Microbial removal and recovery of metals from wastewater

Removal and recovery of metal elements, including metalloids and nonmetals, from wastewater is important for preventing environmental contamination and for sustainable use of resources. Existing physicochemical technologies for metal removal/recovery have some drawbacks such as high cost, material requirements, and environmental impact. Microbial reactions for metal removal and recovery may provide attractive, environmentally friendly alternatives including (i) biomineralization (immobilization of soluble metals to solid phase through microbial metabolisms such as oxidative or reductive transformations)/bioprecipitation, (ii) biovolatilization (conversion of soluble/solid metals into gaseous phase by reduction and methylation/alkylation), (iii) bioleaching (a process where metals in solid phase are solubilized by microbial mechanisms), and (iv) biosorption (binding or accumulation of soluble metals on the surface of microbial biomass or biomolecules like biopolymers). Metals in wastewater can be removed from an aqueous phase and recovered by solid/liquid or gas/liquid separation processes after these microbial reactions. Although various kinds of microbial metal metabolisms and metal-microbe interactions have been studied and their uses for metal removal/recovery from wastewater have been proposed, full-scale practical applications, especially for recovery of metals as resources, are limited. As an example of the research and development necessary for implementing such technologies, research on selenium recovery using a selenate-reducing and selenium-volatilizing bacterium is described. Lastly, future prospects toward environmental conservation and solutions to resource depletion focusing on the strategies of removal/recovery of metals are described.