title: Electrochemical and complexation behavior of neptunium in aqueous perchlorate and nitrate solutions.
abstract: Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.
authors.affiliation: Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, 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: Christoph
authors.initials: C
authors.lastname: Hennig
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