Diffusion of Cesium and Iodine in Compacted Sodium Montmorillonite Under Different Saline Conditions
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Diffusion of Cesium and Iodine in Compacted Sodium Montmorillonite Under Different Saline Conditions Yukio Tachi, Kenji Yotsuji, Yoshimi Seida and Mikazu Yui Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency 4-33 Muramatsu, Tokai-mura, Ibaraki 319-1194, Japan ABSTRACT Diffusion and sorption of cesium (Cs) and iodine (I) were investigated in a purified and moderately compacted sodium montmorillonite (dry density of 800 kg m-3) saturated with 0.01, 0.1 and 0.5M NaCl solutions. The effective diffusivity (De) and capacity factor () for Cs and I were measured by through-diffusion experiments, coupled with multiple curve analyses, including tracer depletion, breakthrough and depth concentration curves, which could be fitted with a conventional diffusion model using only one set of parameters. The De values obtained for Cs were of the order of 10-9-10-10 m2 s-1 and decreased as salinity increased, and those for I were of the order of 10-11-10-12 m2 s-1 and showed the opposite dependency. The distribution coefficient (Kd) of Cs decreased from the order of 100 to 10-2 m3 kg-1 as salinity increased. Diffusion and sorption parameters for Cs were also obtained by in-diffusion and batch sorption experiments and showed good agreement with those obtained by the through-diffusion experiments. The diffusion model, based on homogeneous pore structure and electrical double layer (EDL) theory, predicted the salinity dependence of De reasonably well, showing the effect of cation excess and anion exclusion as a function of salinity. The apparent diffusivity (Da), which includes sorption effects, was also interpreted by a coupled sorption model. INTRODUCTION Compacted bentonite is a candidate engineered barrier material for the geological disposal of radioactive waste. Migration of radionuclides in compacted bentonite is expected to be diffusion-controlled and retarded by sorption processes owing to its low hydraulic conductivity and large surface area with abundant reactive sites, respectively. Many of the previous experimental and modeling studies focused on ionic diffusion and sorption in compacted bentonite indicate cation excess diffusion and anion exclusion are important processes [e.g., 1], which have been subsequently modeled by EDL theory [e.g., 2-3]. For predictive evaluation in safety assessment, the porewater salinity is also a key factor, which will be controlled by the composition of the infiltrating groundwater. Some studies have focused on salinity effects on diffusion in compacted bentonite, considering either cation [1, 4-6] or anion diffusion [7-8]; however, these mechanisms are not yet fully understood. The aim of the present study is to provide a comprehensive experimental and modeling approach to evaluate the ionic diffusion and sorption behavior in moderately compacted montmorillonite as a function of salinity. Cs and I were selected because these are key radionuclides in the safety assessment of radioactive waste disposal, are both monovalent, and are reasonably well known with
