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

Abstract

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.

Keywords: Bentonite; Bisulfide; Deep geological repository; Diffusion; Mackinawite.