PFOA-mineral surface complexation and multicomponent reactive transport under static and dynamic hydrochemistry

Perfluorooctanoic acid (PFOA) is a perfluorinated surfactant that occurs primarily in its anionic form across typical environmental conditions, making its adsorption behavior highly sensitive to surrounding hydrochemistry. In this work, we investigate PFOA adsorption behavior on goethite and goethite-coated sand through a combination of batch and column experiments. Adsorption isotherms and pH envelopes were performed across a wide range of pH values, ionic strength, and initial PFOA concentration. The influence of these parameters on PFOA-goethite interactions was then investigated under static and dynamic flow-through conditions where PFOA plumes were transported in saturated porous media in the presence of spatial and temporal hydrochemical gradients. Measurements of pH and PFOA breakthrough curves at the outlet allowed tracking the propagation of multicomponent reactive fronts. Based on batch observations, we developed a surface complexation model incorporating a monodentate inner-sphere complex between PFOA functional headgroup and goethite hydroxyl groups. When implemented for multicomponent reactive transport simulations, the model accurately captured the coupled displacement of PFOA, pH, and electrolyte fronts. The experimental and modeling outcomes revealed that retention is strongest under acidic, low ionic strength conditions, and that infiltrating salinity fronts with decreasing ionic strength trigger interdependent surface charge and multicomponent effects enhancing PFOA retardation and creating complex eluting fronts. These results demonstrate that mineral surface charge and multisolute electrostatic effects are key controls on PFOA mobility, highlighting the need for mechanistic thermodynamic models to predict PFAS transport behavior under variable hydrochemical conditions.