Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf
Influence of calcium on the uranium(VI) interaction with mineral surfaces
Richter, C.; Brendler, V.; Steudtner, R.; Drobot, B.
Many minerals have a strong tendency to sorb ions from the surrounding environment. An understanding of this process allows a realistic prediction of distribution and transport of the elements in nature. In case of uranium this is of high importance with respect to former mining sites or envisaged deposits. In this work, sorption of uranium(VI) onto orthoclase and muscovite, representing important components (feldspars and micas) of the earth crust, was investigated in absence and presence of calcium under aerobic conditions.
Batch experiments with both minerals were performed as triplicates in 0.01 M NaClO4 in the pH range of 5 to 8, with solid-to-liquid ratios of 1/20 and 1/80 g/mL, uranium concentrations of 10-5 and 10-6 M, with and without 1.5×10-3 M Ca. Furthermore time-resolved laser-induced fluorescence spectroscopy (TRLFS) was performed at batch samples with orthoclase (pH 4 to 9, with 10-5 M U and 1.5×10-3 M Ca). There, the aquatic solutions as well as the mineral suspensions were investigated as described in .
Uranium sorption shows its maximum at circumneutral pH values. At pH ≥ 8 in presence of calcium a reduced uranium(VI) sorption was observed which is due to the formation of the non-sorbing neutral Ca2UO2(CO3)3 complex. The evaluation of the spectroscopic results by Parallel factor analysis (PARAFAC) indicates the formation of three surface species. In detail the sorption of U(VI) onto orthoclase indicate two surface species. Based on the peak positions given in  they can be attributed to the formation of ≡SiO2UO20 and ≡SiO2UO2OHCO33– surface complexes. In presence of Ca2+ the ≡SiO2UO2OHCO33– surface complex disappears and instead the ≡SiO2UO2OH– surface complex also given in  can be observed. It occurs at lower pH values than the carbonate complex and shows different peak positions of the fluorescence bands.
The results improve the basis for a mechanistic modeling of the U(VI) sorption onto orthoclase and muscovite, which is important for long-term safety analysis of nuclear waste repositories.
With regard to the U(VI) sorption onto mica and feldspars not only the binary systems have to be understood because in natural systems many more components are present. For calcium this work already showed a significant influence on the speciation and thus on the potential transport of U(VI). But many other elements leached from surrounding minerals or contained in the groundwater may affect the overall sorption, too. Thus future work has to focus on more complex systems representing natural conditions as well as on the generation of surface complexation parameters for predictions of the U(VI) speciation.
GDCh Wissenschaftsforum 2015, 30.08.-02.09.2015, Dresden, Deutschland