STXM-XANES and computational investigations of adsorption of per- and polyfluoroalkyl substances on modified clay

Abstract

The study investigated molecular mechanisms of per- and polyfluoroalkyl substances (PFAS) on a surfactant modified clay (SMC), which rivals or exceeds the performance of granular activated carbon in removing long-chain PFAS from contaminated water. The adsorption isotherms of four anionic and one neutral PFAS from 1 × 10–9 to 1 × 10–5 mol/L were measured to be piecewise, accompanied by the expansion of the SMC interlayer. Based on the spatial distribution of perfluorocarboxylic acid (PFOA) on SMC at the submicron scale, achieved by synchrotron-based scanning transmission X-ray microscopy (STXM) coupled with X-ray absorption near-edge structure spectroscopy (XANES), PFOA molecules were confirmed to enter the interlayer space of SMC and could form charge-charge interaction with the intercalants (quaternary ammonia cations). Revealed by the all-atom molecular dynamics (MD) simulation and density functional theory-based quantum calculations, the interactions between the positively charged surfactant and representative PFAS include charge-charge, charge-dipole, and dipole-dipole interactions. The adsorption mechanisms vary from one PFAS structure to another in terms of adsorption energy and spatial arrangement. Those interactions overcome the interaction between PFAS and water, allowing PFAS to be retained in the SMC's interlayer space. The study provides fundamental insights into how PFAS interact with clay-based materials, supporting the use of the materials for PFAS treatment and remediation.