A Comparative Study of U(VI) and U(V) Carbonato Complexes in Aqueous Solution

Electrochemical, complexation, and electronic properties of uranyl(VI)- and uranyl(V) carbonato complexes in aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) samples and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry (CV) using three different working electrodes of Pt, Au, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasireversibly. Electrolysis of UO22+ has been performed in the Na2CO3 solutions more than 0.8 M with the limited pH range of 11.7 < pH < 12.0 using a Pt mesh electrode. It produces high purity of uranyl(V) carbonate solution that has been confirmed to be stable at least for two weeks in a sealed glass cuvette. EXAFS measurements revealed the structural arrangement of uranyl(VI)- and uranyl(V) tricarbonato complexes, [UO2(CO3)3]n- (n = 4 for uranyl(VI), 5 for uranyl(V)). The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 Å for the uranyl(VI) complex, and 1.91, 2.50, 2.93, and 4.23 Å for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge XANES spectra have been interpreted in terms of electron transitions and multiple scattering features.