Enhancement separation selectivity of mineral ions and perfluorinated and polyfluoroalkyl substances by nanofiltration membrane through hydrogel-assisted interfacial polymerization

The presence of perfluorinated and polyfluoroalkyl substances (PFAS) in drinking water is a critical concern for water safety and public health. Nanofiltration (NF) membranes have emerged promising technology for the elimination of trace organic contaminants from drinking water, but many previous studies have sacrificed the retention of vital mineral ions in human body in pursuit of efficient removal of PFAS. In this study, hydrogel-assisted interfacial polymerization (IP) strategy was designed to enhance the selectivity of mineral ions over PFAS, optimized pore size and surface characteristics of polyamide layers were obtained by IP process assisted by hydrogel formed by chitosan and glutaraldehyde. This approach facilitated the fabrication of NF membranes with a thinner active layer, enlarged pore size, and a more negatively charged surface. The optimized modified membrane exhibited a remarkable improvement in water permeance (16 LMH/bar, over 200 % than the control membrane) and maintained high rejection rates (>90 %) for PFAS with molecular weights ranging from 214 to 514 Da, while significantly reducing the rejection of Ca2+ and Mg2+ ions (<20 %). Density functional theory calculations revealed that all membranes exhibited reduced adsorption energies for PFAS. The treatment of natural surface water indicated the superior rejection selectivity of the modified membrane for mineral ions over natural organic matter, the average gap value of inorganic ions and natural organic matter in modified membranes was 4.6, while the average gap in commercial membranes was 1.6, improved by 2.6 times in selectivity compared to existing commercial membranes. This study offers valuable insights into the targeted enhancement of mineral ions/PFAS selectivity in NF membranes, thereby paving the way of more efficient and sustainable water treatment processes.