Direct Electron Transfer via Organo-Fe(III)-Hydroperoxide Complexes Dominates Fenton-like Kinetics

Fenton oxidation is widely used to degrade refractory organic compounds such as phenols. However, there is a dispute whether the hydrogen peroxide-induced regeneration of Fe(II) is the rate-limiting step in this process. This study systematically investigates the structure–activity relationship between organic substances and iron species in a Fenton-like system, with a focus on the degradation mechanism of phenolic pollutants. By integration of electrochemical characterization and quantum chemical calculation, three kinetic degradation modes are proposed, which are closely related to the molecular redox properties and coordination capability. The three phenolic–Fe(III) interaction modes include: (i) strong reductive electron transfer (zero-order kinetics), (ii) strong coordination forming ligand-stabilized Fe(III) complexes (autocatalysis), and (iii) synergistic electron transfer-coordination (mixed kinetics). Density functional theory calculations demonstrate that the intramolecular electron transfer pathway within the organo–Fe(III)–hydroperoxide complex exhibits a significantly lower activation energy (0.77 eV less) than traditional radical-mediated pathways, rationalizing the dominance of direct electron transfer over hydroxyl radical generation. This finding provides a unified theoretical framework that resolves long-standing ambiguity in the Fenton-like mechanism and offers new substrate-specific wastewater treatment design guidance.