Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

1 Publication
EXAFS study of neptunium complex structures in aqueous solution
Hennig, C.; Ikeda-Ohno, A.; Tsushima, S.;
Electrochemical and complexation properties of neptunium 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 spectroscopy. 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 (Np3+ and Np4+) to trans-dioxoneptunyl ions (NpO2n+). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate solutions. A detailed analysis of EXAFS spectra suggests that Np(IV) 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 Np(V) and 2 for NpVI) [1].
Structural information on the sulfate coordination of neptunium in aqueous solution is actually rather scarce. We investigated actinide complexes in aqueous sulfate solution by using samples of 10-50 mM of actinide and total sulfate concentrations, 0.05 ≤ [SO42-] ≤ 10 M. In contrast to the coordination of actinides with carbonate where bidentate coordination always prevails, the coordination with sulfate comprises monodentate and bidentate linkage with a wide variety of combinations. In general, with increasing [SO42-]/[Ann+] ratio the bidentate coordination becomes dominant in solution [2]. As example, at low [SO42-]/[UO22+] ratio, where the UO2SO4(aq) species prevails, the sulfate coordinates in a monodentate and only to a less extend in bidentate fashion. At high [SO42-]/[UO22+] ratio, where UO2(SO4)22- species prevails, bidentate sulfate coordination with the species [UO2(SO4)2bid]2- becomes dominant. In oxidation state IV, up to five coordinating sulfate groups have been observed, mostly with predominant bidentate coordination as for example in the high-charged complex [U(SO4bid)3(SO4mon)2]6- [3]. Np(IV) and Np(VI) follows the tendency of U(IV) and U(VI), whereas Np(V) is weakly coordinated by sulfate, in accordance with its low formation constant. The attempt to preserve solution species in crystal structures led in most of the cases to a rearrangement of bidentate sulfate groups in solution to monodentate–bridging coordination in solid state. The coordination of the solution complexes was optimized by DFT calculation and the results are compared with the experimental observations.
Keywords: EXAFS, Neptunium, aqueous solution
  • Lecture (others)
    JAEA Actinide Network Workshop, 29.10.2008, Tokai, Japan

Publ.-Id: 11883 - Permalink