Kinetic and mechanistic constraints on the oxidation of biogenic uraninite by ferrihydrite.
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(2(bio)) (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(2(bio)) 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.
Ginder-Vogel
Matthew
M
Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, USA. mattgv@udel.edu
Stewart
Brandy
B
Fendorf
Scott
S
D005290:Ferric Compounds / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D017974:Uranium Compounds / Q000737:chemistry*
D005290:Ferric Compounds; D017974:Uranium Compounds; C092844:ferric oxyhydroxide; C012597:uranium dioxide