Cross-linker Chemistry Determines the Uptake Potential of Perfluorinated Alkyl Substances by β‑Cyclodextrin Polymers

Per- and poly fluorinated alkyl substances (PFASs), notably perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid, contaminate many ground and surface water resources and are environmentally persistent. Furthermore, there are many other PFASs in use that are persistent and contaminate fresh water resources. A polymer consisting of β-cyclodextrin (β-CD) cross-linked with decafluorobiphenyl (DFB-CDP) has shown promise for sequestering PFOA at environmentally relevant concentrations, though its efficacy to remove other PFASs from water has not yet been explored. Additionally, although the DFB-CDP was designed to sequester PFASs on the basis of favorable fluorous interactions, the rationale for its relatively high affinity for PFOA compared to other previously synthesized β-CD polymers remains unknown. In this study, we explored cross-linker chemistry as a potential determinant of PFAS affinity for β-CD polymers. We synthesized three DFB-CDP derivatives with varying degrees of phenolation in the cross-linker (to evaluate effects of polymer surface charge) along with two β-CD polymers cross-linked by two other chemically distinct strategies, epichlorohydrin and 2-isocyanatoethyl methacrylate. We measured the equilibrium removal of ten PFASs from water by each of the five polymers at environmentally relevant concentrations. We found that β-CD polymers cross-linked by perfluorinated aromatics with low degrees of phenolation are more favorable for PFAS adsorption. These findings provide insight into the mechanism of PFAS adsorption by β-CD-based polymers and will inspire modular designs of β-CD-based adsorbents to target other PFASs and micropollutants.