Calcium-Resistant Membrane Behavior of Polymerized Bentonite

Traditional (unmodified) sodium bentonite (Na-bentonite) has been shown to exhibit semipermeable membrane behavior, which is beneficial in terms of the use of Na-bentonites as barriers or components of barriers for hydraulic containment applications. However, degradation of such membrane behavior due to the diffusion of cations into the bentonite also is known to occur, and this degradation increases with increasing salt concentration and increasing valence (charge) of the principle salt cation (e.g., Ca2+ versus K+). In this study, the membrane behavior of a polymerized bentonite referred to as a bentonite-polymer nanocomposite (BPN) was determined during multistage membrane tests using solutions of CaCl2. The BPN illustrated improved membrane behavior relative to that previously reported for both traditional and other chemically modified bentonites. However, complete degradation of the membrane behavior was ultimately observed for specimens of the BPN exposed to 10 mM CaCl2 for a test conducted in a rigid-wall cell and 20 mM CaCl2 for a test conducted in a flexible-wall cell. The destruction of membrane behavior for the specimen in the rigid-wall cell was attributed to short-circuiting along the side walls of the rigid-wall cell after shrinkage of the BPN, whereas destruction of the membrane behavior in the flexible-wall cell correlated with the time required to reach steady-state diffusion of calcium (Ca2+). Thus, the enhanced calcium-resistant behavior of the BPN was limited and was affected by the type of cell (rigid-wall versus flexible-wall) in which the membrane behavior was measured.