Macroscopic and spectroscopic characterization of uranium(VI) sorption onto orthoclase and muscovite and the influence of competing Ca2+

The uranium(VI) sorption onto orthoclase and muscovite, representing 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 complex. TRLFS measurements on the supernatant confirmed the predominance of this Ca2UO2(CO3)3 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 complex reduces the free UO22+ concentration in the solution. Additional, ATR FT-IR 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, log K≡XO-UO2+ = 1.69 and log K≡XO-UO2CO3− = 8.96 were determined for sorption onto orthoclase, whereas log K≡XO-UO2+ = 0.41 and log K≡XO-UO2CO3− = 8.71 best describe sorption onto muscovite.