Oxidation of triclosan by permanganate (Mn(VII)): Importance of ligands and in situ formed manganese oxides

Experiments were conducted to examine permanganate (Mn(VII); KMnO ₄) oxidation of the widely used biocide triclosan (one phenolic derivative) in aqueous solution at pH values of 5-9. Under slightly acidic conditions, the reactions displayed autocatalysis, suggesting the catalytic role of in situ formed MnO₂. This was further supported by the promoting effects of the addition of preformed MnO₂ colloids on Mn(VII) oxidations of triclosan and two other selected phenolics (i.e., phenol and 2,4-dichlorophenol), as well as p-nitrophenol which otherwise showed negligible reactivity toward Mn(VII) and MnO₂ colloids, respectively. Surprisingly, phosphate buffer significantly enhanced Mn(VII) oxidation of triclosan, as well as phenol and 2,4-dichlorophenol over a wide pH range. Further, several other selected ligands (i.e., pyrophosphate, EDTA, and humic acid) also exerted oxidation enhancement, supporting a scenario where highly active aqueous manganese intermediates (Mn(INT)_aq) formed in situ upon Mn(VII) reduction might be stabilized to a certain extent in the presence of ligands and subsequently involved in further oxidation of target phenolics, whereas without stabilizing agents Mn(INT)_aq autodecomposes or disproportionates spontaneously. The effectiveness of Mn(VII) for the oxidative removal of triclosan in natural water and wastewater was confirmed. Their background matrices were also found to accelerate Mn(VII) oxidation of phenolics.