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5 Bioremediation of Heavy Metals

Prior to the application of biological processes for the removal of heavy metals

from the environment, conventional methods like chemical precipitation, chemical

redox reactions, ion exchange, filtration and reverse osmosis were the method of

choice. These techniques had some demerits, for instance, if the concentration of

heavy metals was below 100 mg/l, then it cannot be removed by these techniques.

Similarly, these techniques were expensive, were difficult to operate, and produce

some secondary contaminants. The field application of conventional metal remedi-

ating methods is often observed to be expensive and inefficient, which lead towards

the development of new methods. Keeping in mind the demerits of these methods,

biological removal of heavy metals can be efficient, easy, cost-effective and environ-

mentally friendly strategy.

5.5

Bioremediation – The Emerging Sustainable

Strategy

Bioremediation is a sustainable, environment friendly strategy that explores the

cellular resistance of microorganisms and plants to clean-up contaminated envi-

ronment. It achieves contaminant decomposition by existing metabolic potential of

microorganisms. Bioremediation as a technology may be introduced in the removal

of xenobiotic compounds from agrochemical and petrochemical industries, oil

spills, heavy metals in sewage, sludge and marine sediments, etc.

Decontamination of heavy metals from polluted environment is of great signif-

icance to local agriculture and the population elsewhere in the affected area. The

disadvantage of the traditional metal decontamination techniques includes lesser

accuracy, particularly, in very low heavy metal concentration and secondary envi-

ronmental pollution due to the chemicals used in the remediation process. The cost

that is involved restricts the utilization of the prevailing techniques.

Bioremediation techniques are often broadly divided into in situ and ex situ biore-

mediation strategies. The in situ technique deals with the treatment of soil and asso-

ciated ground water in its original place without displacing the material, whereas the

ex situ process involves removal of the entire contaminated material for treatment at

different places where the activity of bioremediating agent could be controlled. But

the field selection of the choices depends on three basic principles: the responsive-

ness of environmental pollutant to biological transformation; the accessibility of the

contaminant to bioremediation agent (bioavailability); and the possibilities for the

optimization of biological activity (bioactivity).

5.5.1

Intervention of Metal Contamination by Microbial Adaptation

Microbe-assisted bioremediation of heavy metal involves uptake of heavy metals

by microorganisms either by bioaccumulation, which is an active process, and/or

through adsorption, which is a passive process. Microbial cell wall comprises var-

ious functional groups such as carboxylate, hydroxyl, amino, and phosphate. The

metal ions can easily bind to such groups and be separated from the environment.