Exploring Bisulfide Sorption onto Bentonite under Deep Geological Repository (DGR) Conditions

Abstract

The use of bentonite clay as an engineered barrier is common in the design of deep geological repositories (DGRs) for long-term management of radioactive wastes. In Canada engineered barrier system is planned to include copper-coated used fuel containers (UFCs) surrounded by a highly compacted bentonite (HCB) buffer and bentonite backfill. However, microbiologically influenced corrosion becomes a possible mode of degradation for UFC under anaerobic conditions, where sulfate is reduced to bisulfide (HS^-) by sulfate reducing bacteria and then transported to the UFC surface, may result in copper corrosion. The overall corrosion process is expected to be controlled by HS^- flux to the UFC surface; therefore, understanding the HS^- transport mechanisms through the HCB layer is critical to aid in the safety assessment of DGRs. For this purpose, sorption of HS^- onto bentonite should be studied to gather reliable values of sorption parameters for these models (e.g., sorption efficiency). However, the DGR may also experience geochemical evolutions over its long lifetime, which may affect the bentonite sorption characteristics. Therefore, the objective of this research is to evaluate sorption of HS^- onto bentonite under varying key geochemical conditions, including pH and ionic strength. The results of batch sorption experiments performed suggest that HS^- sorption decreases with increasing pH and ionic strength. In addition, ANOVA (analysis of variance) tests were performed on the experimental data to determine whether the individual effect of each factor is statistically significant or not. Altogether, the study results will be useful for developing a thermodynamic sorption model under a range of expected geochemical conditions anticipated in the Canadian DGR.