Effects of molecular-level component variation of fulvic acid on photodegradation of Microcystin-LR under solar irradiation

Abstract

Microcystin-LR (MC-LR), a monocyclic heptapeptide, is the most commonly occurring cyanotoxin in the eutrophication waters during algal blooms. The dissolved organic matter sensitized photodegradation of MC-LR under solar irradiation plays a key role in MC-LR detoxification. In this study, fulvic acid (FA) was separated into five molecular weight (MW) ranges to evaluate the effect of FA size and its molecular-level compositions on MC-LR degradation under simulated solar irradiation. The rate constants of MC-LR photodegradation decreased as FA MW increased. This was mainly because the generated amounts of the reactive oxygen species (ROS) and the triplet state FA (3FA*) from photolysis of FA decreased with increasing MW of FA. The lower yield of ROS and 3FA* with higher MW FA was attributed to the lower proportion of lignin-like moieties in higher MW FA, which led to smaller quantities of hydroxyl moieties, carbonyl moieties and lower electronic cloud intensity. Moreover, the increase in FA MW and the decrease of ROS steady-state concentration changed the MC-LR degradation pathway from double bond addition to peptide or carbon bond break. All MC-LR degradation products had lower toxicity than MC-LR, where low MW FA has better performance on MC-LR detoxification than high MW one under solar irradiation. The findings of this study provide new insights into the effects of molecular-level component variation of FAs on the photodegradation of MC-LR under solar irradiation.