In situ XANES study of PFAS impacted soils filled with aqueous and non-aqueous phases

Per- and polyfluoroalkyl substances (PFAS) contamination is widespread in our environment and partitioning at interfaces is a major factor that influences their fate and transport within a soil pore space. To develop better PFAS management and remediation strategies, we need to gain a deeper understanding of PFAS soil-porewater partitioning processes in situ. In this study, we investigated the partitioning of PFAS in a spiked agricultural soil and an Aqueous Film-Forming Foam (AFFF) impacted "aged" soil under various soil moisture conditions and when the soil pores were filled with a non-aqueous phase liquid (NAPL). Sulfur (S) K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to probe S speciation in situ, and spectra were analysed applying a Gaussian Curve Fitting (GCF) approach. Six major S species were identified in both soils and PFAS partitioning behaviour was interpreted based on changes in sulfonate fractions, representing sulfonic acid-based PFAS. In the spiked soil, sulfonate fractions increased the most when soil pores had a water saturation ratio (S_W) of 0.25, whereas in the "aged" soil the most significant increase occurred for completely saturated soil pores. These differences were attributed to differing micellar formation behaviours in freshly spiked versus AFFF impacted "aged" soils. A linear correlation was also observed between the ratio of total reduced S and sulfonate fractions against the degree of saturation of "aged" soil. Furthermore, in spiked soil it was observed that a lower percentage NAPL content at S_W = 0.25 led to higher partitioning of sulfonic acid-based PFAS.