Investigation of Re(VII) diffusion in bentonite by through-diffusion and modeling techniques

The interlayer cation of bentonite has obviously influence on the diffusion behavior of radionuclides. The diffusion behavior of Re(VII) in Na+- and Ca2+ -bentonites was investigated by a through-diffusion method at bulk dry densities from 1000 kg/m(3) to 1800 kg/m(3). A modified multi-porosity model (APM), which subdivided the porosity into the interlayer porosity (epsilon(il)), the diffuse double layer porosity (epsilon(ddl)) and free layer porosity (epsilon(free)) in compacted bentonite, is employed to describe the distribution of the three porosities as a function of the bulk dry density and to explain the lower effective diffusion coefficient (D-e) and accessible porosity (epsilon(acc)) of Re(VII) in Na+-bentonite compared to that in Ca2+ -bentonite. With increasing bulk dry density, epsilon(acc) decreased, leading to a decrease of D, for Re(VII). epsilon(ddl) and epsilon(il) are both related to the montmorillonite bulk dry density. Moreover epsilon(ddl )is also related to the external surface area (A(ext)) and the ionic strength. APM was successful in reproducing trends of epsilon(acc) data for Re (this work), Cl-35 and I-125 (data from the literature) as a function of the bulk dry density. Compared to Ca2+-bentonite, Na+-bentonite had higher epsilon(il) and epsilon(ddl) at the same compaction, indicating a less proportion of large pores. After combing with Archie's law, the dependency of D-e on bulk dry density was reproduced with a cementation factor of 2.2 and 2.6 for Na+- and Ca2+-bentonites, respectively.