No abstract

The study deals with sorption and diffusion behaviour of strontium in Czech bentonite B75. The study is a part of a research on reactive transport of radioactive contaminants in barrier materials of a deep geological repository of radioactive waste in the Czech Republic. Series of sorption and diffusion experiments with Sr and non-activated Ca bentonite B75 produced in the Czech Republic were performed in two background solutions (CaCl 2 and NaCl). On the basis of sorption batch experiments the kinetics of strontium sorption on bentonite was assessed and the sorption isotherms for various experimental conditions were obtained. As a result of performed diffusion experiments the parameters of diffusion (i.e. effective diffusion coefficient D e and apparent diffusion coefficient D a ) were determined. The observed discrepancies between sorption characteristics obtained from the sorption and diffusion experiments are discussed.

With the aim to determine the influence of dominant interlayer cation on the sorption and diffusion properties of bentonite, diffusion experiments with Sr on the compacted homoionous Ca- and Na-forms of Czech natural Mg/Ca bentonite using the planar source method were performed. The bentonite was compacted to 1400 kg·m−3, and diffusion experiments lasted 1, 3 or 5 days. Two methods of apparent diffusion coefficient Da determination based on the analytical solution of diffusion equation for ideal boundary conditions in a linear form were compared and applied. The determined Da value for Ca-bentonite was 1.36 times higher than that for Na-bentonite sample. Values of Kd were determined in independent batch sorption experiments and were extrapolated for the conditions of compacted bentonite. In spite of this treatment, the use of Kd values determined by batch sorption experiments on a loose material for the determination of effective diffusion coefficient De values from planar source diffusion experiments proved to be inconsistent with the standard Fickian description of diffusion taking into account only the pore diffusion in compacted bentonite. Discrepancies between Kd and De values were measured in independent experiments, and those that resulted from the evaluation of planar source diffusion experiments could be well explained by the phenomenon of surface diffusion. The obtained values of surface diffusion coefficients Ds were similar for both studied systems, and the predicted value of total effective diffusion coefficient De(tot) describing Sr transport in the Na-bentonite was four times higher than in the Ca-bentonite.

Commercial cementitious material, hydrated cement pastes of type CEM I and CEM III, and a synthetic cementitious phase CSH (calcium-silicate-hydrates) with a Ca/Si ratio equal to 1.0, 1.2, and 1.4 were used in the study of lead sorption. Kinetic and equilibrium experiments were performed under different conditions, namely liquid to solid phase ratio and the presence of organic compounds (EDTA, phthalate). The duration of equilibrium experiments was determined based on kinetic experiments, 1 week for calcium-silicate-hydrates and 3 weeks for hydrated cement paste. Equilibrium experiments were evaluated using the distribution ratio Rd and sorption isotherms. For both hydrated cement pastes, Rd’s in the range 102–103 L kg−1 were found while for calcium-silicate-hydrates the distribution ratio values were an order of magnitude higher under comparable conditions. The difference in the studied cementitious materials was also manifested in the shape of the sorption isotherm, while the behavior of calcium-silicate-hydrates can be fitted linearly and therefore by the Kd-model, for hydrated cement paste as more complex cement material it is necessary to use a non-linear isotherm description. The effect of organic matter, represented here by EDTA and phthalate, described by the sorption reduction factor SRF was more apparent for calcium-silicate-hydrates, where its value was always higher than one and reached up to 500. On the contrary, for hydrated cement paste CEM I and CEM III, all determined SRFs were less than 1.