Competitive reduction of pertechnetate (99TcO4-) by dissimilatory metal reducing bacteria and biogenic Fe(II).

The fate of pertechnetate (99Tc(VII)O4−) during bioreduction was investigated in the presence of 2-line ferrihydrite (Fh) and various dissimilatory metal reducing bacteria (DMRB) (Geobacter, Anaeromyxobacter, Shewanella) in comparison with TcO4− bioreduction in the absence of Fh. In the presence of Fh, Tc was present primarily as a fine-grained Tc(IV)/Fe precipitate that was distinct from the Tc(IV)O2·nH2O solids produced by direct biological Tc(VII) reduction. Aqueous Tc concentrations (<0.2 μm) in the bioreduced Fh suspensions (1.7 to 3.2 × 10−9 mol L−1) were over 1 order of magnitude lower than when TcO4− was biologically reduced in the absence of Fh (4.0 × 10−8 to 1.0 × 10−7 mol L−1). EXAFS analyses of the bioreduced Fh-Tc products were consistent with variable chain length Tc−O octahedra bonded to Fe−O octahedra associated with the surface of the residual or secondary Fe(III) oxide. In contrast, biogenic TcO2·nH2O had significantly more Tc−Tc second neighbors and a distinct long-range order consisten...