Neptunium Carbonato Complexes in Aqueous Solution: An Electrochemical, Spectroscopic, and Quantum Chemical Study

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• 作者：Atsushi Ikeda-Ohno ; Satoru Tsushima ; Koichiro Takao ; Andr Rossberg ; Harald Funke ; Andreas C. Scheinost ; Gert Bernhard ; Tsuyoshi Yaita ; Christoph Hennig
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：December 21, 2009
• 年：2009
• 卷：48
• 期：24
• 页码：11779-11787
• 全文大小：938K
• 年卷期：v.48,no.24(December 21, 2009)
• ISSN：1520-510X

文摘

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 Na₂CO₃ and Na₂CO₃/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 Na₂CO₃ with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO₂(CO₃)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 Na₂CO₃/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [Np^VIIO₄(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 (CO₃²⁻) to hydroxide ions (OH⁻).