Combining solar irradiation with chlorination enhances the photochemical decomposition of microcystin-LR

Microcystin-LR (MC-LR) generated by cyanobacteria is a potent toxin threatening human health. In this study the kinetics and mechanisms of MC-LR elimination from drinking water under solar irradiation with free chlorine (the solar/chlorine process) was evaluated. The rate of MC-LR degradation was dramatically enhanced in the solar/chlorine process (1.1 × 10^-2 s^-1) compared with chlorination alone (2.6 × 10^-3 s^-1) or solar irradiation alone (1.2 × 10^-4 s^-1) with a free chlorine dose of 42 μM (3.0 mg L^-1 as Cl₂). The enhancement was due to the presence of hydroxyl radicals, reactive chlorine species (RCS), and ozone during free chlorine photolysis. The second-order rate constants of Cl ^• and Cl₂^•- reacting with MC-LR were determined to be (2.25 ± 0.07) × 10¹⁰ and (5.58 ± 0.42) × 10⁷ M^-1s^-1, respectively. Cl ^• was the major RCS contributing to MC-LR elimination. The highest MC-LR degradation rate was observed at pH 8.0. Free chlorine, HO ^•, Cl ^• and O₃ together accounted for almost 95% of the MC-LR elimination. Hydroxyl- and chloro-MC-LR were generated in the process, followed by dechlorination, dehydration and cleavage of cyclic structures in MC-LR. Aldehyde- and ketone-MC-LR byproducts were also observed. The destruction of dienes led to a great reduction in MC-LR's toxicity. MC-LR removal in natural water samples under natural sunlight irradiation with free chlorine was demonstrated with limited formation of disinfection byproducts. The solar/chlorine process is an energy-efficient approach for MC-LR control, especially suitable for rural areas or where algal blooming threatens.