Infrared spectroscopic identification of aqueous neptunium species


Infrared spectroscopic identification of aqueous neptunium species

Müller, K.; Foerstendorf, H.; Brendler, V.

In the safety assesment of underground disposal of radioactive waste neptunium (Np) is one of the most relevant radionuclides. Due to its long half-life, its hazardous to human health and its mobility under aerobic conditions Np-237 is considered as a possible long-term pollutant of the ecosystem from the safety point of view. The distribution of aqueous neptunium species, as there are the free neptunyl ions of Np(V) and Np(VI) and various complexes arising from hydrolysis or carbonate complexation, primarily defines its geochemical reactions and migration behavior in the environment.

Current knowledge about Np speciation is mainly based on data obtained by non-structural experiments such as potentiometric titration [1]. A spectroscopic verification of these Np species is still incomplete. There are only few EXAFS and Raman spectroscopic studies [2-4], using very high Np concentrations. Attenuated Total Reflectance Fourier-transform Infrared (ATR-FTIR) spectroscopy is a useful technique for the identification of different molecular species and it allows the direct measurement of liquid samples to obtain important spectral information even at low neptunyl concentrations relevant to environmental conditions.

In this study we investigate the Np speciation in aqueous solution using ATR-FTIR spectroscopy focusing on micromolar neptunyl concentrations. We work with varied experimental conditions, i.e. Np oxidation state (Np(V) and Np(VI)), wide pH range (pH 2 – 10), inert gas atmosphere, ionic strength. We present the first IR spectra of aqueous Np solutions and compare our results critically to thermodynamic calculations.

[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).

  • Lecture (Conference)
    V Ibero-American Congress of Environmental Physics and Chemistry, 14.-18.04.2008, Mar del Plata, Argentina
  • Contribution to proceedings
    V Ibero-American Congress of Environmental Physics and Chemistry, 14.-18.04.2008, Mar del Plata, Argentina
    Las Fronteras de la Física y Química Ambiental en Ibero América: libro de actas del V congreso iberoamericano de física y química ambiental, Buenos Aires, Argentina: Universidad Nacional de San Martin, 978-987-1435-05-0, 39

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Publ.-Id: 11270