title: Uranium(VI) chemistry in strong alkaline solution: speciation and oxygen exchange mechanism.
abstract: 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.
authors.affiliation: Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf (HZDR) , Bautzner Landstraße 400, Dresden 01328, Germany.
authors.forename: Henry
authors.initials: H
authors.lastname: Moll
authors.forename: André
authors.initials: A
authors.lastname: Rossberg
authors.forename: Robin
authors.initials: R
authors.lastname: Steudtner
authors.forename: Björn
authors.initials: B
authors.lastname: Drobot
authors.forename: Katharina
authors.initials: K
authors.lastname: Müller
authors.forename: Satoru
authors.initials: S
authors.lastname: Tsushima