Per- and polyfluoroalkyl substances (PFAS) cause liver damage, disrupt the reproductive system, and affect the immune system. Anion-exchange resins are one of the most effective treatment technologies for removing PFAS from water. However, anion-exchange resins have (i) low selectivity for short- and ultrashort-chain PFAS and (ii) finite capacity before regeneration is required. In this project, we explored the potential for another ion-exchange process, namely Donnan dialysis, to separate PFAS from contaminated water. Donnan dialysis leverages electrochemical potential gradients across ion-exchange membranes to drive target ion (i.e., PFAS) transport from a waste solution into a salt-based draw solution (e.g., NaCl). The objective of this work was to evaluate, for the first time, PFAS separation by Donnan dialysis. Initial experiments were conducted with waste solutions containing 500 µg L^-1 PFBA, PFPeA, and PFHxA and draw solutions with 580 mg L^-1 NaCl. The two solutions were separated by anion-exchange membranes with different thickness and ion-exchange capacity: FAS-50 (45-55 µm, 1.6-2.0 meq g^-1); FAA-3-PK-130 (110-130 µm, 1.1-1.4 meq g^-1); and FAD-PET-75 (70-80 µm, 1.2-1.4 meq g^-1). After 10 h, greater than 99% PFAS removal was observed with all three membranes; however, PFAS levels in the draw solution differed due to variable PFAS retention by the membrane. The thinnest membrane, FAS-50, also exhibited the lowest selectivity coefficients for PFAS over Cl^-. These properties resulted in higher PFAS recovery into the draw solution compared to the other membranes. Longer-chain PFAS, like PFHxA, displayed higher retention in the membrane compared to short-chain PFBA. To increase the rate of PFAS recovery, formate was employed as an alternative draw ion to enhance membrane swelling. The optimized system was evaluated with a suite of 19 PFAS to identify the extent and rate of removal as a function of chain-length and head group. The outcomes of this work support further development of Donnan dialysis systems for PFAS separation and treatment.

Kaylyn Stewart

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