Synergistic and Complementary Electrostatic and Hydrophobic Interactions Drive Per- and Polyfluoroalkyl Substance (PFAS) Adsorption to Cationic β-Cyclodextrin Polymers

Abstract

Styrene-functionalized β-cyclodextrin (StyDex) polymers are structurally tunable adsorbents for removing per- and polyfluoroalkyl substances (PFASs) from water. Although specific functional elements are included in StyDex polymers to enhance PFAS adsorption, their relative contributions are not well understood. This knowledge gap hinders efforts to improve the selectivity of StyDex polymers for PFASs in complex water matrices. In this study, we characterized the adsorption mechanisms of six PFASs on three StyDex polymer derivatives. PFAS adsorption on StyDex polymers involves complementary ion-exchange (IX), host–guest complex formation, and hydrophobic interactions. The data also support the presence of synergistic non-IX mechanisms coupled to IX (IX-coupled, non-IX binding), which facilitate PFAS adsorption and are more resilient to inhibition in the presence of inorganic ions. IX and host–guest complex formation dominate at lower degrees of saturation, whereas other hydrophobic interactions become more relevant at higher degrees of saturation. PFAS adsorption inhibition is primarily attributed to IX-site competition, whereas non-IX mechanisms remain largely unaffected by inorganic ions, resulting in non-IX mechanisms becoming more important under environmentally relevant scenarios. This study establishes a general mechanistic framework for characterizing PFAS adsorption mechanisms and highlights opportunities to enhance synergistic adsorption mechanisms to improve PFAS removal performance in practical applications.