Comparative study of uranyl(VI) and -(V) carbonato complexes in an aqueous solution

Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an 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 using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)(3)](n-) [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-O-ax, U-O-eq, U-C, and U-O-dist are determined to be 1.81, 2.44, 2.92, and 4.17 angstrom for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 angstrom 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 L-I- and L-III-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.