Control of host-guest complexation and release of PFAS with pH changes employing ionizable β-cyclodextrin derivatives

Poly- and perfluoroalkyl substances (PFAS) are organic pollutants whose widespread use, resistance to degradation, and adverse health effects have raised significant environmental and public health concerns. The ability of β-cyclodextrin (β-CD) to form strong inclusion complexes with PFAS has led to its incorporation in adsorbents for treating PFAS-contaminated water. Regenerating these PFAS-laden materials often remains challenging. We report herein the functionalization of β-CD with ionizable groups to control host-guest complexation with PFAS in response to pH changes. Adjusting the solution pH from neutral to alkaline led to a 56 to 98 % reduction in the binding constants (K_CD:PFAS) of short- and long-chain PFAS with amino- and thiol-functionalized β-CDs. The observed reduction in binding constants under alkaline conditions is attributed to enhanced electrostatic repulsion between negatively charged functional groups attached to β-CD (host) and the anionic PFAS polar head group (guest). Incorporation of two pH-dependent ionizable functional groups, phenol and benzylamino, into β-CD to yield 6-(3-hydroxybenzylamino)-6-deoxy-β-cyclodextrin [(3-OH)BnNHβ-CD] enables the β-CD host to transition from a positive to a negative charge as the solution pH increases from neutral to alkaline. Consequently, (3-OH)BnNHβ-CD exhibits a pronounced pH-modulated complexation of PFOA, with an 88 % decrease in the association constant under alkaline conditions. The association constant for (3-OH)BnNHβ-CD with hexafluoropropylene oxide dimer acid (HFPO-DA), a branched perfluoroether carboxylic acid (PFECA), however, decreases by nearly 50 % under alkaline conditions compared to an 81 % and 98 % decrease observed for mono-thiol and mono-amino β-CDs, respectively. A 95 % decrease in binding in PFOA is observed for mono-thiol-β-CD, while heptakis-(6-mercapto-6-deoxy)-β-cyclodextrin, with seven ionizable thiol groups, leads to a modest 23 % decrease for complexation of PFOA with change from neutral to alkaline pH. Steric effects due to chain branching within PFAS in combination with size and number of substituents on the β-CD reduce the impact of pH effects on the complexation. This study demonstrates derivatization of β-CD with pH ionizable functional groups can be used to control the β-CD binding of PFAS as a possible strategy for the removal and recovery of PFAS from contaminated water streams.