Preliminary multi-method spectroscopic approach for the uranium(VI) hydrolysis at temperatures up to 60°C

For the safety assessment of high-level nuclear waste repositories in deep geologic formations, the understanding of actinide migration behaviour is one of the most important issues. In recent decades, the solution chemistry, e.g. hydrolysis [1], complexation with inorganic ligands [1], but also the interactions of the actinides at interfaces with the geo- and biosphere have been intensely investigated [2]. However, because of the experimental difficulties, only few studies have been performed at temperatures outside the range 20 – 30°C which hampers the prediction of actinide reactive transport in the environment of heat generating high-level nuclear waste repositories.
The speciation of (radioactive) metal ions in solution will be affected by the thermal conditions, since the properties of water, e.g. density, dielectric constant, viscosity, ion product, are altered with temperature and pressure [3,4].
The formation and distribution of U(VI) hydrolysis species is predicted to depend strongly on the temperature. In particular the stability of U(VI) polynuclear hydroxo complexes, which are dominant species at 25°C may change. According to experimental studies of other metal ions, namely Al(III) and La(III), the nuclearity of polynuclear complexes decreases upon increasing temperature [5,6]. At 25°C several spectroscopic techniques, namely UV-vis, TRLFS, EXAFS and vibrational spectroscopy have been applied for identification and structural characterization of U(VI) hydroxo species [7-10]. At elevated temperatures, TRLFS was used for the determination of luminescent characteristics of single hydroxo species as a function of the temperature [11,12]. But, approaches to examine alterations in the thermodynamic data itself are rare.
In this study, we investigate the U(VI) hydrolysis reactions up to 60°C using a multi-methodical approach by application of TRLFS and ATR FT-IR spectroscopy. The spectral data is compared to computed speciation patterns based on state-of-the-art thermodynamic models.

1.Guillaumont, R. et al., Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium. Chemical Thermodynamics Vol. 5, OECD Nuclear Energy Agency, Elsevier (2003).
2.Brendler, V. et al., RES3T-Rossendorf expert system for surface and sorption thermodynamics, Journal of Contaminant Hydrology 281-291 (2003).
3.Marshall, W.L. et al., Io product of water substance, 0 - 1000°C, 1-10,000 bars - New international formulation and its background, Journal of Physical and Chemical Reference Data 2, 295-304 (1981).
4.Fernandez, D.P. et al., A formulation for the static permittivity of water and steam at temperatures from 238 K to 873 K at pressures up to 1200 MPa, including derivatives and Debye-Huckel coefficients, Journal of Physical and Chemical Reference Data 4, 1125-1166 (1997).
5.Mesmer, R.E. et al., Acidity measurements at elevated temperatures. 5. Aluminum ion hydrolysis, Inorg. Chem. 10, 2290-2296 (1971).
6.Ciavatta, L. et al., The hydrolysis of the La(II) ion in aqueous perchlorate solution at 60°C, Polyhedron 6, 1283-1290 (1987).
7.Meinrath, G., Uranium(VI) speciation by spectroscopy, J. Radioanal. Nucl. Chem. 1-2, 119-126 (1997).
8.Moulin, C. et al., Time-resolved laser-induced fluorescence as a unique tool for low-level uranium speciation, Appl. Spectrosc. 4, 528-535 (1998).
9.Müller, K. et al., Aqueous uranium(VI) hydrolysis species characterized by attenuated total reflection Fourier-transform infrared spectroscopy, Inorg. Chem. 21, 10127-10134 (2008).
10.Tsushima, S. et al., Stoichiometry and structure of uranyl(VI) hydroxo dimer and trimer complexes in aqueous solution, Inorg. Chem. 25, 10819-10826 (2007).
11.Eliet, V. et al., Time-resolved laser-induced fluorescence of uranium(VI) hydroxo-complexes at different temperatures, Appl. Spectrosc. 1, 99-105 (2000).