Heavy Metal Pollutants: Environmental and Biotechnological Aspects

Microorganisms play important roles in the environmental fate of toxic metals, metalloids, and radionuclides with physicochemical and biological mechanisms mediating transformations between soluble and insoluble phases. Such mechanisms are important components of natural biogeochemical cycles for metals and associated elements, for example, S and P, with some processes relevant to the treatment of contaminated materials. Metal mobilization can be achieved by chemolithotrophic (autotrophic) and chemoorganotrophic (heterotrophic) leaching, complexation by metabolites and siderophores, and methylation. Immobilization can result from sorption to cell components or exopolymers, transport and intracellular sequestration, or precipitation as organic and inorganic compounds, for example, oxalates, sulfides, or phosphates. In addition, microbial reduction of higher valency species may effect either mobilization, for example, Mn(IV) to Mn(II), or immobilization, for example, Cr(VI) to Cr(III); U(VI) to U(IV). For bioremediation, solubilization of metal contaminants provides a means for removal from solid matrices, such as soils and industrial wastes. Alternatively, immobilization processes enable metals to be transformed in situ and in bioreactors into insoluble, chemically inert forms. Biotechnological development of microbial systems may provide an alternative or adjunct to conventional physicochemical treatment methods for contaminated effluents and wastewaters.