Bioremediation of Heavy Metals

Bioremediation of Heavy Metals

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819 Bioremediation of Heavy Metals RENITTA JOBBY AND NEETIN DESAI*

ABSTRACT

Today, man’s greatest challenge is to cope up with metal pollution problem because unlike organic compounds which are degraded naturally, heavy metals cannot be degraded, hence get accumulated at different useful sites. With the growing environmental awareness emphasis is more on the development of environment friendly ways for decontamination procedures. Bioremediation is a good alternative to conventional clean up technologies. Use of microbial systems for metal bioremediation is most preferred because of its low cost and low waste generating techniques. Despite the increased metal toxicity microbes have acquired variety of mechanisms to adapt themselves to these toxic heavy metals, leading to removal, immobilization or detoxification of heavy metals. The present chapter summarises the problems and biotechnological solutions of heavy metal contamination. Key words: of heavy metals. The present chapter summarises the problems and biotechnological solutions of heavy metal contamination. 1. HEAVY METALS

The term ‘Heavy metals’ is a collective name for metallic elements with a density above 5 g/cm3. Kennish (1992) classified heavy metals as elements having atomic weights usually greater than 50. Hence in the periodic table the transition elements from V (but not Sc and Ti) to the half-metal As, from Zr (but not Y) to Sb, from La to Po, the lanthanides and the actinides can be referred to as heavy metals (Nies, 1999). Of the 90 naturally occurring elements, 21 are non-metals, 16 are light metals and the remaining 53 (with as included) are heavy metals (West, 1984). Amity Institute of Biotechnology, Amity University Mumbai, Bhatan, Panvel, Mumbai410206. *Corresponding author: E-mail: [email protected]

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Environmental Sci. & Eng. Vol. 8: Biodegradation and Bioremediation

2. OCCURRENCE OF HEAVY METALS IN THE ENVIRONMENT

Heavy metals naturally occur in the environment in form of chemical and physical weathering caused due to igneous and metamorphic rocks. In addition, decomposition of plant and animal waste matter, precipitation or atmospheric deposition of airborne particles from volcanic activity, wind erosion, forest fire smoke, plant exudates and oceanic spray also contribute heavy metals in the environment (Kennish, 1992). But in the recent years due to rapid industrialization around the world, anthropogenic activities have caused an increase in heavy metal concentration in the environment. Any kind of industrial activity can alter the mobilization and distribution of heavy metals in the environment (Gadd, 2009). Heavy metals can be emitted into the environment in many ways; to the air (e.g., during combustion, extraction and processing), to surface waters (via. runoff and releases from storage and transport) and to the soil. Roadways and automobiles have also become one of the major contributors of metals to the environment (Sahni, 2011). Table 1 lists out the anthropogenic sources of metal contamination. Table 1: Sources of some of the most toxic heavy metals Metal

Industry

Chromium (Cr)

Mining, industrial coolants, Cr salts manufacturing, leather tanning, road runoff.

Lead (Pb)

Mineing, smelting, Lead acid batteries, paints, E-waste, ceramics, bangle industry, road runoff.

Mercury (Hg)

Chlor-alkali plants, thermal power plants, fluorescent lamps, dental amalgams, hospital waste (damaged thermometers, barometers, sphygmomanometers), electrical appliances, etc.

Arsenic (As)

Smelting operations, thermal power plants, fuel burning, arsenical pesticides and wood preservatives.

Copper (Cu)

Mining, electroplating, smelting operations, road runoff.

Nickel (Ni)

Smelting operations,metal plating, combustion of fossil fueals, electroplating, thermal power plants, battery industry, road runoff.

Cadmium (Cd)

Ni/Cd batteries, e-waste, paint sludge, incinerations and fuel combustion, road runoff.

Zinc (Zn)

Smelting, electroplating, road runoff.

3. ADVERSE EFFECTS OF HEAVY METALS

Although the adverse effects of heavy metals are well known, the irony of the use of heavy metals is still wide spread. Three kinds of heavy metals are of concern, including toxic metals (such as Hg, Cr, Pb, Zn, Cu, Ni, Cd, As, Co, Sn, etc.), precious metals (such as Pd, Pt, Ag, Au, Ru, etc.) and radionuclides (such as U, Th, Ra, Am, etc.) (Wang, 2006, 2009). Some metals are essentials (like Cu, Co, Mg, Ni, Zn, etc.) while others are non essential (like Ag, Cd, Pb, As, Au, etc.). Heavy metal contamination of soil is a matter


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of concern and is hazardous to humans and the ecosystem. Contaminated soil can be directly ingested through the food chain and drinking of contaminated groundwater. Metals cause phytotoxicity to plants leading to reduction in food quality, reduction in land usability for agricultural production causing food insecurity, and land tenure problems (Mc Laughlin et al., 2000a,b, Ling et al., 2007). General symptoms of heavy metal toxicity include: headache, short-term memory loss, mental confusion, sense of unreality, distorted perception, pain in muscles and joints, gastro-intestinal upsets, food intolerances, allergies, vision problems, chronic fatigue, fungal infections, etc. Sometimes the symptoms are vague and difficult to diagnose (Sahni, 2011). The Minamata disease is a best example of the devastating effects of Hg. It has been more than 50 years since the first signs of mass Hg poisoning emerged caused by the release of methylmercury in the industrial wastewater from the Chisso Corporation’s chemical factory. Many uncertified victims are still fighting for recognition, whereas the affected ones who are now in their forties and fifties have deteriorating health and no source of care (Blacksmith Institute). 4. HEAVY METAL CONTAMINATION

The environmental quality decides the overall nature of the earth (Garbisu abd Alkorta, 2003). Today man’s greatest challenge is to cope up with the increasing industrialization and its pollution problems (Thassitou and Arvanitoyannis, 2001). Heavy metal pollution is a global issue, although severity and levels of pollution differ from place to place.

4.1. National Status India has been cited as the second most polluted country in the world. Soil contamination is a common problem in many urban and dense cities with significant industrial waste generation. The soil study data of the industrial area of Surat city done by Krishna and Govil (2005) revealed significant contamination with high concentrations of heavy elements like Ba, Cu, Cr, Co, Ni, Sr, V and Zn (http://www.sacriver.org/aboutwatershed/mercury/ california-aspects). In 1984, arsenic contamination was first reported in the Ganga plain, West Bengal, India (Garai et al., 1984). Since then, a number of other states in India like Jharkhand, Bihar, Uttar Pradesh, Assam, Manipur and Chhattisgarh are affected by arsenic contamination and thousands are suffering from arsenic toxicity and millions are at risk. Most of the water resources in West Bengal have arsenic levels higher than that stated by WHO guidelines (10µg/L) (Chakraborti et al., 2004). Table 2 presents a list of some of the heavy metal contaminated sites in India, and its sources. These sites have been tagged by Blacksmith institute for immediate remediation.

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Table 2: Heavy metal contaminated sites in India Location

Pollutant

Aruputo, Kolkata Tangra, Kolkata Tiljala, Kolkata

Cr VI Cr VI Cr VI

Kanpur, UP Ranipet, TN Sukinda, OR Angul Talcher, OR Baroda, GJ Plachimada,KL Ratlam, MP

Cr VI Cr VI Cr VI Cr VI Cr VI Pb, Cd Hg, Pb

Singrauli, MP

Source Product manufacturing Tannery Lead smelting, Battery recycling Tannery operations Tannery operations Mining Chemical manufacturing Chemical manufacturing General industry Pharmaceuticals, General industry, Dye industry Power plant

Transmission Affected people Water, soil 1000 Water 7000 Water, soil, air 200,000 Water Water Water Water, soil, air Water, soil, air Water, soil Water, soil, air

30,000 3,482,000 2.6 million 1140000 1,200,000 4000 300,000

Hg, Water, soil, air 185,500 other HM Parwanoo, HP HM General industry Water, soil, air 8,609 Tarapur, MH HM Chemical manufacturing Water, soil, air 1,114,539 in Thane Dist. Amlakhadi River, GJ HM General industry Water, soil 100,000 Jharkand Cr, HM Mining Water, soil, air 50 tribal villages Tuticorin, TN As Smelting Water, soil, air 222,603 Symbols in table represent: TN-Tamil Nadu, Gj-Gujrat, OR Orissa, KL-Kerala, HPHimachal Pradesh, MH-Maharashtra, UP-Uttar Pradesh, MP-Madhya Pradesh, HM-Heavy metals

The rapid urbanization in Mumbai has brought along numerous industries. The small and medium scale industries (SSI and MSI) in India are considered to be the most careless and worst waste generators. Most SSI in India discharges their effluent directly into nearby depressions, rivers, lakes and even in the open fields. The industrial estates in Thane-Belapur of Maharashtra, Pattancheru in Andra Pradesh, Vapi, Ankleshwar, and Nandseri in Gujarat have a large number of SSI snd MSI. The Thane Belapur industrial area is found to be one of the most contaminated sites identified by Central Pollution Control Board, New Delhi (James and Klein, 2008). Krishna and Govil (2005) have reported the accumulation of Cu, Cr, Co, Ni, and Zn in the soils of the Thane-Belapur belt. Similar heavy metal contamination is also seen for the various water bodies located in the vicinity of the Thane-Belapur industrial area. Sediments of Ulhas river have registered high levels of Hg and As (Hosetti, 2006). 5. HEAVY METAL REMEDIATION

5.1. Physical and Chemical Methods The commonly used procedures for removing metal ions from aqueous


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streams include chemical precipitation, lime coagulation, ion exchange, reverse osmosis, and solvent extraction (Rich and Cherry, 1987). The disadvantages of incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require careful disposal together with the need for more economical and effective methods for the recovery of metals from wastewaters, have resulted in the development of alternative separation technologies (Volesky and Naja, 2007). The development and implementation of cost-effective process for removal/ recovery of metals is essential to improve the competitiveness of industrial processing operations. In recent years, there has been a trend toward the implementation of passive treatment schemes. These take advantage of naturally occurring geochemical and biological processes to improve water quality with minimal operation and maintenance requirements. Biological removal includes the use of microorganisms (fungi, algae, and bacteria), plants (live or dead) and biopolymers and may provide suitable means for heavy metal treatment from wastewater (Macek, 2011).

5.2. Bioremediation Bioremediation includes all those processes and actions that take place in order to return the natural environment altered by contaminants to its original condition (Garbisu and Alkorta, 2003). It primarily uses microorganisms, fungi, green plants or their enzymes to degrade and transform environmental contaminants into harmless or less toxic forms. It uses relatively low-cost, low-technology techniques, which generally have a high public acceptance (Vidali, 2001).

5.3. Metal Microbe Interactions Microbial life has conquered extremely hostile environments (Haferburg and Kothe, 2007). Metals play a role in all aspects of microbial growth, metabolism and differentiation (Gadd, 1992a). Despite the increased toxicity to microbes posed by the increased contamination microbes have acquired a variety of mechanisms for adaptations to the presence of toxic heavy metals (Gadd, 2009a). Microbial resistance has been identified in polluted as well as non polluted environments (Silver and Phung, 2009). The interactions between metals and microbes leading to the removal of metals from solution is based on mechanisms ranging from purely physicochemical interactions like adsorption to cell wall components to those that depend on cellular metabolism such as intracellular compartmentalization and extracellular precipitation (Bhagat et al., 1993; Taniguchi et al., 2000). The former can occur in live and dead cells. It is a metabolism independent rapid process, while the latter one involves metabolism dependent slower transport across the cell membrane.

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Fig. 1: The two systems used for bioremediation of heavy metals

Certain microbial activities result in metal solubilization, whereas others immobilize them and reduce their environmental mobility (Barkay and Schaefer, 2001; Gadd, 2009, 2010). Microorganisms can mobilize metals through autotrophic and heterotrophic leaching, chelation by microbial metabolites, siderophores and methylation, which can result in volatilization (Gadd, 2004). A number of processes can immobilize metals, like sorption to cell components or exopolymers, transport and intracellular sequestration, or precipitation as for example, oxalates, sulfides, or phosphates. Apart from this microbially mediated oxidation-reduction reactions may result in either mobilization, for example, Mn (IV) to Mn (II) and Fe (III) to Fe (II) or immobilization, for example, Cr (VI) to Cr (III) and U (VI) to U (IV) (Lovely and Coates, 1997; Lovely et al., 2003). The various mechanisms of heavy metal resistance by microbes can be summarized as follows: (1) Metals can be excreted via efflux transport systems[2]. Sequestering compounds of the cytosol can bind and detoxify metals inside the cell[3]. The release of chelators into the extracellular milieu leads to bound and fixed metals[4]. Reduction of heavy metals to less toxic state[5]. The structure of the cell envelope is prone to bind large amounts of metals by sorption thus preventing influx (Haferburg and Kothe, 2007; Nies, 1999).


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Fig. 2: The various mechanisms of heavy metal resistance by microbes

These mechanisms may depend on genetic as well as physiological adaptations (Rani et al., 2008). Silver and Misra (1988) reported that metal resistance in most bacteria studied is often a plasmid mediated trait. Bacterial plasmids have been identified bearing resistance genes to many toxic metals and metalloids e.g., Hg, As, Pb, Cd, Co, Cr, Cu, Te, Zn, Ni and Sb.

5.4. Genetic Basis of Heavy Metal Resistance 5.4.1. Arsenic Arsenic is a heavy metalloid which sometimes acts as a metal and sometimes not. In the inorganic form, it mainly exists as arsenate, the oxidized form (AsO43– or HAsO42) and as arsenite, in the reduced form (AsO2– or H2AsO3 – ), both being toxic for cells, but arsenite is 100 times more toxic than arsenate (Misra, 1992). It is a toxic analogue for inorganic phosphorous and enters the cell by two types of phosphate transport system viz. pit (non specific transport) and pst (specific transport) transport systems (Bruins, 2000; Silver and Walderhaug, 1992). Some microorganisms have acquired genes that permit neutralizing the toxic effects of arsenic through the exclusion of arsenic from the cell. Chromosomal and plasmid-based arsenic resistance genes are clustered in an operon, commonly known as the ars operon (Nies, 1999). The first step for arsenate detoxification involves differentiating it from phosphate, by reduction of arsenate to arsenite (Ji and Silver, 1992; Ji et al., 1994). The arsC gene product is the cytoplasmic arsenate reductase enzyme for reduction of arsenate to arsenite (Ji et al., 1994, Oden, et al., 1994). Arsenite then leaves the bacterial cell. Since anion export from bacterial cells is

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always driven by the chemiosmotic gradient, simple arsenite efflux systems are composed of just one efflux protein, the arsB gene product (Wu, 1992). In addition to the efflux only mediated by ArsB, arsenite transporters exist that are composed of an ArsB pore plus an ArsA ATPase. The ArsA protein is a arsenite-stimulated ATPase that couples ATP hydrolysis to the arsenite efflux through the ArsB protein (Cervantes, 1994, Silver et al.1993, Silver and Phung, 1996). 5.4.2. Cadmium Cd is the best known toxic heavy metal (West, 1984)Error! Reference source not found). Although a lot of work has been done on Cd toxicity, no clear mechanism of action has been highlighted. Cd can enter via. The magnesium system (R. eutropha CH34 and S.cerevisiae) or via. manganese uptake systems (Liu et al., 1997; Tynecka and Malm, 1995). Resistance to Cd in bacteria is based on Cd efflux. In gram-positive bacteria, S. aureus a CadA pump was found to export Cd working as a P-type ATPase (Nucifora et al., 1989; Silver et al., 1989). Cd resistance in other gram positive bacteria was also found to be mediated by CadA-like proteins (Liu et al., 1997). On the contrary cadium detoxification in gram negative bacteria seems to be by RND-driven systems like Czc, which is mainly a zinc exporter (Nies, 1995) and Ncc, which is mainly a Ni exporter (Schmidt, 1994). Cyanobacteria, however, contain metallothioneins. Amplification of the smt metallothionein locus increases Cd resistance and deletion of it decreases resistance (Gupta et al., 1993, Turner et al., 1995). Since cyanobacteria contain a variety of RNA and P-type transport systems, transport may also be important for Cd resistance in these bacteria. 5.4.3. Chromium Cr exists in several oxidation states ranging from Cr (II) to Cr (VI). The hexavalent Cr (VI) and the trivalent Cr (III) forms are the most stable and abundant (Cervantes et al., 2001). Oxidation-reduction reactions between the two states are thermodynamically possible under physiological conditions (West, 1984). Cr (VI) is more toxic than Cr (III). In microorganisms, no beneficial role of Cr was found. Chromate enters the cell by means of the sulphate uptake pathway (Nies and Silver, 1995). Chromate resistance is probably based on an interaction of chromate reduction and chromate efflux (Peitzsch et al., 1998; Cervantes and Silver, 1992). This resistance can be mediated by plasmids or chromosomal genes (Nies et al.,1998), Cervantes and Campos-Garcia, 2007). At the molecular level the chr operon mediates resistance against toxic oxyanion chromate (Nies and Silver, 1990). The chromate is effluxed via the ChrAB proteins (Nies et al., 1998). ChrA functions as a chemiosmotic pump that effluxes chromate from the cytoplasm using


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the proton motive force (Alvarez, 1999; Pimentel, 2002). While the chrB gene has a regulatory function and is responsible for inducibilty of resistance (Mergeay, 1991; Silver, 1996). 5.4.4. Copper Cu plays an important role in the active site of various enzymes, including terminal oxidases, monooxygenases, and dioxygenases and is also required for the transport of electrons in several photosynthetic and respiratory pathways. Besides being essential, Cu can also be toxic at increased concentration (Lee et al., 2002). Cu ions can catalyse harmful redox reactions resulting in oxidation of lipid membranes, damage to nucleic acids and generation of free radicals from hydrogen peroxide (Lippert, 1992), Dameron and Harrison, 1998). Generally microorganisms have evolved effective and accurate mechanisms to maintain Cu homeostasis which include Cu sequestration, uptake and efflux. The genes involved in the resistance may be located on plasmids as well as chromosomes (Dameron and Harrison, 1998). The genetic determinant cop from Peudomonas syringae and pco from E. coli show strong similarilites to each other. But they seem to mediate resistance mechanisms that are apparently opposite to each other, wherein uptake and sequestration occurs in P. syringae and reduced uptake and cellular exclusion takes place in E. coli. The system consist of four structural genes pcoA, pcoB, pcoC and pcoD in E. coli involved in efflux mechanism to remove Cu outside the cell, and copA, copB, copC and copD involved in accumulation and compartmentalization of Cu in the periplasm and outer membrane of cells. The P.syringae and E. coli systems both have paired regulatory genes, with a membrane bound Cu2+ sensor (pcoS and copS gene products) coupled with the pcoR and copR gene products that may be DNA binding repressor protein (Silver et al., 1993; Cooksey, 1993; Brown et al., 1994). The pco genes are in turn dependent on the expression of chromosomal cut genes (cutC and cutF) which encode a Cu binding protein and an outer membrane lipoprotein (Gupta et al., 1995). 5.4.5. Mercury Hg ions have no beneficial role in the living system. They are highly toxic due to their affinity for thiol groups. The genes conferring resistance to Hg are clustered in mer operon and are widely distributed in Hg resistant bacterial population (Osborn et al., 1997; Barkay et al., 2003). Resistance to Hg is based on its unique peculiarities: its redox potential, whereby the reduction of Hg (II) to volatile Hg (0) takes place (Mishra, 1992). In the first step of detoxification in gram negative bacteria, the Hg2+ first binds to periplasmic Hg2+ binding protein MerP and then is transported into the

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cytoplasm by MerT (Qian et al., 1998, Hobman and Brown, 1996). Once inside the cell, Hg2+ is reduced with NADPH to Hg(0) by the MerA protein, which is related to glutathione reductase and other proteins (Schiering et al., 1991). Resistance to organomercurials like methylmercury, phenylmercury, etc. is conferred if the mer determinant encodes a MerB organomercurial lyase in addition to the other Mer proteins. After cleavage by MerB, the resulting Hg2+ is reduced by MerA (Silver and Pung, 1996; Silver, 1996). 5.4.6. Nickel In many bacteria Ni is required for enzymes such as urease, dehydrogenase and hydrogenase, but excess of Ni can be toxic as Ni binds to proteins and nucleic acids and frequently inhibits enzymatic activity, DNA replication, transcription and translation (Maier et al., 1993; Valiet et al., 2001). Free Ni occurs mostly in the Ni2+ cationic form. Ni resistance is based on Ni efflux driven by a RND transporter in most bacteria. Two systems have been described; a Ni/Co resistance cnr and a Ni/Co/ Cd resistance ncc (Liesegang, 1993; Schmidt and Schlegel, 1994; Jobby et al., 2015). Both are closely related to the Co/Zn/Cd resistance system czc from strain CH34. Recently, a P-type ATPase (ATPase 439) was described, which binds Ni2+, Cu2+ and Co2+ to its amino terminus (Melchers et al., 1998). This may be the first example of a Ni P-type ATPase in bacteria.

5.5. Biosorption—A Suitable Approach for Heavy Metal Removal Microbial biomass provides a metal sink, by biosorption to cell walls, pigments and extracellular polysaccharides, intracellular accumulation, or precipitation of metal compounds in and around cells (Gadd, 2000 a,b, 2001 a,b, 2007; Baldrian, 2003; Fomina et al., 2007a,b). Algae, bacteria fungi and yeasts have proved to be potential metal biosorbents (Voleskey, 1986). The major advantages of biosorption over conventional treatment methods include: low cost, high efficiency, minimisation of chemical and biological sludge, no additional nutrient requirement, regeneration of biosorbent, and possibility of metal recovery (Kratochvil and Volesky, 1998). The metals from aqueous solution can be removed by passive and/or active forms (Aksu, 2001). Live as well as dead cells can interact with metallic species. Metal removal can occur by either bioaccumulation or biosorption. Bioaccumulation is an active process involving microbial metabolism to capture ionic species. Metal resistance is usually exhibited by the organism in active process when they are in high concentrations and not part of the nutrition (Zouboulis, 2004; Godlewska-Zylkiewicz, 2006).


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Biosorption is non metabolic passive process for metal removal by live and dead biomasses. The metal linkage is based on the chemical properties of the cellular envelope without the requirement of biological activity (Gadd, 2009; Volesky, 2001). The process occurs through interaction among the metals and some active sites (e.g., carboxylate, amine, sulfate, etc.) on cellular envelope. The behaviour of biosorbent towards metallic ions is a function of the chemical makeup of the microbial cells (Volesky and Holan, 1995). Mechanism of biosorption is not well understood. It can involve a combination of ion exchange, complexation, coordination, adsorption, electrostatic interaction, chelation and microprecipitation (Wang and Chen, 2006; Vijayaraghavan and Yun, 2008). Since the main component of biosorption is the cell wall it is important understand the structure and function of the microbial cell. Two different types of microbial cells exist-prokaryotic and eukaryotic. Prokaryotic cells are much simpler and smaller in structure in comparison to the complex eukaryotic cells. The key difference between eukaryotic and prokaryotic cells is that eukaryotes contain true nuclei. Algae, fungi, protozoa, higher plants and animals form eukaryotes while prokaryotes are represented by bacteria and archaea (Wang and Chen, 2009; Prescott et al., 2002). 5.5.1. Microbial biosorbents Different kinds of materials have been studied as biosorbents for metal removal. These include microbes like bacteria (e.g., Bacillus subtilis), fungi (e.g., Rhizopus arrhizus), yeast (e.g., S. cerevisiae), algae, to waste materials like industrial wastes (e.g., S. cerevisiae waste biomass from fermentation and food industry), agricultural wastes (e.g., corn core) and other polysaccharide materials, etc. (Vijayaraghavan and Yun, 2008). Among microorganisms, fungal biomass offers the advantage of having a high percentage of cell wall material which shows excellent metal-binding properties (Gadd, 1990; Paknikar, 1993). Bacterial biomass is found to be more useful and effective in remediation of contaminated sites because of the advantage of low operating cost, minimization of volume of chemical or biological sludge to be disposed and high efficiency in detoxifying very dilute effluent. These advantages have served as the primary incentives for developing full-scale bioremediation process in minimizing heavy metal pollution (Shukla et al., 2006). Numerous studies have identified a number of potential bacterial species capable of accumulating metals. Bacillus sp. has been identified as having high potential for metal sequestration and has been used in commercial biosorbent preparation (Brierley, 1986). There are also reports on biosorption of metals using Pseudomonas sp., Zoogloea ramigera, and streptomyces sp. (Nilanjana

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et al., 2008). Ilhan et al. (2004) reported the removal of Cr, Pb and Cu ions from industrial wastewaters by Staphylococcus saprophyticus. The nitrogen fixing Sinorhizobium sp. was also found to have good potential for bioremediation of nickel, copper and chromium (Jobby et al., 2015; Jobby et al., unpublished data). Bioremediation has also been studied by using sulphate reducing bacteria for Cd accumulation (Rani et al., 2008). Bader et al., studied the potential of aerobic reduction of Cr (VI) by an indigenous soil microbial community and found that Cr (VI) in the soil was reduced by 33% within 21 days (Bader et al., 1999). Jeyasingh and Ligy Philip showed that bacterial strains isolated from the contaminated sites of Tamil Nadu Chromates and Chemicals Limited (TCCL) premises were capable to clean up the Cr contaminated site at TCCL, Ranipet Tamil Nadu, India (Jeyasingh and Philip, 2005). Another report states that different species of Aspergillus, Pseudomonas, Sporophyticus, Bacillus, Phanerochaete, etc. are efficient Cr and Ni reducers (Yan and Viraraghavan, 2003; Gopalan, 1994; Bafana, 2010). Bafana et al. (2010) isolated the multiple metal resistant strain Arthrobacter ramosus from mercuric salt contaminated soil. This strain was found to resist and bioaccumulate several metals, such as Cd, Co, Zn, Cr and Hg. 5.5.2. Commercial microbial biosorbents Several attempts of commercialization of biosorbents have been done. The initial attempts of patenting microbial biosorbents began in 1980’s (Tsezos, 2001). Further in 1990s commercialization began using other biomaterials like AlgaSORB™ (C. vulgaris), AMT-BIOCLAIM (Bacillus biomass) (MRA), Bio-fix, etc., prepared by immobilization technology (Volesky, 1990; Veglio, 1997; Garnham, 1997). B.V. SORBEX Inc. in Montreal, Canada, have produced a series of biosorbents based on different types of biomaterial, including the algae S. natans, A. nodosum, H. opuntia, P. pamata, C. crispus and C. vulgaris (Volesky 1990). Though many attempts for commercialization have been tried they have failed to obtain successful commercial application (Tsezos, 2001; Garnham, 1997).

5.6. Microbial Transformation of Metals Apart from biosorption, microbes can also transform metals and metalloid by redox reactions and methylation (Lovley, 2000). These reactions can bring change in the mobility of the metals and also decrease their toxicity offering bioremediation applications (Lovley, 2001; Finneran et al., 2002). 5.6.1. Metal precipitation Such enzymatic transformations can result in metals with decreased toxicity


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and mobility. Many organisms are known to aerobically or anaerobically reduce Cr(VI) to Cr(III) (Smith and Gadd, 2000; McLean and Beveridge, 2001). Reduction of U(VI) to U(IV) causes reduction in solubility and formation of precipitates at neutral pH. This forms the basis of U removal from contaminated water, leachates and soils (Lovley, 2001; Finneran et al., 2002). SRBs are the most studied organisms in reductive precipitation of toxic metals like U (VI), Cr(VI), Tc(VII) and Pd(II) (Lloyd and Renshaw, 2005). These processes are mediated by membrane bound enzymes that are either c type cytochromes or associated with such cytochromes (Macy et al., 2000). As(V) is more readily adsorbed and less toxic than As(III). The oxidation of As (III) to As(V) from contaminated sites.can be helpful in arsenic removal. In nature this oxidation is microbially driven, since chemical oxidation is slower. Most of the arsenite oxidising bacteria are found to be heterotrophic e.g. Alcaligens faecalis (Langner et al., 2001). But a few aerobic chemolithoautotrphic (Santini et al., 2000) microbes have also been found. Some of these strains exhibit combined tolerance to arsenic as well as other heavy metals like cadmium and hence have good potential for arsenic removal from heavily contaminated sites (Weeger et al., 1999). Such processes can also occur as a consequence of other metabolic reactions, g., sulphate reducing bacteria bring about reduction of Cr(VI) to insoluble Cr(III) due to indirect reduction of Fe 2+ and sulphide (Lloyd et al., 2000). Many bacteria, fungi, algae and yeast have also been isolated which can reduce Au(III) to elemental Au(0) and Ag+ to elemental Ag0 (Holden and Adams, 2003; Southam et al., 2009). Se(VI) can be reduced to elemental insoluble Se(0) by dissimilatory reduction and this process can be used for remediation in contaminated waters and soils (Gadd, 2004). Microbes can also mediate formation of several inorganic and organic biominerals, e.g., oxalates, phosphates, sulfides, oxides and carbonates, which lead to metal immobilization (Gadd, 2007). 5.6.2. Decreased metal toxicity Enzymatic transformations generating less toxic metals species have also gained importance for application in bioremediation. Mercury and arsenic are the best studied examples for the same (Valls et al., 2002). Bacterial mercury resistance is associated with mercuric reductase an enzyme which reduces Hg2+ to volatile and less toxic Hg0 (Mishra, 1992). This reductase activity mobilizes mercury thus aiding its removal to the atmosphere. This is an alternative to metal immobilization strategy (Valls, 2002). Many naturally mercury tolerant isolates have been studied for the same. P. putida was one such strain which was found to efficiently remove 90% of mercury from 40 mg-L of solution in 24 h (Okino et al., 2000). Mer A reductase engineered strains of E. coli and Deinococcus radiodurans were

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