Heavy Metal Pollution and Its Eco-friendly Management Heavy Metal Pollution and Its Eco-friendly...

2 c h a p t e r

Heavy Metal Pollution and Its Eco-friendly Management

Dr. Ashok K. Rathoure*(GM-Consultancy, Ecosystem Resource Management Pvt. Ltd.

Surat, Gujarat, India)

1. Background

The rapid industrial developments have led to the generation of huge quantities of hazardous wastes, which have further aggravated the environmental problems in the country by depleting and polluting natural resources. Therefore, rational and sustainable utilization of natural resources and their protection from toxic releases is vital for sustainable socio-economic development (Chakrabarti et al., 2006). Hazardous waste management is a new concept for most of the Asian countries including India. The lack of technical and financial resources, and the regulatory control for the management of hazardous wastes in the past led to the unscientific disposal of hazardous wastes in India, which poses serious risks to human, animal and plant life. A huge quantity of pollutants in the form of domestic and industrial effluents is discharged directly or indirectly into the waterbodies, which has severe impacts on its biotic and abiotic environment. During rain, surface water with soil, mud and humus enter the river, tanks and other waterbodies. The soil is the target of thousands of contaminants that vary in composition and in concentration. The contaminants enter the system as a result of a wide range of actions, such as intentional applications, inadequate residue disposal, accidental wastes and inappropriate use. Some human activities have resulted in the accumulation of metals in the environment. The inorganic minerals like sodium, potassium, calcium, magnesium and heavy metals like iron, manganese, lead, mercury, chromium, cadmium, nickel, cobalt, beryllium, copper, etc., when reach to the river with water cause water pollution. The use of various types of pesticides and insecticides in agriculture also causes water pollution. The pathogenic organisms of these wastes

* Corresponding Author E-mail: [email protected]

20 Bioremediation: Current Research and Applications

need for educating consumers including the potential threat to public health and the environment posed by their products and for raising awareness for the proper waste management (Kesselman, 2007; Dogbevi, 2007; Beary, 2008; Mcphate, 2008; Toxic Links, 2008; Pinto, 2009).

Liquid waste can be defined as such fluids as wastewater, fats, oils, grease or used oil. The disposal of such waste, such as transmission fluid, cooking oil, spent oil, fats or grease can contaminate the groundwater or negatively impact the wastewater system. The liquid waste is basically the wastewater discharged from the industrial and domestic area. Wastewater is water that has been used for some purpose and is deemed unfit for further use. In fact, wastewater can be used for secondary purposes in most cases. Also, efficient use of water reduces the amount of wastewater generated. Sewage is created by residences, institutions, hospitals and commercial as well as industrial establishments. Raw influent or sewage includes household waste, liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediments, heavy metals, organic compounds, animal waste, oil and grease. It may be originated from various sites and by different uses (US EPA, 2000; Central New York Regional Planning and Development Board, 2004; Massoud and Ahmad, 2005).

Municipal wastewater is the combination of liquid or water carried wastes originating in the sanitary conveniences of dwellings, commercial or industrial facilities and institutions, in addition to any groundwater, surface water and storm water that may be present. Wastewater is the flow of used water from a community. The characteristics of the wastewater discharges vary from location to location depending on the population and industrial sector served, land uses, groundwater levels and degree of separation between storm water and sanitary wastes. Domestic wastewater includes typical wastes from the kitchen, bathroom and laundry, as well as any other wastes that people may accidentally or intentionally pour down the drain. Sanitary wastewater consists of domestic wastewater as well as those discharged from commercial, institutional and similar facilities. Industrial wastes will be as varied as the industries that generate the wastes. Municipal wastewater also contains a variety of inorganic substances from domestic and industrial sources, including a number of potentially toxic elements, such as arsenic, cadmium, chromium, copper, lead, mercury, zinc, etc., (Xie et al., 1996, Ferrari et al., 1999).

3. industrial effluents and soil pollution

Toxic chemicals found in wastewater pass through wastewater treatment facilities that have not been designed to remove them and can interfere with their operation. In a biological treatment process, toxic materials can upset a treatment process or even

Heavy Metal Pollution and Its Eco-friendly Management 21

kill the biological community and make the process ineffective. To remove the toxic pollutants at the treatment facility can be costly. Therefore, it is generally advantageous to remove them at the source. Source control can be achieved by the use of municipal by-laws limiting pollutant discharges to the sewerage system. The removal of toxic pollutants at the source can be achieved by treatment prior to discharge, recycling of waste byproducts, manufacturing process changes and the substitution of raw material (Thorpe, 2009).

The effluent of the industries goes into the water system and changes the physico-chemical quality of water and also makes it unfit for drinking and other uses. Since all natural waterways contain bacteria and nutrients, almost any waste compound introduced to such waterways will initiate biochemical reactions. These biochemical reactions are measured as BOD and COD in the laboratory. Both have been widely adopted as a measure of pollution effect. Disposal of wastewaters from an industrial plant is difficult and costly. Most petroleum refineries, chemical and petrochemical plants have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local or national regulations regarding the disposal of wastewaters into community treatment plants or into rivers, lakes or oceans. The solids can be suspended 30 percent as well as dissolved solids which are about 70 percent. The dissolved solids can be precipitated by chemical and biological processes. Organic components may consist of carbohydrates, proteins, fats and greases, surfactants, oils, pesticides, phenols, etc. From a physical point of view, the suspended solids can lead to the development of sludge deposits and anaerobic conditions when discharged into the receiving environment. Physically, the wastewater is usually characterized by grey colour, musty odour, 0.1 percent solid content and 99.9 percent water content (Tchobanoglous, 1987; Tchobanoglous et al., 2003; Clair, 2003; Maiti, 2004).

The soil is the target of thousands of contaminants that vary in composition and concentration. The contaminants enter the system as a result of wide ranging actions, such as intentional applications, inadequate residue disposal, accidental wastes and inappropriate use. Soil pollution is defined as the buildup in soils of persistent toxic compounds, chemicals, salts, radioactive materials or disease causing agents, which have adverse effects on plant growth and animal health. Soil is the thin layer of organic and inorganic materials that covers the earth’s rocky surface. The organic portion, derived from the decayed remains of plants and animals, is concentrated in the dark uppermost topsoil. The inorganic portion, made up of rock fragments, was formed over thousands of years by physical and chemical weathering of bedrock. Soil pollution is caused by the presence of manmade chemicals or other alterations in the natural soil environment. This type of contamination typically arises from the rupture of underground storage links, application of pesticides and percolation of contaminated surface water to subsurface strata, oil and fuel dumping, leaching of wastes from landfills or direct discharge of industrial wastes to the soil. The most common chemicals involved are petroleum


residues in the soil that in long terms, promote their accumulation. Heavy metals in wastewater come from industries and municipal sewage and they are one of the main causes of water and soil pollution. The majority of the sources are originated by human actions like metal manufacture and mining industries with storage, disposal and transportation problems. Among the metals found more frequently are Cd, Pb, Co, Cu, Hg, Ni, Si and Zn. For Cd, Pb, Cu and Zn, their toxicity increases as follows: Pb < Zn < Cu < Cd, depending on countless abiotic and biotic factors (Lloyd and Lovely, 2001; Glick, 2003; Zenker et al., 2005).

In order to survive in heavy metal polluted environments, many microorganisms have developed means of resistance to toxic metal ions. These mechanisms include metal exclusion by permeability barriers, active transport of the metal away from the cell organism, intracellular sequestration of the metal by protein binding, extracellular sequestration, enzymatic detoxification of the metal to a less toxic form and reduction in metal sensitivity of cellular targets. Most microorganisms are known to have specific genes for resistance to toxic ions of heavy metals. Mostly, the resistance genes are found on plasmids or on chromosomes. Plasmid encoded metal resistance determinants have been reported to be inducible (Nies and Silver, 1995; Rosen, 2002).

The intake of heavy metal ions by microbial strains normally includes a redox reaction involving the metal. Some bacteria use heavy metals for energy and their growth. Bacteria that are resistant to heavy metals also play an important role in biogeochemical cycling of metal ions. Since, the oxidation state of a metal ion may determine its solubility, many scientists have been attempting to use microbes that are capable to oxidize or reduce metals in order to remediate metal contaminated sites. Although, some heavy metals are important and essential trace elements, and others are toxic at high concentrations to microbes, some microbes have adapted to tolerate the presence of metals or even to use them to grow. In addition, a number of interactions between microbial strains and metals have important environmental and health implications. Health problem due to heavy metals pollution in human beings include nausea, vomiting, bone complications, nervous system impairment and even death become a major problem for many countries when metal ions concentration in the environment exceeded the admissible limits. Due to that, various treatment technologies have been searched to reduce the concentration of heavy metals in the environment (Nies, 1999; Lodeiro et. al., 2006).

It is needed to remove the heavy metals from the contaminated sites/waste. Several heavy metal removal technologies including chemical precipitation, ion exchange, reverse osmosis, electrodialysis, ultrafiltration and pyhtoremediation are commonly used in industries. However, these technologies are becoming uneconomical and unfavourable to remove heavy metals from industrial wastewaters. The development of new treatment method, such as bioremediation to remove heavy metal ions from


wastewater and solid waste which could be cost effective and more efficient has spurred to overcome the conventional methods. Bioremediation technology has received much attention as it offered low cost technique and nonhazardous biomaterials (Lovley and Coats, 1997; Ahalya et al., 2003).

5. BioreMediation

The history of bioremediation is considerably short and it reflects many upturns and downturns as a result of political and economic forces. Interest in use of microorganisms to degrade specific hazardous organic chemicals probably dates back to Gayle (1952), who proposed the microbial infallibility hypothesis. Gayle postulated that for any conceivable organic compound, there exists a microorganism that can degrade it under the right conditions. If not, evolution and adaptation would produce such a strain. This hypothesis cannot wrong, because failure to degrade a contaminant can be attributed to the researcher’s failure to use the right strain under the right condition. In 1970s, environmental statutes of unprecedented scope passed, such as the Occupational Safety and Health Act (OSHA) of 1970, the Clean Air Act (CAA) of 1970, the Clean Water Act (CWA) of 1972, the Safe Drinking Water Act (SWA) of 1974 and the Toxic Substance Control Act (TSCA) of 1976. This regulatory pressure stimulated interest in site remediation technologies, including bioremediation. However, bioremediation failed to meet the expectations raised by many technology salespeople, who commonly advocated the addition of specialized bacteria to contaminated site, i.e., bioaugmentation. Early proponents of this approach generally did not recognize that indigenous bacteria already present at a contaminated site were probably better predisposed physiologically and genetically to mediate the degradation of the target pollutants. The first patent for a biological remediation agent was registered in 1974, being a strain of Pseudomonas putida that was able to degrade petroleum. In 1991, about 70 microbial genera were reported to degrade petroleum compounds and almost an equal number has been added to the list in the successive two decades. Bioremediation can occur naturally or through intervention processes. Natural degradation of pollutants relies on indigenous microflora that is effective against specific contaminants and it usually occurs at a slow rate. With intervention processes, the rate of biodegradation is aided by encouraging growth of microorganisms, under optimized physico-chemical conditions. Microorganisms play a vital role in heavy metal contaminated soil and wastewater by the mechanisms of biosorption. Some microorganisms possess an astonishing catabolic versatility to degrade or transform such compounds. The need for economical, effective and safe methods for removing heavy metals from wastewater has resulted in the search for unconventional materials that may be useful in reducing the levels or accumulation of heavy metals in the environment (US Congress, 1991; Agarwal, 1998; Bouwer et al., 1998; Smith et al., 1998).

Bioremediation is the use of living microorganism to reduce the environmental pollution. It is a technology for removing pollution from the environment, restoring


of contaminants, diverse organisms and dissimilar environment (Doyle, 1980; Prescott, 2002). Bioremediation is a multidisciplinary field and researchers from various fields need to integrate their knowledge. The success and efficiency of bioremediation requires the involvement of microbiologists, biochemists, engineers, geologists and soil scientists among others. Knowledge on the combined factors is not enough and mostly these are the rate limiting factors of the process of bioremediation. Interdisciplinary research of at least two fields is unavoidable to advance this technology. Assimilation of scientific ideas across disciplines is most needed to optimize the potential of bioremediation (Blackburn and Hafker, 1993; Boopathy, 2000; Bernard and Jack, 2007). There is no doubt that governmental and private sectors are investing on biotechnology companies, but not much of the revenue goes to the improvement of bioremediation. Lack of revenue is an obstruction to advancement in bioremediation as it prevents further research and discoveries from taking place. Thereupon, progress of bioremediation is delayed as a result of material deficiency (Gadd, 1988; Bouwer and Zehnder, 1993). Infrastructure and materials are needed to carry out research. Accuracy in experimental procedures is crucial to ensure successful treatment when introduced to contaminated site. However, there is inadequacy of infrastructure to analyze the conditions of contaminated sites as well as monitor the process of degradation that has been introduced to the site. Insufficiency of advanced tools specifically designed to aid research on bioremediation would also hold back development of research methodology and prevent discoveries from happening. Despite its shortcomings, its pertinence is unquestionable in the light of present-day environmental hazards. Bioremediation provides a technique for cleaning up pollution by enhancing the same biodegradation processes that occur in nature and potential for significant advances (Bouwer et al., 1998; Boopathy, 2000; Chatterjee et al., 2008; Shazia et al., 2009; Hakeem and Bhatnagar, 2010).

7. Metal toxicity and ManageMent

The pollution of the environment with toxic heavy metals is spreading throughout the world along with industrial progress. Effluents from textile, leather, tannery, electroplating, galvanizing, dyes and pigment, metallurgical industries and other metal processing and refining operations at small and large scale sector contains considerable amounts of toxic metal ions (White et al., 1995). Cadmium, chromium, cobalt, copper, iron, manganese, mercury, molybdenum, nickel, silver and zinc are known to be the most commonly heavy metals used and the more widespread contaminants of the environment (FWPCA, 1998). Traces of these heavy metals are necessary as cofactors of enzymatic reactions, but high levels of them may cause extreme toxicity to living organisms due to inhibition of metabolic reactions. Many compounds that are legally considered to be hazardous can be transformed to harmless products. This eliminates the chance of future liability associated with the treatment and disposal of contaminated material. Instead of transferring contaminants from one environmental medium to


another from land to water or air, the complete destruction of target pollutants is possible (Agarwal, 1998).

Arbitrary and hysterical discharge of industrial and urban wastes into the environmental sink has become an issue of major global concern. Intensification of agriculture and manufacturing industries has resulted in increased release of a wide range of xenobiotic compounds to the environment. Excess loading of hazardous waste has led to scarcity of clean water and disturbance of soil, thus limiting crop production. Although enactment of stringent regulations has led to less indiscriminate disposal of organic and inorganic wastes, challenges remain that require other interventions (Hess et al., 1997). Waste comprises liquid waste discharged by domestic residences, institutions, hospitals, commercial properties, industry or agriculture and can encompass a wide range of potential contaminants and concentrations. In the most common usage, it refers to the municipal wastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewater from different sources. Raw influent includes household waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed to the river via sewers. This affects the BOD, COD, turbidity and also causes the physico-chemical changes in the river. In many areas, sewage also includes liquid waste from industry and commerce. As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediments, heavy metals, organic compounds, animal waste and oil and grease (APHA, 1995; 1998; FWPCA, 1968). The metal ions from metal mining pose problems to the water environment by discharging mine water from underground and open pit mines (Knaebel et al., 1994). Leachate water and runoff water from overburden/waste rock dumps also contaminate nearby water streams (Ferrari et al., 1999). The principle pathways by which leached contaminants can enter groundwater are leakage or spills from storage ponds, leach pad liners, storm water run on/off, uncontrolled leaching from heaps and dumps. These toxic metals ions not only cause potential human health hazards, but also affect other life forms (Malik, 2004). Heavy metal pollution usually results from electroplating, plastic manufacturing, fertilizer, pigment industries, mining and metallurgical processes (Nirmal Kumar et al., 2009).

Heavy metals can be emitted into the environment by both natural and anthropogenic causes. The major causes of emission are the anthropogenic sources specifically mining operations (Hutton and Symon, 1986; Nriagu, 1989). Much of the metal salts within the oceans, lakes, streams and estuaries are in the form of magnesium, calcium, potassium and sodium salts. While many other constituents are clearly available in solution, no element having a concentration less than 2 nmol in the ocean is generally considered essential for life. The metals cannot be degraded to harmless products and hence persist in environmental indefinitely (Silver, 1996). The metals leached out and in sloppy areas, are carried by acid water downstream or run-off to the sea. Through mining activities, waterbodies are most emphatically polluted (INECAR, 2000; European Union, 2002). The heavy metals refer to any metallic element that has a relatively high density than


There are three principal advantages of biological technologies for the removal of pollutants; first, biological processes can be carried out in situ at the contaminated site; second, bioprocess technologies are usually environmentally benign, no secondary pollution and third, they are cost effective. Of the different biological methods, bioaccumulation and biosorption have been demonstrated to possess good potential to replace conventional methods for the removal of metals (Volesky, 1995; King et al., 1997). Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. Researchers develop a feasible method to accelerate the process of decay and removal by encouraging the microbial and associated flora and fauna within the ecosystem to accumulate, degrade and remove the pollutants from the identified sites (Vidali, 2001; Ahalya et al., 2003; Dubey, 2004). Bioremediation is the use of biological systems, almost invariably microorganisms to clean up a contaminated site. In bioremediation processes, microorganisms use the contaminants as nutrient or energy sources. Bioremediation uses naturally occurring or genetically engineered microorganisms such as bacteria and fungi to transform harmful substance into less toxic or non-toxic compounds. Compared to other methods, bioremediation is a more promising and less expensive way for cleaning up contaminated soil and water (Hess et al., 1997; Agarwal, 1998; USEPA, 1999; Kamaludeen et al., 2003; Tang et al., 2007).

In bioremediation, solubilization of heavy metal contaminants provides a means of removal from the solid substance. Some microbial species subject heavy metal ions to enzymatic reduction, the reduced form of heavy metal ions are quite insoluble and they precipitate out of solution (Nies, 1999). Most microorganisms can reproduce rapidly and microbes such as bacteria can also freely exchange genes by conjugation, transformation and transduction between widely divergent species. The organisms that are utilized vary, depending on the chemical nature of the polluting agents and are to be selected carefully as they only survive within a limited range of chemical contaminants (Shazia et al., 2009). Contaminant compounds are transformed by living organisms through reactions that take place as a part of their metabolic processes. Biodegradation of a compound is a result of the actions of multiple organisms. When microorganisms are imported to a contaminated site to enhance degradation, the process is known as bioaugmentation (Agarwal, 1998). Bioremediation is the process whereby organic wastes are biologically degraded under controlled conditions to an innocuous state or to levels below concentration limits established by regulatory authorities. Bioremediation is the elimination or reduction of toxic pollutants from the contaminated sites with the help of biological organisms such as bacteria, fungi and plants by degradation, assimilation or transpiration in the atmosphere. Moreover, the toxicity of heavy metals in wastewater was shown to be dependent on factors like metal species and concentration, pH, wastewater pollution load and solubility of the metal ions (Vidali, 2001; Strong and Burgess, 2008). Organic contaminants may be degraded biologically and CO2 and H2O are the final products, which are not hazardous and may be recycled. In contrast to toxic organic and inorganic substances, the metallic species released into the environment cannot be degraded biologically, chemically or physically (Singh and Ward, 2004).


the cleanup of polluted water and land areas. Bioremediation leads to changes in the solubility, sorption characteristics, transport properties and toxicity metals. It promotes the growth of microorganisms to degrade contaminants by utilizing those contaminants as carbon and energy sources (Teresa and Lee, 1994). The bioremediation systems in operation today rely on microorganisms native to contaminated sites, encouraging them to work by supplying them with the optimum levels of nutrients and other chemicals essentials for their metabolism (Shazia et al., 2009). Microorganisms can be isolated from almost any environmental condition (Holt et al., 1994). The search for alternative and innovative treatment techniques has focused attention on the use of biological materials, such as algae, fungi, yeast and bacteria for the removal and recovery technologies. It has gained importance during recent years because of the better performance and low cost of the biological materials. Since, numerous types of pollutants are to be encountered in a contaminated site, diverse types of microorganisms are likely to be required for effective mediation (Gogoi et al., 2003). Microbes can adapt and grow at subzero temperatures, as well as extreme heat, desert conditions, in water, with an excess of oxygen and in anaerobic conditions, with the presence of hazardous compounds or on any waste stream. The main requirements are, an energy source and a carbon source (Teresa and Lee, 1994). In situ biodegradation involves supplying oxygen and nutrients by circulating aqueous solutions through contaminated soils to stimulate naturally occurring bacteria to degrade organic contaminants. It can be used for soil and groundwater. In situ bioremediation means there is no need to excavate or remove soils or water in order to accomplish remediation. Generally, this technique includes conditions such as infiltration of water containing nutrients and oxygen or other electron acceptors for groundwater treatment. Bioremediation can be carried out on site without causing a major disruption of normal activities (Thassitou and Arvanitoyannis, 2001). This also eliminates the need to transport quantities of waste from the site and the potential threats to human health and the environment that can arise during transportation. Bioremediation can prove less expensive than other technologies that are used for cleanup of hazardous waste (Singh and Ward, 2004; Ramirez et al., 2009). The complex structure of microorganisms implies that there are many ways for the metal to be taken up by the microbial cell (Volesky and Holan, 1995). The presence and activity of microorganisms in biological wastewater treatment are vital to the process. With regard to microbial removal of heavy metals, biosorption phenomenon is crucial (Knauer et al., 1997). The mechanism by which microorganisms remove heavy metals can be divided into three categories; the first mechanism is the biosorption of metal ions on the cell surface, second intracellular uptake of metal ions and third chemical transformation of metal ions by microorganisms (Pardo et al., 2003). Among the different technique employed for metals removal from multi-elemental system, biosorption has been found to be highly selective. Furthermore, the metal accumulating bacteria can be used to remove, concentrate and recover metals from industrial effluents (Loick et al., 2009).


1999). Bacteria, like the more complex eukaryotic organisms, have all the inner workings for cell function, cellular division and growth, genetic transcription, translation and energy production (Silver et al., 1990). All of this activity within a single bacterium typically takes place within a volume of 1.5–2.0 mm3. How the bacteria alter their environment is equally important as how they are impacted by their surroundings. Much of this interaction takes place throughout and external to the outer structures. The enveloping layers that make up the outer structures of the bacterial cell consist of a cytoplasmic membrane, a capsule, sheaths, an S-layer and flagella (Beveridge and Fyfe, 1985). The cell wall consists primarily of a cytoplasmic membrane and a thick layer of peptidoglycan. All cells have a highly selective permeable barrier known as the cytoplasmic membrane, separating the life sustaining metabolic activity of the cell from its environment and allowing all food materials, nutrients and waste products to pass (Silver, 1996). The metals can be captured in the outer portions of the cell such that the organism can be considered to be continuously enveloped in a brine of various ions. This phenomenon of capturing metals can act as the first step in the bacteria’s active accumulation and assimilation of metals at various concentrations. This concentration of metal ions may have developed as a method for accumulating essential cations, if the bacteria find themselves in a more dilute environment. The outer membrane may also immobilize heavy metals, thus preventing their penetration into the cell. Microorganisms’ ability to oxidize metals plays an important role in the biological cycling of metals on a global scale. Microorganisms are also considered for their ability to accumulate metal ions within and on the membrane surface of the cell. Such interactions are so predominant that they can be utilized for economic or ecological gain (Sattelmacher, 2001).

Bacterial and fungal survival in the presence of toxic metals mainly depends on intrinsic biochemical and structural properties, physiological and genetically adaptation, including morphological changes and environmental modification of metal speciation, availability and toxicity, the relative importance of each being difficult to determine. It is probably more appropriate to define resistance by means of a mechanism produced in direct response to the metal species concerned, e.g., synthesis of metallothioneins (Eccles and Hunt, 1986; Gadd, 1992). Arbitrary terms, such as resistance and tolerance which are used rather loosely and interchangeably in the literature are generally based on the ability to grow on a certain metal concentration in laboratory media (Ehrlich, 1997). Metal tolerance may be defined as the ability of an organism to survive metal toxicity by means of intrinsic properties and/or environmental modification of toxicity. Intrinsic properties that can determine survival include possession of impermeable pigmented cell walls, extracellular polysaccharide and metabolite excretion, especially where this leads to detoxification of the metal species by binding or perception (Aksu, 1998; Giller et al., 1999). However, in many cases distinctions are difficult because of the involvement of several direct and indirect physico-chemical and biological mechanisms implicated in fungal survival as distinct from environmental modification of toxicity include extracellular precipitation, complexion and crystallization, transformation of metal


species by oxidation, reduction, methylation and dealkylation, biosorption to cell walls, pigments and extracellular polysaccharide, decreased transport or impermeability, efflux, intracellular compartmentation and precipitation and/or sequestration. A particular organism may directly and/or indirectly rely on several survival strategies (Peng et al., 2010).

The heavy metal removal from water and wastewater is crucial to protect the water environment from the heavy metal pollution. The heavy metal resistant microorganisms have significant role in bioremediation (Zouboulis et al., 2004; Peng et al., 2010). The metal and organic pollutants can be removed by the microbial flora. Bacillus sp. was very much efficient to remove the Au, Cd, Cr, Fe, Mn, Ni, Pb, U and Zn. It was recorded that Bacillus sp. can efficiently remove the metal pollutants from the waste or industrial effluents (Gunasekaran et al., 2003) and some aromatics, long chain alkanes, phenol, cresol (Cybulski et al., 2003). Pseudomonas sp. were reported to Cd, Cr, Cu, Ni, Pb, U and Zn (Sar and D’Souza, 2001; Tarangini, 2009). Ilhan et al., (2004) observed that Staphylococcus saprophyticus reduced Cr, Cu and Pb ions. The isolate was adsorbed 100 percent Pb ions at different pH range of 3–5 and at 27°C temperature. Corynebacterium sp. and Flavobacterium sp. were reported for removal of organic contaminants and halogenated hydrocarbons, phenoxyacetaes and aromatic hydrocarbons (Jogdand, 1995). Corynebacterium glutamicum was able to reduce the thorium and uranium (Tarangini, 2009). The biosorption of heavy metal Cd and Cu by Flavobacterium sp. was reported by Rajbanshi (2008) whereas, biosorption of Cu, Fe and Zn by Corynebacterium sp. was reported by Odokuma, (2009). Many investigators (Mattuschka et al., 1993; Jogdand, 1995; Tarangini 2009) reported that Streptomyces sp. can be used to remove the metal ions of Ag, Cd, Cu, Cr, Ni, Pb, U and Zn from the waste. Streptomyces noursei was able to reduce the metal ions in order Pb>Ag> Cu>Cr (Mattuschka et al., 1993). Aspergillus niger was reported to remove Ag, Au, Cd, Cu, Th, U and Zn (Townsley et al., 1986; Kuyucak and Volesky, 1988; Gunasekaran et al., 2003). S. meliloti has huge potential to reduce the toxicity from the environment reported by Chauhan (2015). Biosorption is being established as a useful alternative to conventional systems for the removal of toxic metals from industrial effluents/waste. Bioremediation provides a technique for cleaning up pollution by enhancing the natural biodegradation processes. So, by developing an understanding of microbial communities and their response to the natural environment and pollutants, expanding the knowledge of the genetics of the microbes to increase capabilities to degrade pollutants, conducting field trials of new bioremediation techniques which are cost effective and dedicating sites which are set aside for long term research purpose, these opportunities offer potential for significant advances (Sutar and Das, 2012).

More research is needed to assess the extent to which these products affect human health. Public awareness should be created. There should be monitoring and control over the concentration of heavy metals in cosmetics. The existence of metals in nano form or otherwise should be determined. Toxicity of metals bearing nano particles is a domain where systematic research needs to be carried out to establish or negate toxic factors.


10. conclusion

While anthropogenic activities are the major source of heavy metal pollution, natural sources contribute significantly to the burden of arsenic and fluoride. Apart from industries, road runoff is also an important source. The toxic elements enter the body mainly through water, food and air. Cosmetics, dental products, some drugs, particularly Ayurved and Unani drugs also contribute. Major pollutants are introduced into the aquatic systems significantly as a result of various industrial operations. The lack of technical and financial resources and the regulatory controls for the management of hazardous wastes in the past had led to the unscientific disposal of hazardous wastes, which posed serious risks to human, animal and plant life. A huge quantity of pollutants in the form of domestic and industrial effluents is discharged directly or indirectly into the soil/waterbodies, which has severe impacts on its biotic and abiotic environment. During rain, surface water with soil, mud and humus enter the rivers, tanks and other waterbodies. The inorganic minerals like sodium, potassium, calcium, magnesium and heavy metals like iron, manganese, lead, mercury, chromium, cadmium, nickel, cobalt, copper, etc., when reach to the soil/river water caused pollution. Heavy metal contamination is becoming a great concern to the environmental awareness and government policies. Several heavy metal removal technologies including chemical precipitation, ion exchange, reverse osmosis, electrodialysis, ultrafiltration and phytoremediation are commonly used in industries. However, these technologies are becoming uneconomical and unfavourable to remove heavy metal from contaminated sites or wastewater. With increasing environmental attention and legal constraint on discharge effluents, a need of cost effective technology is essential. Therefore, the search for efficient, eco-friendly and cost effective remedies for wastewater treatment has been initiated. In recent years, research has been focused on biological method, e.g., bioremediation, is in the process of commercialization. Bioremediation provides a technique for cleaning up pollution by enhancing the natural biodegradation processes. The bioremediation is one of the most promising technological approaches to the problem of hazardous waste. This process relies on microorganisms, such as bacteria or fungi to transform hazardous chemicals into less toxic or nontoxic substances. Such biological transformation is more attractive than direct chemical or physical treatment. Microorganisms directly degrade contaminants rather than merely transferring them from one medium to another, employ metabolic degradation pathways and can be used in situ to minimize disturbance of the cleanup site. Hence, microorganisms can be effective, economical and non-disruptive tools for eliminating hazardous chemicals. There is no doubt that bioremediation is in the process of paving a way to greater pastures. This technology offers an efficient and cost effective way to treat contaminated soil, waste and water. Its advantages generally outweigh the disadvantages, hence may be used as a management tool.


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