Sunlight photolysis mitigates the formation of N-nitrosodimethylamine (NDMA) during the chloramination of methadone

Abstract N-nitrosodimethylamine (NDMA), a well-known human carcinogen, is widely considered to form through the chloramination of amine-containing compounds; however, there is very limited knowledge regarding how sunlight photolysis affects NDMA formation during chloramination. This study selected methadone, a potential NDMA precursor, as a model compound and found that compared to chloramination alone, chloramine sunlight photolysis (sunlight/chloramine) enhances methadone degradation and reduces NDMA formation. The NDMA formation mechanism during the sunlight/chloramine (or chloramination) process was elucidated. The effects of the chloramine species (monochloramine (NH2Cl) and dichloramine (NHCl2)), chlorine-to-ammonia (Cl2/N) molar ratio (0.5–1.5), solution pH (4–9.5) and nitrate concentration (5 and 10 mM) on the sunlight/chloramine degradation of methadone were examined. The methadone degradation under all investigated sunlight/chloramine conditions followed pseudo-first-order kinetics, and the degradation rate constant increased with increasing NHCl2 concentration. Furthermore, NDMA formation increased with increasing Cl2/N molar ratio and solution pH but decreased in the presence of nitrate.