Photolysis of microcystins by UV irradiation and the effects of different environmental factors on efficiency of UV degradation were studied. The results indicated that the rates of the photolytical degradation reactions of microcystin LR and RR follow pseudo first order kinetic process. The results also showed that the concentrations of two microcystin variants decreased significantly by UV-C irradiation; the wavelength and intensitiy of UV irradiation are two very important factors affecting the rate of degradation; temperature and pH value could also affect the half life of degradation rates. When irradiated by weaker UV light, isomerization could be detected in the course of photolytical degradation. The concentrations of two isomers transformed from microcystin LR reached its maximum at the third minute and decreased with the time afterwards. To simulate photolysis of microcystins in the field water body, microcystins with low concentration were used. It was found that UV-C illumination was capable of decomposing over 95% of microcystins within 40 min. In the presence of humic substances the photodecomposition slowed down to a certain extent. These results are valuable in using UV irradiation for elimination microcystins from raw water.

Key words: microcystin, photodegradation, humic acid, kinetic, isomerization, UV irradiation