Per- and polyfluoroalkyl substance separation by NF and RO membranes: a critical evaluation of advances and future perspectives

Abstract

Per- and polyfluoroalkyl substances (PFAS), dubbed “forever chemicals”, are synthetic compounds containing strong carbon–fluorine bonds. They are widely used in various industrial processes and products, and as a result, PFAS pollution is pervasive and has led to persistent contamination of surface and groundwater sources. Due to the adverse impact of PFAS exposure on health, there have been growing concerns among the public, the scientific community, and regulatory bodies, and treating water to an adequate level is essential. Nanofiltration (NF) and reverse osmosis (RO) are two of the candidate technologies for separating PFAS from water. NF and RO systems are easy to operate and require little use of chemicals. In contrast, other water treatment technologies (e.g., chemical oxidation, adsorption, ion exchange, and photocatalytic degradation) are often unsatisfactory due to slow reaction kinetics, generation and release of harmful by-products, or high operating costs. Despite the advantages of NF and RO, a concentrated residual stream is produced which contains high levels of PFAS. This concentrate, which typically accounts for 10 to 20% of the feedwater volume and is 5 to 10 times more concentrated with PFAS, must be managed or further treated appropriately to prevent environmental contamination. In this review, the NF/RO systems for the treatment of PFAS-contaminated water are discussed, focusing on the factors that affect their effectiveness and the mechanisms by which they remove PFAS. Also, advances in NF/RO membranes and systems as well as technical challenges at present are discussed along with an introduction to a total management plan for concentrated residual streams using a novel combination of NF/RO processes coupled with other state-of-the-art methods.