Evaluation of distribution coefficients (KOC and Kd) for per- and polyfluoroalkyl substances

Sammanfattning: The dominating factors affecting sorption of per- and polyfluoroalkyl substances (PFAS) remain subject of research and debate. Traditionally, distribution coefficients (e.g., Kd and KOC) are used to calculate the fractionation of the contaminant between soil and water, to estimate leaching and subsequently the risks it imposes reaching water reservoirs. Research has aimed to establish the sorption mechanisms for PFAS but, due to the complexity of interactions between the substance specific physiochemical properties and geochemical sorbent characteristics, it has shown to be a complicated task. For PFOS, one of the most commonly encountered PFAS, the Swedish Geotechnical Institute (SGI) recommends using the 10th percentile of a small data set for the organic carbon-water distribution coefficient KOC (500 L/kg) and multiply this with the organic content of the in-situ soil to obtain the soil-water distribution coefficient (Kd). The result of this study shows that this method is insufficient to obtain a good approximation of the mobility of PFOS at a contaminated site. With a review of recent research on PFAS sorption and a case study performed at Stockholm Arlanda Airport, this study concludes that as of today, and due to PFAS potent mobility, well measured field coefficients for each soil type present in the soil profile and an elaborate geohydrological model is necessary to estimate PFAS environmental transport, fate and associated risks. It also concludes that parameters such as anionic exchange capacity and soil protein content may be highly relevant to estimate PFAS sorption.