Reactions of amines with ozone and chlorine: Two novel oxidative methods to evaluate the N-DBP formation potential from dissolved organic nitrogen
Karim-Alexandre Essaïed, Lucy V. Brown, Urs von Gunten
Abstract
The composition of oxidant-reactive dissolved organic nitrogen (DON) is poorly characterized, although its ozonation is likely to form a great variety of disinfection by-products containing a nitrogen-oxygen bond (N-DBPs). In this study, two chemical oxidation procedures were developed: continuous ozonation at pH 7.0 and free available chlorine (FAC) titrations at pH 9.2. The formation of two oxidation products (nitrate (NO3−) and chloramines, respectively) was used to quantify and characterize oxidant-reactive nitrogenous moieties in DON. In addition, batch experiments were conducted to study the NO3− yields of 30 selected nitrogenous model compounds upon ozonation. The NO3− yields of 12 primary and secondary amines were highly variable (17–100%, specific ozone dose of 20 molO3/molN), 7 amino acids had high NO3− yields (≥90%), and tertiary amines as well as pyrrole, acetamide and urea had low NO3− yields (≤15%). The mechanisms of NO3− formation were further examined with benzylamine and N-methylbenzylamine as model compounds. Our results show that nitroalkanes are the last intermediate products before the formation of NO3−, both for primary and secondary amines. The presence of an electron-withdrawing group in the vicinity of the N-atom facilitates the formation of NO3− from nitroalkanes. Therefore, the formation of NO3− is attributed to amino acids and activated primary and secondary amines. In contrast, all primary and secondary amines were transformed to chloramines upon chlorination, which was determined by a novel oxidative titration with chlorine. To further support the selectivity of this assay, it was demonstrated by derivatization of amine moieties that chloramine formation could be inhibited. 13–45% of the DON of 4 dissolved organic matter isolates and 2 wastewater effluents formed NO3− and 0–39% formed chloramines, indicating that the potential for N-DBP formation is high (µMN/mgC-level). From differences in the formation of NO3− and chloramines the nature of the precursors can be hypothesized (e.g., activated or non-activated primary and secondary amines, partially oxidized nitrogenous compounds). This study highlights the capacity of two novel methods to characterize the oxidant-reactive DON fraction. Our results suggest that this fraction is significant and could form a variety of potentially toxic N-DBPs.