Kinetics and mechanisms of nitrate and ammonium formation during ozonation of dissolved organic nitrogen.

Dissolved organic nitrogen (DON) is an emerging concern in oxidative water treatment because it exerts oxidant demand and may form nitrogenous oxidation/disinfection by-products. In this study, we investigated the reactions of ozone with DON with a special emphasis on the formation of nitrate (NO3 - ) and ammonium (NH4 + ). In batch ozonation experiments, the formation of NO3 - and NH4 + was investigated for natural organic matter standards, surface water, and wastewater effluent samples. A good correlation was found between NO3 - formation and the O3 exposure (R2  > 0.82) during ozonation of both model DON solutions and real water samples. To determine the main precursor of NO3 - , solutions composed of tannic acid and model compounds with amine functional groups were ozonated. The NO3 - yield during ozonation was significantly higher for glycine than for trimethylamine and dimethylamine. Experiments with glycine also showed that NO3 - was formed via an intermediate with a second-order rate constant of 7.7 ± 0.1 M-1 s-1 while NH4 + was formed by an electron-transfer mechanism with O3 as confirmed from a hydroxyl radical (OH) yield of 24.7 ± 1.9%. The NH4 + concentrations, however, were lower than the OH yield (0.03 mol NH4 + /mol OH) suggesting other OH-producing reactions that compete with NH4 + formation. This study concludes that NO3 - formation during ozonation of DON is induced by an oxygen-transfer to nitrogen forming hydroxylamine and oxime, while NH4 + formation is induced by electron-transfer reactions involving C-centered radicals and imine intermediates.