Synergistic Adsorption at the Air–Water Interface Strongly Influences Transport of PFAS Precursors in Unsaturated Porous Media

Our knowledge of the transport and interfacial behavior of per- and polyfluoroalkyl substance (PFAS) precursors in the vadose zone remains incomplete, with significant knowledge gaps. This study investigated unsaturated transport of two precursors, 6:2 fluorotelomer sulfonate (6:2 FTS) and perfluorohexanesulfonamido propylamine (PFHxSAm), using air–water interfacial tension measurements, column experiments, and mathematical modeling. PFHxSAm exhibited substantially greater surface activity than 6:2 FTS and perfluorooctanoate (PFOA), leading to a pronounced retention in unsaturated columns. PFHxSAm was also determined as volatile, with an air–water partition coefficient of log Kaw = −4.45 (95% C.I. [−4.67, −4.31]) [log(cmw3/cmair3)], yet gas partitioning and diffusion processes were shown to contribute negligibly to its retention at ∼50% water saturation, likely due to the relatively low volatility. Rate-limited mass transfer at the air–water interface (AWI) was observed for 6:2 FTS and PFHxSAm, with desorption occurring 5–9 times more slowly than adsorption. Mixture experiments provide the first experimental evidence of synergistic AWI adsorption between 6:2 FTS and PFHxSAm, contrasting with the competitive behavior reported for perfluoroalkyl acids. Modeling of the transport experiments indicates that synergistic interactions increased AWI adsorption of PFHxSAm and 6:2 FTS by up to 1.9- and 11.3-fold, respectively, relative to their single-solute behavior. These findings demonstrate that synergistic interfacial interactions can substantially increase PFAS retention in the vadose zone, with important implications for their fate, transport, and risk assessment.