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The large environmental mobility and persistence of per- and polyfluoroalkyl substances (PFASs) along with confirmed or suspected toxicities make the substances one of the great challenges of our time in the fields of chemical management and environmental risk assessment. In order to risk assess PFAS-contaminated sites, improved quantitative and mechanistic understanding of the partitioning and retention in soil is crucial. The overall aim of this thesis was to improve the understanding of how PFASs are bound to soil and components therein, including soil organic matter (SOM) and iron (hydr)oxides. The focus was on investigating the role of soil/sorbent net charge and solution pH on binding, and the mechanisms that govern this partitioning behavior. The effect of soil/sorbent net charge, solution pH, and solid-phase properties on binding was investigated in batch experiments. The surface net charge of sorbents was quantified by geochemical modeling and ζ-potential measurements. Spectroscopic techniques (X-ray absorption, 13C nuclear magnetic resonance) were employed to increase mechanistic understanding. The driving force for the overall binding of a certain PFAS substance to mineral soils and organic soil horizons was identified as hydrophobic interaction, whereas electrostatic interaction was the main process responsible for the binding onto the positively charged iron (hydr)oxide ferrihydrite. The quality of SOM influenced the binding of PFASs to organic soil materials, in particular that of longer-chained PFASs. Binding of PFASs to soil and to isolated SOM and ferrihydrite was inversely related to solution pH and soil/sorbent net charge, in a manner that suggests that the electronegative fluorine atoms by charge interaction contribute to the binding´s overall pH-/charge-dependency. Extrapolation of organic carbon-normalized binding in organic soil materials to mineral soils underestimated the binding onto the latter, as did an equilibrium partitioning approach based on the surface net charge of SOM. The possible presence of black carbon or other high-affinity binding sites in the mineral soils could not be ruled out, why a component additivity approach (SOM, ferrihydrite) could not be tested properly for these materials. To conclude, this highlights the need for additional experimental binding data that allow the development of more accurate geochemical models with the ability to simulate and predict the binding and leaching of PFASs in the terrestrial environment. |
