Direct and Fe(II)-mediated reduction of technetium by Fe(III)-reducing bacteria

The dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens reduc ed and precipitated Tc(Vn) by two mechanisms. Washed cell suspensions coupl ed the oxidation of hydrogen to enzymatic reduction of Tc(MI) to Tc(IV), le ading to the precipitation of TcO2 at the periphery of the cell. An indirec t, Fe(II)-mediated mechanism was also identified. Acetate, although not uti lized efficiently as an electron donor for direct cell-mediated reduction o f technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII), Tc(VII) reduction was c omparatively inefficient via this indirect mechanism when soluble Fe(III) c itrate was supplied to the cultures but was enhanced in the presence of sol id Fe(III) oxide. The rate of Tc(VII) reduction was optimal, however, when Fe(III) oxide reduction was stimulated by the addition of the humic analog and electron shuttle anthaquinone-2,6-disulfonate, leading to the rapid for mation of the Fe(II)-bearing mineral magnetite, Under these conditions, Tc( VII) was reduced and precipitated abiotically on the nanocrystals of biogen ic magnetite as TcO2 and was removed from solution to concentrations below the limit of detection by scintillation counting. Cultures of Fe(III)-reduc ing bacteria enriched from radionuclide-contaminated sediment using Fe(III) oxide as an electron acceptor in the presence of 25 mu M Tc(VII) contained a single Geobacter sp, detected by 16S ribosomal DNA analysis and were als o able to reduce and precipitate the radionuclide via biogenic magnetite, F e(III) reduction was stimulated in aquifer material, resulting in the forma tion of Fe(II)-containing minerals that were able to reduce and precipitate Tc(VII). These results suggest that Fe(III)-reducing bacteria may play an important role in immobilizing technetium in sediments via direct and indir ect mechanisms.