Pathways for the Mineralization of Perfluorooctanesulfonic Acid under a Smoldering Reductive Atmosphere

Smoldering has emerged as a promising thermal treatment for perfluorooctanesulfonate (PFOS)-contaminated soils owing to its self-sustaining nature and low energy demand. However, PFOS degradation pathways and underlying mechanisms under smoldering reductive conditions remain unclear. In this study, PFOS smoldering degradation pathways and relative bond cleavage mechanisms were deduced from lab-scale byproduct profiles and supported by reactive force field simulations and quantum-chemical calculations. Smoldering self-sustained at calorific value of 0.79 MJ/kg and air Darcy velocity of 1.5-2.5 cm/s. The removal rate of PFOS reached 98.1%, with only ∼50% energy consumption compared with that of conventional incineration under equivalent degradation conditions. Investigations revealed distinct degradation pathways in smoldering compared with traditional approaches: in a CO atmosphere, early C-C bond scission and CO-coupled sulfur transformation were predicted (potentially evolving toward COS). This pathway played a dominant role in PFOS degradation, providing more efficient sulfur-group transformation, less toxic byproducts, and better compatibility with oxygen-limited conditions. Pilot-scale experiments further validated the feasibility for field applications, achieving 98.6% PFOS removal. These findings provide mechanistic insights into improved smoldering-based PFASs remediation under reductive conditions and optimization of treatment strategies toward higher efficiency and environmental safety.