Improving membrane performance via bentonite polymer nanocomposite

Abstract

Traditional (non-treated) sodium bentonite has been shown to exhibit substantial semipermeable membrane behavior, or the ability to restrict the migration of solutes. However, partial or complete degradation of the membrane behavior due to diffusion of invading salt cations into the sodium bentonite also has been observed. In this study, a polyacrylate modified bentonite, referred to as a bentonite polymer nanocomposite, or BPN, was evaluated as a potential substitute for sodium bentonite for the purpose of providing increased resistance to salt degradation of membrane behavior. The membrane behavior of the BPN was measured in the laboratory by establishing differences in salt concentrations ranging from 4.7 to 54mM KCl across specimens contained in either rigid-wall or flexible-wall cells under closed-system boundary conditions. The measured membrane efficiency coefficients, ω, for the BPN specimens were greater than those previously reported for specimens containing sodium bentonite under similar testing conditions. For example, the measured ω values for the BPN specimens ranged from 109% to 433% of those previously reported for the specimens containing traditional sodium bentonite, depending on the initial porosity or initial effective stress of the specimen, the concentration of KCl, and the type of test cell (rigid vs. flexible), and despite the BPN specimens not being flushed of soluble salts prior to membrane testing as in the case of the specimens containing sodium bentonite. Thus, the BPN exhibited substantially improved membrane behavior under conditions that presumably were more adverse with respect to soluble salts than those previously reported for specimens containing a sodium bentonite. However, the potential role that any excess low molecular weight polymer in the BPN may have played in affecting the results is uncertain, such that additional testing of the BPN is warranted to better understand the long-term behavior of the BPN.