Molecular Insights into Perfluoroalkyl Substance Adsorption onto Montmorillonite and Montmorillonite-Humic Acid Complexes

The mobility and fate of perfluoroalkyl substances (PFAS) in soil are governed mainly by interactions with soil minerals and natural organic matter, but there is still a knowledge gap about the molecular adsorption behavior of PFAS on mineral surfaces and about the mechanisms behind influences of organic matter on PFAS adsorption. In this study, adsorption experiments, density functional theory calculations, and molecular dynamics simulations were combined to elucidate the adsorption and diffusion behavior of PFAS in montmorillonite (MMT) systems with or without humic acid (HA). Our results reveal that the adsorption of PFAS mainly involves Ca²⁺ bridging, hydrophobic interactions, and outer-sphere complexation via water bridging. The higher adsorption percentage of perfluoro sulfonic acids than perfluoro carboxylic acids is probably because sulfonyl groups present stronger interaction energy or higher affinity to the MMT mineral surface via Ca²⁺ bridging. The increasing adsorption percentage with increasing C-F chain length could be attributed to stronger hydrophobic interaction, intermolecular aggregation, and water-bridging interactions with mineral surface sites for longer-chain PFAS. The presence of HA can weaken the adsorption of PFAS via multiple pathways, such as electrostatic repulsion, steric hindrance, interrupting PFAS aggregation, and competition for Ca²⁺-binding sites. These findings offer molecular-level insight into the different adsorption behavior of PFAS with different head groups or chain lengths, improving our understanding of PFAS transport and retention in complex soil systems.