10.1021/es902452u.s001 Matthew Ginder-Vogel Matthew Ginder-Vogel Brandy Stewart Brandy Stewart Scott Fendorf Scott Fendorf Kinetic and Mechanistic Constraints on the Oxidation of Biogenic Uraninite by Ferrihydrite American Chemical Society 2010 decrease uraninite surface passivation uranium mobility bicarbonate concentration Mechanistic Constraints metal respiring bacteria enzymatically FerrihydriteThe oxidation state uraninite dissolution biogenic uraninite oxidation pH values biogenic uraninite Biogenic Uraninite UO 2010-01-01 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Kinetic_and_Mechanistic_Constraints_on_the_Oxidation_of_Biogenic_Uraninite_by_Ferrihydrite/2802847 The oxidation state of uranium plays a major role in determining uranium mobility in the environment. Under anaerobic conditions, common metal respiring bacteria enzymatically reduce soluble U(VI) to U(IV), resulting in the formation of sparingly soluble UO<sub>2(bio)</sub> (biogenic uraninite). The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Here, we demonstrate that biogenic uraninite oxidation by ferrihydrite, an environmentally ubiquitous, disordered Fe(III) (hydr)oxide, appears to proceed through a soluble U(IV) intermediate and results in the concomitant production of Fe(II) and dissolved U(VI). Uraninite oxidation rates are accelerated under conditions that increase its solubility and decrease uraninite surface passivation, which include high bicarbonate concentration and pH values deviating from neutrality. Thus, our results demonstrate that UO<sub>2(bio)</sub> oxidation by Fe(III) (hydr)oxides is controlled by the rate of uraninite dissolution and that this process may limit uranium(IV) sequestration in the presence of Fe(III) (hydr)oxides.