Mechanistic Investigation of Enhanced Photoreactivity of Dissolved Organic Matter after Chlorination

Chlorine is commonly used in disinfection processes in wastewater treatment plants prior to discharge of the effluents into receiving waters. Effluent organic matter and humic substances constitute up to 90% of dissolved organic matter (DOM) in receiving water, which induces photogeneration of reactive species (RS) such as excited triplet state of DOM ((DOM)-D-3*), singlet oxygen (O-1(2)), and hydroxyl radical ((OH)-O-center dot). The RS plays an important role in the attenuation of trace pollutants. However, the effect of chlorine disinfection on the photoreactivity of the DOM has remained unclear. Here, we investigated the physicochemical properties and subsequent RS variation after chlorination of DOM. Solid-state C-13 cross-polarization/ magic angle-spinning NMR and Fourier transform ion cyclotron resonance mass spectrometry verified that the aromaticity, electron-donating capacity (EDC), and average molecular weight of DOM decreased markedly after chlorination. It was found for the first time that the photoproduction of 3DOM*, O-1(2), and (OH)-O-center dot increased markedly after chlorination of DOM upon irradiation of simulated sunlight. The quantum yields of 3DOM*, O-1(2), and (OH)-O-center dot were positively correlated with E2/E3 (ratio of the absorbance at 254 to 365 nm) while negatively correlated with EDC before and after chlorination. These findings highlight the synergetic effect of chlorine disinfection on the photosensitization of DOM under irradiation of sunlight, which will promote the removal of trace pollutants in surface waters.

Automated summary

The study found that chlorine disinfection significantly reduces the aromaticity, electron-donating capacity, and average molecular weight of DOM, leading to an increase in the photoproduction activity of DOM after chlorination. The quantum yields of reactive species produced by chlorinated DOM under sunlight irradiation are positively correlated with the absorbance ratio and negatively correlated with the electron-donating capacity. Overall, chlorine disinfection has a synergistic effect on the photosensitization of DOM, which aids in the removal of trace pollutants in surface waters.