Desorption of Poly- and Perfluoroalkyl Substances from Soil Historically Impacted with Aqueous Film-Forming Foam

Bench-scale experiments were performed to measure and evaluate the desorption kinetics of poly- and perfluoroalkyl substances (PFAS) from a vadose zone soil exposed decades ago to aqueous film-forming foams (AFFFs). Desorption kinetics in the shallow zone ($0.03–0.9\mathrm{m}$ below ground surface) that contained an elevated organic carbon (OC) content, and in an underlying deep zone ($0.9–2.4\mathrm{m}$ below ground surface) that contained a relatively low OC content, were evaluated for a wide range of anionic and zwitterionic compounds. Results showed that, for a given perfluorinated chain length, the head group impacted desorption. For the low-OC deep soil, desorption equilibrium generally occurred rapidly (within $48\mathrm{h}$), indicating that mass transfer limitations were minimal. However, for the high-OC shallow soil, less-hydrophobic and short-chained compounds (including $\mathrm{C}<8$ for the perfluorinated carboxylates, and $\mathrm{C}<7$ for the perfluorinated sulfonates) generally did not reach equilibrium within $400\mathrm{h}$, whereas longer-chained and more-hydrophobic PFAS appeared to reach equilibrium within $48\mathrm{h}$. Kinetic desorption modeling revealed that these observations likely were due to the depletion of shorter-chained PFAS in the rapid equilibrium sorption domain, coupled with their persistence in the kinetically controlled sorption domain. Kinetic modeling also showed that the rate of desorption was proportional to the PFAS aqueous diffusivity, confirming that diffusion limited the rate of release from the soils. Overall, the extent of desorption generally was substantially less than that predicted by published $K_{\mathrm{oc}}{f}_{\mathrm{oc}}$ relationships, suggesting that PFAS desorption from field-aged soils may have a less pronounced impact on underlying groundwater than anticipated, particularly for shorter-chained PFAS.