Transformation of Methylparaben by aqueous permanganate in the presence of iodide: Kinetics, modeling, and formation of iodinated aromatic products.

This work investigated impacts of iodide (I-) on the transformation of the widely used phenolic preservative methylparaben (MeP) as well as 11 other phenolic compounds by potassium permanganate (KMnO4). It was found that KMnO4 showed a low reactivity towards MeP in the absence of I- with apparent second-order rate constants (kapp) ranging from 0.065 ± 0.0071 to 1.0 ± 0.1 M-1s-1 over the pH range of 5-9. The presence of I- remarkably enhanced the transformation rates of MeP by KMnO4 via the contribution of hypoiodous acid (HOI) in situ formed, which displayed several orders of magnitude higher reactivity towards MeP than KMnO4. This enhancing effect of I- was greatly influenced by solution conditions (e.g., I- or KMnO4 concentration or pH), which could be well simulated by a kinetic model involving competition reactions (i.e., KMnO4 with I-, KMnO4 with MeP, HOI with KMnO4, and HOI with MeP). Similar enhancing effect of I- on the transformation kinetics of 5 other selected phenols (i.e., p-hydroxybenzoic acid, phenol, and bromophenols) at pH 7 was also observed, but not in the cases of bisphenol A, triclosan, 4-n-nonylphenol, and cresols. This discrepancy could be well explained by the relative reactivity of KMnO4 towards phenols vs I-. Liquid chromatography-tandem mass spectrometry analysis showed that iodinated aromatic products and/or iodinated quinone-like product were generated in the cases where I- enhancing effect was observed. Evolution of iodinated aromatic products generated from MeP (10 μM) treated by KMnO4 (50-150 μM) in the presence of I- (5-15 μM) suggested that higher I- or moderate KMnO4 concentration or neutral pH promoted their formation. A similar enhancing effect of I- (1 μM) on the transformation of MeP (1 μM) by KMnO4 (12.6 μM) and formation of iodinated aromatic products were also observed in natural water. This work demonstrates an important role of I- in the transformation kinetics and product formation of phenolic compounds by KMnO4, which has great implications for future applications of KMnO4 in treatment of I--containing water.