Interfacial adsorption and dynamics of fluorotelomers with soil minerals – mechanistic insights

Abstract

Following the global regulation of legacy PFAS molecules, fluorotelomer molecules have been widely employed as replacements to PFOS in aqueous film-forming foam (AFFF) and PFOA in other products. Recent field studies indicate that fluorotelomer molecules are increasingly identified in environmental settings including groundwater, soil and sediments. Consequently, gaining a comprehensive understanding of the fate and transport of fluorotelomers in soils and sedimentary environments is vital. In this study, the behavior of two different fluorotelomers, 6 : 2 FTS and 6 : 2 FTC, in three common soil minerals (kaolinite, montmorillonite and illite) having quite different interfacial properties are reported using molecular dynamics simulations. The interfacial adsorption and dynamical characteristics of 6 : 2 FTS and 6 : 2 FTC vary substantially between the three minerals. Irrespective of the mineral composition, 6 : 2 FTS exhibits surface complexation while 6 : 2 FTC coordinates only with neutral and low charged clay minerals. In addition, the fundamental interactions that dictate the adsorption, interfacial structure of 6 : 2 FTS and 6 : 2 FTC are completely different for the three minerals. The large, aggregated clusters of 6 : 2 FTS at the surface experienced greater stability for longer periods of time and restricted mobility than 6 : 2 FTC for all three clay minerals. Importantly, the current study provides cluster size dependent diffusion behavior of surface adsorbed fluorotelomer molecules in each clay mineral. Such detailed mechanistic insights are necessary to understand the environmental footprint of fluorotelomers around contaminated sites.