title: Comparative study of uranyl(VI) and -(V) carbonato complexes in an aqueous solution.
abstract: 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-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A 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 X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.
authors.affiliation: Forschungszentrum Dresden-Rossendorf (FZD), Institute of Radiochemistry, P.O. Box 510119, 01314 Dresden, Germany. a.ikeda@fzd.de
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ikeda
authors.forename: Christoph
authors.initials: C
authors.lastname: Hennig
authors.forename: Satoru
authors.initials: S
authors.lastname: Tsushima
authors.forename: Koichiro
authors.initials: K
authors.lastname: Takao
authors.forename: Yasuhisa
authors.initials: Y
authors.lastname: Ikeda
authors.forename: Andreas C
authors.initials: AC
authors.lastname: Scheinost
authors.forename: Gert
authors.initials: G
authors.lastname: Bernhard