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5 Bioremediation of Heavy Metals

techniques could not serve ecosystem-friendly heavy metal decontamination,

which leads to the development of environment-friendly strategies that may be

employed to clean up the environment. Thus, the conventional decontamination

techniques which often accelerates removal of all microbial activities including the

ecologically important microbial symbionts, for instance, nitrogen-fixing bacteria

and mycorrhizae, enhancing reduction of biodiversity, ecologically sustainable

bioremediation strategies developed in recent time [5, 6]. The bioremediation

strategies utilize metabolism of life forms for viable, safer, more efficient, and less

expensive physiochemical methods for metal decontamination.

Interestingly, microorganisms do require certain metal ions, such as those of

Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺ in very low concentrations, as essential micronutrients

as components of important cofactors in enzymatic reactions. Numerous findings

have reported that some microbes are tolerant of heavy metals with an ability to

either remove them from the environment or breakdown them to a less toxic or

comparatively benign forms [6]. Microbial resistance or tolerance to pollutants is

vital for the process of environmental bioremediation.

The potential for bioremediation of heavy metals by microorganisms is very much

dependent on the nature of the site and the chemicals in the environment. It remains

the most cost-effective process that reduces pollutants to non-hazardous materials.

Over the past decade or so, the use of microorganisms in treating wastewaters con-

taminated with heavy metals has become an attractive technique. Currently, much

work is being done in the removal of nitrogen, phosphorous, and metal ions from

commercial and municipal waste, by bioremediation. Most microorganisms have

their origin in soil and play a direct or indirect role in maintaining the biogeochemi-

cal cycles within the soil ecosystem. They play an important role in recycling mineral

nutrients such as nitrogen, phosphorous, sulfur, and numerous metallic ions of cop-

per, mercury, iron, and aluminum, thereby contributing substantially to life forms

and also influencing various microbial populations and their related functions.

5.2

Ubiquitous Heavy Metal Contamination – The

Global Scenario

Although geogenic activities are the primary cause of the ubiquity of environmental

heavy metals in world, but in recent times anthropogenic activities have become a

serious concern [5]. Anthropogenic activities such as mining, refinement of ore, fuel

combustion, metal-working industries, battery manufacturing, paints and preserva-

tives, insecticides, and fertilizers have led to the emission of heavy metals and their

accumulation in human habitable ecosystem causing serious threat to the environ-

ment [1].

When considering the anthropogenic contribution to heavy metal pollution, it has

been reported that anthropogenic emissions of Cd are in the range of 30 000 ton/year.

In unpolluted soil, Cd is present at a concentration of 0.1–0.5 mg/kg, but in heav-

ily polluted soils of sewage sludge, concentrations of up to 150 mg/kg have been

found. Arsenic, the metalloid ranking twentieth in abundance of elements in the