Investigating Bisulphide Sorption onto Bentonite: Methodology Development and Design of Batch Experiments

The Nuclear Waste Management Organization (NWMO) is planning to develop a deep geological repository (DGR) for safe and long-term management of Canada’s used nuclear fuel in a stable rock formation 500 m below ground surface. Within a DGR, the used nuclear fuel will be encapsulated in an engineered barrier system (EBS), which will include copper-coated used fuel containers (UFCs) surrounded by highly compacted bentonite (HCB). A potential concern towards the long-term EBS performance is the production of bisulphide (HS⁻) by sulphate-reducing bacteria near the rock-bentonite interface. If produced, HS⁻ may diffuse through the bentonite and corrode the copper surface of the UFC. Although it is anticipated that sorption onto bentonite will restrict HS⁻ transport and minimize the risk of corrosion, the sorption phenomenon of HS⁻ onto bentonite has not been systematically investigated in the hydrogeochemical context of a DGR. To address this knowledge gap, this study designed laboratory batch experiments to investigate HS⁻ sorption onto bentonite under the influence of various conditions relevant to a DGR. As a part of this study, a literature review was conducted to select the factors that may influence the sorption behaviour. The study also required robust methodology development to build confidence in the experimental procedure. The preliminary results showed that sorption process required 24 h to reach equilibrium at room temperature (22 ± 2 °C). In addition, a minimum: (i) liquid-to-solid ratio of 100:1 and (ii) 1 ppm initial HS⁻ concentration were required to obtain detectable amounts of aqueous HS⁻ after sorbing onto bentonite. Considering the experimental constraints and the expected range of the key geochemical conditions in the DGR (e.g. temperature, pH, ionic strength), four sets of batch experiments were designed with appropriate quality control to explore the sorption phenomenon (including kinetics, isotherms, thermodynamics) under the key factors. While this study sheds light on the fundamental sorption mechanisms in bentonite; it also provides valuable guidance on sorption experimental methodology, which can be used in other environmental related research. Altogether, this study supports the broader, ongoing effort to assess the long-term EBS performance of Canada’s DGR.