Redox behaviour and coordination of neptunium in presence of inorganic oxoanions


Redox behaviour and coordination of neptunium in presence of inorganic oxoanions

Hennig, C.; Ikeda-Ohno, A.; Tsushima, S.; Takao, K.; Takao, S.; Scheinost, A. C.; Bernhard, G.

Neptunium shows a complex redox chemistry comprising reversible redox couples like Np(VI)/Np(V) and Np(IV)/Np(III), as well as irreversible couples like Np(V)/Np(IV). The reversibility of Np(VI)/Np(V) is rather unique among the actinides, because other actinide redox couples like U(VI)/U(V) and Pu(VI)/Pu(V) are prone to disproportionation effects in weakly complexing media [1,2]. The basics of reversible redox behavior is well explained by electron transfer without changes of the ligand arrangement, whereas the irreversibility of the transition Np(V)/Np(IV) is due to the formation or release of the NpO2+ oxygen atoms of the trans-dioxo moiety. However, also the reversible transitions may become quasi-reversible and irreversible for several reasons, e.g. as function of ligand exchange and electron transfer kinetics. Structural information on the neptunium redox species in aqueous solution is currently rather scarce. Therefore we performed a systematic study based on cyclic voltammetry, electrolysis, UV-Vis-NIR and EXAFS spectroscopy of the Np redox species in presence of the oxoanions ClO4, SO42, NO3 and CO32. The redox couple Np(V)/Np(IV) remains reversible in non-complexing ClO4 by retaining associated H2O molecules, but becomes irreversible already in weakly coordinating NO3 media [2]. The Np(V)/Np(IV) couple becomes more irreversible in presence of stronger ligands like SO42 and CO32. The slow kinetics of the Np(V)/Np(IV) couple in cyclic voltammetry results in a superposition with the reversible Np(IV)/Np(III) couple. The related species were partly separated by electrolysis and their coordination was analyzed independently. The redox couple Np(VI)/Np(VII) could be observed only in CO32 media at high pH. It is also irreversible as result of a rearrangement of the inner-sphere coordination. The neptunium sulfate system comprises a larger variety of isomers due to similar Gibbs free energies for monodentate and bidentate coordination. A comparison along the series Th(IV) - U(IV) - Np(IV) reveals that the monodentate sulfate coordination decreases whereas the bidentate coordination increases. This trend was studied by DFT calculations and will be discussed in terms of solvation energy and covalency of the molecular bonds.

  • Lecture (Conference)
    Migration 2009; 12th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, 20.-25.09.2009, Kennewick, Washington, USA

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Publ.-Id: 12523