The sulfate coordination of Np(IV), Np(V) and Np(VI) in aqueous solution
The sulfate coordination of Np(IV), Np(V) and Np(VI) in aqueous solution
Hennig, C.; IkedaOhno, A.; Tsushima, S.; Scheinost, A.
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 Xray 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 neptuniumsulfur distances RNpS and coordination numbers NS obtained by EXAFS. Np(VI) is coordinated by sulfate in bidentate (R_{NpS} = 3.12 0.02 Å) and monodentate (R_{NpS} = 3.61 0.02 Å) 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) at 2.0 M SO_{4}
^{2} and pH 1.1. Np(V) is coordinated by sulfate in bidentate (R_{NpS} = 3.16 0.02) and monodentate (R_{NpS} = 3.67 0.02 Å) modes. However, sulfate coordination is less pronounced here and does not exceed in total one SO_{4}
^{2} ligand in a solution of 2.0 M SO_{4}
^{2}. The redox couple Np(VI)/Np(V) is reversible at low [SO_{4}
^{2}]/[NpO_{2}
^{2+}] ratio and becomes irreversible at high sulfate concentration due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R_{NpS} = 3.06 0.02 Å) and monodentate (R_{NpS} = 3.78 0.02 Å) coordination modes. The sulfate coordination increases with increasing [SO_{4}
^{2}]/[Np^{4+}] 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.
Keywords: EXAFS; DFT; Neptunium; Sulfat

Inorganic Chemistry 48(2009), 53505360
DOI: 10.1021/ic9003005
Cited 45 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ12429
Publ.Id: 12429