Enhanced formation of trichloronitromethane precursors during UV/monochloramine treatment

This study systematically investigates the formation of trichloronitromethane (TCNM) from 2 natural waters, 6 humic substances and 16 phenolic compounds during UV/monochloramine (UV/NH₂Cl) followed by post-chloramination. Using ¹⁵N-NH₂Cl as an isotope tracer, we found that ¹⁵N-TCNM accounted for 70.7-76.5% of total TCNM during UV/NH₂Cl treated 2 natural waters, which was significantly higher than the proportion of ¹⁵N-TCNM in chloramination (NH₂Cl alone). This is a direct evidence that NH₂Cl, rather than the nitrogenous matters in waters, was the predominant nitrogen source of TCNM during UV/NH₂Cl treatment. Phenol derivatives with meta-substituents and with electron-withdrawing groups facilitated the formation of TCNM precursors during UV/NH₂Cl treatment. Significant correlations were found between Hammett constants (σ) of substituents and TCNM formation potentials. The formation mechanisms of TCNM were revealed using resorcinol as a representative phenolic compound. During UV/NH₂Cl treatment, HO^•, reactive chlorine species and reactive nitrogen species contributed to 28.1%, 29.0% and 19.4% of resorcinol degradation. Five nitro(so)-intermediates were identified as the main TCNM precursors. The formation pathways of TCNM were proposed. Alkaline pH was recommended to reduce the formation of TCNM precursors during UV/NH₂Cl treatment.