Effect of compaction on bisulfide diffusive transport through MX-80 bentonite

Canada's deep geological repository (DGR) design includes an engineered barrier system where highly compacted bentonite (HCB) surrounds the copper-coated used fuel containers (UFCs). Microbial-influenced corrosion is a potential threat to long-term integrity of UFC as bisulfide (HS−) may be produced by microbial activities under anaerobic conditions and transported via diffusion through the HCB to reach the UFC surface, resulting in corrosion of copper. Therefore, understanding HS− transport mechanisms through HCB is critical for accurate prediction of copper corrosion allowance. This study investigated HS− transport behaviour through MX-80 bentonite at dry densities 1070–1615 kg m−3 by performing through-diffusion experiments. Following HS− diffusion, bromide (Br−) diffusion and Raman spectroscopy analyses were performed to explore possible physical or mineralogical alterations of bentonite caused by interacting with HS−. In addition, accessible porosity ε was estimated using extended Archie's law. Effective diffusion coefficient of HS− was found 2.5 × 10−12 m2 s−1 and 5.0× 10−12 m2 s−1 for dry densities 1330 and 1070 kg m−3, respectively. No HS− breakthrough was observed for highly compacted bentonite (1535–1615 kg m−3) over the experimental timeframe (170 days). Raman spectroscopy results revealed that HS− reacted with iron in bentonite and precipitated as mackinawite and, therefore, it was immobilized. Finally, results of this study imply that HS− transport towards UFC will be highly controlled by the available iron content and dry density of the buffer material. [Display omitted] •Bisulfide diffusion coefficient decreased with increasing bentonite dry density.•Immobilization of HS− by geochemical reactions inhibited its diffusive transport.•Raman anawlysis revealed immobilization occurred via formation and precipitation of mackinawite.•Transport of HS− had negligible effects on bentonite pore structure.•Density and iron content of bentonite will control HS− transport towards UFC.