New Insights into the Role of Humic Acid in Permanganate Oxidation of Diclofenac: A Novel Electron Transfer Mechanism

Humic acid (HA) ubiquitously existing in aquatic environments has been reported to significantly impact permanganate (KMnO₄) decontamination processes. However, the underlying mechanism of the KMnO₄/HA system remained elusive. In this study, an enhancing effect of HA on the KMnO₄ oxidation of diclofenac (DCF) was observed over a wide solution pH range of 5-9. Surprisingly, the mechanism of HA-induced enhancement varied with solution pH. Quenching and chemical probing experiments revealed that manganese intermediates (Mn(III)-HA and MnO₂) were responsible for the enhancement under acidic conditions but not under neutral and alkaline conditions. By combining KMnO₄ decomposition, galvanic oxidation process experiments, electrochemical tests, and FTIR and XPS analysis, it was interestingly found that HA could effectively mediate the electron transfer from DCF to KMnO₄ in neutral and alkaline solutions, which was reported for the first time. The formation of an organic-catalyst complex (i.e., HA-DCF) with lower reduction potential than the parent DCF was proposed to be responsible for the accelerated electron transfer from DCF to KMnO₄. This electron transfer likely occurred within the complex molecule formed through the interaction between HA-DCF and KMnO₄ (i.e., HA-DCF-KMnO₄). These results will help us gain a more comprehensive understanding of the role of HA in the KMnO₄ oxidation processes.