Molecular dynamics evidence for bidirectional feedback between PFAS and humic acid during composting: Interaction mechanisms, conformational reorganization, and diffusion regulation.

Per- and polyfluoroalkyl substances (PFAS) can accumulate in municipal sewage sludge, and their interactions with humified organic matter may influence their association state, mobility, and potential re-release during composting-based resource recovery. Humic acid (HA), a key humified product of sludge composting, provides reactive organic domains that may regulate PFAS behavior. Here, we investigated PFAS-HA interactions in a simplified HA-rich aqueous microenvironment under temperature conditions relevant to traditional thermophilic composting (TC, 333 K) and hyperthermophilic composting (HC, 363 K). Compared with TC, HC markedly enhanced PFAS localization near HA clusters and increased nearest-neighbor contacts, while dynamic partitioning between HA-associated and free PFAS persisted. Interaction energy decomposition showed that PFAS-HA association was mainly driven by van der Waals (vdW)/hydrophobic contributions, whereas direct electrostatic interactions were generally unfavorable. PFAS enrichment perturbed hydrogen-bonding networks within HA domains and induced Ca²⁺ redistribution around HA carboxyl sites, leading to temperature-dependent conformational and kinetic responses of HA. Under HC, HA became more compact and showed a pronounced decrease in diffusivity, with diffusion coefficients reduced by 45.26%-57.89%; under TC, interfacial complexation constraints dominated, resulting in a 24.44%-38.89% reduction. HA further exerted feedback effects on PFAS by generally increasing PFAS diffusion coefficients, especially under HC. These findings provide molecular-level evidence that composting-relevant temperature conditions regulate PFAS-HA association, HA microstructure, ion distribution, and PFAS molecular mobility, offering mechanistic support for understanding PFAS migration-related behavior and potential re-release during sludge composting.