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 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 the structural rearrangement from spherical coordinating ions (Np3+ and Np4+) to transdioxo neptunyl ions (NpO2n+, 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 -(VI) exist dominantly as pentaaquo neptunyl complexes, [NpO2(H2O)5]n+ (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms (FTs) of the EXAF spectra for all the Np oxidation states as increasing the nitrate concentration in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.