Structural effect on the adsorption properties of PFAS by Ionic Liquid-Modified Clays

Adsorption is considered an economic and practical approach for the treatment of per- and polyfluoroalkyl substances (PFAS). Previously we have reported that natural clays amended with ionic liquids (ILs) were efficient for the sorption of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonates (PFOS). In this work, we have evaluated IL-modified clays prepared with different clay substrates for the sorption of eight selected PFAS to explore the roles of structure and composition of clay substrate in the adsorption behaviors of PFAS. Specifically, three natural clays with different structures and expansion properties were selected for IL modification, including calcium-rich montmorillonite (CaMT), sodium-rich montmorillonite (NaMT), and vermiculite (VT). Detailed adsorption behaviors of the eight PFAS were determined based on isotherm studies, and the results were fitted with the Langmuir-Freundlich isotherm (i.e., Sips isotherm) model. The adsorption capacity of the IL-modified clays followed the order that IL-VT > IL-CaMT > IL-NaMT, which was related to the cation exchange capacity and interlayer spacing of the clay substrates. Further, the adsorption affinity with PFAS was related to the surface charges of IL-modified clays and followed the order that IL-CaMT≈ IL-VT > IL-NaMT. Analysis of the isotherm results suggested that the adsorption capacity and affinity of PFAS with IL-modified clays may be dependent on the physicochemical properties of PFAS, particularly the octanol-water distribution coefficient (Dow).