Examining the robustness and concentration dependency of PFAS air-water and NAPL-water interfacial adsorption coefficients

Determining robust values for the air-water or NAPL-water interfacial adsorption coefficient, K_IA, is key to characterizing and modeling PFAS transport and fate in several environmental systems. Direct, high-resolution measurements of surfactant adsorption at the fluid-fluid interface were aggregated from the literature. This data set was used to examine the accuracy and applicability of Γ and K_IA measurements determined for three PFAS from transport experiments and surface-tension data. The transport-measured Γ and K_IA data were observed to be fully consistent with the directly-measured data. Specifically, Γ values for the two methods were entirely coincident in the region of overlapping concentrations, which spanned ~4 orders-of-magnitude. Furthermore, the two data sets adhered to an identical Γ-C profile. These results conclusively demonstrate the accuracy of the transport-measured values. Γ and K_IA values determined from the application of the Gibbs adsorption equation to measured surface-tension data were fully consistent with the directly-measured and transport-measured data sets, demonstrating their applicability for representing PFAS transport in environmental systems. The directly-measured data were used to examine the concentration dependency of K_IA values, absent the potential confounding effects associated with the use of surface-tension or transport-measured data. The directly-measured data clearly demonstrate that K_IA attains a constant, maximum limit at lower concentrations. Two separate analyses of the transport-measured data both produced observations of constant K_IA values at lower concentrations, consistent with the directly-measured data. These outcomes are discussed in terms of surface activities, relative surface coverages, and critical concentrations.