Effects of Microplastics on Diclofenac Degradation in the Permanganate–Manganese Dioxide System

Microplastics (MPs) are increasingly recognized as pervasive co-contaminants in aquatic environments, yet their impacts on advanced oxidation processes remain poorly understood. Herein, we systematically investigate the role of representative MPs in diclofenac (DCF) degradation within a permanganate–manganese dioxide (PM-MnO2) catalytic system. Results show that PM alone exhibits limited reactivity toward DCF, while MnO2 significantly enhances DCF degradation. In the absence of MnO2, MPs increase PM consumption but do not influence DCF degradation, indicating that MPs primarily act as competing oxidant sinks. In contrast, under MnO2 catalytic conditions, the effect of MPs strongly depends on their interaction with MnO2. Pre-adhesion of MPs onto MnO2 suppresses DCF degradation by blocking active sites and inhibiting interfacial electron transfer. However, when MPs are introduced without pre-adhesion, no inhibition is observed; instead, a slight enhancement in DCF removal occurs. This promotion is attributed to in situ oxidation of MPs, which consumes PM and simultaneously generates secondary MnO2 colloids that provide additional reactive interfaces. Further analysis reveals that PM consumption is decoupled from DCF degradation due to multi-pathway oxidant partitioning, including DCF oxidation, MP oxidation, and Mn redox cycling. These findings demonstrate that MPs can act as both inhibitors and promoters depending on their interaction mode with catalysts, highlighting the importance of catalyst accessibility and reaction sequence. This study provides new insights into the complex roles of MPs in catalytic oxidation systems and offers guidance for applying PM-based technologies in realistic water matrices.