Radionuclides and metals can be mobilized by chelating
agents typically present in low-level radioactive liquid wastes.
[60Co]EDTA in the form 60CoIIIEDTA- 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, 60CoIIEDTA2-, 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 CoIIIEDTA- 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 CoIIIEDTA- 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 CoIIIEDTA- 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 CoIIIEDTA- 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.
