Ozonation of Microcystins: Kinetics and Toxicity Decrease

The ozonation of six microcystins (MCs) (MC-LR, MC-RR, MC-LA, MC-LF, MC-YR, and MC-LW) was investigated with a focus on the kinetics and decrease in toxicity. Second-order rate constants for the reactions of the six MCs with O 3 and • OH ( k O 3,MC and k • OH,MC ) ranged from 7.1 × 10 5 to 6.1 × 10 6 M –1 s –1 ( k O 3,MC ) and from 1.2 × 10 10 to 1.8 × 10 10 M –1 s –1 ( k • OH,MC ), at pH 7.2 and 20 °C. The activation energies were calculated to be 21.6–34.5 and 11.6–13.1 kJ mol –1 for k O 3,MC and k • OH,MC, respectively. The rate constants did not show an important pH dependency, except for k O 3,MC-YR, which increased at pH > 7. A kinetic model using the determined rate constants and the measured exposures of O 3 and • OH was able to precisely predict the removal of MCs in natural water. The hepatotoxicities of MCs were decreased by ozonation; the toxicities of the four MCs (MC-LR, MC…