Exploring the effects of pH, ionic strength, and temperature on bisulfide sorption onto bentonite via experiments and numerical modelling.

The long-term performance of the Canadian deep geologic repository (DGR) relies significantly on bentonite clay, as sealing materials intended for use in the engineered barrier system (EBS). One particular safety concern is microbiologically influenced corrosion of the used fuel containers (UFCs) which may occur if bisulfide (HS-) transports through the bentonite buffer to reach the UFC surface and corrode the copper coating. Understanding HS- sorption onto bentonite is therefore an important aspect of this problem, as HS- sorption can reduce the extent of copper corrosion. However, sorption dynamics onto bentonite are not yet well-understood. As such, this study performed laboratory batch experiments to investigate HS- sorption onto bentonite slurries as a function of temperature (10-40 °C), pH (9-11), and ionic strength (0.01 M-1 M NaCl). These conditions were aimed to reflect the range of possible DGR geochemical conditions. The experimental results showed that HS- sorption onto bentonite increased with increasing temperature but decreased with increasing pH and ionic strength. A 3-way ANOVA (analysis of variance) showed that the variables' individual and 2-way interaction effects are statistically significant, which implies that they should be incorporated into a sorption mechanism. A thermodynamic-based sorption model was also developed in PHREEQC assuming that sorption was driven by three key processes: (i) redox reaction with the structural Fe3+ sites, (ii) surface precipitation as FeS (mackinawite), and (iii) surface complexation reactions with surface hydroxyl group (OH) at the edge sites of montmorillonite. The model successfully described the main experimental trends and provided valuable insights into the relative contribution of these processes to the total HS- sorption mechanism. Altogether, this study provides novel insights from experimental and numerical modelling findings that enhance the understanding of HS- sorption onto bentonite, in the context of Canadian DGR design as well as other nuclear repositories worldwide.