Oxidation kinetics of anilines by aqueous permanganate and effects of manganese products: Comparison to phenols

Abstract

In this study, the potential applicability of potassium permanganate (Mn(VII)) for anilines elimination was systematically investigated firstly, with a focus on the effect of manganese intermediates on the kinetics of anilines versus phenols. It was found that Mn(VII) could fairly oxidize anilines, where the second-order rate constants (kMn(VII)) values for anilines always decreased as pH increased from 5 to 9. This interesting pH-dependency was successfully described by the kinetic models proposed in literature to account for the unusual pH-rate profiles for phenols, where the formation of intermediates between Mn(VII) and phenols or anilines was likely involved. The effect of manganese products such as MnO2 and Mn(III) on the oxidation of anilines by Mn(VII) was demonstrated. Under slightly acidic conditions, the reactions of Mn(VII) with anilines displayed autocatalysis, suggesting a similar catalytic role of MnO2 formed in situ as compared to phenols. Several ligands (e.g., pyrophosphate) inhibited the formation of MnO2 colloids and lowered the oxidation rates of anilines by Mn(VII) at acidic pH, while these ligands greatly accelerated the kinetics of phenols under similar conditions. The contrasting effects of ligands might be mainly attributed to the different reactivity of ligand-stabilized Mn(III) formed in situ toward anilines vs phenols. The complex effect of humic acid was highly dependent on solution pH, possible due to the dual role of humic acid that it could act as a reductant (competitively consuming Mn(VII) and phenoxy or aniline radical) as well as a ligand (stabilizing manganese intermediates such as Mn(III) species) to affect Mn(VII) reactions.