Chelating Agent-Mediated Pathways for PFOA Degradation and Defluorination in Fe₃O₄‑Catalyzed H₂O₂ Reactions

Ascorbic acid (AA) and oxalic acid (OX) were the most effective, enhancing PFOA degradation to 81.2% and 77.2%, respectively, compared to citric acid (27.3%) > phosphonoacetic acid (23.4%) > <i>N</i>-methyliminodiacetic acid (12.1%). Mechanistic studies revealed that AA accelerates the initial reaction by reducing Fe³⁺ to Fe²⁺, increasing the initial-stage <i>k</i>_{obs,0–10min} from 0.0077 to 0.0304 min^–1.

In contrast, OX facilitates the rapid dissolution of Fe₃O₄, forming a complex with Fe²⁺ that improves H₂O₂ utilization and significantly accelerates the reaction at the initial stage. The observed <i>k</i>_obs,0–2min and <i>k</i>_{obs,2–10min} are 0.4392 and 0.0189 min^–1, respectively. After approximately 10–20 min, the oxidized product of AA, despite losing its reducing capacity, continued to dissolve Fe₃O₄.

This maintains higher reactivity in the Fe₃O₄–H₂O₂-AA reaction (<i>k</i>_{obs,10–180min} = 0.0050 min^–1). In contrast, the Fe₃O₄–H₂O₂–OX reaction almost stopped (<i>k</i>_{obs,10–180min} = 0.0009 min^–1) due to Fe³⁺–OX complex formation. Quenching experiments identified superoxide radicals as the key reactive species responsible for PFOA degradation.

Product analysis further revealed that AA facilitated partial mineralization, as evidenced by the generation of short-chain perfluorocarboxylic acids and measurable fluoride release (∼50% of total fluorine). In contrast, F^– was not detectable in the OX system, suggesting limited defluorination despite substantial PFOA degradation. Similar degradation behaviors were also observed for other PFAS, including perfluorooctanesulfonic acid (PFOS) and perfluorohexanesulfonic acid (PFHxS), indicating that the identified pathways may be applicable to a broader range of PFAS.

These findings highlight the critical role of Fe²⁺ availability in governing degradation pathways and provide insights for optimizing catalytic systems for PFAS remediation.