Uranium(VI) reduction at mineral surfaces by Fe(II): Periodic DFT studies of mechanistic aspects and combined action of Fe(II) with humic acid

Abstract U(VI) reduction to the less soluble U(IV) species at mineral surfaces emerges as an attractive approach to remediate uranium contamination. Herein, mechanisms for U(VI) reduction by Fe(II) at orthoclase surfaces and impacts for coexistence of humic acid (HA) are addressed by DFT+U calculations. Fe(II) causes dual H-transfers and transforms U(VI) to U(IV) facilely. Fe(II) shows higher reactivity than HA, and its role is further examined by changing the Fe(II) content (n) that decides the extent of U(VI) reduction: U(IV), U(V) and U(VI) dominate respectively for n = 2, 1, and 0. Mechanisms with co-existence of HA are rather distinct: 1) Spectator. Only one of HA and Fe(II) acts as reducing agent. HA greatly alters Fe(II) reactivity, and vice versa. Fe(II) participates in charge transfers and significantly enhances HA reactivity, while replacement by Mg(II) causes adverse effects; 2) Combined action. Both HA and Fe(II) act as reducing agents, and their combined action results in not only U(IV), but also U(III) and U(II) of lower oxidation states, which recently become a focus of synthesis and hence U(VI) is turned into useful materials. Results rationalize experimental observations available, and also provide significantly new insights about U(VI) reduction, and uranium disposal and recycling.