Predominant N-Haloacetamide and Haloacetonitrile Formation in Drinking Water via the Aldehyde Reaction Pathway

In drinking water disinfection, switching from free chlorine to alternative chemical disinfectants such as monochloramine may result in the formation of different classes of toxic disinfection byproducts (DBPs). Haloacetonitriles (HANs) and haloacetamides (HAMs) are two currently unregulated nitrogen-containing DBP (N-DBP) groups commonly found in water disinfected with monochloramine that have been shown to be more cyto- and genotoxic than regulated DBPs. For the first time, this study confirms the formation of HAN and HAM dominant species found in disinfected water, dichloroacetonitrile and dichloroacetamide, from the reaction between monochloramine and dichloroacetaldehyde via the aldehyde reaction pathway. Results from experiments with natural water treated with labeled 15 N-monochloramine confirmed the relevance of the aldehyde pathway. Monochloramine reacted quickly with dichloroacetaldehyde reaching equilibrium with the carbinolamine 2,2-dichloro-1-(chloroamino)ethanol ( K1 = 1.87 × 104 M-1 s-1). Then, 2,2-dichloro-1-(chloroamino)ethanol underwent two parallel reactions where, (1) it slowly dehydrated to 1,1-dichloro-2-(chloroimino)ethane ( k2 = 1.09 × 10-5 s-1) and further decomposed to dichloroacetonitrile, and (2) it was oxidized by monochloramine ( k3 = 4.87 × 10-2 M-1 s-1) to form a recently reported N-DBP, the N-haloacetamide N,2,2-trichloroacetamide. At high pH, dichloroacetonitrile hydrolyzed to dichloroacetamide ( k40 = 3.12 × 10-7 s-1, k4OH = 3.54 M-1 s-1). Additionally, trichloroacetaldehyde was also produced from the reaction of dichloracetaldehyde and monochloramine ( k5 = 2.12 × 10-2 M-1 s-1) under the presence of monochlorammonium ion, a product of monochloramine protonation. Within the N-haloacetamide family, N,2,2-trichloroacetamide (LC50 = 3.90 × 10-4 M) was found to be more cytotoxic than N-chloroacetamide but slightly less potent than N,2-dichloroacetamide.