Technetium reduction and reoxidation in aquifer sediments

This study describes the biogeochemical behaviour of the radionuclide technetium (Tc-99) in background area sediments from the US Department of Energy Field Research Center (FRC) in Oak Ridge, TN, USA. Microcosm experiments with trace levels of Tc-99(VII) were used to examine Tc reduction and reoxidation. Efficient removal of 0.5 mu M Tc(VII) from solution was seen under Fe(III)-reducing conditions, and was attributed to a lower valence insoluble form of the radionuclide. Molecular and cultivation-dependent analysis confirmed the presence of known Fe(III)-reducing bacteria (Geothrix and Geobacter species) in these sediments. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic analysis of analogous microcosm experiments, challenged with higher (550 mu M) concentrations of Tc(VII), confirmed the presence of reduced insoluble Tc(IV) as hydrous TcO2 in the Fe(II)-bearing sediments. Reoxidation experiments of pre-reduced microcosms challenged with 0.5 mu M Tc-99 showed very limited (< 3 %) remobilization of the reduced Tc-99 with 100 mM nitrate but significant (ca 80%) remobilization of Tc-99 under air reoxidation conditions. Fe(II) oxidation was, however, significant in all oxidation treatments. EXAFS analyses of Fe(II)-bearing sediments challenged with higher (550 mu M) concentrations of Tc(VII) and then reoxidized with 100 mM nitrate contained both Tc(IV) and Tc(VII) immobile phases. These results suggest that under anaerobic oxidation conditions, Tc(IV) will not remobilize rapidly, even in the presence of high concentrations of nitrate. This has implications for the biogeochemical cycling of technetium in contaminated environments, including those where bioreduction has been stimulated to minimize transport of the radionuclide.