The Structure of Polynuclear Uranyl Sorption Complexes at the Gibbsite/Water Interface

Up to now only occasionally polynuclear sorption complexes of uranyl have been reported. This is undoubtedly surprising, since polynuclear hydrolysis species dominate aqueous uranyl speciation across a wide pH range and at solution concentrations typically used for sorption studies (10-6 - 10-4 M). Using U-LIII EXAFS spectroscopy at 15 K, we demonstrate here that polynuclear sorption complexes occur consistently at the gibbsite surface. They were observed across a pH range of 5.5 to 7.5 in equilibrium with atmospheric CO2, and at pH 8.5 in absence of CO2.
At these conditions, thermodynamic calculations predict the trimeric (UO2)3(OH)5+ complex to be the predominant hydrolysis species in the aqueous phase. This complex has been investigated recently by EXAFS spectroscopy and DFT calculations [1]. Both methods showed a stoichiometry of (UO2)3O(OH)3+ for this complex with a central oxo bridging, and provided a relatively short U-U distance of 3.8 Å. In contrast, coordination numbers and radial distances found for the sorption complex are 2xOaxial @ 1.8 Å, 6xOequatorial @ 2.4 Å and 1xU @ 4.2 Å. Both EXAFS shell fitting and a newly developed approach for the calculation of the radial pair distribution function achieved the same result. The U-U distance of 4.2 Å together with the coordination number of six for equatorial oxygen are in line with formation of uranyl dimers, where the two uranyl units are linked in edge-sharing configuration. This unusual structure is currently further investigated by DFT calculations.

REFERENCES
1. S. Tsushima, A. Rossberg, A. Ikeda, K. Mueller, A. C. Scheinost, Inorganic Chemistry 46, 10819-10826 (2007).