PFAS adsorption at the air-water interface in the presence of salts: Influence of molecular structure, salt composition, and colloidal interactions

Per- and polyfluoroalkyl substances (PFAS) in groundwater present significant risks to both the environment and human health. The adsorption affinity of PFAS determines their mobility and fate in groundwater and is utilized in remediation techniques. This affinity is influenced by both the molecular structure of PFAS and the local environmental conditions. Therefore, understanding how structural variation influences the adsorption affinity of PFAS molecules under relevant environmental conditions is crucial for effective remediation strategies. Here, we investigate the effect of tail length and headgroup of PFAS molecules on their adsorption affinity across various salt compositions that represent the mono- and di-valent cations in groundwater, illustrating the combined effects of molecular structure and colloidal interactions on the surface activity. Surface tension measurements coupled with Langmuir-Szyszkowski modeling reveal that longer PFAS chains significantly enhance adsorption affinity, as illustrated by reduced equilibrium surface tensions and lower Gibbs free energy of adsorption, Δ G ads ∘ . Our results show that in deionized (DI) water, each addition of CF 2 decreases the Δ G ads ∘ by approximately −4.2 kJ/mol. Increasing the ionic strength, either by increasing the salt concentration or varying the valency of the salts, leads to an increase in surface activity and a decrease in the Δ G ads ∘ . These effects are most pronounced for long-chain PFAS. Furthermore, PFAS with sulfonic acid headgroups exhibit higher surface activity than those with carboxylic acid headgroups. These findings offer insights into PFAS surface activity in groundwater and inform the design of effective remediation technologies for both legacy and emerging PFAS molecules. • In DI water, each additional CF 2 in the PFAS tail lowers Δ G ads ∘ by −4.2 kJ/mol. • Higher ionic strength amplifies surface activity due to electrostatic interactions • Salts reduce Δ G ads ∘ by disrupting the hydration shell and increasing the entropy • PFBS shows higher activity than PFBA due to weaker hydration and charge density