Formation of NDMA by chloramination of nitrogenous contaminants: potential role of bromide and dissolved oxygen
Résumé
Disinfection with chloramines is known to significantly reduce the formation of regulated disinfection by-products (i.e. trihalomethanes and haloacetic acids) as compared to chlorination. Moreover, monochloramine can be added to wastewater prior to reuse processes to avoid biofouling of membranes. However, chloramination favors the formation of N-nitrosamines, including N-nitrosodimethylamine (NDMA), a probable human carcinogen. The formation mechanisms of NDMA are not fully understood. Proposed mechanisms used dimethylamine as a model precursor, but some studies indicated that amounts of dimethylamine present in surface waters or secondary municipal wastewaters cannot be sufficient to explain the amount of NDMA formed. The potential importance of tertiary amines presenting dimethylamine functional groups (e.g. ranitidine, a histamine antagonist often used for peptic ulcer treatment) has been pointed out. In this study, NDMA formation potential of several nitrogen-containing organic compounds from the reaction with monochloramine is investigated. All of these compounds are tertiary amines presenting DMA functional groups. The variety of compounds was chosen to understand the role of molecular structures in the formation of NDMA. Simultaneous analysis of nitrosamines in aqueous samples is based on a solid-phase extraction procedure followed by gas chromatography/mass spectrometry analysis in electron impact ionization mode. This method reaches extraction efficiencies of ~85 %, and a method detection limit of 33 ng/L. Preformed monochloramine with a Cl:N molar ratio of 1:1.2 is used for chloramination experiments. Compounds presenting heterocyclic rings substituted with DMA functions (e.g. dimethylaminomethylfurfuryl alcohol) show much higher conversion rates to NDMA than other tertiary amines or DMA. New formation mechanisms have to be proposed to explain the importance of structural characteristics of tertiary amines compounds for NDMA formation. The concentration of dissolved oxygen was found to play a major role for NDMA formation. With ranitidine, in absence of dissolved oxygen almost no NDMA was formed, while the NDMA molar yield was 54 % in saturated O2 solution. The presence of bromide also significantly enhanced the formation of NDMA during the chloramination of dimethylamine and dimethylaminomethyl-furfuryl alcohol. This is of great concern regarding wastewater reuse, because the chloramination of bromide-containing wastewaters could lead to significant amounts of NDMA.