PFOS fate in dynamic and multiphase subsurface environments: exploring electrostatic and chemical properties regulating PFOS accumulation at mineral-water interfaces

Per- and polyfluoroalkyl substances (PFAS) have emerged as a major concern in contaminant management and risk assessment due to their long-term and widespread occurrence in the environment, their resistance to degradation, and their tendency to bioaccumulate in human and animal tissues. With growing evidence linking PFAS to severe health effects, addressing this challenge has become increasingly urgent. To effectively assess the risks associated with PFAS contamination, it is essential to develop a comprehensive mechanistic understanding of the physical and chemical processes that control their transport and sorption behavior in subsurface porous media. The appropriate description of these phenomena ultimately requires process-based reactive transport models capable of incorporating PFAS surfactant-like interfacial mechanisms as a function of the dynamic evolution of the hydrochemical characteristics of multiphase subsurface environments.

The research performed in this PhD thesis addresses the fate and transport of perfluorooctanesulfonate (PFOS), one of the most frequently detected, toxic, and environmentally recalcitrant PFAS compound, within dynamic and multiphase subsurface porous media. The aim of the project is to develop a mechanistic understanding of the electrostatic and chemical properties regulating PFOS interfacial retention mechanisms. By integrating multiple lines of experimental evidence, this investigation linked molecular-level physicochemical processes with macroscopic adsorption phenomena to develop a mechanistic sorption prediction model capable of assessing PFOS accumulation at mineral-water interfaces under variable hydrochemical conditions. The innovative framework of adsorption reactions was further implemented for multicomponent reactive transport simulations of PFOS mobility supporting the model-based interpretation of flow-through laboratory experiments conducted in both saturated and unsaturated porous media.

In a first study, the adsorption behavior of PFOS on a pure crystalline mineral phase, goethite (α-FeOOH), was investigated across a wide range of pH and ionic strength conditions representative of variable groundwater chemistry. It was demonstrated that PFOS accumulation at the mineral-water interface can be accurately predicted under varying hydrochemistry by using a surface complexation model (SCM) built on molecular-level information derived from spectroscopy analyses. In particular, it was shown that PFOS binding to goethite occurs through a strong hydrogen-bonded complex and a weaker outer-sphere complex involving counterions adsorption. The pH and ionic strength of the solution were found as primary controllers of the speciation and relative abundance of these surface complexes. It was demonstrated that acidic pH conditions and low ionic strength promote hydrogen bonding complexation, whereas an increase in pH and ionic strength hinders the formation of strong hydrogen bonds upon the formation of a ternary outersphere complex.

In a second contribution, the applicability and robustness of the PFOS-goethite surface complexation model was investigated by incorporating the surface complexation reactions network into a reactive transport simulator for interpreting experimental datasets obtained from saturated flow-through experiments. The experimental and multicomponent reactive transport modeling outcomes proved that the coupled spatio-temporal evolution of pH and electrolyte fronts exerts a key control on PFOS mobility by determining its adsorption and speciation reactions on goethite surfaces. It was demonstrated that empirically derived solid-water distribution coefficients (Kd) fail to accurately simulate PFOS transport due to the inherent limitations of the Kd approach in accounting for the electrostatic effects arising from multicomponent transport and changes in charge on the mineral surface caused by variations in pore water chemistry.

In a third study, unsaturated flow-through experiments under well-controlled hydrogeochemical conditions were performed to characterize the relative contributions and interplay of multiple interfacial retention processes as a function of ionic strength. The reactive transport modeling framework was extended to incorporate air-water and mineral-water interfacial retention processes within a thermodynamic framework of mass-action reactions that accounted for the chemical and electrostatic effects induced by solution ionic charges. It was demonstrated that multiprocess retention leads to nonideal PFOS transport behavior with plume retardation and spatio-temporal change of mass distribution between the different phases determined by the relative contribution of the individual retention processes and by their electrostatic interplay driven by solution counterions.

In conclusion, the research performed in this PhD thesis has contributed to advance the knowledge on PFOS interfacial adsorption behavior towards a comprehensive understanding of the physical, chemical and electrostatic processes that ultimately regulate its migration rates in saturated and unsaturated flow-through systems. It has been shown that (i) variable solution chemistry exerts a key control on PFOS accumulation at mineral-water interfaces, (ii) surface complexation models that incorporate molecular-level descriptions of the electrical interfacial layer are essential for accurately predicting PFOS adsorption behavior, (iii) multicomponent transport processes, along with and surface charge effects, can considerably influence PFOS mobility, particularly in subsurface environments characterized by spatially and/or temporally dynamic hydrochemical conditions, and (iv) multiprocess retention causes nonideal PFOS transport behavior as determined by the relative contribution of the individual retention processes and by their nontrivial electrostatic and chemical interplay influenced by solution counterions. The results of this study are of particular importance for comprehensively understanding the fate and transport behavior of PFAS in subsurface environments, and to evaluate and improve the benefits and effectiveness of subsurface remediation actions as well as water quality management strategies.