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

Abstract 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/m3 to 1800 kg/m3. A modified multi-porosity model (APM), which subdivided the porosity into the interlayer porosity (eil), the diffuse double layer porosity (eddl) and free layer porosity (efree) 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 (De) and accessible porosity (eacc) of Re(VII) in Na+-bentonite compared to that in Ca2+-bentonite. With increasing bulk dry density, eacc decreased, leading to a decrease of De for Re(VII). eddl and eil are both related to the montmorillonite bulk dry density. Moreover eddl is also related to the external surface area (Aext) and the ionic strength. APM was successful in reproducing trends of eacc data for Re (this work), 35Cl and 125I (data from the literature) as a function of the bulk dry density. Compared to Ca2+-bentonite, Na+-bentonite had higher eil and eddl at the same compaction, indicating a less proportion of large pores. After combing with Archie's law, the dependency of De on bulk dry density was reproduced with a cementation factor of 2.2 and 2.6 for Na+- and Ca2+-bentonites, respectively.