Overlooked Roles of Oxygen in PFAS Degradation by UV/Sulfite: Mechanistic Insights and Engineering Implications

UV/sulfite treatment is a promising approach for PFAS destruction.The first step is C-F bond cleavage by a hydrated electron (eaq -).In contrast, oxygen (O2) was considered detrimental because it scavenges eaq -and consumes sulfite.However, this study found unique contributions by O2 in PFAS degradation.Although excess O2 in the photoreactor headspace could suppress the degradation of long-chain CnF2n+1-COO -and CnF2n+1-SO3 -, it enhanced both decay and defluorination of short-chain fluorotelomers (e.g., C4F9-CH2CH2-X) by generating hydroxyl radicals (HO • ) through sulfite-derived reactions.O2 also enhanced favorable oxidative pathways, such as the formation of CxF2x+1-COO -(x≤n) products from CnF2n+1-CH2CH2-X and the shortening of CnF2n+1-COO -into Cn-1F2n-1-COO -.When we added sulfite at intervals to compensate for the rapid depletion, O2 in the headspace substantially improved the destruction of short-chain CnF2n+1-CH2CH2-COO -(n=1-4) to >90% decay and ~90% defluorination, compared to <3% decay and <2% defluorination under strict O2-free conditions.The same strategy increased the defluorination of PFOA, PFOS, and 8:3 FTCA by another 10% from ~80% under strict O2free conditions.The overall defluorination of a PFAS mixture in synthetic brine also increased from 69% to 86%.Unlike the "common impression" that reductive and oxidative species would quench each other and thus block pollutant degradation, eaq -and O2-induced HO • worked synergistically, resulting in deeper PFAS defluorination than earlier results under O2-limited conditions.Therefore, controlled air exposure, rather than strict O2 removal, could serve as a simple and effective engineering strategy to enhance PFAS destruction by UV/sulfite.