Molecular scale adsorption behavior of per- and poly-fluoroalkyl substances (PFAS) on model surfaces

Per- and poly-fluoroalkyl substances (PFAS) are emerging contaminants of concern owing to their longevity, toxicity, mobility, and bioaccumulation. Alternative products, and chemistries, have failed to replace PFAS due to its unparalleled surfactant properties. Thus, research efforts must shift the focus onto remediation methods and mitigation strategies for PFAS removal. This study aimed to elucidate the adsorption mechanisms of four prevalent PFAS molecules through an integrated approach using a quartz-crystal microbalance with dissipation (QCM-D) and organosilane-functionalized surfaces. Nonpolar-hydrophobic and polar-hydrophilic interfaces formed the basis of the study for surface functionality. QCM-D adsorption experiments were conducted to measure the accumulation profile of each PFAS, determine the interfacial properties and kinetic parameters, and probe its molecular behavior at each proposed interface. While each model surface showed promising results for PFAS adsorption, the mechanisms each surface induced varied. Hydrophobic and electrostatic interactions played the most prominent role in PFAS adsorption. However, electrostatic interactions were highly dependent on charge distribution and molecular size, whereas hydrophobic interactions proved to be less selective and only a function of chain length. A quantitative structure–property relationship (QSPR) analysis was implemented to establish a correlation between adsorption rate, molecular composition, and interfacial physicochemical properties. Molar volume-based QSPR models successfully represented measured behavior for the nonpolar-hydrophobic interfaces, while electrostatic potential-based models could provide robust predictions of PFAS adsorption for both nonpolar-hydrophobic and polar-hydrophilic interfaces. The latter high performance was due to the descriptor being associated with both constituents of the molecule.