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

The mechanism by which oxygen bound in UO22+ 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 Szabo 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.