Investigation of In Situ Strategy Based on Zn/Al-Layered Double Hydroxides for Enhanced PFOA Removal: Adsorption Mechanism and Fluoride Effect

Perfluorooctanoic acid (PFOA) contamination poses serious environmental risks due to its persistence and mobility. Conventional ex situ method using preformed layered double hydroxides (LDHs) shows limited performance, particularly under complex leachate conditions. This study developed an effective in situ Zn/Al LDH strategy for enhanced PFOA removal. Batch experiments, solid-phase characterization, and theoretical simulations were conducted to elucidate the adsorption mechanism and the effect of fluoride ion (F−). The results demonstrated that the in situ method exhibited superior performance in the presence of fluoride, achieving a PFOA adsorption density of up to 54.93 mmol/mol-Al, which is significantly higher than that of the ex situ method (26.76 mmol/mol-Al). Unlike the competitive adsorption observed in the ex situ method, the in situ process relies on synergistic mechanisms: F− participates in LDH formation as an interlayer anion and coordinates with Zn2+ and Al3+ to regulate LDH growth, thereby optimizing the surface chemical environment for PFOA capture. Molecular dynamics (MDs) and density functional theory (DFT) further showed that preferentially adsorbed F− affects hydrogen-bond networks and stabilizes PFOA through inner and outer sphere complexation. Overall, these findings clarify the fluoride-regulated adsorption mechanism and demonstrate the potential of in situ LDH coprecipitation for PFAS remediation in leachates.