Per- and polyfluoroalkyl substances (PFAS) form strong mutual interactions at the water-air interface even when exhibiting a Langmuir adsorption isotherm

Abstract

Per- and polyfluoroalkyl substances (PFAS)—so-called forever chemicals—present an emerging and ubiquitous threat to human and environmental health. Adsorption to the water-air interface is an important phenomenon impacting PFAS fate and transport in natural systems and, also, the efficacy of multiple remediation methods including foam fractionation and micro- and nanobubble-based approaches. Unfortunately, this phenomenon remains poorly understood, particularly in conditions relevant to natural and treatment systems, where water-air interfaces generally carry significant loads of other organic contaminants or natural organic matter. To help understand the impact of organic loading on PFAS adsorption, molecular dynamics simulations of five anionic terminal PFAS (PFOA, PFBA, PFOS, PFHxS, PFBS), one zwitterionic terminal PFAS (5:3 FtB), and one cationic PFAS precursor (PFHxSaAm) as single species and in several 1:1 mixtures were conducted at varying interfacial densities. The results show that PFAS form strong mutual interactions (van der Waals attraction between perfluoroalkyl chains, Coulomb interactions between charged head groups) that give rise to laterally and longitudinally ordered interfacial coatings. Overall, these attractive and repulsive interactions exhibit significant mutual cancellation, in agreement with observations that PFAS often, but not always, exhibit a Langmuir isotherm at the water-air interface.