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Anaerobic microbial dissolution of several crystalline, water-insoluble forms of metal oxides commonly associated with the waste from energy production was investigated. An anaerobic N-fixingClostridium sp. with an acetic, butyric, and lactic acid fermentation pattern, isolated from coal-cleaning waste, solubilized Fe2 O3 and MnO2 by direct enzymatic reduction; CdO, CuO, PbO, and ZnO were solubilized by indirect action due to the production of metabolites and the lowering of the pH of the growth medium. Extracellular heat-labile components of the cell-free spent medium obtained from cultures without oxide solubilized a significant amount of Fe2 O3 (1.7 μmol); however, direct contact with the bacterial cells resulted in the complete dissolution (4.8 μmol) of the oxide. Under identical conditions, the cell-free spent medium solubilized only a small amount of MnO2 (0.07 μmol), whereas 2.3 μmol of the oxide was solubilized by direct bacterial contact. Reduction of Fe2 O3 and MnO2 byClostridium sp. proceeds at different rates and, possibly, by different enzymatic systems. Fe(III) and Mn(IV) oxides appear to be used as sinks for excess electrons generated from glucose fermentation, since there is no apparent increase in growth of the bacterium concomitant with the reduction of the oxides. Dialysis bag experiments with Co2 O3 indicate that there is a slight dissolution of Co (0.16 μmol) followed by precipitation or biosorption. Although Mn2 O3 , Ni2 O3 , and PbO2 may undergo reductive dissolution from a higher to a lower oxidation state, dissolution by direct or indirect action was not observed. Also, Cr2 O3 and NiO were not solubilized by direct or indirect action. Significant amounts of solubilized Cd, Cu, and Pb were immobilized by the bacterial biomass, and the addition of Cu2+ inhibited the growth of the bacterium.

Anaerobic Microbial Dissolution of Transition and Heavy Metal Oxides