Proton-Driven Mechanistic Shift between One- and Two-Electron Transfer Pathways in Contaminant Oxidation by Ferrate(VI)

Role of protons in governing ferrate reactivity has been conventionally attributed to kinetic effects. This study reveals that the protonation of Fe(VI) not only accelerates the reaction rate but also shifts its electron-transfer pathway in the oxidation of phenolic and aniline contaminants. Using acetaminophen as a model contaminant, we observed that decreasing pH from 9.0 to 6.0 reduced the dimer yield from 36% to 9%. By establishing correlations among indicators of the Fe(VI) reduction pathway, contaminant oxidation pathway, and Fe(VI) speciation, we demonstrated a proton-driven mechanistic shift from one-electron hydrogen-atom transfer (HAT) to two-electron oxygen-atom transfer (OAT). The protonation redistributes spin density across the Fe and O atoms, enhancing the electrophilicity of the terminal oxo ligand to facilitate the OAT pathway of HFeO4-, whereas FeO42- preferentially favors the HAT pathway due to a lower energy barrier. This mechanistic divergence dictates the subsequent evolution of reactive iron species, causing the dependence of the apparent rate constant on the reactant molar ratio to switch between positive and negative values. Preliminary evaluations indicate that OAT-derived products may carry a higher risk of acute toxicity than HAT-derived ones. These findings elucidate the proton-driven mechanistic shift of Fe(VI), offering critical strategies for optimizing ferrate-based water treatments.