Occurrence and leaching of per- and polyfluoroalkyl substances in waste incineration residues: Insights into thermodynamic mechanisms and implications for environmental release.

Per- and polyfluoroalkyl substances (PFAS) have been detected in municipal solid waste incineration (MSWI) residues, but their precursor contributions, leaching mechanisms, and release potentials remain unclear. Here, 21 PFAS were analyzed in fly ash (FA) and bottom ash (BA) from five MSWI plants in China. Mean Σ21PFAS concentrations were 75.8 ng·g-1 in FA and 38.4 ng·g-1 in BA, exceeding previous reports. Short-chain and alternative PFAS predominated, with HFPO-DA (GenX) occurring at 2.5-29.1 ng·g-1 and accounting for 3.7%-44.4% of Σ21PFAS. The total oxidizable precursor (TOP) assay indicated limited precursor-derived PFAA formation, although hidden precursors or unidentified organofluorine cannot be excluded. Baseline PFAS leaching ranged from 1.1 to 9.9 ng·g-1 in FA and from 0.4 to 30.7 ng·g-1 in BA. Leaching was governed by desorption and mass transfer, was generally endothermic and thermodynamically unfavorable, and was enhanced by dissolved organic matter. For China in 2025, total ash-associated Σ21PFAS loads were estimated at 916.9, 2858.0, and 6283.4 kg·y-1 under conservative, likely, and extreme scenarios. FA-derived release could account for 1.4%, 13.3%, and 19.2% of the national Σ21PFAS load in landfill leachate, whereas BA showed potential releases of 19.1, 264.2, and 1690.5 kg·y-1. These findings identify MSWI residues as potential secondary PFAS sources requiring improved risk management.