title: 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.
authors.affiliation: Earth System Science Institute, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Gareth T W
authors.initials: GT
authors.lastname: Law
authors.forename: Andrea
authors.initials: A
authors.lastname: Geissler
authors.forename: Christopher
authors.initials: C
authors.lastname: Boothman
authors.forename: Ian T
authors.initials: IT
authors.lastname: Burke
authors.forename: Francis R
authors.initials: FR
authors.lastname: Livens
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Lloyd
authors.forename: Katherine
authors.initials: K
authors.lastname: Morris
mesh_terms: D019015:Geologic Sediments / Q000737:chemistry*; D009566:Nitrates / Q000737:chemistry*; D010084:Oxidation-Reduction; D016133:Polymerase Chain Reaction; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D013667:Technetium / Q000378:metabolism*; D014875:Water Pollutants, Radioactive / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D009566:Nitrates; D012336:RNA, Ribosomal, 16S; D014875:Water Pollutants, Radioactive; D013667:Technetium