NDMA formation by chloramination of nitrogenous organic compounds
Résumé
A large diversity of disinfection by-products (DBPs) are formed during water treatment processes using chlorination, including trihalomethanes (THMs) and haloacetic acids (HAAs). Disinfection with chloramines is known to significantly reduce the formation of regulated DBPs as compared to chlorination. 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 (DMA) 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. NDMA precursors in natural organic matter (e.g. humic substances) are not fully determined. 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.
In this study, the reactivity with monochloramine of several nitrogen-containing organic compounds is investigated. This reactivity is leading to the formation of NDMA and haloacetonitriles (HANs), another type of nitrogenous DBPs. Model compounds investigated include two phenylurea herbicides (diuron, isoproturon) and pharmaceuticals (ranitidine, doxepine, amitriptyline, mifepristone and minocycline). All of them are tertiary amines presenting DMA functional groups.
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 ~70 %, 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. After 5 days at pH 8, ranitidine shows the highest molar conversion rate with 70 % NDMA formed. Conversion rates for other pharmaceuticals are in the range of 0.7 to 5 % and less than 0.5 % for diuron and isoproturon. Ranitidine formed about 10 times more NDMA than HANs. For other substances tested, amounts of HANs formed are quite similar to those of NDMA. The pharmaceutical ranitidine shows much higher conversion rates to NDMA than other tertiary amines or DMA. These variations cannot be explained by current proposed mechanisms, based on DMA as a precursor. Compounds presenting heterocyclic rings substituted with DMA functions form more NDMA than compounds with DMA functions near carbonyl groups (diuron). New formation mechanisms have to be proposed to explain the importance of structural characteristics of tertiary amines compounds for NDMA formation. The influence of chloramination conditions (pH, dose, contact time) will be investigated in further researches.