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

Abstract Perfluorooctanoic acid (PFOA), a widely used per- and polyfluoroalkyl substance, is persistent in the environment and linked to health concerns, leading to strict regulations and the need for remediation strategies. This study explores PFOA degradation using magnetite (Fe3O4)-catalyzed H2O2. To address the slow reaction rate of heterogeneous catalysts, five chelating agents (CAs) were tested as enhancements to investigate their influence on the Fe3O4–H2O2 process. Central composite design results showed that increasing H2O2 concentrations (0 to 2 M) improved PFOA degradation, but Fe3O4 needed to remain within 3–5.5 g/L for optimal performance. Even under the optimized conditions, only 15% of PFOA was degraded after 48 h. However, adding chelating agents significantly improved degradation rates. 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%) > N-methyliminodiacetic acid (12.1%). Mechanistic studies revealed that AA accelerates the initial reaction by reducing Fe3+ to Fe2+, increasing the initial-stage kobs,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 kobs,0–2min and kobs,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 (kobs,10–180min = 0.0050 min–1). In contrast, the Fe3O4–H2O2–OX reaction almost stopped (kobs,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.