Sorption and diffusion behavior of Cs and Sr on Jih-Hsing bentonite

Sorption and diffusion are important processes for the transport of radionuclides through buffer materials such as bentonite. In this study, the sorption and diffusion behaviors of Cs and Sr on Jih-Hsing bentonite are investigated using batch and through-diffusion techniques. The distribution coefficients (Kds) of Cs and Sr from batch experiments are approximately 1200 ml/g and 800 ml/g, respectively. It is found that the Freundlich isotherm model could fit the sorption isotherm with an equilibrium concentration of 10(-7)-10(-1) N. The calculated retardation factors (Rds) for samples at densities of 1.8 g/cm3, 2.0 g/cm3 and 2.2 g/cm3 are 5685, 7744, and 11000 for Cs, and are 3790, 5162, and 7334 for Sr. For the through-diffusion experiments on the compacted samples with the same densities, the corresponding apparent diffusion coefficients for Cs are (2.83+/-0.75) x 10(-13) m2/s, (1.97+/-0.02) x 10(-13) m2/s, and (1.91+/-0.12) x 10(-13) m2/s, respectively. The corresponding apparent diffusion coefficients for Sr are (1.33+/-0.13) x 10(-13) m2/s, (1.51+/-0.15) x 10(-13) m2/s, and (1.34+/-0.10) x 10(-13) m2/s. The Rds obtained from the diffusion experiments for sample densities of 1.8 g/cm3, 2.0 g/cm3 and 2.2 g/cm3 are 1166+/-355, 2113+/-123, 2796+/-171 for Cs, and 713+/-258, 510+/-68, 846+/-158 for Sr. It appears that the retardation factors obtained from the diffusion experiments are about one order of magnitude lower than those derived from the batch experiments. The discrepancy and the possible explanations are discussed in the paper.