Redox mechanism and stability of uranyl phosphites at mineral surfaces: Cooperative proton/electron transfer and high efficacy for Uranium(VI) reduction

Abstract

Uranium phosphites have recently emerged as promising materials to remediate radioactive contamination. In this study, the redox mechanisms of uranyl phosphites at mineral surfaces have been addressed by periodic DFT calculations with dispersion corrections. Different from other ligands, the phosphite anions (H2PO3−, HPO32−) are efficient reducing agents for uranyl reduction, and the redox reactions are divided into three steps, as isomerization between two phosphite anion isomers (Step 1), conformational transition (Step 2) and dissociation of the water molecule (Step 3). A second water molecule is critical to lower the activation barriers of Step 1, and all activation barriers are moderate so that the redox reactions occur favorably under normal conditions, which are further dramatically accelerated by the highly exergonic Step 3. Accordingly, formation of uranyl phosphites becomes an effective approach to manage uranium pollution. Moreover, the lower activation barriers for H2PO3− rather than HPO32− rationalize the superior reduction activities of uranyl phosphites and the enhanced stability of U(IV) products at lower pH conditions. Owing to the cooperative proton/electron transfer, the U(VI) reduction to U(IV) and P(III) oxidation to P(V) are completed within one step, with transition states being featured by the U(V) and P(IV) species.