Fluorinated covalent triazine frameworks for highly efficient removal of per- and polyfluoroalkyl substances in water: Synthesis, performance and mechanism

Abstract

The environmental toxicity, bioaccumulation and persistence of per- and polyfluoroalkyl substances (PFAS) are of global concern, and adsorption is an efficient method to remove PFAS from water. Here, we prepared various novel fluorinated covalent triazine frameworks (CTFs) under ionothermal synthesis using long nitrile-based monomers. The synthesis process of CTFs is a dynamic reversible reaction, in which the temperature and ZnCl₂ ratio affecting the formation of N–based groups, pore size, and BET surface area of CTFs, substantially determining PFAS adsorption. The optimal CTF500-15 showed the best performance in perfluorooctanoic acid (PFOA) removal with a maximum experimental adsorption capacity of 323.39 mg/g, and could treat water contaminated with PFOA at an initial concentration of 1000 ng/L to 55.9 ng/L due to its suitable pore size, high quaternary N content and BET surface area. CTF500-15 also exhibited excellent performance in removing various mixed PFAS from simulated wastewater. A detailed mechanistic study of PFOA diffusion and the role of F atoms in the CTFs was conducted. A large pore size favors the diffusion of PFOA, the F atom and quaternary N can adsorb PFOA via fluorophilic attraction and ion exchange, respectively, which are the underlying mechanisms for the efficient removal of PFAS by fluorinated CTFs. The findings expand the idea of further refining and optimizing the CTFs structure for target pollutants.