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 Gamma and K-IA measurements determined for three PFAS from transport experiments and surface-tension data. The transport-measured Gamma and K-IA data were observed to be fully consistent with the directly-measured data. Specifically, Gamma values for the two methods were entirely coincident in the region of overlapping concentrations, which spanned similar to 4 orders-of-magnitude. Furthermore, the two data sets adhered to an identical Gamma-C profile. These results conclusively demonstrate the accuracy of the transport-measured values. Gamma 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. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study demonstrated that transport-measured Gamma and K-IA values are consistent with directly-measured data, indicating their accuracy and applicability in characterizing and modeling PFAS transport and fate in environmental systems.