Hydrolysis of U(VI). An infrared spectroscopic verification of uranyl species

Detailed information about aqueous species of uranium(VI) is an indispensable prerequisite for a reliable assessment of its migration in the environment. Up to now the identification of distinct U(VI) species in aqueous solution was mostly accomplished by thermodynamical calculations based on non-structural experiments such as potentiometric titration [1] and by spectroscopic investigations of UO22+ solutions in the upper millimolar range at low pH values [2,3].
Attenuated total reflectance (ATR)-FT-IR spectroscopy has shown to be a useful tool for investigations of molecule complexes formed during sorption processes in aqueous solutions [4]. In this study we focus on the hydrolysis reactions of U(VI) under atmospheric conditions throughout a wide pH range (pH 2 – 9).
First, we investigated diluted uranyl(VI) solutions. Due to the low concentration ([UO22+] = 20 µM) the acquisition of infrared spectra of UO22+ solutions have become feasible even at high pH values avoiding the precipitation of solid uranyl phases. Our results clearly demonstrate that already at pH values < 4 not yet identified hydroxo uranyl complexes considerably contribute to the infrared spectra which is in contrast to recent thermodynamical calculations. It is also quite conceivable that we identified new U(VI) species in aqueous solutions. At higher pH values (> 7) the spectra indicate the formation of another uranyl species possibly with contributions of atmospheric carbonate anions which can be derived from model spectra of the pure UO2(CO3)34− complex.
In a second series of experiments we studied the hydroxo species of uranium(VI) at different concentrations (5 mM – 5 µM UO22+) at a constant pH value (pH 4). Upon lowering the concentration below 500 µM the fraction of the uncomplexed uranyl species is significantly reduced and can not be detected at a concentration of 50 µM any longer. Simultaneously two other hydroxo complexes are obviously formed at these lower concentrations.
A critical evaluation of literature data and the obtained spectral data constitute a reference to a modified speciation of U(VI) in particular at lower pH values and at concentrations in the micromolar range.

[1] R. Guillaumont et al. (OECD-NEA TDB), Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, Elsevier, 2003.
[2] H. Moll et al., Radiochim. Acta 88, 411 (2000).
[3] F. Quilès and A. Burneau, Vib. Spec. 23, 231 (2000).
[4] G. Lefèvre, Adv. Colloid Interface Sci. 107, 109 (2004).