Structure and Speciation of U(VI) sulfate in aqueous solution

The local structure of U(VI) sulfato complexes in aqueous solution has been investigated by U L3-edge EXAFS, high energy x-ray scattering (HEXS), x-ray diffraction (XRD) and UV-Vis spectroscopy using samples of 50 mM U(VI) and total sulfate concentrations, 0.05 ≤ [SO42-] ≤ 3 M at pH 1 and 2. At low [SO42-]/[UO22+] ratio (~1), where the UO2SO4(aq) species prevails, the sulfate coordinates in a monodentate fashion. At high [SO42-]/[UO22+] ratio, where UO2(SO4)22- species prevails, bidentate sulfate coordination becomes dominant. In all cases, the equatorial coordination number NOeq, remains 5. According to the bond-valence rule this is in line with the average bond distance between U(VI) and oxygen of ~2.42 Å observed by EXAFS and HEXS. The bidentate sulfate reaches a maximal coordination number NSbid of 2 for a [SO42-]/[UO22+] ratio of 60, hence, related with the structure of [UO2(SO4)2bidH2O]2-. The upper limit of NSbid allows to rule out the existence of a species with three monodentate coordinated sulfate groups, [UO2(SO4)3bid]4-. This observation has been confirmed by MCD measurements. Therefore, the species UO2(SO4)34- might be related with the structure [UO2(SO4)2bid(SO4)mon]4-. The existence of two bidentate coordinated sulfate groups has up to now never been observed in crystal structures. The evaporation of the above mentioned solutions with several [SO42-]/[UO22+] ratios resulted always in crystal structures where the sulfate group acts monodentate-bridging. The coordination of U(VI) by two bidentate coordinated sulfate groups might be restricted to solution species. This observation indicates a structural re-arrangement of SO42- during the crystallization. The potential structures in solution were modeled by DFT calculation and compared with the experimental data from EXAFS and HEXS.