Removal characteristics of perfluorinated and polyfluoroalkyl substances in municipal sludge during thermal treatment

Effective management of perfluorinated and polyfluoroalkyl substances (PFASs) in municipal sludge is crucial for mitigating environmental pollution. In this study, an analytical method based on solid-phase extraction (SPE) coupled with ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was employed to analyze the 17 PFASs in solid matrices. The removal characteristics of the 17 PFASs in sludge under various thermal treatment conditions were quantitatively compared, and the effects of key factors including thermal treatment temperature, atmosphere, and oxygen concentration were systematically analyzed. The results found that thermal treatment at moderate temperatures effectively removed PFASs from sludge under both nitrogen and air atmospheres. At 500 ℃, nearly complete removal of 17 PFASs (>99%) was achieved within 30 min. Temperature is the primary controlling factor for the removal of PFASs in municipal sludge during thermal treatment. Within the 100–300 ℃ range, incineration exhibited superior removal performance compared to pyrolysis, whereas the difference in removal efficiency between incineration and pyrolysis narrowed when the temperature exceeded 400 ℃. Below 400 ℃, increasing the oxygen concentration can further improve the removal efficiency of total PFASs in sludge. At the same thermal treatment temperature, an air atmosphere can achieve near-optimal PFASs removal efficiency in sludge. The removal of PFASs from the solid phase of the sludge observed in this study is attributed to the combined effects of volatilization and thermal degradation. This study analyzes in depth the critical “temperature-atmosphere window” for PFASs removal, providing theoretical support for the effective removal of PFASs during the thermal treatment of municipal sludge.