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From radical/electron competition to interfacial shielding: How PFOA and its derivatives obstruct electrochemical degradation of microplastics
Summary
Microplastics and "forever chemicals" (PFAS) often pollute water together, and this study found that PFAS actually make it harder to clean up microplastics using an advanced water treatment method. The PFAS chemicals form a protective coating on the plastic particles' surface, shielding them from breakdown—and this problem persisted even at very low, real-world PFAS levels. This matters because it suggests current water treatment technologies may be less effective than expected when these two common pollutants show up together, meaning we may need better strategies to fully remove microplastics from our water supply.
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.