Revealing the Molecular-Level Interactions between Cationic Fluorinated Polymer Sorbents and the Major PFAS Pollutant PFOA

The development of new technologies for the removal of a family of manufactured chemicals, the per- and polyfluoroalkyl substances (PFAS), from the environment is urgently needed to safeguard public and environmental health. Here we report a fundamental study of the binding mechanisms driven by fluorine-fluorine and electrostatic interactions between perfluorooctanoic acid (PFOA), an important PFAS molecule, and three types of block copolymer sorbents containing individually perfluoropolyether (PFPE) or quaternized ammonium groups, or both functional segments in combination. The results show that both the fluorine-fluorine interactions between the PFPE segment of the block copolymer and the fluorinated tail of the PFOA as well as electrostatic attraction between the quaternized ammonium group and the anionic PFOA headgroup are crucial to achieve effective PFOA sorption from aqueous solutions. The fluorine-fluorine interactions contribute to recognition of PFOA molecules via fluorophilicity, with fast exchange between bound and free PFOA being observed, while the electrostatic interactions can tightly bind PFOA, thus precluding such exchange. Both types of interaction are observed to be rapidly established within 5 min. We show that the sorbents containing both fluorinated and cationic groups have a higher PFOA removal efficiency with potentially improved sorption capacity compared with the sorbents with a single functional group and that the electrostatic attraction is stronger and dominates the fluorine-fluorine interactions when the sorbent is highly charged. Overall, these results provide important insights into designing novel sorbents for rapid and efficient PFAS removal from contaminated environments.