Biotransformation of heavy metal(loid)s in relation to the remediation of contaminated soils

The dynamics of trace elements in soils is dependent on both their physicochemical interactions with inorganic and organic soil constituents and their biological interactions linked to the microbial activities of soil-plant systems. Microorganisms control the transformation (microbial or biotransformation) of trace elements by several mechanisms that include oxidation, reduction, methylation, demethylation, complex formation, and biosorption. Microbial transformation plays a major role in the behavior and fate of toxic elements, especially arsenic (As), chromium (Cr), mercury (Hg), and selenium (Se) in soils and sediments. Biotransformation processes can alter the speciation and redox state of these elements and hence control their solubility and subsequent mobility. These processes play an important role in the bioavailability, mobility, ecotoxicity, and environmental health of these trace elements. A greater understanding of biotransformation processes is necessary to efficiently manage and utilize them for contaminant removal and to develop in situ bioremediation technologies. In this chapter, the key microbial transformation processes, including biosorption, redox reactions, and methylation/demethylation reactions controlling the fate and behavior of As, Cr, Hg, and Se, are addressed. The factors affecting these processes in relation to the bioavailability and remediation of trace elements in the environment are also examined, and possible future research directions are recommended.