Analysis of degradation kinetic modeling and mechanism of chlorinated-halonitromethanes under UV/monochloramine treatment

Chlorinated-halonitromethanes (Cl-HNMs) including chloronitromethane (CNM), dichloronitromethane (DCNM), and trichloronitromethane (TCNM) are nitrogenous disinfection by-products, which have high cytotoxicity and genotoxicity to human. This study aimed to investigate the degradation kinetic modeling and mechanism of Cl-HNMs under monochloramine activated by ultraviolet of 254 nm (UV/NH₂Cl) treatment. The first-principle kinetic model of UV/NH₂Cl process was developed to simulate Cl-HNMs degradation. Of note, the second-order rate constants of Cl-HNMs reacting with HO• (∼10⁸ M^-1 s^-1), Cl• (k_{Cl•,CNM or DCNM} = ∼10¹⁰ M^-1 s^-1, k_Cl•,TCNM = ∼10² M^-1 s^-1), Cl₂•^- (k_{Cl•,CNM or DCNM} = ∼10⁹ M^-1 s^-1, k_Cl•,TCNM = ∼10¹ M^-1 s^-1), ClO• (∼10⁵-10⁶ M^-1 s^-1) and CO₃•^- (∼10⁶-10⁷ M^-1 s^-1) were obtained by the first-principle kinetic model. Overall, Cl-HNMs degradation under UV/NH₂Cl treatment was successfully predicted by the kinetic model under various conditions. It was found that UV (>60%) was dominant in Cl-HNMs degradation, followed by HO• (3.8%-24.5%), reactive chlorine species (RCS, 0.9%-28.8%) and CO₃•^- (0-26.1%). Among the contributions of RCS, Cl• and Cl₂•^- were main radicals in the degradation of CNM and DCNM, while ClO• was responsible for the abatement of TCNM. The minimum EE/O values under UV/NH₂Cl treatment were approximately 30% lower than those under UV treatment. Finally, the possible degradation pathways were proposed, including hemolytic/heterolytic cleavage of Cl-HNMs by UV irradiation, hydrogen abstraction/electron transfer of CNM and DCNM and adduct reaction of TCNM by free radicals. This study based on the kinetic model is beneficial to predict and control the concentrations of Cl-HNMs under UV/NH₂Cl treatment.