Treatment of Per- and Polyfluoroalkyl Substances (PFAS)-Contaminated Hypersaline Brine by Membrane Distillation

This research evaluates membrane distillation (MD) to treat problematic hypersaline per- and polyfluoroalkyl substances (PFAS)-laden waste streams. Results are shown for four commercially available membranes (unlaminated polytetrafluoroethylene (PTFE), polypropylene laminated PTFE, polyether ether ketone (PEEK), and polyvinylidene difluoride (PVDF)) that were tested with a model short-chain PFAS compound, perfluoropentanoic acid (PFPeA) at a concentration of 10 mg/L in the presence and absence of ion-exchange resin spent brine (10% NaCl). For each test, a new membrane with an area of 140 cm² was used, with a constant permeate temperature of 20 °C (cold) and varying feed temperatures of 50 °C, 60 °C, or 70 °C (hot). The unlaminated PTFE membrane demonstrated the best performance in treating the PFPeA-contaminated brine. The water flux through the unlaminated PTFE membrane was 50% higher than the flux through the PEEK membrane and 25% higher than that through the PVDF and laminated PTFE membranes. The laminated and unlaminated PTFE membranes achieved the highest rejection of NaCl and PFPeA (>99.7%) compared to 95 and 97% obtained by the PEEK and the PVDF membranes, respectively. During the 48-h extended experiments, the laminated PTFE membrane exhibited greater stability and mechanical strength than the other membranes, while the PEEK and PVDF membranes proved fragile. The laminated PTFE membrane was then selected for a 300-h experiment with ethanol cleaning cycles to test long-term durability. PFPeA caused reversible fouling in all tested membranes and reduced the membrane's hydrophobicity; however, ethanol cleaning was effective in removing PFPeA, indicating that with further optimization, membrane distillation may be useful for concentrating PFAS for ultimate destruction or disposal.