Combined
chlorine is increasingly being used as an alternative
disinfectant to free
chlorine to maintain a residual in
drinking water distribution systems mainly because it would reduce the formation of regulated disinfection byproducts (
DBPs)
trihalomethanes and haloacetic
acids. However, the use of combined
chlorine could promote the formation of currently unregulated nitrogenous
DBPs (N-
DBPs) such as haloacetonitriles and haloacetamides that are found to be more cyto- and genotoxic than regulated
DBPs.
Monochloramine quickly reacts with
chloroacetaldehyde, a DBP formed during primary disinfection with free
chlorine, forming and reaching pseudoequilibrium (equilibrium constant K1 = 1.87 × 10(3) M(-1)) with the carbinolamine 2-chloro-1-(chloroamino)ethanol. 2-Chloro-1-(chloroamino)ethanol undergoes slow
dehydration to form the
imine 1-chloro-2-(chloroimino)ethane that decomposes at a faster rate to
chloroacetonitrile. 2-Chloro-1-(chloroamino)ethanol is also oxidized by
monochloramine to produce the previously unreported DBP N,2-dichloroacetamide. The carbinolamine
dehydration step was found to be
acid/base catalyzed (k2(0) = 3.30 × 10(-6) s(-1), k2(H) = 2.43 M(-1) s(-1), k2(
OH) = 3.90 M(-1) s(-1)). In contrast, N,2-dichloroacetamide formation was observed to be only base catalyzed (k3(
OH) = 3.03 × 10(4) M(-2) s(-1)). N,2-dichloroacetamide cytotoxicity (LC50 = 2.56 × 10(-4) M) was found to be slightly lower compared to that reported for
chloroacetamide but higher than those of di- and
trichloroacetamide.