Sorption of Polyfluoroalkyl Surfactants on Surface Soils: Effect of Molecular Structures, Soil Properties, and Solution Chemistry

Abstract

Zwitterionic, cationic, and anionic per-and polyfluoroalkyl substances (PFASs) are identified in aqueous film-forming foam (AFFF) concentrates and AFFF-impacted sites. However, the mobility potential of zwitterionic and cationic PFASs is poorly understood, preventing reliable site assessment. The study aimed to elucidate the mobility behaviors of PFASs of various charge states in saturated soil−water systems and assess critical influencing factors. Five anionic, three zwitterionic, and one cationic PFASs were investigated in five soils through batch sorption experiments. Pairwise comparison revealed that the quaternary ammonium group imparted a strong affinity of cationic perfluorooctaneamide ammonium compound (PFOAAmS) for soils. The influence of the quaternary ammonium group is mitigated in polyfluoroalkyl betaines, yet perfluorooctane sulfonamidoalkyl betaine (PFOSB) showed strong sorption in selected soils. Two soil bulk properties showed some correlations with the soil−water distribution coefficient (K d ). A positive correlation with the fraction of soil organic carbon was found only for anionic PFASs, whereas cation exchange capacity had an approximate positive correlation with K d only for PFOAAmS. Water chemistry (Ca 2+ and pH) influences the sorption of nonanionic PFASs in very distinct fashions or even in opposite trends to what was known for anionic PFASs. Sorption was insensitive to pH changes except for PFOSB; PFOSB underwent profound sorption reduction because its speciation occurs around neutral pH, while the two other betaines and PFOAAmS have pK a values that are outside of the environmentally relevant range. The lack of correlations suggests that the transport potential of PFASs is probably highly site-specific. It remains a challenge in deciphering PFAS sorption mechanisms and predicting how AFFF plumes migrate.

Multidimensional simulation of PFAS transport and leaching in the vadose zone: impact of surfactant-induced flow and soil heterogeneities

“…We recognize that foc in the first meter below the land surface may be much greater than the foc employed in the present study. Additionally, more complex sorption processes such as electrostatic interactions may lead to enhanced solid-phase adsorption in particular for cationic and zwitterionic PFAS (Barzen-Hanson et al, 2017;Mejia-Avendaño et al, 2020;Nickerson et al, 2020;Xiao et al, 2019). Under these conditions, our simulations would overestimate PFAS leaching.…”

Limitations of Current Approaches for Predicting Groundwater Vulnerability from PFAS Contamination in the Vadose Zone

“…The degree to which soil was characterized differed widely between the studies and, while not all of the measured parameters are recorded here, effort was made to record those that were common to multiple studies or indicated by prior research to influence PFAS sorption. These properties included concentration of PFAS in solution, pH of aqueous and solid phases, soil composition (sand, silt, clay), percent organic carbon, soil concentrations of K, Ca, Mg, Na, Fe, and Al, cation exchange capacity (CEC), and anion exchange capacity (AEC) (Ahrens et al 2011;Anderson et al 2019;Campos Pereira et al 2018;Ferrey et al 2012;Higgins and Luthy 2006;Jeon et al 2011;Knight et al 2019;Li et al 2018;Milinovic et al 2015;Mejia-Avendaño et al 2020;Oliver et al 2020).…”

“…For example, the frequently encountered chlorinated solvents like tetrachloroethylene (PCE), trichloroethylene (TCE), and 1,1,1-trichloroethane (1,1,1-TCA) are neutral, hydrophobic organic compounds and sorption is expected to be a function of the OC content in sediments and soils, with only limited interaction with mineral surfaces (Allen-King et al 1997;Lu et al 2011). This approach was initially applied to PFAS chemicals as well; however, studies suggest that a wide range of sorption mechanisms are important in environmental systems (Johnson et al 2007;Ferrey et al 2012;Guelfo and Higgins 2013;Chen et al 2016;Barzen-Hanson et al 2017;Hale et al 2017;Dalahmeh et al 2018;Anderson et al 2019;Brusseau 2019;Mejia-Avendaño et al 2020;Oliver et al 2020). The characteristic C-F chain moiety of PFAS molecules results in strong hydrophobicity and expected interaction with the organic matter fraction of soils (Li et al 2018).…”

The role of soils in the disposition, sequestration and decontamination of environmental contaminants

“…The organic matter is typically negatively charged and enhances the release of anionic hydrophobic organic contaminants such as PFAS and polycyclic aromatic hydrocarbons (PAHs) owing to electrostatic repulsion. However, electrostatic attraction of PFAS occurs at low pH of soil medium with small organic carbon content [50]. Furthermore, the PFAS adsorption onto organic matter occurs due to hydrophobic interactions for larger retention of PFAS in soil [51].…”