Influence of grain size, organic carbon and organic matter residue content on the sorption of per- and polyfluoroalkyl substances in aqueous film forming foam contaminated soils - Implications for remediation using soil washing.

A soil that was historically contaminated with Aqueous Film Forming Foam (AFFF) was dry sieved into size fractions representative of those produced during soil washing. Batch sorption tests were then conducted to investigate the effect of soil parameters on in situ per- and polyfluoroalkyl substances (PFAS) sorption of these different size fractions: < 0.063 mm, 0.063 to 0.5 mm, 0.5 to 2 mm, 2 to 4 mm, 4 to 8 mm, and soil organic matter residues (SOMR). PFOS (513 ng/g), 6:2 FTS (132 ng/g) and PFHxS (58 ng/g) were the most dominant PFAS in the AFFF contaminated soil. Non-spiked, in situ K values for 19 PFAS ranged from 0.2 to 138 L/Kg (log K -0.8 to 2.14) for the bulk soil and were dependant on the head group and perfluorinated chain length (spanning C to C). The K values increased with decreasing grain size and increasing organic carbon content (OC), which were correlated to each other. For example, the PFOS K value for silt and clay (< 0.063 mm, 17.1 L/Kg, log K 1.23) were approximately 30 times higher compared to the gravel fraction (4 to 8 mm, 0.6 L/Kg, log K -0.25). The highest PFOS K value (116.6 L/Kg, log K 2.07) was found for the SOMR fraction, which had the highest OC content. K values for PFOS ranged from 6.9 L/Kg (log K 0.84) for the gravel fraction to 1906 L/Kg (log K 3.28) for the silt and clay, indicating that the mineral composition of the different size fractions also influenced sorption. The results here emphasize the need to separate coarse-grained fractions and fine-grained fractions, and in particular the SOMR, to optimize the soil washing process. Higher K values for the smaller size fractions indicate that coarser soils are better suited for soil washing.