Effects of Temperature and Ionic Strength on Diffusion and Swell Behavior of Bentonite for Contaminant Barrier Applications

Sodium bentonite (NaB) is a high-swelling, low permeable clay commonly used in engineered containment barriers to limit fluid flow and advection of contaminants into the surrounding environment. Such NaB-based containment barriers include geosynthetic clay liners, compacted soil- and sand-bentonite liners, buffers for high-level radioactive waste (HLRW) disposal, and soil-bentonite cutoff walls. NaB barriers exhibit low hydraulic conductivity (k) to water and low ionic strength solutions (e.g., k < 10-11 m/s), such that diffusion becomes the dominant transport mechanism for contaminants escaping through the barrier. Therefore, accurate prediction of long-term containment performance requires knowledge of diffusion coefficient values for the bentonite under expected environmental conditions. However, NaB barriers for solid, mining, and radioactive waste may be frequently exposed to temperatures ≥ 50°C and a wide range of solution/leachate concentrations. Further, the swelling and transport properties of NaB are highly sensitive to ionic strength of the fluid, presence of multivalent cations, and temperature, such that comprehensive testing is required to evaluate diffusion behavior for the range of conditions expected during the lifespan of the barrier. This experimental study measured swell index (SI) and solute diffusion coefficients for MX-80 NaB for a range of temperatures (20 - 50o C) and ionic strengths (5 - 100 mM) for both KCl and CaCl2 solutions. Diffusion tests were performed using the dialysis leaching test and through-diffusion methods. Values of apparent diffusion coefficients (Da) and effective diffusion coefficients (D*) generally ranged from 10-11 to 10-9 m2/s for both test methods. As KCl concentration increased, measured values of Da and D* increased and SI values decreased, indicating decreasing barrier performance. In addition, the highest temperature conditions corresponded to the highest measured Da values for all of the tests. The results of the study enhance understanding of the impact of temperature and salt type on contaminant containment performance of MX-80 bentonite.