Adsorption of perfluorooctanoic acid and perfluorooctanesulfonic acid to iron oxide surfaces as studied by flow-through ATR-FTIR spectroscopy

The kinetics and mechanisms of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption to nanoparticulate hematite (α-Fe₂O₃) from aqueous solutions were examined using in situ, flow-through attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Results indicate that both PFOA and PFOS molecules are retained at the hydrophilic hematite surface and the adsorption shows strong pH dependence. However, ATR-FTIR data reveal that PFOA and PFOS are bound to the iron oxide by different mechanisms. Specifically, in addition to electrostatic interactions, PFOA forms inner-sphere Fecarboxylate complexes by ligand exchange, whereas the PFOS sulfonate group forms outer-sphere complexes and possibly hydrogen-bonds at the mineral surface. Both solution pH and surface loading affect adsorption kinetics. Faster adsorption was observed at low pH and high initial PFC concentrations. Sorption kinetics for both compounds can be described by a pseudo-second-order rate law at low pH (pH 3.0 and 4.5) and a pseudo-first-order rate law at high pH (pH 6.0). Sorption isotherm data for PFOA derived from spectroscopic results exhibit features characteristic of ionic surfactant adsorption to hydrophilic charged solid surfaces.