Effect of bromide ion on the formation of N-nitrosodimethylamine (NDMA) in waters treated with monochloramine

摘要

The main objectives of this dissertation were: (i) to evaluate the effect of bromide on the formation of N-nitrosodimethylamine (NDMA) in chloraminated waters; (ii) to test different active bromine compounds in order to determine if they lead to the direct formation of NDMA as a result of reacting with an NDMA precursor; (iii) to develop a kinetic model to predict the formation and decomposition of bromochloramine, which is the major product in the reaction of monochloramine and bromide; and (iv) to obtain kinetic data for NDMA formation under various conditions and determine whether a correlation exists between NDMA formation and bromochloramine. NDMA formation was found to be enhanced in the presence of bromide when waters containing the NDMA precursor dimethylamine (DMA) were treated with monochloramine. Significant enhancement of NDMA formation was observed at high pH (8 – 9) conditions, when the reaction kinetics between monochloramine and bromide are sufficiently slow so that oxidant depletion is not an important factor. The hypothesis in the literature that bromamines are the species responsible for the enhanced NDMA formation was tested and found to not be valid. Additional active bromine species such as hypobromous acid, hypobromite ion, and tribromide ion were also tested and found to form relatively low levels of NDMA, two orders of magnitude lower than in the experiments with monochloramine and bromide at pH 9. Kinetic studies of the reaction between monochloramine and bromide showed that the main product of reaction is the dihaloamine bromochloramine. A kinetic model was developed to predict the concentration of bromochloramine as a function of reaction time between bromide and monochloramine. The model was used at various water quality conditions to study the correlation between bromochloramine concentration and the formation kinetics of NDMA. The results show that at pH 9 when the oxidant concentrations are stable, increasing the bromide ion concentration increases both the NDMA and bromochloramine concentrations, in a seemingly proportional manner; however, initial rate studies show that bromochloramine does not directly react with the NDMA precursor, DMA. Overall, this study shows that when monochloramine is used as a disinfectant, bromide removal should be practiced in water treatment facilities whose feed waters contain bromide in order to minimize the formation of the carcinogen NDMA. Additionally, since higher pH values lead to a higher enhancement of NDMA formation by the presence of bromide, treating waters at lower pH values would require a lower level of bromide removal. Finally, the good agreement between experimental data and the predictions of the model developed to describe the kinetics of formation and decomposition of bromochloramine indicate that the model can serve as a useful tool in the study of systems containing bromide and monochloramine.