Adsorption of Perfluorobutanesulfonic Acid (PFBS) onto Montmorillonite Clay Surfaces

Abstract

Organic contaminants include a broad range of compounds that are environmentally persistent, bioaccumulative, and/or toxic even at low concentrations. One group of emerging organic contaminants that has been of much interest recently is poly- and perfluoroalkyl substances, or PFAs. Formerly the main ingredient in 3M’s Scotchgard product, perfluorooctanesulfonic acid (PFOS), one of the most studied PFAs, has since been phased out of 3M products and been replaced by perfluorobutanesulfonic acid (PFBS). In order to better understand the fate and transport of PFBS, this thesis studies the adsorption of PFBS onto calcium-montmorillonite clay surfaces through molecular dynamics simulations and batch adsorption experiments. Molecular dynamics simulation of PFBS showed that PFBS quickly sorbs onto the basal surfaces of the montmorillonite, with the hydrophobic fluorine group adjacent to the surface while the negative sulfonate group points away from the clay. This result indicates that the hydrophobic attraction is sufficiently strong to overcome the electrostatic repulsion between the negatively charged solute and the clay surface. Furthermore, the simulations show PFBS entering the interlayer of the clay sheets. Batch adsorption experiments validated the sorption observed in the simulations, revealing that even at a very low solid-water ratio of 0.001 kg/L, approximately 15 to 25% of PFBS sorbs onto the Ca-montmorillonite surface. The linear adsorption coefficient obtained experimentally (Kd value of 183 L/kg) shows that in a typical soil with ~30% montmorillonite, on the order of 99.3% of PFBS may be adsorbed onto the surface.