Potential-Dependent SEIRAS Study of PFAS Surface Adsorption

Per- and polyfluoroalkyl substances (PFAS) and their bioaccumulation are growing priorities for water remediation efforts. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), our measurements allowed observations of PFAS adsorption in response to surface charge. Increased adsorption of perfluorooctanoic acid (PFOA) was observed at positive potentials via the molecule's hydrophobic long-chain carbon-fluorine tail, despite the electronegativity of the carboxylic acid head. This behavior was observed at pH 2, 3, and 4, with a continued lay-flat orientation of the surface molecules due to the dominance of the hydrophobic interaction versus electrostatic interaction. Potential cycling demonstrated weak adsorption hysteresis, and a disruption of surface interactions was observed in the presence of potassium bromide. For comparison, perfluorooctanesulfonate (PFOS) was also tested and observed to adsorb via the hydrophobic carbon-fluorine tail. Time-dependent adsorption behavior for PFOS was noted, with a higher pH solution showing a higher rate of surface adsorption. These findings demonstrate the complexities involved in the competition between electrostatic and hydrophobic interactions and contribute to future innovation in electrosorption methods for remediation of PFAS contamination.