Degradation of benzylamines during chlorination and chloramination†

Abstract

As widely used industrial ingredients and products of the biodegradation of benzalkonium chloride disinfectants, benzylamines are expected to occur in municipal wastewater effluents and other wastewater-impacted waters, but their fate during chlorine or chloramine disinfection is unclear. This study characterized the degradation pathways of benzylamine, N-methylbenzylamine and N,N-dimethylbenzylamine during chlorination and chloramination. The dominant reaction pathways during chlorination involved chlorine transfer to the benzylamine nitrogen followed by hydrochloric acid elimination to form an imine and hydrolysis of the imine to form an aldehyde and lower order amine. Benzylamine formed benzaldehyde in preference to benzonitrile. For N-methylbenzylamine and N,N-dimethylbenzylamine, hydrochloric acid elimination between the benzyl nitrogen and the methyl substituent formed formaldehyde and either benzylamine or N-methylbenzylamine, while elimination between the nitrogen and the benzyl substituent formed benzaldehyde and either monomethylamine or dimethylamine. Similar products were observed during chloramination, but over longer timescales. Formation of products involving halogenation of the aromatic ring was not observed. Of highest toxicological concern was the 34% molar yield of NDMA that formed during chloramination of N,N-dimethylbenzylamine in concert with benzyl alcohol by a pathway occurring in parallel to the imine formation and hydrolysis pathway. Based on these reaction pathways, a strategy to reduce NDMA formation within potable reuse facilities was validated using laboratory-scale versions of the reverse osmosis and ultraviolet light processes used in potable reuse trains. The strategy involved treating fully nitrified wastewater influents to these facilities with free chlorine for 5 min to degrade N,N-dimethylbenzylamine and other potent NDMA precursors prior to the addition of ammonia to form chloramines used to control biofouling within these facilities.