Dialysis Method to Control Exchangeable Sodium and Remove Excess Salts From Bentonite

摘要

Control of the cation species on the exchange complex of bentonite and removal of excess soluble salts may be desirable when preparing specimens for research focused on fundamental clay behavior as well as for improving containment properties of bentonite (e.g., swell, hydraulic conductivity, diffusion, and membrane behavior). The dialysis method has been used extensively by soil scientists to prepare homo-ionized clays (i.e., clays with a single cation species on the exchange complex) and to rinse soils of excess salts. However, achievement of homo-ionization and removal of soluble salts often is assumed without verification via measurement of the exchangeable and soluble cations after dialysis. Therefore, a dialysis procedure was evaluated as a method to prepare bentonite with a controlled exchangeable sodium percentage (ESP) and a low concentration of soluble salts, as verified by measurement of the ESP and soluble cations before and after dialysis. The procedure consisted of two stages: (1) dialysis with strong (>= 0.1 M) NaCl solution to increase the percentage of Na+ on the exchange complex, and (2) subsequent removal of excess soluble salts via dialysis with de-ionized water (DIW). The ESP of the bentonite increased from 47 % for the untreated bentonite to 69 %, 80 %, and 89 % after dialysis with 0.1 M, 0.5 M, and 1.0 M NaCl solutions, respectively. Dialysis with DIW also removed soluble salts within a shorter time frame than other methods currently used to flush bentonite specimens of excess salts for testing (2 weeks versus several months).