Sorption of Anionic and Zwitterionic Per- and Polyfluoroalkyl Substances Present in Legacy Aqueous Film-Forming Foams

Aqueous film-forming foams (AFFF) are a major source of polyfluoroalkyl substances (PFAS) in the environment. Many PFAS in AFFF formulations lack analytical standards, which has challenged the evaluation of their fate and transport. In this study, AFFF was used to spike batch sorption reactors, and experimental samples were analyzed with high-resolution mass spectrometry to yield the first experimental K_d values (1195 total) for 50 anionic and zwitterionic AFFF PFAS in 23 US soils. Sorption of most anionic, substituted, and unsaturated perfluoroalkyl sulfonates (PFSAs) was comparable to PFSAs of the same chain length. Zwitterionic PFAS with net neutral charge or terminal positive functional groups showed log K_d values >2 log units higher than anionic PFAS of the same chain length, whereas those with one or two terminals, negative functional groups, and net negative charge showed log K_d values closer to anionic AFFF PFAS. The influence of PFAS charge and charge position was validated using hydrophilic interaction liquid chromatography (HILIC) retention times collected under conditions that isolated PFAS electrostatic interactions. These findings provide a conceptual model of AFFF PFAS retention and mobility that demonstrates a range of expected behaviors─from highly mobile PFAS likely to undergo downgradient transport to less mobile PFAS likely to be more highly retained in source zones.