Light-driven breakdown of microcystin-LR in water: A critical review

Abstract

• Light-driven oxidation technologies for removing microcystin-LR from water were reviewed. • Factors affecting the microcystin-LR degradation were probed. • Microcystin-LR was removed using photocatalysis/oxidation under real scenarios. • Conjugated dienes, double bonds, free carboxyl groups, and peptide ring were toxicity-induced structures. • Several new pathways of attacking peptide ring of microcystin-LR were introduced. Cyanobacterial algal blooms are ubiquitous around the globe and lead to severe cyanotoxin pollution, which poses a severe threat to the safety of drinking water. Microcystins (MCs), the monocyclic heptapeptides, are the most commonly occurring cyanotoxin, which consist of five D-amino acids and two variable L-amino acids. There exist over 100 MC variants, and microcystin-leucine arginine (MC-LR) is one of the most hazardous and prevalent variants. This review comprehensively discussed the status quo of investigations light-driven oxidation technology for degradation of MC-LR, including photolysis, heterogenous photocatalysis and homogeneous oxidation that are driven by UV, visible light and solar light (main information of homogeneous oxidation are shown in Supplementary materials). The effects of key parameters on these systems are critically clarified, including light wavelength, photocatalyst/oxidant concentration, initial MC-LR concentration, pH values, and coexisting organic or inorganic matter. An increasing number of studies are conducted on real water matrix at toxin concentrations and pH values that are similar to water contaminated by cyanobacterial algal blooms. However, very few studies have investigated the mineralization and detoxification of treated water, and little is known about the fragments of MC-LR that are responsible for its toxicity. Thereafter, reaction mechanism, ecotoxicological effects and operational costs of the MC-LR photodegradation process in all systems are also comprehensively elucidated. This paper will help to better understand the MC-LR decomposition process and provide improved perspectives for future research in this field.