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

• Published in: Inorganic chemistry Vol. 46; no. 15; pp. 5882 - 5892
• Main Authors: Hennig, Christoph, Schmeide, Katja, Brendler, Vinzenz, Moll, Henry, Tsushima, Satoru, Scheinost, Andreas C
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 23-07-2007
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0619759

The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L3 and Th-L3 EXAFS spectroscopy for total sulfate concentrations 0.05 ≤ [SO4 2-] ≤ 3 M and 1.0 ≤ pH ≤ 2.6. The sulfate coordination was derived from U−S and Th−S distances and coordination numbers. The spectroscopic results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar [SO4 2-]/[UO2 2+] solution, a U−S distance of 3.57 ± 0.02 Å suggests monodentate coordination, in line with UO2SO4(aq) as the dominant species. With increasing [SO4 2-]/[UO2 2+] ratio, an additional U−S distance of 3.11 ± 0.02 Å appears, suggesting bidentate coordination in line with the predominance of the UO2(SO4)2 2- species. The sulfate coordination of Th(IV) and U(IV) was investigated at [SO4 2-]/[M(IV)] ratios ≥8. The Th(IV) sulfato complex comprises both, monodentate and bidentate coordination, with Th−S distances of 3.81 ± 0.02 and 3.14 ± 0.02 Å, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate U−S distances of 3.67 ± 0.02 and 3.08 ± 0.02 Å, respectively. By increasing the pH value to 2, a U(IV) sulfate precipitates. This precipitate shows only a U−S distance of 3.67 ± 0.02 Å in line with a monodentate linkage between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate sulfate coordination in aqueous solutions can now be explained by differences of the [SO4 2-]/[M] ratio. At low [SO4 2-]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at higher ratios.