Formation of N-Nitrosodimethylamine by Sunlight Irradiation of Dimethylamine Compounds in the Presence of Nitrite

N-Nitrosodimethylamine (NDMA) is a disinfection byproduct and a probable human carcinogen formed during water treatment processes. While its generation in engineered systems has been extensively explored, its formation in natural environments is inadequately understood. This study demonstrates that compounds with dimethylamine (DMA) functionality exposed to sunlight irradiation in the presence of nitrite (NO2–) react to form NDMA. The molar yields of NDMA ranged from 0.006 to 0.035% after 6 h irradiation. The radicals generated by the photolysis of NO2–, notably •OH, NO•, and NO2•, play critical roles in this process. Three reaction pathways are involved in the NDMA formation. In the first and second pathways, nitrogen oxide radicals couple or dimerize to form the nitrosating agents N2O3 and N2O4, which attack the DMA moiety, yielding NDMA. In the third pathway, the DMA moiety is oxidized by •OH via H-abstraction to produce a nitrogen-centered radical (CH3N•CH3) which, followed by the addition of a NO•, yields NDMA. Radical quenching experiments combined with theoretical calculations suggest that the •OH/H-abstraction pathway is more favorable. Given that many environmental contaminants contain DMA functionalities, the photochemical formation pathway of NDMA in the sunlit surface waters may be important for aquatic environments.