Electrochemical and Complexation Behavior of Neptunium in Aqueous Perchlorate and Nitrate Solutions

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 LIII-edge X-ray absorption spectroscopies. The redox reactions of NpIII/NpIV and NpV/NpVI couples are reversible or quasi-reversible, while the electrochemical reaction between NpIII/IV and NpV/VI is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np3+ and Np4+) to transdioxoneptunyl ions (NpO2 n+, n = 1 for NpV and 2 for NpVI). The redox reaction of the NpV/NpVI 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 NpIV forms a decaaquo complex of [Np(H2O)10]4+ in 1.0 M HClO4, while NpV and NpVI exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5] n+ (n = 1 for NpV and 2 for NpVI). 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.