Carbon-Based Microbial-Electro-Fenton Systems for Microplastic-Contaminated Wastewater: Mechanistic Basis, Photo-Bio Hybrid Strategies, and Research Priorities.

The widespread occurrence of microplastics (MPs) in aquatic environments has become a major environmental concern because conventional wastewater treatment plants primarily retain these particles in sewage sludge rather than achieving their degradation or mineralization. Microbial-electro-Fenton systems (MEFSs) have recently emerged as a promising bio-electro-chemical advanced oxidation technology that integrates microbial extracellular electron transfer (EET) with electro-Fenton (EF) chemistry for the in situ generation of hydrogen peroxide (H2O2) and hydroxyl radicals (•OH). This review critically evaluates recent advances in carbon-based electrode engineering, microbial electron transfer mechanisms, molecular regulation of the two-electron oxygen reduction reaction (2e- ORR), and photo-bio hybrid enhancement for the treatment of MP-contaminated wastewater. Emphasis is placed on the distinct roles of anodic biofilms and cathodic electrocatalysts, polymer-specific degradation pathways, and the successive stages of MP transformation, including surface oxidation, polymer-chain scission, fragmentation, and mineralization. The influence of reactor configuration, wastewater matrix, and carbon electrode properties on H2O2 electro-generation, reactive oxygen species production, and degradation efficiency is comprehensively discussed. In addition, current challenges associated with electrode fouling, long-term durability, real wastewater treatment, scale-up, and techno-economic and environmental sustainability are critically assessed. Rather than presenting MEFS as a mature treatment technology, this review identifies the key mechanistic and engineering challenges that must be addressed before practical implementation. Finally, future research priorities are proposed to facilitate the transition of MEFS from laboratory-scale investigations toward pilot-scale validation and sustainable wastewater treatment for MP remediation.