HDTMA-Modified Bentonite Vertical Barrier for Retaining Per- and Polyfluoroalkyl Substances

Perfluoroalkyl substances (PFAS), as a class of environmentally persistent emerging contaminants, are widely present in groundwater at military bases, landfills, and industrial sites, posing a serious threat to ecosystems and human health. Traditional soil-bentonite vertical barrier materials exhibit low adsorption capacity for PFAS, which limits their effectiveness in controlling PFAS-contaminated groundwater. To address this issue, this study modified bentonite using hexadecyltrimethylammonium bromide to produce HDTMA-modified bentonite (HDTMA-NaB). Material characterization confirmed the successful intercalation of HDTMA into the bentonite structure, resulting in a reversal of the zeta potential on the bentonite particle surfaces from negative to positive. Batch adsorption tests demonstrated a significant increase in the adsorption capacity of HDTMA-NaB for four typical PFAS compounds compared to unmodified bentonite, primarily attributed to the synergistic enhancement of hydrophobic interactions and electrostatic attraction. Subsequently, HDTMA-NaB was incorporated into a soil-bentonite mixture to fabricate a vertical barrier material. Flexible-wall permeameter tests revealed that the adsorption capacity of the barrier material for perfluorooctanesulfonic acid (PFOS) increased significantly with higher HDTMA-NaB content. When the HDTMA-NaB content was 2.5%, no PFOS was detected in the effluent even after permeation of up to 38 pore volumes (equivalent to 155 days), indicating excellent PFAS retardation performance of the barrier material. However, the hydration swelling capacity of HDTMA-NaB was notably lower than that of unmodified bentonite. Tests showed that incorporating HDTMA-NaB led to an increase in the hydraulic conductivity of the vertical barrier material. The HDTMA-modified bentonite vertical barrier material proposed in this study can provide technical support for the development of barrier materials aimed at controlling PFAS-contaminated groundwater.