Role of Nitrate in Conditioning Aquifer Sediments for Technetium Bioreduction

Abstract

Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence of pH and nitrate on bioreduction processes. The systems studied include unamended-, carbonate buffered-, low nitrate-, and high nitrate-groundwaters. During anaerobic incubation, terminal electron accepting processes (TEAPs) in the circumneutral pH, carbonate buffered system progressed to sulfate reduction, and Tc(VII) was removed from solution during Fe(III) reduction. In the high-nitrate system, pH increased during denitrification (pH 5.5 to 7.2), then TEAPs progressed to sulfate reduction. Again, Tc(VII) removal was associated with Fe(III) reduction. In both systems, XAS confirmed reduction to hydrous Tc(IV)O(2) like phases on Tc removal from solution. In the unamended and low-nitrate systems, the pH remained low, Fe(III) reduction was inhibited, and Tc(VII) remained in solution. Thus, nitrate can have complex influences on the development of the metal reducing conditions required for radionuclide treatment. High nitrate concentrations stimulated denitrification and caused pH neutralization facilitating Fe(III) reduction and Tc(VII) removal; acidic, low nitrate systems showed no Fe(III)-reduction. These results have implications for Tc-cycling in contaminated environments where nitrate has been considered undesirable, but where it may enhance Fe(III)-reduction via a novel pH "conditioning" step.

The Impact of Gamma Radiation on Sediment Microbial Processes

“…Analysis of the bacterial community in the oxic starting sediment revealed a relatively diverse community with 16 phyla detected through pyrosequencing of 16S rRNA gene amplicons. Communities were dominated by species representing the acidobacteria (47%) and proteobacteria (32%), a finding consistent with previous studies conducted on Sellafield-type sediments (38,39). Of the most dominant individual species, an uncharacterized acidobacterium and a bacterium of the Bradyrhizobiaceae family (proteobacteria) represented 5 and 4% of the complex microbial community, respectively.…”

“…This site was selected because our group has extensive experience studying the biogeochemistry of sediment from this area. Samples were collected from the shallow subsurface at a locality ϳ2 km from the Sellafield site, in the Calder Valley, Cumbria (38)(39)(40). Samples were transferred to sterile containers, sealed, and stored in the dark at 4°C prior to use.…”

Redox interactions of technetium with iron-bearing minerals

“…For the reduced minerals, the biosiderite, bio-vivianite and Fe(II)-gel all showed significantly better fits with a backscatterer in the second shell, and with marginally better fits apparent with a Tc-backscatterer. This is similar to past work on sediments and indicates a short range environment for Tc in these systems Law et al, 2010a;Plymale et al, 2011). The bio-magnetite sample showed a poor fit to the model with a Tc-backscatterer in the second shell; here the Tc-backscatterer model required an unrealistically short bond length (2.46 Å ) or showed a high DebyeÀWaller factor.…”

“…Further, bioreduction (where indigenous sediment microorganisms are stimulated to produce reducing conditions by the addition of an electron donor) has been shown to be an effective means of removing Tc(VII) from solution in microcosm (Burke et al, 2005;Begg et al, 2007;Law et al, 2010a), column (Michalsen et al, 2006;Lear et al, 2010) and field studies (Istok et al, 2004) over a wide range of conditions. A typical feature in these bioreduction experiments is that the removal of Tc(VII) is associated with the development of microbially-mediated Fe(III) reduction in the sediments (Burke et al, 2005;Fredrickson et al, 2004;Li and Krumholz, 2008;Michalsen et al, 2006;Wildung et al, 2004).…”

“…A typical feature in these bioreduction experiments is that the removal of Tc(VII) is associated with the development of microbially-mediated Fe(III) reduction in the sediments (Burke et al, 2005;Fredrickson et al, 2004;Li and Krumholz, 2008;Michalsen et al, 2006;Wildung et al, 2004). Furthermore, removal is thought to be dominated by abiotic electron transfer from sediment associated biogenic Fe(II) to Tc(VII) in all but Fe-poor environments (Lloyd et al, 2000;Begg et al, 2007;Fredrickson et al, 2004;Plymale et al, 2011;Law et al, 2010a;Zachara et al, 2007). Indeed, in experiments using different sediment types, Tc has been found to be associated primarily with Fe-rich (Lear et al, 2010, Fredrickson et al, 2004 or Fe(II)-rich nano particles.…”