Reduction of U(VI) by Fe(II) during the Fe(II)-Accelerated Transformation of Ferrihydrite

X-ray absorption spectroscopy has been used to study the reduction of adsorbed U­(VI) during the Fe­(II)-accelerated transformation of ferrihydrite to goethite. The fate of U­(VI) was examined across a variety of pH values and Fe­(II) concentrations, with results suggesting that, in all cases, it was reduced over the course of the Fe­(III) phase transformation to a U­(V) species incorporated in goethite. A positive correlation between U­(VI) reduction and ferrihydrite transformation rate constants implies that U­(VI) reduction was driven by the production of goethite under the conditions used in these studies. This interpretation was supported by additional experimental evidence that demonstrated the (fast) reduction of U­(VI) to U­(V) by Fe­(II) in the presence of goethite only. Theoretical redox potential calculations clearly indicate that the reduction of U­(VI) by Fe­(II) in the presence of goethite is thermodynamically favorable. In contrast, reduction of U­(VI) by Fe­(II) in the presence of ferrihydrite is largely thermodynamically unfavorable within the range of conditions examined in this study