Predicting the long-term PFAS sorption performance in a colloidal activated carbon barrier using laboratory column experiments

At Örnsköldsvik aiport in northern Sweden historical firefighting training activities has resulted in a strongly contaminated hotspot with per- and polyfluoroalkyl substances (PFAS), with total concentrations exceeding 100000 ng/L in the groundwater. As part of a governmental assignment on PFAS mitigation, the Swedish Geotechnical Institute and Swedish Geological Survey have installed a large pilot-scale colloidal activated carbon (CAC) barrier intercepting the PFAS plume. Field observations show that the CAC barrier has effectively reduced PFAS mobility, with strongly reduced downstream concentrations and no notable breakthroughs detected within two years after barrier installation. To predict the long-term performance and carefully evaluate sorption processes within the barrier, controlled laboratory-scale column experiments were designed using soil from the barrier and the natural PFAS-contaminated groundwater. The spatial variation of injected carbon in the barrier has been characterized by soil coring. Soil from two representative locations with different CAC contents (0.037% and 0.103% by weight) as well as natural soil from before CAC injection were examined in flow-through column experiments monitoring the breakthrough of different PFAS. The groundwater taken from upstream the barrier contained several perfluoroalkyl carboxylic acid (PFCA), perfluoroalkyl sulfonic acids (PFSA), fluorotelomer sulfonates as well as unknown precursors indicated by total oxidizable precursor (TOP) analyses. The results provide quantitative estimates of PFAS adsorption capacity, retardation factors, competition effects and breakthrough characteristics in relation to injected CAC content. The findings can be used to estimate the breakthrough of PFAS at the field site and predict the longevity and sorption capacity of the CAC barrier. The data can further be used to refine, develop and calibrate PFAS transport models and serve as a basis to optimise the long-term effectiveness of in-situ sorbent-based remediation.

Keywords: PFAS, groundwater, contamination, remediation, activated carbon