Investigating the effect of ionic strength and pH on bisulfide sorption onto bentonite through laboratory batch experiments

The Nuclear Waste Management Organization (NWMO) plans to develop a deep geological repository (DGR) to safely isolate Canada's used nuclear fuel in a stable rock formation 500 m below the ground surface. Within the DGR, the used nuclear fuel will be encapsulated in an engineered barrier system, which will include copper-coated used fuel containers (UFCs) surrounded by highly compacted bentonite (HCB). A potential concern regarding the long-term DGR performance is the production of bisulfide (HS-) by sulfate-reducing bacteria near the rock-bentonite interface. If produced, HS- may diffuse through the bentonite and corrode the copper surface of the UFC. However, it is anticipated that sorption onto bentonite will restrict HS- transport and minimize the risk of corrosion. As such, the HS- sorption onto bentonite is a critical aspect of the DGR safety assessment but is not well-understood in the hydrogeochemical context of the DGR. This study aims to address this knowledge gap by investigating HS- sorption onto bentonite through a set of batch experiments varying pH (9 to 11) and ionic strength of background NaCl (0.01, 0.1, 1 M). Experiments were performed using MX-80 bentonite (supplied by NWMO), with all other key experimental conditions remaining constant, e.g., temperature (at 22 ± 2°C), liquid to solid mass ratio (200), HS- initial concentration (5 mg/L) and contact time (28 hours). The results show that increasing pH, decreased HS- sorption (around 25-60%), which may indicate that as pH increased, the bentonite edge surfaces might have become more negatively charged (due to increasing deprotonation), and led to decreased HS- sorption due to electrostatic repulsion. In addition, the sorption efficiency decreased from 68-61% to 38-33% with increasing ionic strength from 0.01 M to 1 M. These findings will be used to develop a thermodynamic sorption model to better understand the fundamental sorption mechanisms that may inhibit HS- transport through bentonite under various DGR conditions. Altogether, this study will support the broader, ongoing effort to assess the long-term performance of Canada's DGR.