Oxidation of Microcystins by Permanganate: pH and Temperature-Dependent Kinetics, Effect of DOM Characteristics, and Oxidation Mechanism Revisited

Oxidative degradation of six representative microcystins (MCs) (MC-RR, -LR, -YR, -LF, -LW, and -LA) by potassium permanganate (KMn0(4); Mn(VIl)) was investigated, focusing on the temperature- and pH-dependent reaction kinetics, the effect of dissolved organic matter (DOM), and the oxidation mechanisms. Second-order rate constants for the reactions of the six MCs with Mn(VII) (kappa(Mn(VII),MC)) were determined to be 160.4-520.1 M-1 s(-1) (MC-RR > -LR approximate to -YR > -LF approximate to -LW > -LA) at pH 7.2 and 21 degrees C. The kappa(Mn(VII),MC) values exhibited activation energies ranging from 15.1 to 22.4 kj mol(-1). With increasing pH from 2 to 11, the kappa(Mn(VII),MC) values decreased until pH 5, and plateaued over the pH range of 5-11, except for that of MC-YR (which increased at pH > 8). Species-specific second-order rate constants were calculated using predicted p(K)a values of MCs. The oxidation of MCs in natural waters was accurately predicted by the kinetic model using kappa(Mn(VII)),MC and Mn(VIl) exposure (integral[Mn(VII)]dt) values. Among different characteristics of DOM in natural waters, UV254, SUVA(254,) and the abundance of humic-like substances characterized by fluorescence spectroscopy exhibited good correlation with integral[Mn(VIl)]dt. A thorough product study of MC-LR oxidation by Mn(VII) was performed using liquid chromatography-mass spectrometry.