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1 PublicationUranium(VI) Chemistry in Strong Alkaline Solution: Speciation and Oxygen Exchange Mechanism
Moll, H.; Rossberg, A.; Steudtner, R.; Drobot, B.; Müller, K.; Tsushima, S.
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.
Involved research facilities
- Rossendorf Beamline at ESRF DOI: 10.1107/S1600577520014265
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Permalink: https://www.hzdr.de/publications/Publ-19321