Dechlorination kinetics at alkaline pH of N-chloropiperidine, a genotoxin in chlorinated municipal wastewater

N-chloropiperidine in chlorinated municipal wastewaters is worthy of attention both because of its genotoxicity and because of the acute general toxicity displayed by all chloramines. Here, the rate at which it is reduced and thus detoxified by sulfite is determined in alkaline solutions. Three parallel reaction paths, with rate constants at 25°C are: SO32−+ClNC5H10+H3BO3 (buffer)→products, k1=565 M−2 s−1; SO32−+ClNC5H10+H3O+→products, k2=1.13×1011 M−2 s−1 and SO32−+ClNC5H10+H2O→products, k3=4.1 M−1 s−1. The activation energy associated with the last (uncatalyzed) pathway is 30±2 kJ mol−1. Exploratory experiments suggest that N-chlorodiethylamine reacts at a rate similar to N-chloropiperidine whereas N-chloropyrrolidine reacts about 5 times faster. These are the first systematic measurements of the rate of dechlorination of secondary N-chloramines. At the slightly acidic pH of ordinary municipal wastewaters, such compounds will be reduced quickly by standard dechlorination practices and they cannot contribute significantly to the slowly reduced residual chlorine that has been discovered in dechlorinated effluents from wastewater treatment plants. On the other hand, above pH 8 their lifetime in dechlorinated effluents could become long enough to affect aquatic wildlife.