Evaluating the efficiency of nanofiltration and reverse osmosis membrane processes for the removal of per- and polyfluoroalkyl substances from water: A critical review

• There are 26 types of NF/RO membranes in use or under investigation for PFAS removal. • Under environmentally relevant pH and concentration, NF and RO can effectively remove long chain PFAS from water. • More study should focus on overlooked compounds with smaller size, shorter C-F chain, and higher hydrophilicity. • Novel membrane materials, modifications, and hybrid treatment trains with improved PFAS selectivity/fouling resistance are expected. Per- and polyfluoroalkyl substances (PFAS) have been ubiquitously detected globally in the aquatic environment which poses a risk for human exposure. The recalcitrant behavior of PFAS to most traditional water treatment processes calls for development of novel or modified treatment approaches for effective PFAS elimination. Currently, high-pressure membrane systems, including nanofiltration (NF) and reverse osmosis (RO) are among the most promising and readily applied technologies for PFAS removal. In this review, we summarized peer-reviewed scientific articles on the development and use of 20 different types of commercial and 6 synthesized/modified NF and RO membranes in terms of PFAS removal efficacy. Synthesizing of existing knowledge shows that under environmentally relevant pH and concentration, NF and RO can effectively remove PFAS from water. The impact of PFAS molecular weight (MW) and functionality, membrane characteristics, feed water constitution, natural organic matters and operational conditions were systematically reviewed and evaluated. Moving forward, we recommend additional research dedicated to understanding the behaviors of PFAS alternatives and other overlooked compounds (e.g., PFAS with chain length C2 and C3) during NF and RO processes. Further, it is critical to develop or optimize existing membrane products with strong selectivity for specific PFAS species, while being resistant to fouling. Overall, this review is expected to assist researchers and water quality managers investigating membrane processes for PFAS removal.