Anaerobic microbial dissolution of transition and heavy metal oxides

Abstract

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-fixing Clostridium sp. with an acetic, butyric, and lactic acid fermentation pattern, isolated from coal-cleaning waste, solubilized Fe(2)O(3) and MnO(2) 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 Fe(2)O(3) (1.7 mumol); however, direct contact with the bacterial cells resulted in the complete dissolution (4.8 mumol) of the oxide. Under identical conditions, the cell-free spent medium solubilized only a small amount of MnO(2) (0.07 mumol), whereas 2.3 mumol of the oxide was solubilized by direct bacterial contact. Reduction of Fe(2)O(3) and MnO(2) by Clostridium 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 Co(2)O(3) indicate that there is a slight dissolution of Co (0.16 mumol) followed by precipitation or biosorption. Although Mn(2)O(3), Ni(2)O(3), and PbO(2) may undergo reductive dissolution from a higher to a lower oxidation state, dissolution by direct or indirect action was not observed. Also, Cr(2)O(3) 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 Cu inhibited the growth of the bacterium.