Photo-transformation of wastewater effluent organic matter reduces the formation potential and toxicity of chlorinated disinfection byproducts

Sunlight exposure can degrade and transform discharged wastewater effluent organic matter (EfOM) in aquatic systems, potentially enhancing the feasibility of reusing wastewater for drinking purposes. However, there remains a lack of comprehensive understanding regarding the sunlight-induced changes in the molecular-level composition, characteristics, and chlorine reactivity of EfOM. Herein, we investigated the impact of sunlight on the optical properties, chemical composition, and formation of disinfection byproducts of EfOM using multiple spectroscopic analyses, high-resolution mass spectrometry, chlorination experiments, and in vitro bioassays. Upon natural sunlight exposure, we observed significant decreases in ultraviolet-visible absorbance and fluorescence intensity of EfOM, indicating the destruction of chromophores and fluorophores. Photolysis generally yields products with lower molecular weight and aromaticity, and with higher saturation and oxidation levels. Moreover, a shift within the EfOM from condensed aromatic-like compounds to tannin-like components was observed. Furthermore, sunlight exposure reduced the reactivity of EfOM toward the formation of trihalomethanes and haloacetonitriles during chlorination, while there was a slight increase in the specific formation potential of haloketones. Importantly, the disinfection byproducts resulting from chlorination of the irradiated EfOM exhibited reduced microtoxicity. Overall, this study provides new insights into alterations in EfOM under sunlight exposure and aids in predicting the health risks of effluent discharge in water environments.