RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The uranium(VI) sorption onto orthoclase and muscovite, representing the mineral groups of feldspars and micas as important components of the earth crust, was investigated in the presence and absence of Ca²⁺ under aerobic conditions. Batch experiments were accompanied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) as well as in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. The results indicate that the U(VI) sorption is reduced by Ca²⁺ at pH ⩾ 8 up to 30% due to the formation of the neutral aqueous Ca₂UO₂(CO₃)₃ (aq) complex. TRLFS measurements on the supernatant confirmed the predominance of this Ca₂UO₂(CO₃)₃ (aq) complex in accordance with thermodynamic calculations. Furthermore, TRLFS measurements on the mineral suspension as a function of pH (4–9) and Ca²⁺ revealed the existence of several species. Parallel factor analysis (PARAFAC) indicated the formation of three surface species totally. In the absence of Ca²⁺, the ≡XO–UO₂⁺ and ≡XO–UO₂CO₃⁻ surface complexes were formed, whereas the presence of Ca²⁺ leads to the formation of ≡XO–UO₂⁺ and ≡XO–UO₂OH as the formation of the aqueous Ca₂UO₂(CO₃)₃ (aq) complex reduces the free UO₂²⁺ concentration in the solution. In addition, ATR FT-IR spectroscopy confirmed an outer-sphere surface species in the absence of Ca²⁺. These experimental results were used for the assessment of surface complexation parameters to improve the basis for a mechanistic modeling of the sorption processes of U(VI) onto orthoclase and muscovite including the influence of Ca²⁺. Namely, logK_{≡XO–UO2⁺} = 1.69 and logK_{≡XO–UO2CO3⁻} = 8.96 were determined for sorption onto orthoclase, whereas logK_{≡XO–UO2⁺} = 0.41 and logK_{≡XO–UO2CO3⁻} = 8.71 best describe sorption onto muscovite in the absence of Ca²⁺.