Salt Diffusion Through Sodium Bentonite and Bentonite Polymer Composite

Abstract

Sodium bentonite (NaB) commonly is used in engineered barriers due to the advantageous properties of the bentonite for waste containment, such as high swell, low hydraulic conductivity, and low diffusion coefficients. However, the sensitivity of NaB to chemical incompatibility has motivated development of enhanced bentonites, such as bentonite polymer composites (BPC), for improved chemical resistance and superior barrier performance. Although recent studies have indicated that BPC may maintain lower values of hydraulic conductivity (k) than NaB when exposed to solutions with high ionic strength and/or concentrations of multivalent species, data to compare diffusion performance remains limited, even though diffusion is a significant contaminant transport mechanism in clays for the reported values of $k( < 10^{ - 9} {\text{m}}/{\text{s}})$. Therefore, the goal of this study was to measure diffusion through BPC and NaB specimens using a series of dialysis-leaching tests and potassium chloride (KCl) solutions ranging in concentration from 0 to 200 mM. Apparent diffusion coefficients, D_a, for chloride (Cl⁻) for the BPC increased from 1.1 × 10⁻¹² m²/s to 1.1 × 10⁻⁹ m²/s as the average concentration of Cl⁻ in the specimen (C_ave) increased from 25 to 211 mM, These results were consistent with trends reported in a previous study based on the through-diffusion test method. The D_a values for NaB increased from 3.3 × 10⁻¹² m²/s to 1.4 × 10⁻⁹ m²/s as the C_ave for Cl⁻ increased from 5.6 to 177 mM, which was consistent with expectations based on diffuse double-layer theory. Similar values of D_a were measured for chloride for both NaB and BPC, indicating equivalent diffusive performance of the two materials for strong monovalent solutions.