Water bodies are being overwhelmed with bacteria and waste matter. Among toxic substances reaching hazardous levels are heavy metals (Regine and Volesky, 2000). Heavy metals of concern include lead, chromium, mercury, uranium, selenium, zinc, arsenic, cadmium, silver, gold, and nickel (Ahalya et al., 2003). Heavy metal pollution in the aquatic system has become a serious threat today and of great environmental concern as they are non-biodegradable and thus persistent. Metals are mobilized and carried into food web as a result of leaching from waste dumps, polluted soils and water. The metals increase in concentration at every level of food chain and are passed onto the next higher level–a phenomenon called bio-magnification (Paknikar et al., 2003). Heavy metals even at low concentrations can cause toxi-city to humans and other forms of life, its adverse effects on human health are quite evident from Table 1. The toxicity of metal ion is owing to their ability to bind with protein molecules and prevent replication of DNA and thus subsequent cell division (Kar et al., 1992). To avoid health hazards it is essential to remove these toxic heavy metals from waste water before its disposal. Main sources of heavy metal contamination include urban industrial aerosols, solid wastes from animals, mining activities, industrial and agricultural chemicals. Heavy metals also enter the water supply from industrial and consumer water or even from acid rain which breaks down soils and rocks, releasing heavy metals into streams, lakes and ground water. Techniques presently in existence for removal of heavy metals from contaminated waters include: reverse osmosis, electrodialysis, ultrafiltration, ion-exchange, chemical precipitation, phytoremediation, etc. However, all these methods have disadvantages like incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require careful disposal (Ahalya et al., 2003). With increasing environmental awareness and legal constraints being imposed on discharge of effluents, a need for cost–effective alternative technologies are essential. In this endeavor, microbial biomass has emerged as an option for developing economic and eco-friendly waste water treatment process.

Biosorption can be defined as “a non-directed physicochemical interaction that may occur between metal/radionuclide species and microbial cells” (Shumate and Stranberg, 1985). It is a biological method of environmental control and can be an alternative to conventional contaminated water treatment facilities. It also offers several advantages over conventional treatment methods including cost effectiveness, efficiency, minimization of chemical/biological sludge, requirement of additional nutrients, and regeneration of biosorbent with possibility of metal recovery.

The biosorption process involves a solid phase (sorbent or biosorbent; usually a biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, a metal ion). Due to higher affinity of the sorbent for the sorbate species the latter is attracted and bound with different mechanisms. The process continues till equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution. While there is a preponderance of solute (sorbate) molecules in the solution, there are none in the sorbent particle to start with. This imbalance between the two environments creates a driving force for the solute species. The heavy metals adsorb on the surface of biomass thus, the biosorbent becomes enriched with metal ions in the sorbate.