Sorption of perfluoroalkyl substances (PFASs) to an organic soil horizon – Effect of cation composition and pH

Accurate prediction of the sorption of perfluoroalkyl substances (PFASs) in soils is essential for environmental risk assessment. We investigated the effect of solution pH and calculated soil organic matter (SOM) net charge on the sorption of 14 PFASs onto an organic soil as a function of pH and added concentrations of Al³⁺, Ca²⁺ and Na⁺. Often, the organic C-normalized partitioning coefficients (K_OC) showed a negative relationship to both pH (Δlog K_OC/ΔpH = -0.32 ± 0.11 log units) and the SOM bulk net negative charge (Δlog K_OC = -1.41 ± 0.40 per log unit mol_c g^-1). Moreover, perfluorosulfonic acids (PFSAs) sorbed more strongly than perfluorocarboxylic acids (PFCAs) and the PFAS sorption increased with increasing perfluorocarbon chain length with 0.60 and 0.83 log K_OC units per CF₂ moiety for C₃-C₁₀ PFCAs and C₄, C₆, and C₈ PFSAs, respectively. The effects of cation treatment and SOM bulk net charge were evident for many PFASs with low to moderate sorption (C₅-C₈ PFCAs and C₆ PFSA). However for the most strongly sorbing and most long-chained PFASs (C₉-C₁₁ and C₁₃ PFCAs, C₈ PFSA and perfluorooctane sulfonamide (FOSA)), smaller effects of cations were seen, and instead sorption was more strongly related to the pH value. This suggests that the most long-chained PFASs, similar to other hydrophobic organic compounds, are preferentially sorbed to the highly condensed domains of the humin fraction, while shorter-chained PFASs are bound to a larger extent to humic and fulvic acid, where cation effects are significant.