Sorption and desorption of anionic, cationic and zwitterionic polyfluoroalkyl substances by soil organic matter and pyrogenic carbonaceous materials

The fraction of pyrogenic carbonaceous materials (PCMs) left in aqueous film-forming foams (AFFFs) source zones may strongly affect the persistence of perfluoroalkyl and polyfluoroalkyl substances (PFASs). To examine the hypothesis and gain an additional perspective on the potential contributions of different organic phases present in soil, we measured sorption and desorption of five AFFF relevant PFASs, one cationic (perfluoroctaneamido ammonium iodide, PFOAAmS), two anionic (perfluorooctane sulfonate, PFOS; perfluorooctanoic carboxylate, PFOA), and two zwitterionic (perfluorooctane amido betaine, PFOAB; 6:2 fluorotelomer sulfonamido betaine, 6:2 FTAB), by three types of PCMs (biochar, soot, and oil-free soot) and soil organic matter (SOM, presented by Pahokee peat) by single-solute batch sorption experiments. It was found that sorption to PCMs is substantially stronger and nonlinear than SOM, especially for the cationic PFOAAmS. Strong sorption to PCMs discovered in this study suggests that such phenomenon can lead to high retardation of PFASs in the AFFF source zone and decreased mobility of PFASs in groundwater, especially for the precursor compounds to the legacy perfluoroalkyl acids. Fouling of PCMs by unburnt oil would hinder the surface activity of PCMs and consequently lower the sorption of PFAS, thereby facilitating PFAS mobility. Moreover, sorption hysteresis was observed in all the systems with sorption of 6:2 FTAB by biochar being the most significant. This study for the first time provided the evidence that PCMs are potentially a significant sink of PFASs in AFFF-impacted sites.