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 pre sent in low-level radioactive liquid wastes. [Co-60]EDTA in the form (60)Co (III)EDTA(-) represents a;highly stable [log K-Co(III)EDTA = 43.9] and mobi le form of this radionuclide. By contrast, the reduced form of this metal-l igand complex, (60)Co(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 t heir oxidation state. We conducted a series of column experiments to provid e 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 de monstrate the sustained reduction of Co(III)EDTA- in column flow experiment s with the desired result that a less stable, less mobile product was forme d. 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 reduct ion reaction. Even in the presence of a strong mineral oxidant (beta-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 ac tivity 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 termi nal electron acceptor. These results demonstrate that DMRB can he effective in the manipulation of redox-sensitive metals and radionuclides in a syste m characterized by the advective transport of solutes.