Arsenite can adsorb

or coprecipitate with metal sulfides and has a high affinity for other sulfur compounds (Evanko and Dzombak, 1997). Since arsenic is often present in anionic form, it does not form complexes with simple anions such as Cl− and SO

42−.

Arsenic speciation also includes organometallic forms such as methylarsinic acid (CH

3)HAsOOH and dimethylarsinic

acid (CH3)

2AsO

2H (Evanko and Dzombak, 1997).

Zinc usually occurs in the +2 oxidation state and forms complexes with a number of anions, amino acids and organic acids. Zn may precipitate as solid Zn (OH)

2, solid

ZnCO3, solid ZnS, or solid Zn(CN)

2. Cadmium (Cd) occurs

naturally in the form of CdS or CdCO3. The form of cadmium

encountered depends on solution and soil chemistry as well as treatment of the waste before disposal. The most com-mon forms of cadmium include Cd2+, cadmium–cyanide complexes, or Cd (OH)

2 solid sludge. Hydroxide [Cd (OH)

2]

and carbonate [CdCO3] solids dominate at high pH whereas

Cd2+ and aqueous sulfate species are the dominant forms of cadmium at lower pH (<8) (Smith et al., 1995).

Solution and soil chemistry strongly influence the spe-ciation of copper in ground-water systems. In aerobic, suf-ficiently alkaline systems, CuCO

3 is the dominant soluble

copper species. The cupric ion, Cu2+, and hydroxide com-plexes, CuOH+ and Cu (OH)

2, are also commonly present.

Copper forms strong solution complexes with humic acids. The affinity of Cu for humates increases as pH increases and ionic strength decreases. In anaerobic environments,

when sulfur is present, solid CuS will be formed (Dzombak and Morel, 1990). Mercury, after release to the environ-ment usually exists in mercuric (Hg2+), mercurous (Hg

22+),

elemental (Hg°), or alkylated form (methyl/ethyl mercury). The redox potential and pH of the system determine the stable forms of mercury that will be present. Hg

22+ and Hg2+

are more stable under oxidizing conditions. When mildly reducing conditions exist, organic or inorganic mercury may be reduced to elemental mercury, which may then be converted to alkylated forms by biotic or abiotic processes (Smith et al., 1995).

Bioremediation of metal pollutionSeveral methods are available for tackling the problem of heavy metal pollution. But Conventional treatment technolo-gies simply transfer these pollutants from one environmental compartment to another, therefore creating new waste such as incineration residues. This does not eliminate the problem. It also requires high energy inputs. The ideal solution for pol-lution abatement is bioremediation. This has been described as a most effective, innovative technology to come along with this century that uses biological systems to sequester heavy metals (Schnoor, 1997; NABIR Primer, 2003).

Use of biological methods is derived from the fact that the biological response of an organism depends simply not only on total concentrations but also on the activities of the metal ions and their complexes and on the concentration of labile metal species in solution (Hudson and Morel, 1990; Morel and Hering, 1993).

There are several chemical groups that would attract and sequester the metals in biomass such as acetamido groups of chitin, structural polysaccharides of fungi, amino and phosphate groups in nucleic acids, amido, amino, sulphy-hydryl, and carboxyl groups in proteins and hydroxyls in polysaccharides. These groups can interact selectively with different forms of the metals, which in some cases can be facilitated due to steric, conformational or other barriers (Ahalya et al., 2003).

The affinity of various metals varies with respect to the biomolecular ligand. The bond character in biosorption can be explained partially by Pearson’s concept of hard and soft metallic ions. This scale is based on the binding strength of the ions with F− and I−. The metallic ions, which form strong binding with F− and I−, are referred to as “hard” while those forming relatively weaker bonds are referred to as “soft” ions (Pb, Hg, etc.). Avery and Tobin (1993) have studied the appli-cability of hard and soft principle in predicting metal sorption by Saccharomyces cerevisiae. There also exists a class of ions with intermediate degree of hardness (Zn, Cu, etc.), and are referred to as “transition” metals. Among the ligand atoms “O” and “F” are considered hard “S” and “P” are considered soft while “N” stays in the intermediate category. The hard ions in biological system form stable bonds with hydroxyl, phosphonate, phosphate, carboxyl, and carbonyl group all of which contain “O” atoms. While the soft ions form very strong bonds with sulfhydryl, amine, imidazole, amide and imine groups, that is, groups rich in S and N atoms.

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Page 3: Biological methods for speciation of heavy metals: different approaches

Different Approaches to Biological methods for speciation of heavy metals 309

Nieboer and Richardson (1980) have classified the metal ions into class A (oxygen seeking), class B (nitrogen, sulphur seeking) and class C (borderline or intermediate character), based on their binding preferences toward the O, N, or S containing ligands of biomolecules. These characters also help in the speciation of metals.

Different approaches to speciation of heavy metals

Microorganisms in speciation of heavy metalsMicroorganisms can either immobilize or convert metals to less toxic forms. The immobility of metals is primarily caused by reactions that cause metals to precipitate or chemical reactions that keep metals in a solid phase. When a micro-organism oxidizes or reduces metallic species, the metal precipitates. Mercury is an example of a metal that can be precipitated. The process begins with mercury (Hg2+) being reduced to mercuric sulfide causing mercury to transform to a precipitated form. Chromium is another metal that can be converted to a precipitated form with the use of microorgan-isms. The process involves the reduction of hexavalent chro-mium to trivalent chromium, which then can precipitate to chromium oxides, sulfides or phosphates. Arsenic can also be converted similarly to less toxic form by microorganisms. Reactions between metals and microorganisms can be used for making inference for the speciation of metals (Gihring et al., 2001).

A variety of microorganisms including fungi, yeast, algae, and organelles (erythrocytes) have been employed as ana-lytical preconcentrators for speciation purposes. The studies by Baldrian (2003) reveal that the ability of white-rot fungi to adsorb and accumulate metals together with the excellent mechanical properties of fungal mycelial pellets provide an opportunity for application of fungal mycelia in selective sorption of individual heavy metal ions from polluted water. Bag et al. (1999) developed an adsorption-elution method for the preconcentration of Cu, Zn, and Cd followed by flame atomic absorption spectrometry (FAAS) using the adsorb-ent Saccharomyces cerevisiae immobilized on sepiolite. Recoveries were 98.3 ± 0.4% for Cu, 94.2 ± 0.3% for Zn, and 99.04 ± 0.04% for Cd at 95% confidence level obtained by the column method. The influence of sea water matrix elements on the separation of the trace elements was also assessed by them using the column procedure. The breakthrough capac-ities were found to be 74 μmol/g for copper, 128 μmol/g for zinc, and 97 μmol/g for cadmium. After optimization the proposed method was applied to the trace metal determina-tion in sea and river water. Mahan et al. (1989) used fresh water heat killed lyophilized, blue-green algae strains for the accumulation of heavy metals and reported that the algal strains exhibited relatively high adsorption affinities for Fe, Pb, and Cu. The investigations by Neidhart et al. (1990) on selective uptake of trace metals like dissolved chromates beside Cr (III) from aqueous solutions by human erythrocytes under physiological conditions stated that for sampling, the mechanical stability of the red blood cells is

increased by immobilization in Ca-alginate beads without loss of the biochemical activity against Cr (VI). Radiotracer studies were performed by the group in order to determine the influence of temperature, pH, and concentration and solubility of chromates on the accumulation. Similar stud-ies on various microorganisms including yeast (Maquieira et al., 1994; Madrid et al., 1995; Martin-Esteban et al., 1997; Bag et al., 1998) algae (Darnall et al., 1986; Pappas et al., 1990; Elmahadi and Greenway, 1994; Gunther et al., 2007) and organelles (erythrocytes) (Rohling and Neidhart, 1999) have been carried out by different groups.

Speciation of As has been studied using microbes, namely; Staphylococcus aureus, Bacillus subtilis, Escherichia coli (Tauriainen et al., 1997; Turpeinen et al., 1999; 2002). It is known that inorganic As (V) is subjected to microbial reduction and methylation leading to volatilization as ars-ines (Alexander, 1977; Gao and Barau, 1997). However, the reduction and/or methylation rates of arsenic, which are necessary pre-requisites for production of arsine, vary greatly depending on the properties of the matrix, such as tempera-ture, different species of arsenic, and microbial populations. In the above studies, the microbial transformation rate of water soluble As (V) under both aerobic and anaerobic con-ditions to volatile trimethylarsine form represented 0.02% to 0.3%. Microorganisms such as Gracilaria sp. and Eisenia sp. have been used for separation of gold (Zhao et al., 1994). Investigations by Jiang and Fan (2008) were carried out on the use of sulfate reducing bacteria (SRB) for bioremediation and speciation of cadmium contaminated soil. Their results indicated that the concentration of exchangeable fraction of Cd decreased significantly. The concentrations of the carbon-ate fraction of Cd and organic matter fraction of Cadmium both increased slightly. However, the residue fraction of Cd did not change. Cr (VI) reduction, available Cr and Cr frac-tion in soil were studied by inoculating the soil with Cr (VI) reducing strain, Bacillus sp. XW-4 and incubating at 28°C (XU et al., 2009). The results showed that addition of Bacillus sp. XW-4 could promote Cr (VI) reduction but inoculation of this strain had a negative effect on the decrease of available Cr content in soil.

Biofilms have a promising future for biospeciation of metals (Singh et al., 2006) due to some physico-chemical properties of microorganisms such as biosurfactant produc-tion and chemotaxis. Biofilms are clusters of microbial cells that are attached to a surface. They play a significant role in the remediation of heavy metal and radionuclide. In a study (White and Gadd, 2000), sulphate reducing bacterial bio-films grown in continuous culture and exposed to a medium containing 20 to 200 μM Cu were found to accumulate it in the form of Cu sulfide.

Aller and Castro (2006) have reviewed the application of live bacterial cells as analytical tools for speciation analysis. Bacterial cells act as an efficient extraction system due to their high surface–volume ratios and availability of abun-dant potentially active chemosorption sites in their walls Bacteria have a complex membrane that can act as a selec-tive extractant by mimicking the best extraction conditions.

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310 Neha Singh and Ranu Gadi

Gram-negative bacteria have a cell envelope comprising a cytoplasmic membrane, a peptidoglycan layer, and another membrane with lipopolysaccharides (LPSs) which act as a molecular sieve while the hydrophilic LPS layer forms a bar-rier for lipophilic species (Beveridge and Graham, 1991). The Gram-negative bacterial cell surface can thus host mul-tiple proteins which are functionally and structurally differ-ent (Faraldo-Gomez and Samson, 2003; Rahn et al., 2003; Wernerus and Stahl, 2004). Gram-positive bacteria lack the outer membrane. However, the cytoplasmic (or inner) membrane contains specific carrier proteins, which allow the selective uptake of species and play a crucial role in dif-ferent energy-transducing processes. Surface-display appli-cations of Gram-positive bacteria seem to be very attractive, as they are robust by nature and possess a functionally very active C-terminal anchoring tail (Wernerus and Stahl, 2004).Gram positive bacterial walls also contain an external thick layer of peptidoglycan with two main components: a poly-mer of sugars (N-acetylglucosamine and N-acetylmuramic acid); and, cross links of short peptides. Other cell envelope constituents of Gram-positive microorganisms are teichoic acids, which support a whole negative charge (Wilson et al., 2001) and are based on polymers up to 30 units long of glyc-erol or ribitol joined to phosphate groups. They also tend to bind sugars and amino acids, being covalently linked to the peptidoglycan layer through N-acetymuramic acid and to plasma membrane lipids (lipoteichoic acids) from the cyto-plasmic membrane.

Plants in speciation of heavy metalsNumerous plant species have been identified and tested for their traits in the uptake and accumulation of different heavy metals. The aquatic moss accumulates metals in a time-integrated way that reaches an equilibrium related to the metal concentration in the surrounding water (Bengtsson and Lithner, 1981). The uptake of metals can be divided into two main phases based on the cellular compartment in which the metal accumulates; extracellular followed by intracellular uptake (Brown and Beckett, 1985). Extracellular uptake is dominated by exchange adsorption to the cell walls. This is a rapid uptake (minutes to a few hours) and the metals are readily exchangeable if the concentration levels in the water change. The intracellular uptake is a slow accumulation (hours to several weeks) within the cell, and the metal concentrations in this compartment are not as greatly affected by a change in the surrounding water concentration (Salt et al., 1995).

Retention of particulate metal or precipitation of metal oxides to the surface of the plant may also contribute to the measured concentrations (Figueira and Ribeiro, 2005). If metal concentrations in the water decrease, the moss re-equilibrates to the new levels. This release rate is, however, slower than the uptake rate (Bengtsson and Lithner, 1981). The accumulation in the moss is affected by several factors including temperature, light intensity and physico-chemical water parameters.

Sequestering metal ions using living or dead plants (Eisler, 2003) is a proposed economical means of removing gold and

encouraging speciation via intracellular accumulation or surface adsorption. However, in the case of live plants, this is frequently a relatively slow and time consuming process. Nonliving plant material for surface adsorption offers several advantages over live plants, including reduced cost, greater availability, easier regeneration, and higher metal specificity (Gardea-Torresdey et al., 2000). Bagasse and coconut jute have been used as adsorbents for the removal of chromium (VI) from aqueous waste (Chand et al., 1994). They reported that 97% of Cr (VI) was removed using coconut jute carbon as adsorbent at natural pH, whereas other conventional adsorb-ents showed much lower activities. Dried ground shoots of alfalfa Medicago sativa accumulated gold by the reduction of Au3+ to colloidal Au+ (Gardea-Torresdey et al., 2000).

Effects of Pine (Pinus sylvestris) and liming (pH-change with CaCO

3) on the solubility, mobility and bioavailability

of lead in soil were examined in laboratory microcosms (Turpeinen et al., 2000). The results showed that pine seed-lings had a major role in the immobilization of the lead in the contaminated soil. Cotter-Howells and Capron (1996) found that the roots of Agrostis capillaris, growing in highly contaminated lead wastes caused the formation of lead phosphate. Plants have been used to transform lead metal in the soil to less toxic forms or to reduce mobility (Meagher, 2000; Garbisu and Alkorta, 2001; Van del Lelie et al., 2001). Plants species Brassica junecea (Watanabe, 1997), Vetiveria zizanioides (Chen et al., 2000) have capacity to take up large concentration of lead (Pb2+). During the investigations on the use of Thlaspi caerulescens plant species for the removal of Zn, it was found that the uptake of Zn

3 (PO

4)

2 and ZnSO

4

was 1.5 times more than ZnS when they were present in the same matrix (Lone et al., 2008).

Biopolymers in speciation of heavy metalsA biopolymer is a biodegradable material. These types of materials can be degraded by landfill processes. In recent years, a number of biopolymers have been suggested as potential materials for removal of heavy metals (Crist et al., 1981; Majeti and Kumar, 2000). However, a little work has been done on their use in speciation of heavy metals. A natural biopolymer, Chitosan was evaluated for its capac-ity to remove hexavalent chromium from synthetic as well as field samples obtained from chrome plating facilities. Chitosan coated on alumina exhibited greater adsorption capacity for Cr (VI) (Boddu et al., 2003). It has also been used for the remediation and speciation of As (III) and As (V) at pH 4.0, under equilibrium and dynamic conditions (Boddu et al., 2008). The adsorption capacity of the biopolymer was found as 56.50 and 96.46 mg/g of chitosan for As (III) and As (V), respectively. The adsorption of As (V) on chitosan may be due to ionic attraction which takes place between NH

3+ in chitosan and H

2AsO

4− species of As (V) which exists

along with H3AsO

4 at pH 4.0. Other factors for adsorption

are formation of metal containing nodules on the surface and absorption. In case of As (III), its species H

3AsO

3 exists

uncharged at pH 4 therefore it cannot undergo ionic attrac-tion with the adsorbent. However such species can interact

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Different Approaches to Biological methods for speciation of heavy metals 311

with the unprotonated amino groups and hydroxyl groups present in the chitosan. Therefore, higher adsorption of As (V) than As (III) may be attributed to the stronger interaction of As (V) with the adsorbent through electrostatic forces of attraction (Boddu et al., 2008).

Conclusion

Biological methods are reliable, cost effective, efficient, and environment-friendly for the speciation of heavy met-als. These methods can be of high application in probing the distribution, mobility and toxicity of metals in different environmental compartments. Different Microorganisms such as fungi, yeast, algae, bacteria, and organelles have been employed in speciation analysis of heavy metals. Several plants (aquatic as well as nonaquatic) accumulate heavy metals through extracellular and intracellular uptake. Preferential accumulation of a particular species of any metal on extracellular or intracellular sites can help in speciation of metals. Bioploymers either in natural or in modified form can be used for speciation of metals under specific condi-tions. However, the application of biological methods needs integration of information from all disciplines and involve-ment of scientists, engineers, and agriculturists.

Acknowledgements

The authors wish to express their sincere thanks to the refe-rees for their valuable comments. Authors are grateful to Prof D.K. Bandyopadhyay, Vice Chancellor, GGS Indraprastha University and Prof. Ashwini Kumar, Principal, IGIT and Dean, University School of Engineering and Technology, GGS Indraprastha University for their encouragement and support.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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