Coexisting Anilines Accelerate the Oxidation of Phenolics by Manganese Oxides

This work discovered that various anilines could accelerate the oxidation of phenolics by permanganate (Mn^VIIO₄⁻) or birnessite (δ-MnO₂) under ambient conditions. Taking phenol as a probe compound, the coexisting anilines increased the apparent kinetic constants of phenol oxidation by 1.73–18.14 times, that depends on the substituted groups on the anilines and pH conditions. Besides, taking 4-hydroxybenzoic acid (4-HBA) as a probe (a phenolic compound that inert toward permanganate), the coexisting anilines increased the apparent kinetic constants of 4-HBA oxidation by 1.46–210.82 times. Aromatic anilines could be oxidized by manganese oxides via electron-transfer process, leading to the formation of metastable intermediates known as aniline radicals, which could rapidly oxidize phenolics due to their high reactivities. Additionally, the oxidation products containing aromatic amino or imino groups, formed in the phenolics/anilines binary-contaminants system, can also induce the aforementioned acceleration process, such as benzidine and the cross-coupling products derived from phenolics and anilines. Therefore, even the anilines were consumed up, the accelerating oxidation of 4-HBA by permanganate could be sustained due to the in-situ formed oxidation products. Our study emphasized that the oxidized intermediates and products might influence the reaction pathways involving oxidants and contaminants among the environmental processes, especially in combined pollution system, thereby rendering the transformation pathways of contaminants significantly more intricate than initially anticipated.