Evaluating the saturated paste extraction method for determining PFAS solid–liquid partitioning coefficients (Kd) in low-organic-carbon sand aquifers

Per- and polyfluoroalkyl substances (PFAS) is a chemical family that contains many mobile compounds, including through groundwater transport. As much of the world’s water supply originates from groundwater, it is crucial to predict PFAS transport through these systems, governed by their solid-liquid partitioning coefficient ( K d ). In this study, K d of PFAS in low (<0.2%) organic carbon (OC) sandy aquifers from PFAS-contaminated sites were determined using i) batch leaching tests ii) field-derived K d values based on analyses of drill cores and groundwater, and iii) the saturated paste extraction method (SPEM). A strong correlation was observed between SPEM-derived K d values and PFAS perfluoroalkyl chain length, demonstrating that SPEM is sufficiently sensitive to resolve small differences in PFAS K d even at low absolute values. Extraction time of SPEM was critical, with K d values increasing during prolonged extraction, likely due to air entering the soil column, increasing the air-water interface area. A suitable extraction time was identified as 10-15 s, corresponding to the extraction of ∼40% of the total liquid volume. The mean K d values from the 10-15 s SPEM extraction ranged from 0.0034 (perfluoropentanoic acid (PFPeA)) to 1.4 (perfluorooctanoic sulfonic acid (PFOS)), with an increase in K d of ∼0.41 log units per perfluoroalkyl carbon. 6:2 fluorotelomer sulfonamido betaine (FTAB) exhibited a higher K d of 6.3, likely due to its bulky and zwitterionic head group. The SPEM-derived K d agreed with the field-derived values. This study highlights SPEM as a promising method for determining PFAS K d in low sorption environments and the high mobility of PFAS in low-OC sand aquifers.