EXAFS investigation of U(VI), U(IV) and Th(IV) sulfato complexes in aqueous solution

The structures of U(VI), U(IV) and Th(IV) sulfato complexes were investigated by LIII edge EXAFS spectroscopy in aqueous solutions with total sulfate concentrations, [SO42-]total, ranged from 0.05 to 3 M in the pH range 1.0-2.7. In equimolar U(VI)/[SO42-]total solutions, the species distribution is dominated by monodentate sulfate coordination with a U-Smon distance of 3.57±0.02 Å. With increasing U(VI)/[SO42-]total ratio bidentate coordination becomes dominant with a U-Sbid distance of 3.11±0.02 Å. The aqueous Th(IV) sulfate comprises both, monodentate and bidentate coordination with Th-S distances of 3.14±0.02 and 3.81±0.02 Å, respectively. A similar coordination 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. By increasing the pH value to 2, a U(IV) precipitate occurs, where sulfate act exclusively as bridging ligand with a U-S distance 3.67±0.02 Å.