The uranium citric acid system and its sorption onto hematite

The determination and verification of thermodynamic and kinetic parameters of complexation, redox and sorption processes will improve the safety assessment of nuclear waste disposal sites. On one side, this study focusses on uranium complexation and redox reactions in aqueous citric acid solution as a function of reaction parameters (pH, c, T, atm.) and visible light irradiation. To evaluate the impact of these reaction parameters on the uranium – citric acid – water system i we used UV-Vis, ATR FT-IR and TRLF spectroscopy. In all systems the uranium citrate complexes were detected as dissolved species. We identified and characterized the formed complexes for uranium(VI) – (UO2)2(cit)22–, (UO2)2(cit)2(OH)3–, (UO2)2(cit)2(OH)24– and (UO2)3(cit)3(OH)55– as well as for uranium(IV) – U(H2cit)3+, U(Hcit)20 (aq.) and U(cit)22– and determined the corresponding complex formation constants. The determination and verification of thermodynamic and kinetic parameters of complexation and redox processes were consequently applied to the uranium sorption on hematite in the presence of citric acid.
The uranium reactions at the iron mineral − water interface were comprehensively investigated by in situ vibrational spectroscopy and classical batch sorption experiments. The uranium retention on hematite depends on pH value and citric acid concentration. By infrared spectroscopy, we observe a typical chemical binding of uranium by formation of an inner-sphere sorption complex in absence of citric acid. In presence of citric acid the sorption process is suppressed due to simultaneous complex formation in aqueous solution.