The sulfate coordination of Np(IV), Np(V), and Np(VI) in aqueous solution.
The coordination and redox behavior of Np(IV), Np(V), and Np(VI) sulfate in aqueous solution were investigated by Np L(3)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances R(Np-S) and coordination numbers N(S) obtained by EXAFS spectroscopy. Np(VI) is coordinated by sulfate in the bidentate (R(Np-S) = 3.12 +/- 0.02 A) and monodentate (R(Np-S) = 3.61 +/- 0.02 A) modes at a low sulfate concentration of [SO(4)(2-)]/[NpO(2)(2+)] = 1. At higher [SO(4)(2-)]/[NpO(2)(2+)] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) with 2.0 M SO(4)(2-) and at pH 1.1. Np(V) is coordinated by sulfate in the bidentate (R(Np-S) = 3.16 +/- 0.02 A) and monodentate (R(Np-S) = 3.67 +/- 0.02 A) modes. However, sulfate coordination is less pronounced and does not exceed one SO(4)(2-) per Np(V) with 2.0 M SO(4)(2-). The redox reaction between the Np(VI)/Np(V) couple can be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R(Np-S) = 3.06 +/- 0.02 A) and monodentate (R(Np-S) = 3.78 +/- 0.02 A) coordination modes. The sulfate coordination increases with an increasing [SO(4)(2-)]/[Np(4+)] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th(IV)-U(IV)-Np(IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons.
Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany. hennig@esrf.fr
Christoph
C
Hennig
Atsushi
A
Ikeda-Ohno
Satoru
S
Tsushima
Andreas C
AC
Scheinost