Uranium Chemistry in Citric Acid Solution

For the long-term safety assessment of radioactive waste disposal sites, detailed knowledge on the migration behavior of the different actinides as a function of pH value and redox potential of the solution, concentration of inorganic or organic complex partners and temperature is important. The majority of the studies on the uranium chemistry in presence of carboxylic acids deal with complexation reactions performed at lower pH value and at room temperature. Thermodynamic data of the complexation of U(IV) and U(VI) by citric acid are summarized up to the year 2005 by Hummel et al. [1] and since then comprehensively studied by Bonin et al. [2], Steudtner [3] and Guenther et al. [4]. The photoreduction of U(VI) in citric acid solutions was studied in the presence of visible light by Ohyoshi and Ueno [5]. However, the mechanistic understanding of the basic interaction processes is very fragmentary.
Thus, the study is focused on the mechanism and kinetics of the uranium complexation and redox reactions as a function of carbonate concentration and visible light in citric acid solution. To evaluate the influence of these reaction parameters on the uranium – citric acid – system we used UV-Vis, ATR FT-IR and TRLF spectroscopy. The aqueous complexes of the U(VI) and U(IV) compounds are spectroscopically characterized to gain information on their molecular structures. The knowledge of the spectral properties is indispensable for the interpretation of the spectral changes occurring during the redox reactions. The variation of reaction parameters and the change of different reaction paths strongly influence the redox reactions. The derivation and verification of thermodynamic and kinetic parameters of the complexation and redox processes will improve the safety assessment of nuclear waste disposal sites.