Bifunctional Organosilane-Grafted Kaolinite for Enhanced PFAS Adsorption via Synergistic Mechanisms

The widespread presence of per- and polyfluoroalkyl substances (PFAS) at industrial and military sites threatens subsurface environments and human health. Conventional barrier materials, such as those used in cutoff walls and compacted clay liners, retain PFAS poorly due to their low adsorption affinity. To address this, we engineered a robust functionalized clay (DMOAP-Kao) by covalently grafting dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium onto kaolinite. This grafting strategy ensures superior stability compared to modification by cation exchange. The resulting DMOAP-Kao exhibited a high PFAS adsorption capacity and sustained strong adsorption performance under various aqueous chemistry conditions. Permeation tests further confirmed that incorporating DMOAP-Kao as an amendment into soil-bentonite barrier materials enhances PFAS retardation performance. Molecular dynamics simulations identified three distinct PFAS adsorption morphologies, corresponding to their binding with the outer organosilane shell, the inner organosilane shell, and the kaolinite surface. Free energy calculations revealed a three-stage adsorption process synergistically governed by electrostatic attraction and hydrophobic interactions. The positively charged quaternary ammonium groups of the grafted DMOAP enable long-range electrostatic attraction, effectively "capturing" anionic PFAS species from water and drawing them toward the modified clay surface for subsequent binding. This study establishes a conceptual framework for designing high-affinity clay adsorbents by selecting functionalized organosilanes based on the specific physicochemical properties of target contaminants and grafting them via condensation with the hydroxyl groups on clay surfaces and edges.