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)-O-center dot (k(O3,MC) and k(center dot OH,MC)) ranged from 7.1 x 10(5) to 6.1 x 10(6) M-1 s(-1) (k(O3,MC)) and from 1.2 x 10(10) to 1.8 x 10(10) M-1 s(-1) (k(center dot OH,MC)), at pH 7.2 and 20 degrees C. The activation energies were calculated to be 21.6-34.5 and 11.6-13.1 kJ mol(-1) for k(O3,MC) and k(center dot OH,MC), respectively. The rate constants did not show an important pH dependency, except for k(O3,MC-YR), which increased at pH > 7. A kinetic model using the determined rate constants and the measured exposures of O-3 and (OH)-O-center dot 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-RR, MC-LA, and MC-LF) decreased nearly concurrently with decreases in their concentrations. However, MC-YR and MC-LW showed a gap between the concentration and toxicity as a result of the incomplete destruction of the Adda moiety (a key amino acid expressing the hepatotoxicity of MCs). A product study using liquid chromatography-mass spectrometry identified a number of oxidation products with an intact Adda moiety produced by the ozonation of MC-YR and MC-LW.