Stress and temperature regulated ion exchange selectivity in smectite within deep geological repository environments

AbstractUnderstanding the thermal-hydrological-mechanical-chemical interactions in smectite is crucial for its performance in the engineered barriers of geological nuclear waste repositories. This study explores the effect of stress and temperature on ion exchange in montmorillonite, a type of smectite mineral. Smectite samples treated with deionized (DI) water, CsCl, and LiCl were subjected to stress-induced dehydration under various temperatures and then characterized for ion exchange selectivity and structural changes. The CsCl-treated smectite sample showed a lower dry density due to its porous and disordered structure, whereas DI water- and LiCl-treated smectite samples showed higher dry density due to the well-ordered particle arrangement and lower porosity. Because Cl− is a conservative ion that exists only in the outer space, the cation concentration in the outer space pore fluid was normalized by the Cl− concentration to eliminate the influence of fluid freshening caused by interlayer water release. In CsCl-treated smectite, the normalized Cs+ concentration in the outer space pore fluid decreased with increasing effective stress due to interlayer space compaction, which promoted Cs+ adsorption into the interlayer space because of its lower hydration energy. In contrast, Li+ remained mostly in the outer space under stress because of its higher hydration energy. We also studied the concentration evolution of inherent exchangeable cations, such as K+, Ca2+, Mg2+, and Na+, in the outer space pore fluid during dehydration. CsCl-treated smectite samples favored K+ uptake into the interlayer space, whereas LiCl-treated smectite samples favored the adsorption of exchangeable divalent cations, such as Mg2+ and Ca2+, into the interlayer space. In addition, temperature further influenced ion exchange. These experimental findings demonstrate how stress-induced interlayer space dehydration, associated with the influence of the temperature, regulate ion exchange between the interlayer space and outer space in smectite, which is critical for the assessment of nuclear waste barrier performance.