Diffusion in Sodium and Enhanced Bentonites at Elevated Temperatures

Coupled thermal-hydraulic-chemical behavior of bentonite plays a critical role in the grand challenges of growing energy demand and protection of water and the environment. The challenge of long-term waste containment (e.g. radioactive waste disposal) has motivated experimental and theoretical research evaluating chemical transport properties of bentonite clay barriers under elevated temperature conditions. The purpose of a bentonite barrier is to limit advective transport, often resulting in diffusion becoming the dominant transport mechanism for contaminants. However, diffusion in sodium bentonite (NaB) under variable temperatures and chemical conditions remains poorly understood, especially for recently-developed chemically-enhanced bentonites. This knowledge gap is due to both the complexity and duration of traditional diffusion test methods, as well as the evolving development of enhanced bentonites. In this study, an experimental investigation was conducted using a newly developed dialysis-leaching test to measure diffusion properties of NaB and enhanced (polymerized) bentonite for a range of temperature and chemical conditions. Apparent diffusion coefficients (D_a) of the bentonites were measured at temperatures ranging from 20°C to 50°C. Tests were performed using calcium chloride (CaCl₂) solutions to evaluate chemical incompatibility effects on D_a. A wide range of concentrations were used (5 to 100 mM CaCl₂) to evaluate the impact of the average porewater concentration (C_ave) on rates of diffusion. For both the NaB and polymerized bentonite, values of D_a were impacted by solution temperature and generally increased as C_ave increased. This study represents the first experimental measurement of diffusion coefficients of enhanced bentonites at elevated temperatures and also demonstrates the importance of considering coupled thermal-chemical behavior in the prediction of long-term performance of bentonite barriers.