The Sulfate Coordination of Np(IV), Np(V), and Np(VI) in Aqueous Solution

Abstract

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.