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 x 10(-2) s(-1)) compared with chlorination alone (2.6 x 10(-3) s(-1)) or solar irradiation alone (1.2 x 10(-4) s(-1)) with a free chlorine dose of 42 mu M (3.0 mg L-1 as Cl-2). 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 (center dot) and Cl-2(center dot-) reacting with MC-LR were determined to be (2.25 +/- 0.07) x 10(10) and (5.58 +/- 0.42) x 10(7) M(-1)s(-1), respectively. Cl (center dot) was the major RCS contributing to MC-LR elimination. The highest MC-LR degradation rate was observed at pH 8.0. Free chlorine, HO (center dot), Cl (center dot) and O-3 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. (C) 2019 Elsevier Ltd. All rights reserved.