Sung Eun Lim
Nitrogen-containing moieties are widespread in natural waters in dissolved organic nitrogen and micropollu-tants. They are often susceptible to an electrophilic attack of ozone because of the electron-rich nature of the neutral form of nitrogen in organic compounds. This indicates that reactions of nitrogen-containing compounds with ozone inevitably occur in water and wastewater treatment where ozonation is applied for disinfection or oxidation. Despite the relevance to ozone chemistry, the knowledge on reactions of ozone with some principal nitrogen-containing functional groups is limited. The aim of this thesis was to elucidate reaction mechanisms of nitrogen-containing compounds with ozone for common nitrogen-containing functional groups: aliphatic amines and azoles. The primary objectives were (1) to determine the second-order rate constants for the reactions of nitrogen-containing compounds with ozone (kO3), (2) to identify/quantify transformation products and reactive oxygen species, (3) to perform kinetic simulations and quantum chemical computations to supplement or corroborate empirical evidences. Reaction mechanisms were proposed by compiling the results from all aspects, providing a better understanding of nitrogen-ozone chemistry. The investigation of triethylamine, diethylamine, and ethylamine yielded kO3 of the neutral form of amines ranging from 9.310^4 M-1 s-1 to 2.210^6 M-1 s-1. The apparent kO3 at pH 7 for potential or identified transformation products were 6.810^5 M-1 s-1 for N,N-diethylhydroxylamine, ~10^5 M-1 s-1 for N-ethylhydroxylamine, 1.910^3 M-1 s-1 for N-ethylethanimine oxide, and 3.4 M-1 s-1 for nitroethane. All amines predominantly underwent oxygen-transfer pathways to form major transformation products containing nitrogen-oxygen bonds: triethylamine N-oxide (88% per abated triethylamine) and nitroethane (69% per abated diethylamine and 100% per abate ethylamine). N,N-diethylhydroxylamine was a potential primary transformation product during the diethylamine-ozone reaction. Its formation was not confirmed due to its high ozone reactivity, but supported by measurements of reactive oxygen species, kinetic simulations, and quantum chemical computations. Pyrrole and imidazole reacted fast (kO3 > 10^3 M-1 s-1 at pH 1 â 12), whereas pyrazole reacted moderately fast with ozone (kO3 = 56 M-1 s-1 at pH 7). All studied azoles underwent an addition of ozone to the C-C double bond in the ring. Subsequent pathways after the initial ozone addition varied considerably among the azoles. Pyrrole reacted with ozone via a Criegee-type and an oxygen-addition mechanism. The major identified products (yield per abated pyrrole) were maleimide (34%), formamide (14%) and formate (54%). Imidazole predominantly reacted via a Criegee-type mechanism with ring cleavage and formation of the three fragments, formamide, cyanate, and formate (~100 % yields, respectively), completely closing the mass balance. For pyrazole, only carbonous products were identified (126% formate and 34% glyoxal per abated pyrazole). Products containing hydroxypyrazole and hydrazide moieties were postulated, possibly formed via an oxygen addition and a Criegee-type mechanism, respectively. The nitrogenous transformation products identified during this study may have different environmental implications. Some of them (e.g., nitroalkanes, N-formyl compounds) require further efforts to understand their impact on the aquatic environment and human health.
EPFL2020
