Mechanistic study of PFOS adsorption on kaolinite and montmorillonite

Abstract

Adsorption of perfluorooctane sulfonate (PFOS) on clay minerals is pivotal in the fate and transport of PFOS in the environment. Understanding the PFOS adsorption mechanisms, however, presents challenges, especially in the presence of natural organic substances. In this study, the adsorption mechanism of PFOS on kaolinite and montmorillonite, with and without humic acids (HA), were investigated using batch experiments, ATR-FTIR spectroscopy, zeta potential, NEXAFS, and XRD. The results showed that PFOS had a higher adsorption capacity on kaolinite than on montmorillonite, both with a rapid formation of an outer-sphere complex followed by a slow ligand exchange reaction with the elimination of adsorbed H2O molecules. In addition to nonspecific hydrophobic and electrostatic interactions, specific chemisorption also exists between sulfonate group of PFOS and hydroxyl group on surfaces of clay minerals. The HA coating on clay materials inhibited PFOS adsorption due to its occupation of adsorption sites and increased electrostatic repulsion. The insights gained from this study improve our understanding on the mechanism of multilateral forces on PFOS adsorption at the molecular level.