The role of microorganisms in biosorption of toxic metals and radionuclides

A multiplicity of physico-chemical and biological mechanisms determine the removal of toxic metals, metalloids and radionuclides from contaminated wastes. Physico-chemical mechanisms of removal, which may be encompassed by the general term "biosorption", include adsorption, ion exchange and entrapment which are features of living and dead biomass as well as derived products. In living cells, biosorption can be directly and indirectly influenced by metabolism. Metabolism-dependent mechanisms of metal removal which occur in living microorganisms include metal precipitation as sulphides, complexation by siderophores and other metabolites, sequestration by metal-binding proteins and peptides, transport and intracellular compartmentation. In addition, transformations of metal species can occur resulting in oxidation, reduction or methylation. For metalloids such as selenium, two main transformation mechanisms are the reduction of oxyanions to elemental forms, and methylation to methylated derivatives which are volatilized. Such mechanisms are important components of natural biogeochemical cycles for metals and metalloids as well as being of potential application for bioremediation.