Neptunium (93Np) is one of the most critical elements for the geological disposal of high-level radioactive wastes because of its considerable content in such wastes, and the high radioactivity, half-lives and radiotoxicity of its nuclides. From a chemist’s point of view, it is also a very interesting element because of its diversity of oxidation states from III to VII [1]. Whether Np may be retained in the waste repository for millions of years, or whether it will migrate to the biosphere, depends heavily on its chemical forms (speciation). We assume that oxidation state will have a strong influence on the speciation. Therefore detailed knowledge about the interrelation between oxidation state and the structure of chemical species is critical for the development of safe nuclear waste repositories. This motivated us to perform X-ray absorption fine structure (XAFS) studies to determine the complex structure of Np species in aqueous solutions at different oxidation states. The experiments were performed at the Rossendorf Beamline (BM20), the only beamline at the ESRF where such studies with aqueous Np samples can be performed.

The solution samples investigated in this study were prepared by dissolving NpVIO2(ClO4)2·nH2O into aqueous solutions, to give a Np concentration of 0.04 M. The oxidation state of Np in the samples was electrochemically adjusted. Np LIII-edge (17.625 keV) XAFS measurements were carried out in transmission mode using a Si(111) double-crystal monochromator, and two Pt-coated mirrors for rejection of higher harmonics.

Fig. 121: a) k3-weighted Np-LIII edge EXAFS spectra, and b) their corresponding Fourier transforms for Np(IV), -(V), and -(VI) in 1.0 M HClO4. Solid lines represent experimental data and dotted lines represent theoretical fit. The line colours reflect the actual colour of sample solutions.

Figure 121 shows the k3-weighted Np LIII-edge extended X-ray absorption fine structure (EXAFS) spectra for Np(IV), -(V), and -(VI) in 1.0 M HClO4 (left), and their corresponding Fourier transforms (right). Perchlorate is known to be a noncomplexing ligand to actinide ions in aqueous solution, and hardly coordinates to the primary coordination sphere of actinide ions [2]. Accordingly, the chemical species formed in aqueous HClO4 solution are considered to be pure hydrate species. The EXAFS spectrum of Np(IV) is composed of a single oscillation pattern, giving a single peak at 1.9 Å in the Fourier transform. The structural parameters determined by shell fitting are a Np-O distance (RNp–O) of 2.40 Å with a coordination number (CN) of 10.4, corresponding to water molecules of the primary coordination sphere. The EXAFS spectra for the two higher oxidation states, V and VI, exhibit more intricate oscillation patterns and the corresponding Fourier transforms show several significant backscattering peaks. The most prominent peak at around 1.4 Å arises from the single scattering of the double-bond axial oxygen atoms (Oax), indicative of the neptunyl unit (NpO2n+). The following two peaks at around 1.9 Å arise from oxygen atoms of the water molecules in the neptunyl equatorial plane (Oeq(H2O), while the small but sufficiently distinguishable peak at 2.9 Å is due to the multiple scattering of Oax, respectively. The shell fitting results reveal that the neptunyl(V) ion is coordinated in its equatorial plane by 5.2 water molecules with O atoms at a distance of 2.49 Å, whereas the neptunyl(VI) ion is coordinated by 5.3 water molecules with a shorter interatomic distance of 2.42 Å.

Fig. 122: Structural rearrangement of Np hydrate species through the redox reaction.

Our EXAFS data show therefore a structural rearrangement of Np hydrate species with oxidation state as illustrated in Figure 122: both Np(V) and (VI) ions exist predominately as pentaaquo neptunyl complexes, [NpO2(H2O)5]n+ (n = 1 for Np(V) and 2 for Np(VI)), whereas the Np(IV) ion forms a spherically coordinated decaaquo complex, [Np(H2O)10]4+. It is this drastic change in complex structure between Np(IV) and Np(V), which makes the transition between these two redox states almost irreversible (E ~0.9 V), while the transition between Np(V) and Np(VI) requires no structural change, hence is quasi-reversible (E = ~0.2 V). These results support our initial hypothesis that there is a strong relationship between the electrochemical behaviour of Np and its complex structure.


Principal publication and authors

A. Ikeda-Ohno (a,b), C. Hennig (a), A. Rossberg (a), H. Funke (a), A.C. Scheinost (a), G. Bernhard (a), T. Yaita (b), Inorg. Chem. 47, 8294 (2008).
(a) Institute for Radiochemistry, Forschungszentrum Dresden-Rossendorf (Germany)
(b) Synchrotron Radiation Research Center, Japan Atomic Energy Agency (Japan)


[1] Z. Yoshida, S.G. Johnson, T. Kimura, J.R. Krsul, in The Chemistry of the Actinide and Transactinide Elements (3rd Ed.), L.R. Morss, N.M. Edelstein, J. Fuger (Eds.), Springer, Dordrecht, The Netherlands, 699 (2006).
[2] L. Sémon, C. Boehme, I. Billard, C. Hennig, K. Lützenkirchen, T. Reich, A. Rossberg, I. Rossini, G. Wipff, ChemPhysChem 2, 591 (2001).


This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Contract HE2297/2-1.