Efficient sorption of perfluoroalkyl acids by ionic liquid-modified natural clay

Abstract

Contamination caused by per- and polyfluoroalkyl substances (PFAS), including perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), is a big concern worldwide due to their persistence in the environment. Removal of PFAS represents a special challenge for water/wastewater treatment because of their diverse structures and unique physicochemical properties such as high water solubility and high stability of the C-F bond. In this study, we primarily investigated the removal of PFOA and PFOS from water through sorption with ionic liquid (IL)-modified natural clay. A series of ILs with varied alkyl chain lengths were applied for clay modification, and the successful intercalation of ILs was verified with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), zeta potential measurement, and elemental analysis. The structure and properties of IL-modified clays were strongly affected by the IL chain lengths. Specifically, long-chain IL not only changed the surface potential of clay, but also expanded the interlayers of clay, both of which greatly facilitated PFOA and PFOS sorption. Clay modified with the long-chain IL exhibited both fast kinetics for PFOA and PFOS sorption and high PFOA and PFOS removal efficiency under various water chemistry conditions. Furthermore, long-chain IL-modified clay was very effective for the removal of a suite of perfluorinated carboxylic and sulfonic acids with various chain lengths of the perfluoroalkyl moiety. IL-modified natural clay may be potentially applied for PFAS pollution control.