(701e) Nanofiltration of per- and Polyfluoroalkyl Substances (PFAS): Rejection Mechanisms

Perfluoroalkyl and Polyfluoroalkyl substances (PFAS) are an emerging and extremely pervasive contaminant with a potentially high health risk. They are currently being found throughout the United States in ground and surface waters above the EPA’s limit of 70 ppt. Membrane separation is a potentially effective process to remove PFAS from water sources. Past research has shown that nanofiltration (NF) membranes are capable of removing PFAS from water, however the rejection mechanism for removal is unknown. Dead-end filtration experiments were conducted to infer the rejection mechanism used by nanofiltration membranes to remove PFAS from water. Nanofiltration membrane NF270 was found to intrinsically reject the highest risk PFAS molecules: PFOA (Perfluorooctanoic acid), PFOS (Perfluorooctanesulfonic acid), PFNA (Perfluorononanoic acid) and PFHxS (Perfluorohexanesulfonic acid). Rejections of 95-99.8% were observed at a neutral pH. A direct relationship was found between both molecular weight and radius and rejection, demonstrating the presence of the size exclusion mechanism. The electrostatic interaction rejection mechanism was investigated by changing testing solution pH. Changing pH was found to significantly impact PFAS rejection, with highest rejections at a basic pH of 10 (99.8%) and lowest rejections at an acidic pH of 3 (94.2%). At basic pH the zeta potential, and thus the electric charge, of the NF270 membrane surface is negative repelling the negatively charged PFAS molecules. These findings indicate that both electrostatic interactions and size exclusion are important for nanofiltration rejection of PFAS.