Retention and transport of PFAS precursor 6:2 FTS in heterogeneous variably saturated porous media

Many widely detected perfluoroalkyl contaminants are transformation products of perfluoroalkyl acids (PFAA) precursors, whose transport behavior has received minimal attention. This study provides the first experimental investigation of spatially resolved retention of 6:2 fluorotelomer sulfonate (6:2 FTS) in a variably saturated porous medium, influenced by adsorption at the air-water interface (AWI) and porous-medium heterogeneities. An instrumented column apparatus was constructed, equipped with 5 ports to collect in-situ porewater samples and 5 accompanying sensors to measure water content. This approach allowed quantification of in-situ retardation factors (R) at multiple depths across a wide range of water saturations (S_w, 0.2-1.0). The R values remained consistently low (1.5) under saturated conditions, whereas they were significantly greater under unsaturated conditions (e.g., R = 19.7 at S_w = 0.21). The enhanced retention was due to adsorption at the AWI, which is the dominant retention mechanism for this system. The spatial variation in S_w led to a sharp decrease in R with depth, decreasing from ∼20-2, demonstrating spatially heterogeneous retention even over small distances, a phenomenon that has not been reported before. Non-ideal transport (early breakthrough and tailing) was observed at low S_w in contrast to ideal transport at higher S_w, which was attributed to rate-limited air-water interfacial adsorption (AWIA) due to constrained access to AWI under low saturations. This interpretation was supported by the results of a Damkohler Number analysis.