Fluorinated quaternary ammonium covalent organic frameworks for selective and efficient removal of typical per- and polyfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFAS) are widely distributed in water and pose a potential threat to ecological security and environmental health, and PFAS removal from water is becoming an urgent need. In this study, we prepared the novel covalent organic frameworks (COFs) through structural design and mixed edges combination with high adsorption selectivity and capacity for PFAS. The COFs were designed with appropriate pore size, and the edge of COFs was replaced with different ratios of fluorinated edge and quaternary ammonium edge to create ideal fluorinated quaternary ammonium COFs. The prepared COFs had high adsorption capacity of 300.2 μmol/g and 879.8 μmol/g for PFHxS and PFOS, respectively, and exhibited a relatively stable adsorption performance at different solution pH, ionic strength and humic acid concentrations. More significantly, the fluorinated quaternary ammonium COFs can selectively adsorb PFAS in the presence of various hydrocarbons with similar structures, and efficiently remove multiple PFAS from actual electroplating wastewater simultaneously, much better than powdered activated carbon and anion exchange resins. Density functional theory (DFT) calculations revealed that the PFAS were efficiently adsorbed on the C-F chain of COFs, while the C-H chain of hydrocarbons were repelled by the C-F chain on COFs. Additionally, the COFs was well regenerated by the mixture of NaCl and methanol solution, and showed relatively stable adsorption capacity for PFAS within 5 cycles. These results offer an alternative strategy for bifunctional modification to selectively and efficiently remove PFAS from water.