UV photolytic mechanism of N-nitrosodimethylamine in water: Roles of dissolved oxygen and solution pH

Abstract

The photolytic mechanism of N-nitrosodimethylamine (NDMA) in aqueous solution was investigated under ultraviolet (UV) irradiation at 253.7 nm, by determining the quantum yield of NDMA photolysis and analyzing the photolysis products in the pH range of 2-11 under O-2 or N-2 saturation. Several important photolytic behaviors of NDMA photolysis regarding the roles of dissolved oxygen and solution pH were newly observed, and novel photolytic mechanisms were proposed based on these observations. First, the presence of dissolved oxygen clearly increased the quantum yield of the NDMA photolysis, and promoted the photolytic pathway leading to the formation of methylamine and nitrate. The direct oxidation of photoexcited NDMA by dissolved oxygen (photooxidation) was suggested as a novel photolytic pathway to explain the observed effect of dissolved oxygen. This photooxidation mechanism was supported by the detection of superoxide radicals during the photolysis of NDMA under 02 saturation. Second, the dependence of the quantum yield of NDMA photolysis andtheyields of the known photolysis products (methylamine, dimethylamine, nitrite, and nitrate) on the solution pH were different under O-2 and N-2 saturation. N-Methylformamide was observed as the novel photolysis product of NDMA in the neutral pH region under N-2 saturation. These pH-dependent photolytic behaviors of NDMA were successfully explained by the interaction of the photoexcited NDMA with a proton, and the pH-dependent secondary reactions between the primary photolysis products.