The organic matter in International Humic Substances Society Natural Organic Matter (IHSS NOM) water exposed to ultraviolet (UV) irradiation and chlor(am)ine sequentially under practically relevant conditions was characterized based on fluorescence spectra. IHSS NOM water exposed to UV irradiation or chlor(am)ine alone was also evaluated. Both chlor(am)ine alone and UV/chlor(am)ine exposure showed similar chlor(am)ine demand and fluorescence spectra. UV irradiation and UV/chlorine exposure diminished the fluorescence intensity of IHSS NOM water, while UV/chloramine exposure resulted in increased fluorescence intensity. When compared with the results obtained following chlor(am)ination alone, IHSS NOM water showed decreased chlorine decay and increased chloramine decay after UV irradiation/chlor(am)ination. Additionally, IHSS NOM water exposed to UV/chloramine and chloramine showed less disinfection by-product (DBP) formation than water subjected to UV/chlorine and chlorine. Overall, these findings indicate that UV irradiation degrades NOM molecules to low-molecular-weight fractions, facilitating the subsequent reaction with chlor-(am)ine. However, chlorine and chloramine play different roles in the reaction. Chlorine degradation and substitution dominates the process of UV/chlorine exposure, while chloramine substitution is the major reaction during UV/chloramine exposure.

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