Molecular transformation of algal organic matter during sequential ozonation-chlorination: Role of pre-ozonation and properties of chlorinated disinfection byproducts.

Formation of unknown chlorinated disinfection byproducts (Cl-DBPs) during chlorination gradually raised great concern, and pre-oxidation was considered as an efficient method to minimize Cl-DBP formation. In this study, pre-ozonation of algal organic matter was investigated, to explore its impacts on Cl-DBP formation and acute toxicity during subsequent chlorination. With fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, the conversion of algal organic matter in chlorination with/without pre-ozonation was tracked. The results show that pre-ozonation reduced the formation of trichloromethane (TCM), yet the species and intensity of unknown Cl-DBPs were significantly increased in subsequent chlorination. Meanwhile, the solution acute toxicity was higher in chlorination with pre-ozonation than in chlorination only. Besides, molecular properties of these unknown Cl-DBPs were further explored and featured. One-chlorine-containing DBPs were unsaturated high molecular-weight compounds with more CH2 structures, while two or three-chlorine-containing DBPs were mainly oxidized or saturated compounds. Of note, large amounts of one-chlorine-containing DBPs related to polycyclic aromatics and polyphenols compositions were generated, which may contribute to the high potential toxicity. Overall, the findings of this study could provide new insights into the impacts of pre-ozonation on the formation of unknown Cl-DBPs and potential toxicity during chlorination for actual application.