Diffusion Of Calcium Chloride Through Polymerized Bentonite

Sodium bentonite (Na-bentonite) commonly is used in chemical containment barriers to limit transport of contaminants into the environment. The potential for decreased performance of Na-bentonite barriers due to cation exchange with multivalent species (chemical incompatibility) has led to the development of chemically modified bentonites (CMBs), such as polymer-modified bentonites, for improved chemical resistance. Results of experimental research to date generally indicate that CMBs may maintain values of hydraulic conductivity, k, less than 10(-9) m/s, even when permeated with solutions with high ionic strength and concentrations of multivalent species (e.g., 500 mM calcium chloride (CaCl2)). However, there is limited data available regarding diffusion through CMBs, even though diffusion has been shown to be a significant to dominant contaminant transport mechanism in clays when k is less than 10(-9) m/s. In this paper, results are presented for diffusion of chloride (Cl-) through a bentonite-polymer composite (BPC) for source concentrations of CaCl2 ranging from 0.005 M to 0.1 M. Effective diffusion coefficients (D*) for Cl-through BPC were measured using a multistage through-diffusion method as well as a single-stage, dialysis-leaching test (DLT) method. The values of D* for Cl-increased from 3.3 x 10(-11) m(2)/s to 3.5 x 10(-10) m(2)/s as the average Cl-concentration in the BPC increased from 0.005 M to 0.173 M. In addition, there generally was good agreement between the results of the two test methods over the range of Cave evaluated, despite significant differences in the procedures and test durations (e.g., > 2 months for through-diffusion test, <= 2 days for DLT). The results of the study are compared with available literature for diffusion in Na-bentonite and other CMBs, and recommendations for future research are provided.