Algal blooms negatively impact water quality by releasing algal organic matter (AOM), including extracellular organic matter (EOM) from metabolic secretions and intracellular organic matter (IOM) from cell lysis. These organic matters complicate water treatment processes and increase treatment costs. 

During chlorine-based disinfection, free available chlorine (FAC) reacts with AOM and alters its structural composition. The chlorination behavior and photochemical properties of AOM differ markedly from those of natural organic matter (NOM). However, how the chlorinated AOM influences the generation of reactive oxygen species (e.g., ^•OH) and facilitates the degradation of organic pollutants remains poorly understood. 

In a study published in Water Research, a research group led by Prof. BI Yonghong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences revealed that the chlorination of AOM significantly enhances its capacity to generate ^•OH upon irradiation, thereby accelerating the photodegradation of organic pollutants.

Researchers investigated the effects of chlorination on the photophysical and photochemical properties of AOM derived from Microcystis aeruginosa. Chlorinated AOM showed reduced aromaticity, lower molecular weight, and a pronounced blue shift in fluorescence spectra. 

Under 365 nm light-emitting diode irradiation, researchers found that the ^•OH quantum yield of chlorinated EOM showed an approximately 200-fold increase, and both EOM and IOM exhibited enhanced excited triplet-state activity and singlet oxygen (¹O₂) production, indicating that chlorination significantly enhances AOM's photochemical reactivity.

Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed that nitrogen-rich constituents within AOM served as major electron donors, reacting with FAC to primarily form nitrogen-containing chlorinated species. In contrast, chlorinated NOM generated predominantly CHO-class chlorinated phenolic compounds. Notably, chlorinated EOM exhibited a significant increase in highly oxygenated unsaturated compounds, which were identified as key sources of ^•OH.

Furthermore, researcher found that chlorinated EOM markedly accelerated the photodegradation of N,N-diethyl-meta-toluamide (DEET). Quenching experiments confirmed ^•OH as the primary reactive species responsible. Spectral and product analysis suggested that DEET degradation was mediated by ^•OH-induced hydrogen abstraction and hydroxylation, with deeper ring-opening and bond cleavage products observed in chlorinated systems.

This study demonstrates the intrinsic link between molecular evolution and photochemical activity in chlorinated AOM. It offers valuable insights into optimizing water treatment strategies in algal bloom-impacted environments, and provides a basis for predicting the environmental behavior of pollutants in chlorinated effluents or algicide-treated waters.