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 (KMnO 4; Mn(VII)) 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) ( k Mn(VII),MC ) were determined to be 160.4–520.1 M –1 s –1 (MC-RR > -LR ≈ -YR > -LF ≈ -LW > -LA) at pH 7.2 and 21 °C. The k 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 k 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 …