Sustained bacterial reduction of Co(III)EDTA^- in the presence of competing geochemical oxidation during dynamic flow

Radionuclides and metals can be mobilized by chelating agents typically present in low-level radioactive liquid wastes. [⁶⁰Co]EDTA in the form ⁶⁰Co(III)EDTA^- represents a highly stable [log K(Co(III)EDTA) = 43.9] and mobile form of this radionuclide. By contrast, the reduced form of this metal - ligand complex, ⁶⁰Co(II)EDTA^2-, is much less stable [log K(Co(II)EDTA) = 18.3] and less mobile. There is an increasing awareness that dissimilatory metal-reducing bacteria (DMRB) can be used to mediate redox transformations of metals and radionuclides whose stability and mobility are governed by their oxidation state. We conducted a series of column experiments to provide an improved understanding of Co(III)EDTA^- reduction by the facultative anaerobe Shewanella alga BrY (BrY). Experiments were conducted under growth conditions using lactate as a carbon and energy source. We were able to demonstrate the sustained reduction of Co(III)EDTA^- in column flow experiments with the desired result that a less stable, less mobile product was formed. The amount of reduction varied directly with the fluid residence time in the columns. In the presence of a suitable mineral sorbent [Fe(OH)₃], Co-EDTA transport was delayed as a direct consequence of the bacterial reduction reaction. Even in the presence of a strong mineral oxidant (β-MnO₂) the net reduction of Co(III)EDTA^- dominated the fate and transport of this species. The system was stable after flow interrupts, and metal-reducing activity could be revived after flushing the columns for 5 days with nutrient-free solution. Furthermore, we demonstrated that BrY could grow and carry out sustained reduction using geochemically derived Co(III)EDTA^- as terminal electron acceptor. These results demonstrate that DMRB can be effective in the manipulation of redox-sensitive metals and radionuclides in a system characterized by the advective transport of solutes.