Microbial Reduction of CobaltIIIEDTA- in the Presence and Absence of Manganese(IV) Oxide

Codisposal of 60 Co 2+ and EDTA has promoted the transport of radioactive 60 Co 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(III) to Co(II) decreases the stability of the radionuclide−chelate complex and can limit the transport of the 60 Co 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 be 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 reduc tion 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.