Speciation of Np(V) in aqueous solution investigated by FT-IR spectroscopy.

The distribution of aqueous species of neptunium, one of the most relevant radionuclides for long-term emplacement of nuclear waste, primarily defines its geochemical reactions and migration behavior. Current knowledge about Np(V) species is mainly based on thermodynamical calculations relying on data obtained by non-structural experiments such as potentiometric titration [1]. A spectroscopic verification of these Np(V) speciations is still lacking. Furthermore, there is still considerable uncertainty about the predominating neptunyl species, whether they arise from hydrolysis or carbonate complexation.
Vibrational spectroscopy is a useful tool for the identification of different molecular species. Due to the difficulty to obtain IR spectra from aqueous solutions mainly Raman studies of high concentrated Np(V) solutions were carried out [2-4]. Attenuated Total Reflectance Fourier-transform Infrared (ATR-FTIR) spectroscopy allows the direct measurement of liquid samples and provides additional important spectral information even at low neptunyl concentrations.
In this study we investigate Np(V) hydrolysis and neptunyl carbonate complexation using ATR-FTIR spectroscopy. We focus on micromolar neptunyl concentrations and present infrared spectra recorded over a wide pH range (pH 2-10). The speciation of Np(V), i.e. its hydrolysis at lower pH (< 6) and its complexation with atmospheric carbonate at higher pH values is compared to previous thermodynamic data.

[1] R. Guillaumont et al. (OECD-NEA TDB), Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, Elsevier, 2003.
[2] Gregoire-Kappenstein, A. C. et al., Radiochim. Acta 91, 665 (2003).
[3] Jones, L. H. et al., J. Chem. Phys. 21, 542 (1953).
[4] Madic, C. et al., Inorg. Chem. 22, 1494 (1983).