Electrostatic and Chemical Interplay between Air-Water and Mineral-Water Interfaces during PFOS Transport in Unsaturated Porous Media

Perfluorooctanesulfonate (PFOS) migration from vadose zone sources to groundwater is determined by multiple interfacial retention processes and their dependency on hydrochemistry. This study investigates the impact of air-water and mineral-water interfacial retention on PFOS transport under different hydrochemical conditions to assess their adsorption magnitudes and feedback dynamics as a function of ionic strength. Flow-through experiments were conducted in unsaturated quartz and goethite-coated quartz sands equilibrated with different background electrolyte concentrations to distinguish between air-water and goethite-water interfacial adsorption contributions to PFOS retardation. Measurements of PFOS breakthrough curves at column outlets allowed tracking the differences in spatio-temporal evolution of the PFOS plumes between the two porous media. Process-based reactive transport simulations, incorporating a thermodynamic framework of mass-action reactions accounting for multiple interfacial retention processes, allowed the quantitative interpretation of physical and geochemical processes. Experimental and modeling results reveal that multiprocess retention causes nonideal PFOS transport, with plume retardation and spatio-temporal mass transfer between the different phases determined by the relative contribution of the individual retention processes and their electrostatic interplay driven by solution counterions. These findings illuminate the interplay between air-water and mineral-water sorption and emphasize the need for reactive transport simulators implementing interdependent interfacial retention processes, influenced by water chemistry conditions.