UV254 irradiation of N-chloro-α-amino acids: Kinetics, mechanisms, and N-DBP formation potentials

This study explores the degradation kinetics and mechanisms of N-chloro-α-amino acids and the changes in the formation potential of nitrogenous disinfection byproducts (N-DBPs) upon UV254 irradiation. UV254 irradiation significantly accelerated the degradation of all the tested N-chloro-α-amino acids compared to those in the dark. Both direct photolysis-induced cleavage of the N-Cl bonds and radical oxidation (e.g., Cl• and Cl2•−) involved reactions that contributed to their enhanced degradation. The fluence-based photolysis rate constants of the N-chloro-α-amino acids varied in the range of (1.06—5.47) × 10−3 cm2 mJ−1 at pH 6.0 and (0.74—2.79) × 10−3 cm2 mJ−1 at pH 8.0. The apparent quantum yields (Φapp) of the majority of the N-chloro-α-amino acids were in the range of 0.41−0.95 at pH 6.0 and 0.22−0.79 at pH 8.0, except N-chloroaspartic acid, N-chlorohistidine, and N-chloroalanine. UV254 irradiation significantly enhanced the formation of trichloronitromethane (TCNM) from the tested N-chloro-α-amino acids after post-chlorination, but exhibited various effects on the formation of dichloroacetonitrile (DCAN). A longer UV254 irradiation time generated more TCNM, and a lower pH produced more DCAN from the N-chloro-α-amino acids. The degradation pathways of N-chlorotyrosine, as a representative N-chloro-α-amino acid, are proposed, and the β-scission and 1,2-H shift pathways led to the formation of different precursors of TCNM and DCAN. The results of this study improve our understanding of the fate of N-chloro-α-amino acids under UV254 irradiation and post-chlorination.