Photochemistry of Lake Taihu dissolved organic matter: Reactive species generation and effects on microcystin photodegradation

Abstract

Dissolved organic matter (DOM) plays crucial roles in photochemical processes through light-induced transformations and reactive species generation. Four types – cyanobacteria extract from Lake Taihu (CDOM), reed leaf extract from Lake Taihu (RDOM), sodium humate (HA), and fulvic acid (FA) – were studied. The main component of all four DOM was humic acid-like, with fluorescence intensity reduced under light. Fourier-transform ion cyclotron resonance mass spectrometr (FT-ICR MS) revealed CDOM, RDOM, and FA contained abundant CHON and CHONS compounds, while HA was rich in CHO. Light exposure increased the number of molecular formulae in CDOM, RDOM, and FA. The unsaturation of CDOM, RDOM, and HA increased, while FA decreased. All DOM had high lignin abundance, with CDOM and RDOM also high protein and lipid. Lignin-like compounds decreased after 10 h light. DOM undergoes photochemical reactions under light to produce various reactive species, mainly excited triplet DOM (³DOM^⁎), hydroxyl radicals (·OH) and singlet oxygen (¹O₂), CDOM produces the most ∙OH and ³DOM^⁎, and HA produces the most ¹O₂. All four DOM types enhanced microcystin-LR (MCLR) photodegradation compared to no DOM, with FA showing the strongest promotion (29.2 % degradation). FA photolysis yielded higher ·OH and ¹O₂, it was speculated that the conversion of ³DOM^⁎ to ·OH and ¹O₂ may significantly impacts MCLR degradation. This study elucidates the differential effects of DOM from different sources on the photodegradation efficiency of MCLR due to their molecular composition and photochemical properties.