Infrared spectroscopic comparison of the aqueous species of uranium(VI) and neptunium(VI)


Infrared spectroscopic comparison of the aqueous species of uranium(VI) and neptunium(VI)

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

The distribution of aqueous species of actinide(VI) ions primarily defines their geochemical reactions, e.g. complexation in solution, sorption onto mineral and biological phases, and the formation of colloids, and thus, influences the migration behaviour in the environment. In aqueous solution under normal conditions both uranium and neptunium exist as dioxoactinyl ions AnO2n+ (An = U, Np). They form different complexed species depending on their concentration level, pH range and the presence of potential ligands, such as carbonate.
In recent years, the aqueous U(VI) system has been investigated intensively, in contrast to Np(VI). However, the stability constants used for thermodynamic calculations often arise from non-structural experiments, such as potentiometry and ion exchange, performed at defined sample parameters (1,2). Up to now, a spectroscopic verification of uranyl and neptunyl species is lacking in particular at low concentrations and at neutral pH conditions, reasonably in an environmental context. Thus, speciation modelling using extrapolated data, might be inadequate for the assessment of actinide migration.
In this study we used Attenuated Total Reflectance Fourier-transform Infrared (ATR-FTIR) spectroscopy to compare hydrolysis and carbonate complexation reactions of both U(VI) and Np(VI). Such comparative vibrational study is feasible since the ions NpO22+ and UO22+ are linear and symmetrical. The results are relevant for a comprehensive understanding of actinyl(VI) complexation in aqueous solution.
The experiments were performed at a micromolar concentration level (≤ 100 µM) and in the pH range 2 – 7 under both oxic and anoxic atmosphere. In the infrared spectra the asymmetric stretching vibrations ν3 of the free ions, UO22+ and NpO22+, found in solutions at very acidic pH, are observed at similar wavenumbers, 961 and 964 cm–1. Upon increasing the pH the IR spectra provide evidence for the formation of hydroxo complexes of both actinyl(VI) ions at pH ≥ 3, which is contradictive to current thermodynamic speciation modelling. Since the comparison of the spectral differences between the two actinides show red-shifted bands to a similar extent it can be assumed that analogous hydrolysis species are formed.

(1) Guillaumont, R.; Fanghänel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H. Update on the Chemical Thermodynamics of U, Np, Pu, Am and Tc.; Elsevier: Amsterdam, 2003.
(2) Grenthe, I.; Fuger, J.; Lemire, R. J.; Muller, A. B.; Nguyen-Trung, C.; Wanner, H. Chemical Thermodynamics of Uranium. ; 1st ed.; Elsevier Science Publishers B. V.: Amsterdam, 1992.

  • Contribution to proceedings
    NRC 7 - Seventh International Conference on Nuclear and Radiochemistry, 24.-29.08.2008, Budapest, Hungary
    Book of Abstracts, 978-963-9319-80-6, 275
  • Poster
    NRC7 - Seventh International Conference on Nuclear and Radiochemistry, 24.-29.08.2008, Budapest, Hungary

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