The influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces

The objective of this research is to examine the influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces. Surface tensions were measured for select PFAS, as well as a representative hydrocarbon surfactant. These data are supplemented with data sets collected from the literature. The influence of surfactant headgroup charge, specifically for zwitterionic PFAS, was investigated. The impacts of surfactant counterion for ionic PFAS and the influence of headgroup size for nonionic PFAS were also investigated. In addition, the influence of solution ion composition, ionic strength, and pH was examined. The impact of co-solutes, specifically ethanol, humic acid, and trichloroethene, was also examined, as well as the behavior of PFAS mixtures and fluorocarbon-hydrocarbon surfactant mixtures. The data were interpreted within the framework of a QSPR model recently developed to predict fluid-fluid interfacial adsorption coefficients (K_i) of PFAS. The results demonstrate that all of the factors investigated have some degree of impact on K_i values. Thus, the composition of soil-pore water and groundwater is likely to affect the magnitude of PFAS adsorption at air-water and organic liquid-water interfaces. However, the influence on K_i of most of the factors investigated is small for lower PFAS concentrations (less than ∼1-10 mg/L). Hence, their impacts on fluid-fluid interfacial adsorption are likely to be relatively minor at the low PFAS concentrations representative of many environmental systems, especially compared to the impact of other factors such as fluid saturations, porous-medium properties, and PFAS molecular structure. The results of this study indicate that the revised QSPR model provides reasonable first-order predictions of K_i for a wide range of PFAS in environmental systems.