Electron-transfer management at carbon–iron–oxidant interfaces: carbon architectures bridging radical/non-radical pathways for selective, self-sustained advanced oxidation

Abstract

Advanced oxidation processes (AOPs) mediated by iron are unified by a governing concept: electron-transfer management at carbon–iron–oxidant interfaces. This review explores how carbonaceous scaffolds—such as biochar, activated carbon, crystalline MOF-derived carbons, and single-atom catalysts—facilitate Fe³⁺/Fe²⁺ cycling to activate oxidants (e.g., H₂O₂, persulfate) and direct the selective activation of radical (˙OH, SO₄˙⁻, O₂˙⁻) and non-radical (¹O₂, direct electron transfer, Fe^IVO) pathways. Three levers emerge. (i) Redox-active carbons, enriched with quinone/phenolic motifs and persistent free radicals, act as electron shuttles to promote Fe³⁺ reduction and generate reactive oxygen species (ROSs), thereby enhancing pollutant degradation while minimizing metal leaching. (ii) Electronic structure engineering, achieved through heteroatom doping, defect incorporation, and the precise dispersion of Fe–N₄-like sites, optimizes adsorption geometries, enhances the energetics of Fe^IVO, facilitates efficient electron transfer, and improves catalytic reactivity and selectivity, thereby enhancing the overall catalytic performance and stability. (iii) Spatial and electrostatic field control expands the operating envelope: nanoconfinement enriches reactants and lowers transition-state barriers; contact-electro-catalysis harvests triboelectric fields to produce H₂O₂ and bias water oxidation reaction/oxygen reduction reaction without external power, enabling self-sustained Fenton chemistry. Integrating operando spectroscopy, electrokinetics, and density functional theory (DFT), we map the sequence of structure → states → interfacial charge flow → selectivity. This approach also includes abiotic–biotic bridges, where carbon mediates microbial direct interspecies electron transfer. Despite significant advancements, challenges remain in scaling these systems for practical applications, particularly concerning catalyst recyclability, metal leaching, and the optimization of radical versus non-radical pathways. The manuscript concludes with a perspective on future directions, including the potential for hybrid catalytic-biological systems and the importance of sustainable, scalable approaches in addressing global water contamination issues.