Dialysis Method to Measure Diffusion in Sodium and Enhanced Bentonites

Sodium bentonite (NaB) and enhanced bentonites are used extensively in engineered containment barriers. These high-swelling clays are relied upon to provide long-term, low hydraulic conductivity, k (e.g., 10^-10 m/s), to restrict advective transport of contaminants. Although diffusion becomes a significant contaminant transport mechanism relative to advection in barriers with k<10^-9 m/s, diffusion data for NaB and enhanced bentonites is limited due to the complexity and long test durations of traditional diffusion test methods. Thus, a dialysis-leaching test (DLT) method was developed and refined for measuring diffusion properties of bentonite specimens with porosities (n) similar to geosynthetic clay liners. The DLT method requires short test durations (<7 d), uses inexpensive and commercially-available equipment (<$300 per setup), and has shown good reproducibility of results between different laboratories and researchers. The DLT method, including recent improvements to the technique and testing limitations, is described in detail in this paper. Example results are provided for chloride diffusion in NaB and bentonite polymer composite (BPC) specimens (n=0.82–0.89). Apparent diffusion coefficients (D_a) were measured using 200 mM potassium chloride (KCl) and 100 mM calcium chloride (CaCl₂) solutions. The diffusion results were consistent with expectations based on the literature and diffuse-double layer theory. Tests were performed at both full-scale (7 L) and half-scale (3 L) sizes, and there were no significant effects of test scale on the results. Recommendations are provided for using the DLT method to measure chloride diffusion in bentonites for different test conditions and available laboratory resources.