Thermodynamic Constraints on the Oxidation of Biogenic UO₂ by Fe(III) (Hydr)oxides

Uranium mobility in the environment is partially controlled by its oxidation state, where it exists as either U(VI) or U(IV). In aerobic environments, uranium is generally found in the hexavalent form, is quite soluble, and readily forms complexes with carbonate and calcium. Under anaerobic conditions, common metal respiring bacteria can reduce soluble U(VI) species to sparingly soluble UO₂ (uraninite); stimulation of these bacteria, in fact, is being explored as an in situ uranium remediation technique. However, the stability of biologically precipitated uraninite within soils and sediments is not well characterized. Here we demonstrate that uraninite oxidation by Fe(III) (hydr)oxides is thermodynamically favorable under limited geochemical conditions. Our analysis reveals that goethite and hematite have a limited capacity to oxidize UO_2(biogenic) while ferrihydrite can lead to UO_2(biogenic) oxidation. The extent of UO_2(biogenic) oxidation by ferrihydrite increases with increasing bicarbonate and calcium concentration, but decreases with elevated Fe(II)_(aq) and U(VI)_(aq) concentrations. Thus, our results demonstrate that the oxidation of UO_2(biogenic) by Fe(III) (hydr)oxides may transpire under mildly reducing conditions when ferrihydrite is present.