PFAS Degradation by UV/Sulfite: Mechanisms, Pathways, and Products after the Initial Reductive Defluorination

The global pollution by per- and polyfluoroalkyl substances (PFAS) requires solid advances in technology development and mechanistic understanding for PFAS degradation. The UV/sulfite method that produces hydrated electron (eaq−) has demonstrated excellent performance, but the mechanisms remained largely elusive. Although an eaq− is required to initiate the degradation, the reductive hydrodefluorination (C–F→C–H) is merely a minor pathway. Some extensively observed phenomena, such as chain-shortening of RF–CF2–COO– into RF–COO– (RF=CnF2n+1), “incidental” detections of very-short-chain RF–COO– products, and the substantially enhanced performance at high pH, had not received convincing explanations. This study provides a comprehensive understanding via systematic analyses of transformation products (TPs) across three legacy PFAS families: RF–CH2CH2–COO– (FTCA), RF–SO3– (PFSA), and RF–COO– (PFCA) of various RF chain lengths. TPs from FTCAs evidence hydroxyl radical (HO•) from water photolysis. After the first C–F bond cleavage by eaq−, the fluorocarbon undergoes hydrogenation, sulfonation, hydroxylation (followed by C–C bond cleavage), or unsaturated bond formation by reacting with H+ (and another eaq−), SO3−•, HO•, or HO–, respectively. The TPs provide strong experimental evidence for the electron-transfer-limiting theory. More importantly, results demonstrate the critical role of C–C bond cleavage to generate new –COO– groups in TPs and elucidate the preference for alkaline pH toward deep defluorination. The “decarboxylation” for PFCA chain-shortening produces HCOO– rather than CO2 or CO postulated in recent literature. This work significantly updates fundamental chemical insights for (i) tackling PFAS pollution and (ii) designing readily degradable fluorochemicals.