The influence of solution chemistry on air-water interfacial adsorption and transport of PFOA in unsaturated porous media

There is great interest in the transport behavior of PFAS in the vadose zone, and the impact of leaching on groundwater contamination. Air-water interfacial adsorption is an important process for PFAS retention in unsaturated porous media, and it is influenced by many factors including solution conditions such as ionic strength. The present study employed miscible-displacement column experiments to investigate the impact of ionic strength and pH on perfluorooctanoic acid (PFOA) retardation and transport under dynamic water-flow conditions. The results showed that retardation under unsaturated conditions was affected significantly by changes in ionic strength, whereas there was minimal impact for saturated conditions. This indicates that air-water interfacial adsorption, which was a major source of retardation, was influenced significantly by changes in ionic strength while they had a minor impact on solid-phase adsorption. The impact of changes in ionic strength on the magnitude of air-water interfacial adsorption observed for the column experiments was consistent with measured surface-tension data. The impact of changes in pH was less significant compared to that of ionic strength for transport under unsaturated conditions. These results illustrate the influence of solution chemistry on PFAS adsorption and transport under unsaturated conditions. This solution-dependent behavior should be considered when characterizing PFAS transport in soils and the vadose zone.