Probing the Microscopic Behavior of PFOS Clustering and Adsorption at LNAPL Interfaces; a PFOS-Water-Cyclohexane System.

Per- and polyfluoroalkyl substances (PFAS) are common soil and groundwater contaminants that persist in the environment. Perfluorooctanesulfonic acid (PFOS) has been a key component in aqueous film-forming foams, extensively used for firefighting in both military and civilian applications, including at oil refineries. During firefighting training, fuels have been historically used to ignite fires, and because of its use in foams, PFOS is often found alongside light nonaqueous phase liquid (LNAPL) petroleum hydrocarbons in subsurface soils and groundwater. The film-forming foams contain supersaturated PFOS, and upon contact with LNAPL and soil fluids, PFOS partitions between the phases. This phenomenon is challenging to investigate by using mesoscale approaches. Here, we use molecular-dynamics simulations to study the behavior of supersaturated PFOS, with a focus on micellization and partitioning at LNAPL-water interfaces. We demonstrate that large quantities of PFOS adsorb at LNAPL-water interfaces, suggesting that such interfaces may serve as major retention sites and long-term sources of PFOS contamination. Moreover, we show that both adsorption and micellization are considerably affected by the counterions used (sodium and hydronium). This may suggest a possible avenue for controlling the partitioning process through gaining a better understanding of the effect of water chemistry on PFOS.