Enhanced formation of 2,6-dichloro-4-nitrophenol during chlorination after UV/chlorine process: Free amino acid versus oligopeptide

[Display omitted] •UV/chlorine process resulted in higher yields of DCNP after chlorination.•Combined AAs exhibited higher formation potential of DCNP than free Tyr.•Low pH, high chlorine dose and UV fluence enhanced the formation of DCNP.•Quick yield of N-Cl bond in OCs occurred through reactions between AAs and chlorine.•N-Cl bond cleavage produced R-HN∙ that hydrolyzed to release NH3 and form NH2Cl. Halonitrophenols (HNPs) are emerging aromatic nitrogenous disinfection by-products (N-DBPs) with high cyto- and genotoxicity. However, limited information is available regarding the formation mechanism of HNP during chlorination after the UV/chlorine process. In the past, dissolved organic nitrogen (DON) constitutes a vast array of N-DBP precursors due to its heterogeneous nature. Amino acids (AAs) are an important class of DON in water, where combined AAs (i.e., peptides and proteins) make up a larger identifiable section. In this study, three AAs including free tyrosine (Tyr) and two oligopeptides (i.e., tyrosine-alanine (Tyr-Ala) and tyrosine-tyrosine-tyrosine (Tyr-Tyr-Tyr)) were selected as model AAs to evaluate the effect of the UV/chlorine process on the formation of 2,6-dichloro-4-nitrophenol (DCNP). We found an enhanced formation of DCNP after the UV/chlorine process in comparison with direct chlorination, which indicated the nitrogen sources in AAs can contribute to the formation of HNPs. The presence of NO• in the UV/chlorine process was demonstrated by electron spin-resonance spectroscopy (ESR) test. The effects of three typical factors (i.e., UV fluence, pH, and Cl2/AA) on the formation of DCNP in the UV/chlorine process were investigated to optimize parameters to minimize the formation of HNPs. Furthermore, it was observed that oligopeptides exhibited higher yields of DCNP than free Tyr. Based on this, we propose a possible formation pathway of DCNP from 2 oligopeptides to elucidate the formation mechanisms more intuitively. We observed and confirmed a convertible tendency from HNPs to aliphatic DBPs (e.g., trichloronitromethane). Overall, this study provided a reliable theoretical basis for a comprehensive understanding of the precursors, formation mechanisms and control methods of HNPs.