The photoactivity of complexation of DOM and Fe(Ⅱ)/Mn(Ⅱ) in aquatic system: Implication on the photodegradation of MCLR

Lake eutrophication leads to the excessive proliferation of cyanobacteria in aquatic environments, resulting in microcystin-LR (MCLR) concentrations exceeding regulatory limits. The associated environmental risks and health concerns attract widespread attention. As one of the most active chemical substances in aquatic environments, dissolved organic matter (DOM) plays a critical role in the phototransformation of MCLR. In environments where Fe(II) and Mn(II) coexist, the phototransformation process of MCLR may be influenced. Among fulvic acid (FA), humic acid (HA), cyanobacteria-derived DOM (CDOM), and reed leaf-derived DOM (RDOM), FA has the highest apparent reaction rate constant for MCLR at 0.03567   h -1 . Using isopropanol, furfuryl alcohol, and sorbic acid as quenchers, the contributions of different photochemically produced reactive intermediates(PPRIs) to MCLR photodegradation follow the order: 3 DOM⁎ > ·OH > 1 O 2 . As pH increases, changes in the conformation and charge distribution of DOM and MCLR suppress MCLR photodegradation. The addition of Fe(II) or Mn(II) enhances the photodegradation of MCLR by DOM. Electron paramagnetic resonance tests and radical quenching experiments indicate that the introduction of Fe(II)/Mn(II) increases ·OH production. The enhanced ·OH promotes MCLR photodegradation more effectively than the suppression caused by Fe(II)/Mn(II) quenching of ³DOM⁎. Product analysis reveals that demethylation, hydroxylation, decarbonylation, decarboxylation, denitration, and bond cleavage are the main pathways of MCLR photodegradation. These findings provide valuable insights into the migration and transformation of MCLR in complex environments, contributing to a deeper understanding of the fate of pollutants in natural water systems, particularly in the presence of metal ions.