Laboratory assessment of the relationship between membrane behaviour and migration of PFAS through geosynthetic clay liners

Abstract

Besides their low hydraulic conductivity (typically ≤ 10 -11 m/s), the reduction in the transport rate of aqueous-phase contaminants through geosynthetic clay liners (GCLs), compared to mineral barriers comprising non-active clays, is ascribed to the ability of the bentonite component of GCLs to exhibit semipermeable properties, i.e., to be restrictive with regard to the electrically charged solutes while allowing unimpeded migration of the water molecules. Although the semipermeable membrane behaviour of bentonites has been the subject of extensive investigation over the past 20 years from both experimental and theoretical perspectives, its practical relevance in the presence of emerging contaminants (e.g., perfluoroalkyl substances) at trace concentrations has received limited attention so far. The objective of this paper is thus to illustrate the results of a multi-stage membrane test, which was conducted on a natural bentonite specimen in equilibrium with aqueous mixtures of sodium chloride (NaCl) and sodium perfluorooctanoate (C8F15NaO2). While the concentration gradient of C8F15NaO2 was kept constant throughout the test, the concentration of NaCl in the solution bathing the upper side of the bentonite specimen was increased step-wise from 1 to 100 mM, thereby causing a gradual destruction of the semipermeable properties of the tested specimen. The hydraulic head difference, which built up across the bentonite specimen under closed-system boundary conditions, and the concentration of perfluorooctanoate ions in the collected liquid samples were simultaneously monitored for all the testing stages, highlighting a correlation between the measured reflection coefficient and mass flux of perfluorooctanoate ions.