Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced Transformation of Triclosan by Mn(VII) in the Presence of Ligands?

In previous studies, we interestingly found that several ligands (e.g., pyrophosphate, nitrilotriacetate, and humic acid) could significantly accelerate the oxidation rates of triclosan (TCS; the most widely used antimicrobial) by aqueous permanganate (Mn(VII)) especially at acid pH, which was ascribed to the contribution of ligand-stabilized Mn(III) (defined Mn(III)L) formed in situ as a potent oxidant. In this work, it was found that the oxidation of TCS by Mn(III)L resulted in the formation of dimers, as well as hydroxylated and quinone-like products, where TCS phenoxy radical was likely involved. This transformation pathway distinctly differed from that involved in Mn(VII) oxidation of TCS, where 2,4-dichlorophenol (DCP) was the major product with a high yield of ∼80%. Surprisingly, we found that the presence of various complexing ligands including pyrophosphate, nitrilotriacetate, and humic acid, as well as bisulfite slightly affected the yields of DCP, although they greatly enhanced the oxidation kinetics of TCS by Mn(VII). This result could not be reasonably explained by taking the contribution of Mn(III)L into account. Comparatively, the degradation of TCS by manganese dioxide (MnO2) was also greatly enhanced in the presence of these ligands with negligible formation of DCP, which could be rationalized by the contribution of Mn(III)L. In addition, it was demonstrated that DCP could not be generated from Mn(VII) oxidation of unstable phenoxy radical intermediates and stable oxidation products formed from TCS by Mn(III)L. These findings indicate that manganese intermediates other than Mn(III) are likely involved in the Mn(VII)/TCS/ligand systems responsible for the high yields of DCP product.