The aim of this study was to assess the soil–water partitioning behavior of a wider range of per- and polyfluoroalkyl substances (PFASs) onto soils covering diverse soil properties. The PFASs studied include perfluoroalkyl carboxylates (PFCAs), perfluoroalkane sulfonates (PFSAs), fluorotelomer sulfonates (FTSs), nonionic perfluoroalkane sulfonamides (FASAs), cyclic PFAS (PFEtCHxS), per- and polyfluoroalkyl ether acids (GenX, ADONA, 9Cl-PF3ONS), and three aqueous film-forming foam (AFFF)-related zwitterionic PFASs (AmPr-FHxSA, TAmPr-FHxSA, 6:2 FTSA-PrB). Soil–water partitioning coefficients (log K d values) of the PFASs ranged from less than zero to approximately three, were chain-length-dependent, and were significantly linearly related to molecular weight (MW) for PFASs with MW > 350 g/mol (R 2 = 0.94, p < 0.0001). Across all soils, the K d values of all short-chain PFASs (≤5 −CF2– moieties) were similar and varied less (<0.5 log units) compared to long-chain PFASs (>0.5 to 1.5  log units) and zwitterions AmPr- and TAmPr-FHxSA (∼1.5 to 2 log units). Multiple soil properties described sorption of PFASs better than any single property. The effects of soil properties on sorption were different for anionic, nonionic, and zwitterionic PFASs. Solution pH could change both PFAS speciation and soil chemistry affecting surface complexation and electrostatic processes. The K d values of all PFASs increased when solution pH decreased from approximately eight to three. Short-chain PFASs were less sensitive to solution pH than long-chain PFASs. The results indicate the complex interactions of PFASs with soil surfaces and the need to consider both PFAS type and soil properties to describe mobility in the environment.

Soil contaminated with aqueous film-forming foams (AFFFs) containing per- and polyfluoroalkyl substances (PFASs) at firefighting training sites has become a major concern worldwide. To date, most studies have focused on assessing soil–water partitioning behavior of PFASs and the key factors that can affect their sorption, whereas PFASs leaching from contaminated soils have not yet been widely investigated. This study evaluated the leaching and desorption of a wide range of PFASs from twelve contaminated soils using the Australian Standard Leaching Procedure (ASLP), the U.S. EPA Multiple Extraction Procedure (MEP), and Leaching Environmental Assessment Framework (LEAF). All three leaching tests provided a similar assessment of PFAS leaching behavior. Leaching of PFASs from soils was related to C-chain lengths and their functional head groups. While short-chain (CF2 ≤ 6) PFASs were easily desorbed and leached, long-chain PFASs were more difficult to desorb. PFASs with a carboxylate head group were leached more readily and to a greater extent than those with a sulfonate or sulfonamide head group. Leaching of long-chain PFASs was pH-dependent where leaching increased at high pH, while leaching of short-chain PFASs was less sensitive to pH. Comparing different leaching tests showed that the results using the alkaline ASLP were similar to the cumulative MEP data and the former might be more practical for routine use than the MEP. No single soil property was adequately able to describe PFAS leaching from the soils. Overall, the PFAS chemical structure appeared to have a greater effect on PFAS leaching from soil than soil physicochemical properties.

Kinetics of pyrolysis of the pollutant perfluorooctanesulfonic acid (PFOS) in inert bath gases have been studied in two flow reactors constructed of α-alumina and of stainless steel (SS) at temperatures between 400 and 615 °C. Results from the SS reactor give support to previous and our own quantum chemical calculations based on smaller perfluorinated sulfonates, according to which initiation of decomposition of PFOS first takes place by elimination of HF to form an unstable α-sultone with a rate constant, k 1 . The sultone then rapidly liberates SO 2 and forms perfluorooctanoyl fluoride with a rate constant, k 2 with k 2 ≫ k 1 such that the overall rate constant k′ ≈ k 1 . Products observed from both reactors in the above temperature range comprised HF, SO 2 , and perfluorooctanoyl fluoride. The value of the rate constant for the formation of HF and SO 2 measured in the SS reactor was found to be k 1 = (1.3 ± 0.5) × 10 14 exp(−253 ± 5 kJ/mol/RT) s −1 .

This study investigated the mobilization of a wide range of per- and polyfluoroalkyl substances (PFASs) present in aqueous film-forming foams (AFFFs) in water-saturated soils through one-dimensional (1-D) column experiments with a view to assessing the feasibility of their remediation by soil desorption and washing. Results indicated that sorption/desorption of most of the shorter-carbon-chain PFASs (C ≤ 6) in soil reached greater than 99% rapidlyafter approximately two pore volumes (PVs) and were well predicted by an equilibrium transport model, indicating that they will be readily removed by soil washing technologies. In contrast, the equilibrium model failed to predict the mobilization of longer-chain PFASs (C ≥ 7), indicating the presence of nonequilibrium sorption/desorption (confirmed by a flow interruption experiment). The actual time taken to attain 99% sorption/desorption was up to 5 times longer than predicted by the equilibrium model (e.g., ∼62 PVs versus ∼12 PVs predicted for perfluorooctane sulfonate (PFOS) in loamy sand). The increasing contribution of hydrophobic interactions over the electrostatic interactions is suggested as the main driving factor of the nonequilibrium processes. The inverse linear relationship (R 2 = 0.6, p < 0.0001) between the nonequilibrium mass transfer rate coefficient and the Freundlich sorption coefficient could potentially be a useful means for preliminary evaluation of potential nonequilibrium sorption/desorption of PFASs in soils.

Pyrolysis of perfluorooctanesulfonic acid (PFOS) (200−450 PPMV) was studied in an α-alumina flow reactor under plug flow conditions at temperatures from 450 to 1000 °C and helium flow velocities of 100, 200, and 300 mL min −1 . Major products at the lowest temperatures were HF, SO 2 , and perfluorooctanyl fluoride (C 8 F 16 O). A new low-temperature product, C 2 F 4 , was detected in significant quantities from the decomposition of PFOS. The pyrolysis mechanism was studied by quantum chemical calculation at the b3lyp/GTLarge//b3lyp/6-31G(d,p) and G4MP2 levels of theory to develop a thermochemical analysis and preliminary chemical kinetic model for the decomposition. Alongside the previously postulated initiation of pyrolysis via an α-sultone intermediate to C 8 F 16 O, we have discovered a competitive direct fission route for PFOS into C 8 F 17 radicals and HOSO 2 . C 8 F 17 rapidly fissions CF 2 radicals, which are the source of the observed C 2 F 4 . At the lowest temperatures, an acceleration in the rates of production of C 2 F 4 , HF, and SO 2 was observed in the α-alumina reactor when compared with predictions of the kinetic model. An alumina nanocluster was subjected to quantum chemical analysis to show that PFOS can be both chemisorbed and physisorbed on an alumina surface, a process that might explain the experimental observation of an acceleration of decomposition in an α-alumina reactor at low temperatures.