Speciation and coordination of U(VI) and U(IV) sulfate in aqueous solution

The abandoned uranium mine Königstein near Dresden/Germany, where uranium ore has been leached by sulfuric acid, is even nowadays concerned from the problem of heavy metal mobility affecting adjacent aquifers used for recovering drinking water. A rising interest can be observed worldwide to use such in situ leaching techniques to obtain uranium from ore mines. At the other hand, the thermodynamics of uranium in presence of high sulfate concentration is actually not well understood, especially under reduced conditions. The extended Debye-Hückel formalism, commonly used to estimate the species distribution and to correct activity coefficients, is above the validity limit in case of high ionic strength originated by the sulfuric acid. The specific ion interaction theory (SIT) and the Pitzer model are useful at higher ionic strengths but partly not to the extent in the directly concerned rock areas. In this study were combined therefore EXAFS, HEXS, XRD, TRLFS and UV-vis spectroscopy to extract the species distribution and the coordination of U(VI) and U(IV) sulfate complexes from model systems with low pH values and high sulfate concentrations [1-3]. For that purpose a spectro-electrochemical cell has been constructed that allowed the investigation of the solutions under controlled redox conditions. The study revealed that in contrast to neutral pH conditions, where U(VI) is highly soluble but U(IV) is nearly insoluble, even U(IV) becomes highly soluble at low pH by forming stable complexes resulting in an enlarged migration capacity.

[1] C. Hennig, K. Schmeide, V. Brendler et al.: EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution. Inorg. Chem. 46, 5882 (2007).
[2] C. Hennig, W. Kraus, F. Emmerling et al.: The coordination of a U(IV) sulfate monomer in aqueous solution and in solid state, Inorg. Chem. 47, 1634, (2008)
[3] C. Hennig, A. Ikeda, K. Schmeide et al.: The relationship of monodentate and bidentate coordinated uranium(VI) sulfate in aqueous solution. Radiochim. Acta, submitted.