Ultraviolet photosensitized transformation mechanism of microcystin-LR by natural organic matter in raw water

• Published in: Chemosphere (Oxford) Vol. 209; pp. 96 - 103
• Main Authors: Sun, Qiyuan, Zhang, Tianfang, Wang, Feifeng, Liu, Changqing, Wu, Chunshan, Xie, Rong-rong, Zheng, Yuyi
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.10.2018
• Subjects: Benzopyrans - pharmacology; Dissolved organic matter; Drinking water; Kinetics; MC-LR; Microcystins - radiation effects; Photodegradation; Photolysis; Photosensitizing Agents - pharmacology; Reactive oxygen species; Ultraviolet; Ultraviolet Rays; Water - chemistry; Water Pollutants, Chemical - chemistry; Water Pollutants, Chemical - radiation effects; Water Purification - methods; Photodegradation; MC-LR; Drinking water; Reactive oxygen species; Dissolved organic matter; Ultraviolet
• ISSN: 0045-6535; 1879-1298
• DOI: 10.1016/j.chemosphere.2018.06.091

Microcystins (MCs), produced by cyanobacterial blooms in eutrophic water, are common toxic metabolites and a potential threat to human health. However, the mechanism of MC photodegradation by photosensitizers in raw water remains unclear. In photodegradation and quenching experiments, this study investigates the photosensitized degradation of microcystin-LR (MC-LR) by fulvic acid (FA, a kind of dissolved organic matter with natural photosensitizing properties) under ultraviolet (UV) light irradiation. The photodegradation mechanisms of FA are also explored. The photodegradation process of MC-LR by FA was consistent with second-order reaction kinetics. The degradation rate of MC-LR in FA decreased from 80% to 55% as the pH increased from 3 to 9, because the binding ability of FA to MC-LR reduces as the pH increases. Given that FA can both inhibit and promote MC-LR degradation depending on its concentration, the optimum initial FA concentration for degrading MC-LR was determined as 9.86 mgC·L−1. The excited triplet state of FA (3FA∗) accounted for 50.12% of the MC-LR loss; the remaining loss (49.88%) was contributed by reactive oxygen species and direct photolysis. This implies that the main pathway of MC-LR degradation is reaction with 3FA∗. The MC-LR degradation rate is 36% higher under UV irradiation than that under simulated sunlight irradiation. •High pH decreases MC-LR degradation rate by weakening FA–MC binding ability.•Low (high) initial FA concentration promotes (inhibits) MC-LR degradation.•3FA∗ dominates the MC-LR photodegradation reaction under UV irradiation.•MC-LR degradation rate is 36% higher under UV than under simulated sunlight.