Transport of perfluoroalkyl acids at trace concentrations through engineered bentonite-based barriers

ABSTRACT With the aim of assessing the relation between the transport of perfluoroalkyl acids (PFAA) and the semipermeable properties of bentonites, which are widely used as part of pollutant containment systems, a multi-stage membrane test was conducted on a natural bentonite in equilibrium with aqueous mixtures of sodium chloride (NaCl) and sodium perfluorooctanoate (C 8 F 15 NaO 2 ). The NaCl concentration at the contaminated-solution side of the bentonite specimen was varied stepwise from 1 to 100 mM, thereby simulating a broad spectrum of ionic strengths that may be encountered in field scenarios. In contrast, the C 8 F 15 NaO 2 concentration was maintained constant at 0.1 mM, which was low enough for C 8 F 15 NaO 2 to be regarded as a trace contaminant and the ionic selectivity of the bentonite specimen to be primarily controlled by the NaCl concentration. The hydraulic head difference, which built up across the specimen at null volumetric liquid flux, and the ionic concentrations in the liquid samples were monitored throughout the testing programme, allowing the reflection coefficient and the mass flux of perfluorooctanoate ions to be simultaneously determined. The test results showed a decrease in the reflection coefficient from 0.495 to 0.046, accompanied by an increase in the steady-state molar flux of perfluorooctanoate ions from 0.823·10 -10 to 3.400·10 -10 mol·m -2 ·s -1 . An original theoretical framework was developed to model the transport of trace ions via diffusion and electromigration in the presence of a single dominant salt, and the interpretation of the test results provided evidence that the semipermeable properties of bentonites contribute significantly to restricting the migration of PFAA.

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