Per- and polyfluoroalkyl substances (PFAS) are ubiquitous contaminants with significant human and ecological health impacts. Their unique hydrophobic and oleophobic properties give rise to complex adsorption behaviors that have not been fully understood. In this study, two representative PFAS perfluorononanoic acid (PFNA) and perfluorooctanesulfonate (PFOS), both featuring eight-carbon chain length but different ionic head groups, were examined for their adsorption behavior on -CH3 terminated self-assembled monolayer (SAM) using quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). QCM-D experiments showed that PFNA adsorption on -CH3 terminated SAM is greater than PFOS at comparable solution concentrations. For example, with 500 mg/L solutions, 389 ± 130 ng cm-2 (0.84 ± 0.28 nmol cm-2) PFNA and 253 ± 32 ng cm-2 (0.47 ± 0.06 nmol cm-2) PFOS were adsorbed, respectively. Series injection of either PFAS solution at concentrations ranging from 50 to 500 mg/L yielded Langmuir-type adsorption isotherms. Force-displacement (FD) curves obtained using AFM are employed to resolve the hierarchical structures of the adsorbed PFAS molecules. The breakthrough force and stiffness of the film and the step size of the ruptured film were measured by quasi-statically approaching the PFAS structures. Despite the higher PFNA adsorption than PFOS, fewer hierarchical structures were observed on the PFNA-adsorbed SAM (&lt;5%) than those observed on PFOS-adsorbed SAM (~17%), suggesting that PFNA mostly adsorbs as flat-lying molecules on SAM. The FD curves obtained from PFOS-adsorbed SAM predominantly featured 2-3 nm step sizes, indicating the adsorbed PFOS molecules may have formed bilayer or micelle type of structures. AFM tapping mode imaging of PFAS-adsorbed SAM also displayed few globular aggregates deposited on the grain boundaries or defects of the underlying SAM. This study demonstrates the applicability of QCM-D and AFM as tools to investigate the adsorption behavior of PFAS on well-controlled surfaces. The results contribute to improved understanding of the fate and transport of PFASs in the environment and inform the design of engineered surfaces for PFAS removal.

Adsorption behavior of long-chain PFAS on hydrophobic self-assembled monolayer

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