Mechanism and Reaction Pathways for Microcystin-LR Degradation through UV/H2O2 Treatment

Microcystin-LR (MCLR) is the most common cyanotoxin in contaminated aquatic systems. MCLR inhibits protein phosphatases 1 and 2A, leading to liver damage and tumor formation. MCLR is relatively stable owing to its cyclic structures. The combined UV/H2O2 technology can degrade MCLR efficiently. The second-order rate constant of the reaction between MCLR and hydroxyl radical (center dot OH) is 2.79(+/- 0.23)x10(10) M-1 s(-1) based on the competition kinetics model using nitrobenzene as reference compound. The probable degradation pathway was analyzed through liquid chromatography mass spectrometry. Results suggested that the major destruction pathways of MCLR were initiated by center dot OH attack on the benzene ring and diene of the Adda side chain. The corresponding aldehyde or ketone peptide residues were formed through further oxidation. Another minor destruction pathway involved center dot OH attack on the methoxy group of the Adda side chain, followed by complete removal of the methoxy group. The combined UV/H2O2 system is a promising technology for MCLR removal in contaminated aquatic systems.