From radical/electron competition to interfacial shielding: How PFOA and its derivatives obstruct electrochemical...

The pervasive co-occurrence of microplastics (MPs) and per- and polyfluoroalkyl substances (PFAS) poses a significant challenge for remediation technologies. Electrochemical advanced oxidation processes like electro-Fenton (EF) are promising for MPs degradation, yet their efficacy in complex, co-contaminant systems remains poorly understood. This work revealed a previously overlooked mechanism by which perfluorooctanoic acid (PFOA) and its derivatives severely inhibited the electrochemical degradation of polyethylene terephthalate (PET)-MPs in a pyrite-modified heterogeneous EF system. The inhibition evolved through two distinct phases: an initial phase (0-5 h) dominated by reactive oxygen species (ROS) and electron competition, followed by a subsequent phase (5-10 h) governed by interfacial shielding. This shielding arose from the formation of a persistent fluorine-rich layer on the MPs' surface, facilitated mainly by hydrogen bonding between the oxidized MPs' surface and PFOA-derived intermediates (i.e., short-chain fluorotelomer carboxylic acids), which blocked further ROS attack. Such inhibition led to a drastic reduction in the degradation efficiency of PET-MPs by > 50% at 50 mg/L PFOA and was still effective under more environmentally relevant conditions (1-10 μg/L PFOA). These findings underscore that in heterogeneous co-contaminant systems, interfacial interactions could induce a more profound and persistent inhibitory effect than homogeneous competition alone, providing critical insights for designing effective remediation strategies for complex environmental matrices.