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

Sorption and diffusion are important processes for the transport of radionu clides through buffer materials such as bentonite. In this study, the sorpt ion and diffusion behaviors of Cs and Sr on Jih-Hsing bentonite are investi gated using batch and through-diffusion techniques. The distribution coeffi cients (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 mode l could fit the sorption isotherm with an equilibrium concentration of 10(- 7)-10(-1) N. The calculated retardation factors (Rds) for samples at densit ies of 1.8 g/cm(3), 2.0 g/cm(3) and 2.2 g/cm(3) are 5685, 7744, and 11000 f or Cs, and are 3790, 5162, and 7334 for Sr. For the through-diffusion exper iments on the compacted samples with the same densities, the corresponding apparent diffusion coefficients for Cs are (2.83+/-0.75) x 10(-13)m(2)/s, ( 1.97+/-0.02)x 10(-13)m(2)/s, and (1.91+/-0.12) x 10(-13)m(2)/s, respectivel y. The corresponding apparent diffusion coefficients for Sr are (1.33 +/- 0 .13) x 10(-13)m(2)/s, (1.51 +/- 0.15) x 10(-13)m(2)/s, and (1.34+/-0.10) x 10(-13)m(2)/s. The Rds obtained from the diffusion experiments for sample d ensities of 1.8 g/cm(3), 2.0 g/cm(3) and 2.2 g/cm(3) 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 ex periments are about one order of magnitude lower than those derived from th e batch experiments. The discrepancy and the possible explanations are disc ussed in the paper. (C) 2001 Elsevier Science Ltd. All rights reserved.