XAFS Study of Uranyl Carbonate Complexes in Aqueous Solution

The carbonate complex of uranium is one of the most fundamental chemical species for the migration study on the geological disposal of radioactive wastes or the remediation study of closed uranium mining. In this study, the carbonate complexes of uranyl(VI)- and uranyl(V) ions were investigated in aqueous solution by using electrochemical (cyclic voltammetry and electrolysis) and spectroscopic (UV-visible and XAFS) techniques.
From the electrochemical study, it was found that the UVO2+-carbonate complex was stable only in the limited concentration range of 0.8 M < [Na2CO3] < 1.4 M. Therefore, the UVO2+ samples for XAFS measurements were prepared in 1.4 M-Na2CO3 solution by using a Pt-mesh working electrode. The results of XAFS measurement indicated that both uranyl(V) and uranyl(VI) ions form the tricarbonate complex, [UO2(CO3)3]n- (n = 5 for uranyl(V) and 4 for uranyl(VI)). The coordination occurs in a bidentate manner and, in general, all bond lengths for the uranyl(V) complex were longer than those for the uranyl(VI) complex. These results are in good agreement with the DFT calculations at least for the uranyl(VI) complex. Additionally, a chemical shift was also observed in the U LI- and LIII-XANES spectra, which can be interpreted as the combination of electronic transitions and multiple scattering features.