Uranium(VI) Chemistry in Strong Alkaline Solution: Speciation and Oxygen Exchange Mechanism

The mechanism by which oxygen bound in UO₂ ²⁺ exchanges with that from water under strong alkaline conditions remains a subject of controversy. Two recent NMR studies independently revealed that the key intermediate species is a binuclear uranyl(VI) hydroxide, presumably of the stoichiometry [(UO₂(OH)₄ ²⁻)(UO₂(OH)₅ ³⁻)]. The presence of UO₂(OH)₅ ³⁻ in highly alkaline solution was postulated in earlier experimental studies, yet the species has been little characterized. Quantum-chemical calculations (DFT and MP2) show that hydrolysis of UO₂(OH)₄ ²⁻ yields UO₃(OH)₃ ³⁻ preferentially over UO₂(OH)₅ ³⁻. X-ray absorption spectroscopy was used to study the uranium(VI) speciation in a highly alkaline solution supporting the existence of a species with three U−O bonds, as expected for UO₃(OH)₃ ³⁻. Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO₂(OH)₄ ²⁻)(UO₃(OH)₃ ³⁻)] by quantum-chemical calculations. Assuming that the rate-dominating step is proton transfer between the oxygen atoms, the activation Gibbs energy for the intramolecular proton transfer within [(UO₂(OH)₄ ²⁻)(UO₃(OH)₃ ³⁻)] at the B3LYP level was estimated to be 64.7 kJ mol⁻¹. This value is in good agreement with the activation energy for “yl”−oxygen exchange in [(UO₂(OH)₄ ²⁻)(UO₂(OH)₅ ³⁻)] obtained from experiment by Szabó and Grenthe (Inorg. Chem. 2010, 49, 4928−4933), which is 60.8 ± 2.4 kJ mol⁻¹. Both the presence of UO₃(OH)₃ ³⁻ and the scenario of an “yl”−oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study.