Microbial reduction of cobaltIIIEDTA- in the presence and absence of manganese(IV) oxide

Codisposal of Co-60(2+) and EDTA has promoted the transport of radioactive Co-60 in the environment as (60)CoEDTA complexes. Chemical oxidation of Co(II)EDTA(2-) to highly stable and mobile Co(III)EDTA(-) by manganese(VI) oxide minerals can occur under aerobic and anaerobic conditions. Reduction of Co(lll) to Co(ll) decreases the stability of the radionuclide-chelate complex and can limit the transport of the Co-60 in subsurface environments. This study investigated the microbial reduction of Co(III)EDTA(-) in the presence and absence of reactive manganese(IV) oxides. The metal-reducing bacterium Shewanella alga strain BrY enzymatically reduced Co(III)EDTA(-) to Co(II)EDTA(2-) with a 1:1 stoichiometry. Reduction of Co(III)EDTA(-) was not affected by radioactive (60)Co(III)EDTA(-) at concentrations exceeding those recorded in contaminated environments. Bacterial reduction of Co(III)EDTA(-) could he coupled to the chemical oxidation of Co(II)EDTA(2-) by the manganese(IV) oxide mineral pyrolusite, resulting in biotic-abiotic cycling between Co(II)EDTA(2-) and Co(III)EDTA(-). Co(III)EDTA(-) significantly increased the rate and extent of manganese(IV) oxide reduction in the presence of metal reducing bacteria, and the Co(II)EDTA(2-) complex did not dissociate in these anoxic studies. Direct reduction of Co(III)EDTA(-) by microorganisms and geochemical oxidation of Co(II)EDTA(2-) by manganese(IV) oxides are important components of a complex set of coupled microbial and geochemical reactions that may influence the fate and transport of (60)Co(III)EDTA(2-) in the environment.