The sulfate coordination of Np(IV), Np(V) and Np(VI) in aqueous solution

Coordination and redox behavior of Np(IV), Np(V) and Np(VI) sulfate in aqueous solution has been 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, i.e. monodentate or bidentate, has been determined from neptunium-sulfur distances RNp-S and coordination numbers NS obtained by EXAFS. Np(VI) is coordinated by sulfate in bidentate (R_Np-S = 3.12  0.02 Å) and monodentate (R_Np-S = 3.61  0.02 Å) modes at a low sulfate concentration of [SO₄ ^2-]/[NpO₂ ²⁺] = 1. At higher [SO₄ ^2-]/[NpO₂ ²⁺] ratios bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) at 2.0 M SO₄ ^2- and pH 1.1. Np(V) is coordinated by sulfate in bidentate (R_Np-S = 3.16  0.02) and monodentate (R_Np-S = 3.67  0.02 Å) modes. However, sulfate coordination is less pronounced here and does not exceed in total one SO₄ ^2- ligand in a solution of 2.0 M SO₄ ^2-. The redox couple Np(VI)/Np(V) is reversible at low [SO₄ ^2-]/[NpO₂ ²⁺] ratio and becomes irreversible at high sulfate concentration due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R_Np-S = 3.06  0.02 Å) and monodentate (R_Np-S = 3.78  0.02 Å) coordination modes. The sulfate coordination increases with increasing [SO₄ ^2-]/[Np⁴⁺] ratio. A comparison of other tetravalent actinides reveals 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 covalency of the molecular bonds.