Inhibition of bacterially promoted uranium reduction: Ferric (Hydr)oxides as competitive electron acceptors

The reduction of uranyl (U(VI)) to the relatively insoluble tetravalent form (U(IV)) by Shewanella alga (BrY), a dissimilatory metal-reducing bacteria, was studied in the presence of environmentally relevant iron hydrous oxides. Because this process is dependent on U(VI) being used as the terminal electron acceptor (TEA) during anaerobic respiration, it is important to understand how other potential TEAs might affect this process. When cell suspensions of BrY were added to uranyl acetate (400 μM), uranyl was removed-from solution within 10 h. Similarly, uranyl in the presence on goethite (11.1 μmol of U/m² of solid) underwent dramatic reduction (>90%)with active BrY cells. In contrast, when ferrihydrite was available (0.67 μmol of U/m² of solid) only 48% of the initial U(VI) was removed after 10 h. When varying ratios of goethite and ferrihydrite were incorporated into cell suspensions, the extent of uranyl reduction was inversely related to the fraction of ferrihydrite present. Increasing uranyl concentrations retarded the inhibition, but the effects were transient. Using Raman spectroscopy, we observed that the initial solid product was UO_2.17, but with continued exposure to a reducing environment a relatively pure uraninite phase resulted.