Chromium toxicity and bioaccumulation by chromium tolerant bacillus and arthrobacter species 2

Chapter II: Literature Review CHAPTER II LITERATURE REVIEW 2.1 Heavy Metals: A broad definition Duffus (2002) described the term ‗heavy metals‘ as meaningless and misleading in an IUPAC technical report, due to contradictory definitions and its lack of coherent scientific basis. He collated different terminology for heavy metal in terms of specific gravity, relative atomic mass, atomic number, chemical properties or toxicity from several publications. According to him, heavy metal has never been defined by any authoritative body such as IUPAC and has been given a wide range of meaning by different authors. On the basis of specific gravity, metals with density above 5 gm/cm3 are classified as heavy metals (Streit 1994). On the basis of relative atomic mass, some define it as a metal of atomic weight above sodium (Lyman 1995) while others extend this definition as those metals above sodium but forms soaps on reaction with fatty acids. US EPA (2000) defines it as metals with high atomic weight that can damage living things at low concentrations and tend to accumulate in food chain. On the basis of atomic number, metals above calcium (Venugopal and Luckey 1975; Phipps 1981) or metals between scandium (atomic number 21) and uranium (atomic number 92) (Lyman 1995) are considered as heavy metals. On the basis of other chemical properties, any metal that reacts readily with dithizone (C6H5N) or with fatty acids to form soaps is called as heavy metal. Hence, the definition of heavy metal is rather non-coherent scientifically. In the context of this study, the focus is on the toxicity or ecotoxicity associated with heavy metals. 5 Chapter II: Literature Review 2.2 Chromium: A heavy metal In 1797, the French chemist, Louis Nicolas Vauquelin discovered chromium in the mineral crocoites (lead chromate). In Greek, chroma means ―color‖ and hence it was named chromium because of the many different colors found in its compounds (Mohan and Pittman, 2006). Chromium stands as earth‘s 21st most abundant element with an average concentration of 100 ppm and the sixth most abundant transition metal (Mohan and Pittman, 2006). Ferric chromite, FeCr2O4, is the principal chromium ore found mainly in South Africa (with 96% of the world‘s reserves), Russia and the Philippines (Mohan and Pittman, 2006). Less common sources include crocoites, PbCrO4, and chrome ochre, Cr2O3. The gemstones emerald and ruby owe their colors to traces of chromium. Figure 2.1 shows the native form of chromium with a silvery metallic appearance. Figure 2.1: Chromium (native form) appearance: silvery metallic The concentrations range in soil is between 1 and 3000 mg/kg, in sea water is between 5 to 800 µg/L, and in rivers and lakes is between 26 µg/L to 5.2 mg/L (Kotas and Stasicka 2000). Chromium occurs in 2+, 3+ and 6+ oxidation states but Cr2+ is unstable and very little is known about its hydrolysis. The hydrolysis of Cr (III) is complicated. It produces 6 Chapter II: Literature Review mononuclear species CrOH2+, Cr (OH)2+, Cr(OH)4−, neutral species Cr(OH)30 and polynuclear species Cr2(OH)2 and Cr3(OH)45+ (Ravodic et. al., 2000; Mohan et. al., 2005; Mohan et. al., 2006). The hydrolysis of Cr6+ produces only neutral and anionic species, predominately CrO42−, HCrO42−, Cr2O72− (Mohan et. al., 2005; Mohan et. al., 2006). Cr2O72− predominates at low pH and high chromium concentrations, while Cr (IV) exists in the form of CrO42− at a pH greater than 6.5 (Mohan et. al., 2005). Cr (III) forms relatively strong complexes with oxygen and donor ligands. It is classified as a hard acid. Due to high water solubility and mobility, chromium (VI) compounds are more toxic than Cr (III). On the other hand, trivalent chromium is insoluble and thus immobile under ambient conditions. Chromate and dichromate are the most soluble, mobile and toxic forms of hexavalent chromium in soils. Under aerobic conditions, the hexavalent form is rapidly reduced to trivalent chromium. The speciation of Cr (VI) that exists in solution is influenced by the pH of the environment, as shown in Figure 2.2 (Aoyama et. al., 2007). Figure 2.2: Speciation for Cr (VI) complexes in an aqueous solution of Cr(VI) The relation between Cr (III) and Cr (VI) strongly depends on pH and oxidative properties of the location, but in most cases, Cr (III) is the dominating species (Kotas and Stasicka, 2000). 7 Chapter II: Literature Review 2.2.1 Chromium in biological systems and industrial applications Chromium (III) is an essential element needed in trace concentration in mammalian metabolism. In addition to insulin, it is responsible for reducing blood glucose levels, and is used to control certain cases of diabetes (Mohan and Pittman, 2006). It has also been found to reduce blood cholesterol levels by diminishing the concentration of (bad) low density lipoproteins ―LDLs‖ in the blood. Chromium (III) deficiency results in insulin resistance. According to Mertz (1993), who has listed the importance of Cr (III) in human nutrition, Cr (III) deficiency is a risk factor for a spectrum of disturbances that are almost identical to those of Syndrome X (Reaven 1992). Marginal chromium deficiency may increase the risk for diabetes (Mertz 1969) and, possibly, coronary heart disease (Schroeder 1968). Cr (III) is supplied in a variety of foods such as brewer‘s yeast, liver, cheese, whole grain breads and cereals, and broccoli. Chromium is claimed to aid in muscle development. In fact, dietary supplements containing chromium picolinate (its most soluble form) are very popular with body builders. The Food and Nutrition Board of the U.S. National Research Council suggested a range of safe and adequate intake for chromium of 1 to 4 μmol/d (Food and Nutrition Board 1989). Studies of World Health Organization Expert Committee (WHO Expert Committee 1973) and of the International Program on Chemical Safety (IPCS International Program on Chemical Safety 1988) described trivalent chromium as an essential nutrient with typical intakes of from 1-4 μmol/d (Mertz 1993). In contrast, Cr (VI) is hazardous by all exposure routes. Chromium, being a corrosion resistant metal, finds important usage in a range of applications such as electroplating, metal finishing, magnetic tapes, pigments, leather tanning, wood protection, chemical manufacturing, brass, electrical and electronic 8 Chapter II: Literature Review equipment and catalysis (Kimbrough et. al., 1999). Chromium can be polished to form very shiny surface and is often plated to other material to form a protective and attractive covering. It is added to steel to harden it and to form stainless steel, a steel alloy that contains at least 10% chromium. Other chromium-steel alloys are used to make armor plate, safes, ball bearings and cutting tools. Chromium compounds find specific applications at different sectors. Lead chromate (PbCrO4), also known as chrome yellow, has been used as a yellow pigment in paints. Chromic oxide (Cr2O3) is widely used as green pigment. Rubies and emeralds also owe their colors to chromium compounds. Potassium dichromate (K2Cr2O7) is used in the tanning of leather while other chromium compounds are used as mordants, materials which permanently fix dyes to fabrics. Chromium compounds are also used to anodize aluminum, a process which coats aluminum with a thick, protective layer of oxide. Chromite, chromium's primary ore, is used to make molds for the firing of bricks because of its high melting point, moderate thermal expansion and stable crystal structure. 2.2.2 Problems / Health Impacts associated with the use of Chromium The major environmental issue with chromium is the release of contaminants from industrial wastewater containing this heavy metal. The total quantity released into the environment remains very high even though control technologies have been applied to many industrial and municipal sources. Nriagu and Pacyna (1988) estimated global discharge of trace elements (1000 metric tonnes/ year), in which chromium has a value of 142 in water, 30 in air and 896 in soil. Cr (VI) is highly toxic, carcinogenic and mutagenic (Mclean and Beveridge, 2001; Costa, 2003) and is hence of grave concern. Acute exposure to Cr (VI) causes nausea, diarrhea, liver and kidney damage, dermatitis, internal 9 Chapter II: Literature Review hemorrhage, and respiratory problems (Mohan et. al, 2006). Inhalation may cause acute toxicity, irritation and ulceration of the nasal septum and respiratory sensitization (asthma) (Kimbrough et. al., 1999; Mohan et. al., 2005; Mohan et. al., 2006). Ingestion may affect kidney and liver functions. Upon contact with skin, systemic poisoning damage can happen or even severe burns, and interference with the healing of cuts or scrapes. If not treated promptly, this may lead to ulceration and severe chronic allergic contact dermatitis (Mohan and Pittman, 2006). Eye exposure may cause permanent damage (Manahan, 1994). It is also associated with decrease in plant growth and changes in plant morphology (James and Bartlett, 1984). The drinking water guideline recommended by Environmental Protection Agency (EPA) in US is 100μg/L (Mohan and Pittman, 2006). The discharge of Cr (VI) into surface water is regulated to below 0.05 mg/l by the U.S. EPA, while total Cr (including Cr (III), Cr (VI) and its other forms) is regulated at below 2 mg/l. 2.3 Conventional technologies for chromium removal Conventional methods adopted for removing dissolved heavy metal (Chromium) include the following: 2.3.1 Chemical precipitation Chemical precipitation has traditionally been the most popular method. The most often used precipitation processes, include hydroxide precipitation, sulfide precipitation, carbonate precipitation and phosphate precipitation (Paterson, 1975). The chemicals used for precipitation are lime, caustic and sodium carbonate (Nriagu and Nieboer, 1988). Treatment is two stage processes: first, the reduction of hexavalent chromium to trivalent form and second, the precipitation of trivalent form. Reducing agents most commonly 10 Chapter II: Literature Review employed are gaseous sulfur dioxide or a solution of sodium bisulphite. Because the reaction proceeds rapidly at low pH, an acid (for example, sulphuric acid or hydrochloric acid) is added. The presence of oxidizing agent can prolong the treatment time by competing with hexavalent chromium for reducing agent. Other suitable reducing agents include sodium sulfite, sodium hydrosulfite, and ferrous sulfate. The disadvantage of precipitation is the production of sludge. This constitutes a solid waste disposal problem. 2.3.2 Electrochemical Reduction Reduction of hexavalent chromium can also be accomplished with electrochemical units. The electrochemical chromium reduction process uses consumable iron electrodes and an electric current to generate ferrous ions that react with hexavalent chromium to give trivalent chromium as follows (Nriagu and Nieboer, 1988): The reaction occurs rapidly and requires a minimum retention time. If the pH of the wastewater is maintained between 6 and 9, the ferric and trivalent ions will precipitate as hydroxides. The major disadvantage of this process is increased quantity of sludge; additional sludge results from the precipitated iron hydroxide. 2.3.3 Ion-Exchange Rengaraj et. al. (2003) described this process in which metal ions from dilute solutions are exchanged with ions held by electrostatic forces on the exchange resin. Petruzzelli et. al., (1995) described this process, which is based on a macroporous carboxylic resin (i-e., Purolite C106), that retains the metal of reference; together with other trace metals, like aluminum and iron. Alkaline hydrogen peroxide (pH 12) is used in a first regeneration step. There is formation of anionic species chromate and aluminate. 11 Chapter II: Literature Review They are then quantitatively eluted from the cation resin and separated. Ferric species are eluted with 1 M sulfuric acid in a second polishing step of the resin. The resulting ferric and aluminum sulfate solutions are recycled as flocculating agents. The residual that remains is the chromate solution, which can be directly reused in the plating industry or, after reduction to Cr (III), in the same tannery (Gode and Pehlivan 2007). The disadvantage in this process is that, due to oxidation of Cr (VI) the ion exchange resin becomes unstable (Park et al., 2005). There is partial removal of ions and involves high operational costs. 2.3.4 Ultrafiltration Ultrafiltration is a pressure driven membrane operation that uses porous membranes for the removal of heavy metals. Hydrostatic pressures forces a liquid against a semipermeable membrane. Ghosh and Bhattacharya (2006) suggested a new version of ultrafiltration, called Micellar Enhanced Ultra Filtration (MEUF). In this process, the surfactant having charge opposite to target ions, is added to the effluent stream containing the metal ions at a concentration greater than the critical micellar concentration (CMC), so that they form aggregates of around 50–150 of monomer molecules, called micelles (Christian et. al., 1989). Hence, a high amount of metal ions get electrostatically attached to the micelle surface. Now, if the resulting solution is passed through an ultrafilter, having pore size smaller than the micelle diameter, retention of such metal ions attached to the micelles is possible (Scamehom et. al., 1988). 2.3.5 Dialysis/ Electrodialysis Mohammadi et. al. (2005) described this process in which the ionic components (heavy metals) are separated through the use of semi-permeable ion selective membranes. 12 Chapter II: Literature Review Application of an electrical potential between the two electrodes causes a migration of cations and anions towards respective electrodes. Because of the alternate spacing of cation and anion permeable membranes, cells of concentrated and dilute salts are formed. Electrodialysis has the capability of (1) separating ionic chemicals from nonionic chemicals in process or waste streams to achieve product purity or eliminate waste, (2) concentrating the separated chemicals relative to concentrations in the initial process or waste streams to aid in reusing the chemicals and (3) being used as a reactor, both electrolytic and otherwise, to convert chemicals at high efficiencies to more desirable products. Further, it has the capability to meet these functions simultaneously in a single equipment assembly. Electrodialysis often results in low levels of pollution generation, with high energy efficiencies relative to other conventional treatment methods (Blackburn, 1999). 2.3.6 Reverse Osmosis Ozaki et. al. (2002) described reverse osmosis (RO) as a pressure driven membrane process in which a feed stream under pressure is separated into a purified permeate stream and a concentrate stream by selective permeation of water through a semi-permeable membrane. Padilla and Tavani (1999) have described this process in which a fraction of tanning water waste was passed through an adequate membrane (polyamide) under sufficient pressure to overcome the osmotic pressure. The fraction that passes through the membrane (permeate) is constituted basically by water with a low content of dissolved salts and the fraction retained by the membrane (concentrate) contains most of the dissolved salts in the original effluent. 13 Chapter II: Literature Review 2.3.7 Solvent Extraction Salazar et. al., (1992) described solvent extraction as a process that involves an organic and an aqueous phase. The aqueous solution containing the metal or metals of interest is mixed intimately with the appropriate organic solvent, and the metal passes into the organic phase. In order to recover the extracted metal, the organic solvent is contacted with an aqueous solution whose decomposition is such that the metal is stripped from the organic phase and is re-extracted into the stripping solution. Once the metal of interest has been removed, the organic solvent is recycled. Mauri et. al., (2001) demonstrated the use of a solvent molecule (a ligand or chelator) that exhibit affinity towards the heavy metals and hence get adhered to the solvent and can be filtered out. In this process, solvents that are soluble with water are added to extract the solutes, and then the resulting mixture is separated into two phases by either changing the temperature (Ullmann et. al., 1995) or the composition (Gupta et. al., 1996). 2.3.8 Cementation Cementation is the displacement of a metal from solution by a metal higher in the electromotive series. This technique offers an attractive possibility for treating any waste stream containing reducible metallic ions. Nriagu and Nieboer, (1988) reported the reduction of hexavalent chromium to trivalent state by contact with iron. In the presence of sufficient clean scrap steel, the system is capable of rapidly converting chromium to the trivalent form at pH less than 2.5 and can reclaim a considerable amount of chromium. The disadvantage is that it is suited only for small wastewater flows. It is not very practical for large flows because of the long contact times required. 14 Chapter II: Literature Review 2.3.9 Membrane Separations Solid membrane separation is based on the usage of semi-permeable membrane of different pore-size. On that basis it is classified as Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis (Kozlowski and Walkowiak, 2002). Nriagu and Nieboer, (1988) described liquid membrane separation as a process in which a thin liquid film selectively permits the passage of a specific constituent from the mixture. Separation occur based on chemistry rather size, so it quite similar to the process called solvent extraction. There are two types of liquid membranes: Supported and Unsupported. With supported membranes, the liquid is impregnated into the pores of solid membrane. Unsupported membranes, also known as emulsion membranes or liquid surfactant membranes are usually in the form of double emulsion drops. The problem with the use of these types of membranes is long term non-stability. Also efficient breakup of microspheres for product recovery is one of the difficulties encountered with emulsion separations. 2.3.10 Evaporation In electroplating industries, evaporators are chiefly used to concentrate and recover valuable plating material. Recovery is accomplished by boiling off sufficient water from the collected rinse stream to allow the concentrate to be returned to the plating bath. Generally, there are four types are used in electroplating industry: Rising film evaporators Flash evaporators using waste heat Submerged tube evaporators Atmospheric evaporators 15 Chapter II: Literature Review Evaporators have high efficiencies for removing plating chemicals, and the rinsewater purified by distillation can be recycled to rinse tanks. Low pressure steam provides the energy by which rinse-water vaporizes and drives the climbing film of the concentrate into the vapor-liquid separator where the heavier metallic materials settle out while the water vapor flows through a filter into a condenser. When the recovered solution reaches the desired concentration, it is drained into a storage vessel and retained for reuse. 2.3.11 Adsorption/Filtration Several adsorbents have been reported for adsorption of heavy metals. Activated carbon, charcoal, peat, zeolites, clay are some of the adsorbents used for removing heavy metals (Babel and Kurniawan, 2003; Mohan et. al., 2006). Nriagu and Nieboer, (1988) reported 45% recovery of hexavalent chromium, 95% of lead, 95% of copper, 70% of tin from synthetic effluent (containing 100 mg/L of metals) by addition of 5 gm of peat. Fly ash samples fuelled with lignite coal were used to remove heavy metals including chromium from 100mg/L solution. Silica gel is also a promising adsorbent. 50-98% removal of the metal cations from solution of cadium, chromium, copper, lead manganese, mercury, nickel, and zinc have been reported by Nriagu and Nieboer, (1988) when silica gel is used. 2.3.12 Electrocoagulation Parga et. al., (2005) described electrocoagulation as a process in which an electric current is used in the waste water to destabilize the suspended, emulsified or dissolved chemicals in an electrolyte solution. The electromotive force developed drives numerous complex chemical reactions that breakdown the wastes, converting soluble and toxic metals such as Cr (VI) to less toxic solid chromium (III) hydroxide (Cr(OH)3) that 16 Chapter II: Literature Review precipitates out of the wastewater solution. Electrocoagulation uses an anode-cathode process (Guertin et. al., 2004). 2.3.13 Electro-kinetic Extraction Roundhill and Koch (2002) described the use of electro-kinetic extraction in which a potential difference is applied by inserting electrodes into the soil. In this technique the application of an in situ direct current produces an electro-osmotic water flow, and the H+ and OH- fronts move through the soil in opposite directions. The acid front that moves from the anode to the cathode dissolves metals that are absorbed in the soil. By contrast, the highly alkaline interstitial water near the cathode causes these heavy metals to precipitate as their hydroxides. 2.3.14 Soil Washing/ Flushing The removal of toxic metals from soils by washing/ flooding with highly alkaline or acid water has been reported. Roundhill and Koch, (2002) reported chromium removal from soil by using a washing solution at a pH of 10.4. 2.3.15 Electrochemical Precipitation The electrochemical precipitation (ECP) process is also used for removal of heavy metals from the waste water. The ECP unit consists of an electrolytic cell made up of two steel plates representing anode and cathode. Roundhill and Koch, (2002) reported chromium removal efficiencies greater than 99%, and the residual chromium concentrations of less than 0.5 mg/L. The ECP process uses a current of 0.5–5.0 amperes and an initial pH of 4.5. 17 Chapter II: Literature Review 2.3.16 Chelation Tels (1987) described some chelating agents that forms complex with the heavy metals and precipitate out. Some of the example of chelating agents are Quadrol and EDTA. The disadvantage of this process is the formation of sludge. 2.4 Biological methods / Bioremediation for heavy metal removal Bioremediation is a process involving detoxification (where the waste is made less toxic) and mineralization (where the waste material is converted into inorganic compounds such as carbon dioxide, water and methane) (Martello, 1991). Bioremediation has been known for several decades for its use in treating oil sludges arising from refineries (Arujanan and Yee, 2004). Microorganisms in soils have been to degrade hydrocarbon for more than a century. They are known for degrading aromatic and aliphatic compounds, hydrocarbons, chlorinated solvents and pesticides. Several bioremediation technologies have been developed to treat toxic waste. Many factors, such as site conditions, indigenous microorganism population, and the type, quantity, and toxicity of contaminant chemicals present determine the bioremediation technology most suitable for a specific site. In order to stimulate the activity of microorganism the concentration of nutrients plays an important role. There is need to optimize environmental conditions, that can enhance the growth of microorganisms and increase microbial population resulting in improved degradation of hazardous substances. Bioremediation can be classified into two main categories: In-situ and Ex-situ bioremediation techniques: 18 Chapter II: Literature Review 2.4.1 In-situ bioremediation: This method involves the remediation of contaminated material within the confines of the area in which it was originally contaminated. In-situ techniques do not require excavation of the contaminated soils. The process starts with introduction of nutrients and oxygen and relying on indigenous microflora to destroy the unwanted molecules. Sometimes, oxygen is provided by pumping air into the soil above the water level. This process is called bioventing. This technique is less expensive, create less dust, and cause less release of contaminants. However, the technique has a few drawbacks like: it is a slow process, and is most effective at sites with permeable soil. It doesn‘t work well in clays or in high layered subsurface environment. This occurs because oxygen cannot be evenly distributed throughout the treatment area and hence the remediation cannot be effective. 2.4.2 Ex-situ bioremediation: These procedures require excavation and treatment of the contaminated soil before and, sometimes, after the actual bioremediation step. Ex-situ techniques include slurryphase bioremediation and solid-phase bioremediation. 2.4.2.1 Slurry-phase bioremediation: A large tank called as bioreactor is used in this process. The content of the tank involves combination of contaminated soil, water and other additives. Nutrients and oxygen is supplied in the bioreactor and an optimum condition for the growth of microorganism is maintained for the degradation of the contaminants. On completion, water is removed from the solid, which either disposed or further treated if it contains further contamination. 19 Chapter II: Literature Review 2.4.2.2 Solid-phase bioremediation: This is a process that treats soils in aboveground treatment areas equipped with collection systems to prevent any contaminant from escaping the treatment. Parameters like moisture, heat, nutrients, or oxygen are controlled to enhance biodegradation. Examples of solid-phase soil treatments include composting and land-farming. Solid phase bioremediation processes are easy to operate and maintain. The disadvantage is that this process require large amount of space and clean-ups require more time to complete when compared to slurry-phase bioremediation. 2.5 Major types of Bioremediation: The three major types of bioremediation are Biosorption, Bioaccumulation and Phytoremediation. Of these different biological methods, bioaccumulation and biosorption have been demonstrated to possess good potential to replace conventional methods for the removal of dyes/metals (Volesky and Holan, 1995; Malik, 2004). Bioaccumulation is defined as the phenomenon in which heavy metal removal takes place using living cells; whereas, biosorption mechanisms are based on the use of dead biomass. Detailed description of both the processes is as follows: 2.5.1 Biosorption (Passive Uptake) Biosorption is defined as the passive uptake of toxicants/metals by dead/inactive biological materials or by materials derived from biological sources. Biosorption occurs due to a number of metabolism-independent processes that take place in the cell wall, where the mechanisms responsible for the pollutant uptake differ according to the biomass type. 20 Chapter II: Literature Review Biosorption process involves a solid phase (sorbent or biosorbent; biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, metal ions). Due to higher affinity of the sorbent for the sorbate species, the latter is attracted and bound by different mechanisms. The process continues till equilibrium is established between the amount of solid-bound sorbate species and its concentration remaining in the solution. 2.5.1.1 Biosorbents: There are certain biomaterials that have proved to be a good biosorbent. Biosorption by different types of biomass including algae, fungi, bacteria, agricultural waste, compost, peat moss, polysaccharide materials and other biosorbent materials has been extensively reviewed (Kratochvil and Volesky 1998; Davis et. al., 2003; Gavrilescu, 2004). The details of all the biosorbents specifically used for biosorption of chromium are given below: 2.5.1.1 (a) Algae: Algae are classified on the nature of the chlorophyll(s), the cell wall chemistry, and flagellation. All algae contain chlorophyll a. However, the presence of phyto-pigments other than chlorophyll a is characteristic of each particular algal division. Algae that have reportedly been used as biosorbents for biosorption process include Spirogyra (Gupta et. al., 2001), Chlamydomonas reinhardtii (Arica and Bayramoglu, 2005), Dunaliella (Donmez and Aksu, 2002), Chlorella vulgaris (Aksu and Acikel, 1999; Aksu et. al., 1999; Donmez et. al., 1999; Aksu and Acikel, 2000), Clodophara crispate (Nourbakhsh et. al., 1994; Nourbakhsh et. al., 2002), 21 Chapter II: Literature Review Chlorella miniata (Han et al., 2006), Sargassum wightii (Aravindhan et. al., 2004), Ecklonia sp. (Yun et. al., 2001; Park et. al., 2004) and Spirulina species (Chojnacka et al., 2004). 2.5.1.1 (b) Fungi: Fungi and yeast are well known for removal of non-essential metals such as cadmium, mercury, lead, chromium, etc., in substantial amounts. Both living and dead fungal cells have the ability for toxic and precious metals uptake from water/wastewater. Fungal biosorbent used in heavy metals removal has been reviewed by Kapoor and Viraraghavan, (1995) and Sag, (2001). Several fermentation processes in industry produces byproducts that can be used as biosorbents. These processes could serve as economical biomass supply sources for the removal of metal ions. Various types of fungal biomass have been used for the removal and recovery of tri and hexavalent chromium from water/wastewater. These include unmethylated and methylated yeast (Seki et. al., 2005), R. arrhizus (Sag and Kutsal, 1996; Prakasham et. al., 1999), Penicillium chrysogenum (Deng and Ting, 2005; Park et. al., 2005), dead fungal biomass (Sekhar et. al., 1998), Lentinus sajorcaju mycelia (Arica and Bayramoglu, 2005; Bayramoglu et. al., 2005), R. nigricans (Bai and Abraham, 2001; Bai and Abraham, 2002; Bai and Abraham, 2003), Neurospora crassa (Tunali et. al., 2005), Ganoderm lucidum (Krishna and Philip, 2005), Aspergillus niger, Rhizopus oryzae, and Saccharomyces cerevisiae (Park et al., 2005). Park et al., (2005) proposed a new Cr (VI) removal mechanism for the dead 22 Chapter II: Literature Review A. niger fungal biomass. Aqueous Cr (VI) can be removed through the two mechanisms: I and II (see figure 2.3). In mechanism I, aqueous Cr (VI) is directly reduced to Cr (III) upon contact with the biomass. Mechanism II consists of three steps which include: (1) binding Cr (VI) to positively charged groups such as protonated amines present in the chitin and chitosan fungal cell wall components; (2) reduction of Cr (VI) to Cr (III) by adjacent functional groups having lower reduction potentials than that of Cr (VI); (3) the release of reduced Cr (III) into the aqueous solution by electrostatic repulsion between the positively charged groups and the Cr (III) ion. Figure 2.3: Cr (VI) removal mechanism by dead fungal biomass. Source: Park et al. (2005) 2.5.1.1 (c) Bacteria The use of bacteria as biosorbent in metal remediation is a promising technology because of their ubiquity, ability to grow under 23 Chapter II: Literature Review controlled conditions and smaller size, which leads to high surface area and fast rates. Zoogloea ramigera (Nourbakhsh et. al., 1994), Bacillus sp. (Nourbakhsh et. al., 2002), Aeromonas caviae (Loukidou et al., 2004a, b), Bacillus thuringiensis (Şahin and Öztürk, 2005), Bacillus licheniformis (Zhou et al., 2007), Bacillus coagulans (Srinath et al., 2002), Bacillus megaterium (Srinath et al., 2002), Staphylococcus xylosus (Ziagova et al., 2007), Chryseomonas luteola (Ozdemir and Baysal, 2004), Pseudomonas sp. (Ziagova et al., 2007), Zoogloea ramigazera (Sağ and Kutsal, 1989), Pantoea sp. (Ozdemir et. al., 2004) have been used for chromium remediation. 2.5.1.1 (d) Agricultural waste, compost and peat moss: Peat (natural humic substance) has ability to sequester metals from waste water stream. Peat‘s hydroxyl, carboxyl and phenol functional groups play important role in complexation and ion exchange during metal ions fixation. The ability of four common plant-derived products—wood, grass, compost, and peat moss to remove cadmium, chromium and lead from dilute aqueous solutions was investigated (Hardin and Admassu, 2005). Depending on the type of peat used, mechanisms of cation uptake vary greatly (Viraraghavan and Ayyaswami, 1987; Wase and Forster, 1997; Brown et. al., 2000). Ma and Tobin, (2004) reported Cr3+, Cu2+ and Cd2+ biosorption onto peat in batch experiments. The order of maximum uptake was Cr >Cu > Cd. Sharma and Forster, (1995) reported that peat moss could be a useful biosorbent for treating metal contaminated 24 Chapter II: Literature Review wastewaters. Deepa et al. (2006) reported batch sorption of aqueous Cr(VI) using live and pretreated Aspergillus flavus biomass. Carrillo-Morales et al. (2001) utilized the pulp of cactus (CACMM2) for the adsorption of aqueous Cd2+, Cr3+, Cu2+, Fe3+, Ni2+, Pb2+ and Zn2+. The adsorption capacities decreased in the order Cd(II) > Ni(II) > Cr(III) > Pb(II) > Cu(II) > Fe(II) > Zn(II). 2.5.1.1 (e) Polysaccharide materials: Chitin and chitosan are excellent natural adsorbents (Crini, 2005; Mcafee et. al., 2001) with high selectivity because: Large numbers of hydroxyl and amino groups give chitosan high hydrophilicity; Primary amino groups provide high reactivity; The chitosan polymer chains provide suitable configurations for efficient metal ion complexation. Chitin, the most widely occurring natural carbohydrate polymer next to cellulose, is a long, unbranched polysaccharide derivative of cellulose (figure 2.4 (a)) Chitosan is a product of deacetylation of chitin using concentrated alkali at high temperature (figure 2.4 (b)). Figure 2.4 (a): Chitin. Source: Mohan and Pittman, (2006) 25 Chapter II: Literature Review Figure 2.4 (b): Chitosan. Source: Mohan and Pittman, (2006) Crini, (2005) reviewed the developments in the synthesis of polysaccharide-containing adsorbents, in particular modified biopolymers derived from chitin, chitosan, starch and cyclodextrin. Chromium adsorption onto cross-linked chitosan was investigated by Rojas et al., (2005). Spinelli et al., (2004) synthesized quaternary ammonium salt of chitosan (QCS) for aqueous Cr(VI) removal. Castro Dantas et al., (2001) reported Cr(III) removal from aqueous solutions by chitosan impregnated with a microemulsion. Hasan et al., (2003) demonstrated Cr(VI) remediation using chitosan-coated perlite beads. 2.5.1.1 (f) Other biosorbent materials: Some other waste materials from food and agricultural industries have also reported as effective sorbents such as animal bones (Chojnacka, 2005), papaya wood (Saeed et al, 2005), Lentinus edodes (Zeng et al., 2006). Some other biosorbents such as Tamarindus indica seed (Chaudhari et al., 2005), Azadirachta indica (Neem) leaf powder (Sharma and Bhattacharyya, 2005) have also been reported. 26 Chapter II: Literature Review 2.5.2 Phytoremediation Phytoremediation is a process whereby plants are used for the cleanup of heavy metal contamination in soil and water. This process preserves the topsoil and reduces the amount of hazardous materials generated during cleanup. Figure 2.5 depicts bacteria adhering around the roots of a plant that helps in phytoremediation. Figure 2.5: Bacteria around the roots of a plant help in phytoremediation. Source: Arujanan and Yee, (2004) There are many types of phytoremediation as explained below: 2.5.2.1 Phytoextraction: Phytoextraction is a process in which metal-accumulating plants that can transport and concentrate metals from the soil to the roots and aboveground shoots are used. The process has the potential to be used in large scale and on-site treatment is possible. It is a cost-effective method, in which top-soil can be preserved. 27 Chapter II: Literature Review 2.5.2.2 Rhizodegradation: This is a type of phytodegradation that occurs in the root zone. The rhizosphere is the area of soil around plant roots that contains higher populations of microbes and hence more microbial activity. Plants‘ roots release exudations such as short chain organic acids, phenolics, enzymes, and proteins that affect the enzyme systems of the bacteria already living in the soil. Plants also make the soil in the rhizosphere a habitat for mycorrhizae fungi that metabolize organic pollutants. 2.5.2.3 Rhizofiltration: In this process, the plant roots are used to absorb, concentrate as well as precipitate heavy metals from water. Dushenkov et. al., (1995) reported the use of roots of sunflower to treat waste water. Water hyacinth and eastern cotton are some other plants with the potential for metal sequestration. 2.5.2.4 Phytostabilization : In this process, plants are used to immobilize metals and radionuclides in the soil (and thus minimize their mobility in water or dust). It involves the use of plants and its roots to prevent contaminant migration via wind and water erosion, leaching, and soil dispersion. 2.5.2.5 Photovolatilization: In this process, plants uptake and transpires the contaminant to the atmosphere. It is mainly applied to ground water to remove chlorinated solvents, but has shown promising results in the removal of Selenium, Mercury and Arsenic (Arujanan and Yee, 2004). 28 Chapter II: Literature Review 2.5.2.6 Phytodegradation: In this process, there is uptake and breakdown of organic contaminant through metabolic processes occurring in the plant. This method is mainly for removal of organic contaminant and not for removing any metal as they cannot be degraded. 2.5.3 Limitations of bioremediation processes  Mostly, during the dead/pretreated biomass, metal is not taken up into the cells, rather it is just adsorbed at the cell surface (Torres et al., 1998) and hence only a small fraction of metal removal takes place.  Current methods of biosorption are sensitive to ambient conditions such as pH, ionic strength and the presence of organic or inorganic ligands and hence have certain limitations (Malik, 2003)  Biosorption lacks specificity in metal binding (Baudet et. al., 1988)  Final recovery of metal is difficult in the biosorption process.  Reusability of biomass in biosorption is possible only if weak chemicals are used for desorption, which will further lead to low concentration of metal in the eluent.  In biosorption process, the biomass may be used for a maximum of 5–10 sorptions–desorption cycles.  Actually batch processes are commonly used. The batch is then regenerated when saturated and new column is then used.  Most of the biosorption studies are conducted in synthetic effluents (Ahuja et al., 1999; Mehta and Gaur, 2001), but with real industrial effluent; it was found that biosorption efficiency was very low. For example, Corder and Reeves (1994) used 29 Chapter II: Literature Review autoclaved cyanobacteria biomass for the removal of nickel from actual effluent sample and found that none of the species could bind efficiently due to presence of high concentration of Na ions.  Reports by Matsunaga et. al., (1999) and Perez-Rama et. al., (2002) indicate higher intracellular accumulation of cadium in the live cells.  Phytoremediation processes, though cheap, requires a long time period for the remediation. 2.5.4 Bioaccumulation (Active Uptake) Bioaccumulation can be defined as the uptake of toxicants by living cells. The toxicant initially absorbed on the cell wall or membrane is accumulated intracellularly across the cell membrane and through the cell metabolic cycle (Malik, 2004). After understanding the limitations of biosorption, it is proved that there is another additional method required for effective metal remediation. Hence, application of active or live cells seems to be a better approach due to its ability of self-replenishment, continuous metabolic uptake of metals after physical adsorption and the potential for optimizing through development of resistant species and cell surface modification (Wilde and Benemann, 1993; Sandau et al., 1996). The metals diffused inside the cell gets bound to the intracellular proteins or chelatins before being incorporated into vacuoles and other intracellular sites. This process if often irreversible and ensure less risk of metal releasing back to the environment (Gekeler et. al., 1988). The use of live cultures during the removal process by-passes the need for a separate biomass production process, e.g. cultivation, harvesting, drying, processing and storage prior to the use. When compared to conventional chemo-physical and biosorptive methods, employment of active 30 Chapter II: Literature Review microorganisms may allow development of a single stage process for removal of most of the pollutants present in industrial effluents. Another advantage in using these growing cells is that, they have unlimited capacities to cleave organo-metallic complexes, degrade organic compounds, as well as take up other inorganic ions such as ammonium, nitrate and phosphate. Further, dissolved and fine-dispersed metallic elements can also be removed via immobilization. 2.5.4.1 Limitations of bioaccumulation Though bioaccumulation has shown to be a promising technology, it suffers some practical limitations. Donmez and Aksu, (2001) highlighted the sensitivity of the system to extreme pH, high metal/salt concentration and requirement of external metabolic energy. 2.5.5 Proposed strategy to overcome this problem- Microbial resistance with Bioaccumulation Malik (2003) have reviewed possible solution to the existing problem in bioaccumulation. The proposed strategy incorporates the isolation and selection of metalresistant strains to overcome the prime constraint of employing living cell systems. It has been shown that resistant cells bind substantially more metals, which in turn is a prerequisite for enhanced bio-precipitation/ intracellular accumulation. There are certain examples where this strategy has gained importance. Significant reduction of cadmium was observed in Cd- resistant bacterial strains, Bacillus strain H9 and Pseudomonas strain H1 (Roane and Pepper, 2000). Haq et. al., (1999) isolated three strains of bacteria from industrial effluents, Enterobacter cloacae and Klebsiella sp. They were found to be resistant to high concentrations of Cd, Pb and Cr and could remove approximately 85% 31 Chapter II: Literature Review Cd during growth. Not only bacteria, marine algae like Tetraselmis suecica also have high resistance and heavy metal (Cd removal) capacity (Perez-Rama et al., 2002). Wong et. al., (2000) reported that the alga, Chlorella vulgaris for nickel resistance and hence enhanced bioaccumulation. Donmez and Aksu, (2001) has isolated yeast (Candida sp.) from sewage samples, which was found to accumulate Ni (57-71%) and Cu (52-68%). Cu- resistant yeast strains Kluveromyces marxianus, Candida sp. and Saccharomyces cerevisiae remove 73–90% of Cu during their growth (Donmez and Aksu, 1999; Donmez and Aksu, 2001). Brady and Duncan, (1994) and Krauter et. al., (1996) have reported the mechanism behind the metal uptake in yeasts. It involves an initial rapid biosorption of metal ions to negatively charged sites on the cell wall followed by a slower, energy-dependent entry into the cell. It was found that both the outer mannan–protein layer of the yeast cell wall as well as the inner glucan–chitin layer play important role in heavy-metal accumulation (Avery and Tobin, 1993; Brady et al., 1994). A majority of intracellular metals become bound to polyphosphate granules localized in and near the vacuoles or may also get detoxified via binding to specific low-molecular weight proteins, namely, metallothioneins and phytochelatins (Volesky et al., 1992; Volesky and May-Philips, 1995). Viable cells of S. cerevisiae were able to remove metals (Cu, Cr, Cd, Ni and Zn) from electroplating effluents (Stoll and Duncan, 1996). Aspergillus niger could remove significant quantities of Cu and Pb from growth media but was less resistant against Cr (Dursun et al., 2003). An efficient Zn uptake by growing cells of Aspergillus sp. isolated from industrial waste has been reported (Sharma et al., 2002; Sharma et al., 2000). Bacteria have also been known for metal resistance and accumulation. Pseudomonas 32 Chapter II: Literature Review aeruginosa isolated from waste water was able to accumulate Cr (Hassen et al. 1998). Curesistant Pseudomonas putida strain S4 isolated from smelter drainage of copper mines accumulated not only metals from metal-supplemented growth medium but also removed Cu and Zn from mine effluents, low-grade ore and ore tailings (Saxena et al., 2001; Choudhury and Srivastava, 2002). 2.5.5.1 Relation of EPS (Exo-polysaccharide) with microbial resistance: Exo-polysaccharides are extra-cellular polysaccharides secreted by microorganism into the surrounding environment. EPS may contain polysaccharides, proteins, phospolipids, teichoic and nucleic acid, and other polymeric substances hydrated to 85 to 95% water (Costerton and Irving, (1981); Sutherland, (1983)). In gram negative bacteria these polysaccharides are neutral or polyanionic, while for gram-positive bacteria it may be primarily cationic (Evans, 2000). EPS is highly hydrated because it can incorporate large amount of water into its structure by hydrogen bonding (Kumar and Anand, 1998). EPS are both hydrophilic and hydrophobic and may vary in its solubility. EPS may associate with metal ions, divalent cations, and other macromolecules such as proteins, DNA, lipids, and even humic substances (Sutherland, 2001). Microbial resistance and production of EPS are proportionally related. Priester et. al., (2006) reported the enhanced production of EPS when Pseudomonas putida were exposed to chromium. It resulted in elevated amount of extracellular carbohydrates, proteins and EPS sugars. Similarly Kiran and Kaushik, (2008) have examined the production of EPS by cyanobacterium Lyngbya putealis HH-15 on 33 Chapter II: Literature Review chromium exposure and studied chromium removal. Hence EPS plays an important role when studying the resistance of the microbes to heavy metals. 2.5.5.2 Overall relation of EPS, microbial resistance with bioaccumulation The rich EPS content from the microorganisms may also be beneficial for both entrapping dispersed solids and adsorption of dissolved metals. Further, it provide a microenvironment (like alkaline pH, high concentrations of CO2), which could be very beneficial for metal precipitation. The positive interaction between constituent species may also facilitate the survival of sensitive strains (Bradshaw et al., 1998). Kiran and Kaushik, (2008) presents the chromium adsorptive potential of EPS by cyanobacterium Lyngbya putealis HH-15. Roane et. al., (2001) have isolated four Cd resistant bacterial isolates. Out of which, Pseudomonas strain IIa and Arthrobacter strain D9 showed the evidence of EPS production. It was found that the EPS production enhanced the accumulation of metal by additionally absorbing the metal to the cells externally. Roane, (1999) isolated Pseudomonas marginalis from a contaminated soil that showed higher resistance towards Pb with high amount of EPS production. Basnakova et. al., (1998) indicated the role of EPS in the creation of intracellular deposits with Citrobacter sp. cells immobilized in polyacrylamide gel. Kazy et. al., (2002) has reported enhanced accumulation of copper due to higher amount of EPS produced in Pseudomonas aeruginosa. 34 Chapter II: Literature Review 2.6 Analytical tools / techniques to evaluate location of accumulated metals in cell 2.6.1 X-ray diffraction (XRD) Analysis It is an analytical technique which reveals information about the crystallographic structure, chemical composition and physical properties of the material. Here in this project, it was used to analyze the chemical nature before and after exposure to heavy metals. In case of Aspergillus niger, Ni was found to be associated with the cell wall as well as inside the cell (Magyarosy et. al., 2002). The chemical nature was analyzed by Xray and electron diffraction analysis and found that Ni was accumulated as nickel oxalate dehydrate crystals. Sar et al., (2001) have demonstrated potential role of phosphoryl and carboxyl/carbonyl group of cell wall for Ni accumulation by Pseudomonas aeruginosa. Ni forms compounds like nickel phosphide (Ni5P4, NiP2, Ni12P5) and nickel carbide crystals (Ni3C). Lopez et al., (2000) shows Pseudomonas fluorescens 4F39 forming Ni (OH)2 upon bioaccumulation of Ni. 2.6.2 X-ray photoelectron spectroscopy (XPS) It is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material. Yang and Chen, (2008) have reported that involvement of alcohol, carboxyl, amino, and sulphonic groups in the biosorption of Cr by Sargassum sp. seaweeds. Silva et al., (2008) have reported the removal of Cr (III) and Cr (VI) from aqueous solution by Arthrobacter viscosus biofilm supported on NaY zeolite and has provided information about the presence of chromium through the surface layer of NaY as well as about the oxidation state of the metal. 35 Chapter II: Literature Review 2.6.3 Microscopy Analysis 2.6.3.1 Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) gives the surface analysis of the morphological changes that occurs in the microorganism upon exposure of heavy metals. SEM is a type of microscope that images the sample surface by scanning it with high energy beam of electron in a raster scan (rectangular scanning) pattern. AFM consists of a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. Depending on the situation, forces that are measured in AFM include mechanical contact force, vander waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, casimir forces, solvation forces, etc. The deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes and scanner that depicts the topography of the sample. There are reports that have used SEM and AFM analysis to confirm morphological changes upon exposure to metal. Yin et. al., (2008) showed that considerable amount of metals was precipitated on cell surface (Candida tropicalis and Candida lipolytica). Chourey et al., (2006) have also shown the morphological changes that occur when Shewanella oneidensis MR-1is exposed to chromium. Edward et al., (2006) have also depicted the morphological changes that occur when metal resistant Pseudomonas aeruginosa is exposed to Cd, Ni, Cr and Pb. 2.6.3.2 Transmission Electron Microscopy (TEM) is a microscopic technique whereby a beam of electrons is transmitted through an ultra thin specimen, 36 Chapter II: Literature Review interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device and image is captured. Roane, (1999) has showed localization of lead in Pseudomonas marginalis and Bacillus megaterium. Similarly, Suh et. al., (1998) have shown 3-step mechanism of lead accumulation in Saccharomyces cerevisiae. White and Gadd (1996) have demonstrated the localization Co (vacuole), Cd, and Cu (soluble fraction) in Saccharomyces cerevisiae. Torres et. al., (1998) have shown Zn, Pb, Mn and Al localization in polyphosphate bodies by Plectonema boryanum. TEM has shown nickel accumulation was restricted to periplasm and cell membrane of Pseudomonas aeruginosa (Sar et al., 2001). Lopez et al., (2000) on the other hand has shown Ni decomposition occurred mere on the cell surface of Pseudomonas fluorescens 4F39. 37 [...]... high surface area and fast rates Zoogloea ramigera (Nourbakhsh et al., 1994), Bacillus sp (Nourbakhsh et al., 20 02) , Aeromonas caviae (Loukidou et al., 20 04a, b), Bacillus thuringiensis (Şahin and Öztürk, 20 05), Bacillus licheniformis (Zhou et al., 20 07), Bacillus coagulans (Srinath et al., 20 02) , Bacillus megaterium (Srinath et al., 20 02) , Staphylococcus xylosus (Ziagova et al., 20 07), Chryseomonas... 20 05; Bayramoglu et al., 20 05), R nigricans (Bai and Abraham, 20 01; Bai and Abraham, 20 02; Bai and Abraham, 20 03), Neurospora crassa (Tunali et al., 20 05), Ganoderm lucidum (Krishna and Philip, 20 05), Aspergillus niger, Rhizopus oryzae, and Saccharomyces cerevisiae (Park et al., 20 05) Park et al., (20 05) proposed a new Cr (VI) removal mechanism for the dead 22 Chapter II: Literature Review A niger fungal... al., 1999; Donmez et al., 1999; Aksu and Acikel, 20 00), Clodophara crispate (Nourbakhsh et al., 1994; Nourbakhsh et al., 20 02) , 21 Chapter II: Literature Review Chlorella miniata (Han et al., 20 06), Sargassum wightii (Aravindhan et al., 20 04), Ecklonia sp (Yun et al., 20 01; Park et al., 20 04) and Spirulina species (Chojnacka et al., 20 04) 2. 5.1.1 (b) Fungi: Fungi and yeast are well known for removal... resistance and heavy metal (Cd removal) capacity (Perez-Rama et al., 20 02) Wong et al., (20 00) reported that the alga, Chlorella vulgaris for nickel resistance and hence enhanced bioaccumulation Donmez and Aksu, (20 01) has isolated yeast (Candida sp.) from sewage samples, which was found to accumulate Ni (57-71%) and Cu ( 52- 68%) Cu- resistant yeast strains Kluveromyces marxianus, Candida sp and Saccharomyces... biomass Carrillo-Morales et al (20 01) utilized the pulp of cactus (CACMM2) for the adsorption of aqueous Cd2+, Cr3+, Cu2+, Fe3+, Ni2+, Pb2+ and Zn2+ The adsorption capacities decreased in the order Cd(II) > Ni(II) > Cr(III) > Pb(II) > Cu(II) > Fe(II) > Zn(II) 2. 5.1.1 (e) Polysaccharide materials: Chitin and chitosan are excellent natural adsorbents (Crini, 20 05; Mcafee et al., 20 01) with high selectivity... the removal and recovery of tri and hexavalent chromium from water/wastewater These include unmethylated and methylated yeast (Seki et al., 20 05), R arrhizus (Sag and Kutsal, 1996; Prakasham et al., 1999), Penicillium chrysogenum (Deng and Ting, 20 05; Park et al., 20 05), dead fungal biomass (Sekhar et al., 1998), Lentinus sajorcaju mycelia (Arica and Bayramoglu, 20 05; Bayramoglu et al., 20 05), R nigricans... at high temperature (figure 2. 4 (b)) Figure 2. 4 (a): Chitin Source: Mohan and Pittman, (20 06) 25 Chapter II: Literature Review Figure 2. 4 (b): Chitosan Source: Mohan and Pittman, (20 06) Crini, (20 05) reviewed the developments in the synthesis of polysaccharide-containing adsorbents, in particular modified biopolymers derived from chitin, chitosan, starch and cyclodextrin Chromium adsorption onto cross-linked... and ion exchange during metal ions fixation The ability of four common plant-derived products—wood, grass, compost, and peat moss to remove cadmium, chromium and lead from dilute aqueous solutions was investigated (Hardin and Admassu, 20 05) Depending on the type of peat used, mechanisms of cation uptake vary greatly (Viraraghavan and Ayyaswami, 1987; Wase and Forster, 1997; Brown et al., 20 00) Ma and. .. food and agricultural industries have also reported as effective sorbents such as animal bones (Chojnacka, 20 05), papaya wood (Saeed et al, 20 05), Lentinus edodes (Zeng et al., 20 06) Some other biosorbents such as Tamarindus indica seed (Chaudhari et al., 20 05), Azadirachta indica (Neem) leaf powder (Sharma and Bhattacharyya, 20 05) have also been reported 26 Chapter II: Literature Review 2. 5 .2 Phytoremediation... hyacinth and eastern cotton are some other plants with the potential for metal sequestration 2. 5 .2. 4 Phytostabilization : In this process, plants are used to immobilize metals and radionuclides in the soil (and thus minimize their mobility in water or dust) It involves the use of plants and its roots to prevent contaminant migration via wind and water erosion, leaching, and soil dispersion 2. 5 .2. 5 Photovolatilization: ... al., 20 02) , Aeromonas caviae (Loukidou et al., 20 04a, b), Bacillus thuringiensis (Şahin and Öztürk, 20 05), Bacillus licheniformis (Zhou et al., 20 07), Bacillus coagulans (Srinath et al., 20 02) , Bacillus. .. predominately CrO 42 , HCrO 42 , Cr2O 72 (Mohan et al., 20 05; Mohan et al., 20 06) Cr2O 72 predominates at low pH and high chromium concentrations, while Cr (IV) exists in the form of CrO 42 at a pH greater... and 3000 mg/kg, in sea water is between to 800 µg/L, and in rivers and lakes is between 26 µg/L to 5 .2 mg/L (Kotas and Stasicka 20 00) Chromium occurs in 2+ , 3+ and 6+ oxidation states but Cr2+