Sustained Bacterial Reduction of Co^IIIEDTA^- in the Presence of Competing Geochemical Oxidation during Dynamic Flow

Abstract

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.

Metal reduction at cold temperatures by Shewanella isolates from various marine environments

“…60 Co has been identified as a priority pollutant by the United States Department of Energy, and its migration in contaminated subsurface environments is influenced by the presence of chelating agents, as well as local mineralogy (Szecsody et al 1994, Brooks et al 1999). W3-6-1 indicates that the cobalt cycle should remain active at low temperatures, similar to iron cycling in marine sediments.…”

Environmental Assessment for Selection and Operation of the Proposed Field Research Centers for the Natural and Accelerated Bioremediation Research (NABIR) Program

“…Injection of electron donors or acceptors or nutrients into the contaminated area would be at levels more consistent with those used at the Schoolcraft site rather than the levels encountered during the guar gum situation. For example, as part of another program, ORNL scientists are planning to inject less than 700 ppm of lactate (an electron donor) to stimulate the microbial community in another field site on the Oak Ridge Reservation to examine whether cobalt contamination can be mitigated (Jardine I999b; Brooks et al 1999). Another reason for injecting only low levels of electron donors or acceptors or nutrients is that at high concentrations, the injection of electron donors or acceptors or nutrients could overstimulate microbial reproduction and result in well clogging.…”

“…Based on the discussion of the results from work at another Oak Ridge Reservation field site, as presented in Section 4.1.3.1.3, it does not appear that injection of chelators would significantly affect the groundwater characteristics of the contaminated area (Jardine et al 1999). Chelators would not be added to the background area.…”

Dimensionless parameters to summarize the influence of microbial growth and inhibition on the bioremediation of groundwater contaminants

“…Bioattenuation involving organic substrates and redoxsensitive species has also been the focus of several studies (e.g. Prommer et al 1998;Brooks et al 1999;Saiers et al 2000;Vainshtein et al 2003;Curtis 2003;Khan and Husain 2003;Guha 2004;Yu et al 2007;Bauer et al 2008). To model these systems, it is critical to capture the biogeochemistry.…”