Algal blooms deteriorate 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, altering its structural composition. The chlorination behavior and subsequent photochemical properties of AOM differ markedly from those of natural organic matter. However, the specific mechanisms through which chlorinated AOM influences the generation of reactive oxygen species (e.g., ^•OH) and facilitates the degradation of organic pollutants remain insufficiently understood. 

Recently, 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 enhanced its capacity to generate ^•OH upon irradiation, thereby accelerating the photodegradation of organic pollutants. This study was published in Water Research.

In this study, the researchers extracted EOM and IOM from Microcystis aeruginosa (FACHB-905) and subjected them to simulated chlorination conditions (FAC/TOC = 1.0, 2.0). Chlorinated AOM displayed reduced aromaticity, lower molecular weight, and a pronounced blue shift in fluorescence spectra. Under 365 nm LED irradiation, the ^•OH quantum yield of chlorinated EOM increased by approximately 200-fold relative to unchlorinated controls. Both EOM and IOM exhibited enhanced excited triplet-state activity and singlet oxygen (¹O₂) production, indicating that chlorination significantly augments AOM’s photochemical reactivity. 

Using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, the researchers revealed that nitrogen-rich constituents within AOM serve as major electron donors, reacting with FAC to form nitrogen-containing chlorinated species. In contrast, chlorinated NOM predominantly yielded CHO-class chlorophenols. Notably, chlorinated EOM exhibited a significant enrichment in highly oxygenated unsaturated compounds, which were identified as key sources of ^•OH.

Furthermore, chlorinated EOM markedly accelerated the photodegradation of N,N-diethyl-meta-toluamide (DEET), a model organic pollutant. Quenching experiments confirmed ^•OH as the primary reactive species responsible. Spectral and product analyses suggested that DEET degradation proceeded via ^•OH-induced hydrogen abstraction and hydroxylation, with deeper ring-opening and bond cleavage products observed in chlorinated systems. 

This study systematically clarifies the intrinsic link between molecular evolution and photochemical activity in chlorinated AOM. The results offer valuable insights for optimizing water treatment strategies in algal bloom-impacted environments and provide a scientific basis for predicting the environmental behavior of pollutants in chlorinated effluents or algicide-treated waters.

(Online: 17 April 2025)

(Editor: MA Yun)