Single- and Multi-component Transport of Per- and Polyfluoroalkyl Substances (PFAS) in Unsaturated Porous Media

Abstract

Per- and polyfluoroalkyl substances (PFAS) are highly persistent contaminants that pose a risk to both humans and the environment. Understanding the transport of these compounds in groundwater systems is crucial to risk characterization and remediation efforts. An important part of this understanding is the adsorption of PFAS at the air-water interface. The goal of this research was to improve the overall understanding of PFAS transport by examining the retention and release of individual PFAS and PFAS mixtures in a trapped gas (quasi-saturated) system. One-dimensional, small-scale laboratory experiments were completed using 5.1 cm- or 7.6 cm-diameter, 20 cm-long columns packed with sand and containing trapped gas. Various PFAS solutions, including single- and multi-component mixtures, were pumped through the unsaturated columns and effluent samples were analyzed to develop breakthrough curves. The results of these experiments indicated that PFAS air-water interfacial adsorption is enhanced under low concentration conditions, which is captured by the Freundlich isotherm. Increases in PFAS surface activity also enhanced interfacial adsorption. The release of single-component PFAS exhibited tailing, which was similar in PFAS mixtures. Tailing has implications for source zone longevity and should be considered at the field scale. PFAS mixture experiments showed competitive sorption effects, where the more surface active component(s) outcompeted the less surface active component(s) for adsorption at the air-water interface. These effects led to accelerated arrival of the less surface active component(s). The presence of hydrocarbon surfactants in AFFF experiments also accelerated less surface active PFAS arrival. Additionally, representing a PFAS system as ‘total PFAS’ will overestimate the arrival of certain compounds and underestimate the arrival of others. These effects were observed for isomers of PFOS and among different PFAS compounds. Overall, this research highlights the importance of identifying PFAS and other surface active components during the characterization of PFAS-impacted sites, as compounds will be transported according to their individual surface activities in concert with mixture effects.