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

Abstract

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 <stack>(K)</stack> 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.