Salinity effect on bisulfide diffusive transport through compacted MX-80 bentonite

Understanding solute transport behaviour in bentonite barriers is critical for evaluating the long-term safety of deep geological repositories. This study investigated the effect of salt (NaCl) concentrations on the diffusive transport of bisulfide (HS − ) and bromide (Br − , a conservative analogue), through MX-80 bentonite at a dry density of 1090 kg m −3 by performing through-diffusion experiments. Our previous study showed that at low NaCl ionic concentration (<0.05 M), the effective diffusion coefficient of HS − was nearly an order of magnitude lower than that of Br − . As NaCl ionic concentration increased up to 0.5 M, the effective diffusion coefficient of HS − increased by an order of magnitude, while that of Br − doubled. However, the diffusivity values for both HS − and Br − remained comparable when the NaCl ionic concentration was further increased from 0.5 to 1 M. The results also indicated that salinity influenced diffusivity through affecting the diffusion-accessible porosity and modifying the diffuse double layer, and possibly by influencing pore water viscosity. Furthermore, the findings indicate that Br − may serve as a reliable analogue for estimating HS − diffusivity at high ionic strengths (> 0.5 M), enabling simpler and shorter experimental studies. Finally, this study's insights offer improved predictions of HS − diffusivity under Canadian repository conditions, facilitating more accurate assessments of HS − induced corrosion within the repository environment. • HS − diffusivity in compacted bentonite increased tenfold as salinity increased from 0 to 0.5 M. • No notable variation in HS − and Br − diffusivity observed with a salinity change from 0.5 to 1 M. • Salinity influenced diffusion primarily by altering accessible porosity.