EXAFS Investigation of U(VI), U(IV) and Th(IV)Sulfato Complexes in Aqueous Solution

The structures of aqueous U(VI), U(IV) and Th(IV) sulfato complexes were investigated by LIII edge EXAFS spectroscopy in solutions with total sulfate concentrations ranging from 0.05 to 3 M. U(VI), U(IV) and Th(IV) sulfate samples comprises both, sulfate in monodentate and bidentate co-ordination. In equimolar [SO4 2-]total/U(VI) solutions, the species distribution is dominated by monodentate sulfate co-ordination with a U-Smon distance of 3.57±0.02 Å. With increasing [SO4 2-]total/U(VI) ratio, bidentate co-ordination becomes dominant with a U-Sbid distance of 3.11±0.02 Å. In general, in all systems the bidentate co-ordination becomes dominant with increasing sulfate co-ordination. An exclusively bidentate co-ordination was observed only for U(VI) with high total sulfate concentration. The aqueous Th(IV) sulfate comprises both, monodentate and bidentate co-ordination with Th-S distances of 3.14±0.02 and 3.81±0.02 Å, respectively. A similar co-ordination is obtained for U(IV) sulfato complexes at pH 1 with U-S distances of 3.08±0.02 Å and 3.67±0.02 Å. These parameters were used to identify the structure of some corresponding thermodynamic species. The EXAFS data suffer from double-electron resonances that will be discussed in a second contribution.