The Effect of Cations on Perfluorobutane Sulfonate-Montmorillonite Interaction

Abstract

Environmental partitioning of organic contaminants into air, water, and soil has been widely studied, due to the risks that these pollutants pose to human and ecosystem health. Organic contaminant partitioning into soil has been largely characterized by organic matter content; however, clay minerals may also play a significant role in this process. Because the EPA has recently designated certain perfluoroalkyl substances (PFASs) as contaminants of interest, researchers have begun to characterize adsorption of these pollutants onto clays. I examined perfluorobutane sulfonate (PFBS) adsorption onto montmorillonite clays rich in calcium (Ca2+), sodium (Na+), and potassium (K+) cations, comparing molecular dynamics (MD) simulations to batch adsorption and X-ray diffraction results (XRD). Simulations show that hydrophobic attraction strongly influences PFBS adsorption to homoionic clay minerals; furthermore, simulations and XRD results both suggest intercalation of PFBS. Simulation results and experimental results each yield partition coefficients (Kd) for the three homoionic clay minerals on the same order of magnitude, although the results from the two methods differ by an average factor of 10 (experimental log Kd values were between 2 and 2.5; simulated log Kd values were between 3 and 3.5). These results suggest that hydrophobic interaction, independent of cation influence, makes clay minerals effective adsorbents for PFBS.