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

The reduction of uranyl (U(VI)) to the relatively insoluble tetravalent for m (U(IV)) by Shewanella alga (BrY), a dissimilatory metal-reducing bacteria , was studied in the presence of environmentally relevant iron hydrous oxid es. Because this process is dependent on U(VI) being used as the terminal e lectron acceptor (TEA) during anaerobic respiration, it is important to und erstand how other potential TEAs might affect this process. When cell suspe nsions of BrY were added to uranyl acetate (400 mu M), uranyl was removed f rom solution within 10 h. Similarly, uranyl in the presence on goethite (11 .1 mu mol of U/m(2) of solid) underwent dramatic reduction (>90%) with acti ve BrY cells. In contrast, when ferrihydrite was available (0.67 mu mol of U/m(2) 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 su spensions, the extent of uranyl reduction was inversely related to the frac tion of ferrihydrite present. Increasing uranyl concentrations retarded the inhibition, but the effects were transient. Using Raman spectroscopy, we o bserved that the initial solid product was UO2.17, but With continued expos ure to a reducing environment a relatively pure uraninite phase resulted.