The presence of polyfluorinated alkyl substances (PFASs) in ground and surface water is widely considered as a major environmental and health emergency in the world. The most common PFASs are perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) that are extensively used in the production of fluoropolymers and manufacturing of fire-fighting foams. These pollutants are environmentally persistent and demonstrate significant bioaccumulation as a consequence of thermodynamically stable C-F bond. In response to such a crisis, treatment processes based on oxidation, photochemical reduction, and electrochemical degradation have been proposed to break the strong C-F bond. However, these methods are expensive and generate further byproducts with unknown toxicity. On the other hand, filtration using adsorption remove PFASs instead of degrading them and is comparatively a cheap technique. The Dichtel and Helbling group proposed a breakthrough cross-linked cyclodextrins (CDs) based adsorbents in order to remove the PFAS from water with the adsorption rate constant of 15 to 200 times greater than activated carbon1. However, the exact understanding of adsorption mechanisms at molecular scale is missing in the literature. In this study, we investigate a two-dimensional network of β-CDs with rigid aromatic fluorinated group as a cross-linker and performed molecular dynamics (MD) simulation to study the adsorption of PFO-/NH4+ surfactants. Through this work, we investigated the role of CD by artificially modifying the dipole moment of hydroxyl group, and inducing additional repulsion between the counter ion. These modifications elucidate the significance of H-bonding between the hydroxyl group and ammonium group on PFO- adsorption. In order to understand the role of the linker, we compared the adsorption efficiency of hydrogenated and fluorinated linkers namely, decafluorobiphenyl and biphenyl, and confirmed the importance of strong hydrophobic-hydrophobic interaction. These results provide new insight into the PFOA adsorption mechanism and will facilitate designs of the next-generation of adsorbents.

Adsorption mechanism of PFOA on cross-linked β-cyclodextrin network

ACS Spring Meeting 2021

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