Hexadecyltrimethylammonium-modified bentonite for enhanced adsorption of per- and polyfluoroalkyl substances: Experiment and thermodynamic mechanism

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental and human health concerns due to their extreme persistence and bioaccumulation potential, and widespread contamination. Conventional clay-based barriers, such as geosynthetic clay liners, exhibit limited PFAS containment capabilities owing to their inherently low adsorption capacity. To overcome this challenge, this paper synthesized hexadecyltrimethylammonium-modified bentonite (HDTMA-NaB) as a high-performance adsorbent for engineered containment applications. Comprehensive batch adsorption experiments demonstrated that HDTMA-NaB achieves substantially greater PFAS adsorption capacity compared to pristine bentonite, with adsorption kinetics following a pseudo-second-order model and isotherms were well-described by both Langmuir and Freundlich models. Molecular dynamics simulations unveiled a distinctive “head-attracted, tail-parallel” adsorption configuration, where PFAS molecules align with HDTMA surfactants within montmorillonite interlayers. Chain-length-dependent adsorption patterns were observed, with short-chain PFAS preferentially adsorbing at clay particle edges while long-chain PFAS penetrated deeper into interlayer spaces. Potential of mean force analysis quantitatively verified stronger adsorption affinity for long-chain PFAS, consistent with adsorption experiment observations, through deeper and more numerous energy minima. Thermodynamic analysis indicated that PFAS adsorption was driven by both enthalpic and entropic contributions, with entropy playing the dominant role. The entropic contribution primarily arose from hydrophobic interactions between PFAS fluorinated carbon chains and HDTMA alkyl chains, providing a mechanistic explanation for the observed chain-length-dependent adsorption behavior in adsorption experiments. Meanwhile, the enthalpic contribution arose from electrostatic attraction between PFAS anionic headgroups and HDTMA cationic trimethylammonium groups. These molecular-scale insights provide a fundamental basis for designing high-performance clay-based containment systems for PFAS mitigation.