title: Neptunium carbonato complexes in aqueous solution: an electrochemical, spectroscopic, and quantum chemical study.
abstract: The electrochemical behavior and complex structure of Np carbonato complexes, which are of major concern for the geological disposal of radioactive wastes, have been investigated in aqueous Na(2)CO(3) and Na(2)CO(3)/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na(2)CO(3) with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO(2)(CO(3))(3)](n-) (n = 5 for Np(V), and 4 for Np(VI)). Hence, the electrochemical reaction of the Np(V/VI) redox couple merely results in the shortening/lengthening of bond distances mainly because of the change of the cationic charge of Np, without any structural rearrangement. This explains the observed reversible-like feature on their cyclic voltammograms. In contrast, the electrochemical oxidation of Np(V) in a highly basic carbonate solution of 2.0 M Na(2)CO(3)/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [Np(VII)O(4)(OH)(2)](3-), indicating that the oxidation reaction from Np(V) to Np(VII) in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO(3)(2-)) to hydroxide ions (OH(-)).
authors.affiliation: Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, P.O. Box 510119, 01314 Dresden, Germany. a.ikeda-ohno@spring8.or.jp
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ikeda-Ohno
authors.forename: Satoru
authors.initials: S
authors.lastname: Tsushima
authors.forename: Koichiro
authors.initials: K
authors.lastname: Takao
authors.forename: André
authors.initials: A
authors.lastname: Rossberg
authors.forename: Harald
authors.initials: H
authors.lastname: Funke
authors.forename: Andreas C
authors.initials: AC
authors.lastname: Scheinost
authors.forename: Gert
authors.initials: G
authors.lastname: Bernhard
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Yaita
authors.forename: Christoph
authors.initials: C
authors.lastname: Hennig