Repeated application of aqueous film-forming foams (AFFF) in designated firefighting training areas has caused severe groundwater contamination by per- and polyfluoroalkyl substances (PFASs). Many research efforts are currently engaged for the effective removal of these chemicals from environmental waters. In this study, we demonstrate that modified clay produced by intercalating quaternary ammonium cations in the exchangeable interlayer sites of smectite clay can effectively remove PFAS pollutants in real groundwater via strong adsorption. The performance of the modified clay (with removal efficiencies 95~99%) is superior to those of granular activated carbon or hard-wood biochar and comparable to an ion exchange resin. Removal efficiency is not impacted by potential organic co-contaminants (e.g., diesel, BTEX, TCE, and 1,4 dioxane) or water chemistry (Ca²⁺ and Na⁺) at environmentally relevant concentrations. Furthermore, piecewise isotherms are identified to represent the uptake of PFASs by the modified clay. Based on molecular dynamics simulations, the anionic PFASs first occupy the highly polarized bare interlayer edge sites leading to a linear isotherm and then the interlayer surface sites resulting in a Langmuir isotherm. The ionic interactions between the cationic intercalant (N⁺) and the terminal oxygen atoms of carboxylate or sulfonate groups of PFASs play a dominant role in adsorption, and the lateral interaction in particular fluorophilic attraction among PFASs accelerate the adsorption. The strength of these interactions is quantified using Density Functional Theory calculations. Simulation results match reasonably well with the experimentally determined basal spacing and Fourier transform infrared spectroscopy of the modified clay loaded with PFASs. Overall, the combined experimental and molecular simulation studies elucidate the adsorption mechanism of PFASs on the modified clay and provide critical information to guide the use of modified clays for PFAS water treatment in the field.

在指定的消防训练区反复使用水性成膜泡沫（AFFF）已导致全氟烷基物质和多氟烷基物质（PFAS）严重污染​​地下水。当前正在进行许多研究工作以从环境水中有效去除这些化学物质。在这项研究中，我们证明了通过在蒙脱石粘土的可交换夹层位置插入季铵阳离子而产生的改性粘土可以通过强吸附有效去除实际地下水中的PFAS污染物。改性粘土的性能（去除率在95〜99％之间）优于颗粒状活性炭或硬木生物炭，可与离子交换树脂媲美。去除效率不受潜在的有机污染物（例如，柴油，BTEX，TCE和1）的影响²⁺和Na ⁺）在与环境相关的浓度下。此外，确定了分段等温线以代表改性粘土对PFAS的吸收。基于分子动力学模拟，阴离子PFAS首先占据高度极化的层间裸露边缘位点，从而导致线性等温线，然后层间表面位点产生Langmuir等温线。阳离子嵌入剂之间的离子相互作用（N ⁺），PFAS的羧酸根或磺酸根基团的末端氧原子在吸附中起主要作用，并且PFAS之间的侧向相互作用（尤其是亲氟性吸引）加速了吸附。这些相互作用的强度使用密度泛函理论计算来量化。模拟结果与实验确定的基面间距和PFAS改性粘土的傅里叶变换红外光谱法吻合得很好。总的来说，结合实验和分子模拟研究阐明了PFAS在改性粘土上的吸附机理，并为指导改性粘土在现场PFAS水处理中的应用提供了关键信息。