Environmental Science Water Research & Technology PAPER Cite this: DOI: 10.1039/c9ew01078e Received 4th December 2019, Accepted 22nd January 2020 DOI: 10.1039/c9ew01078e rsc.li/es-water Reactions of pyrrole, imidazole, and pyrazole with ozone: kinetics and mechanisms† Agnes Tekle-Röttering,‡ a Sungeun Lim, ‡ bc Erika Reisz, d Holger V. Lutze, efgj Mohammad Sajjad Abdighahroudi, e Sarah Willach, e Winfried Schmidt, af Peter R. Tentscher, h Daniel Rentsch, i Christa S. McArdell, b Torsten C. Schmidt * efg and Urs von Gunten* bc Five-membered nitrogen-containing heterocyclic compounds (azoles) belong to potential moieties in com- plex structures where transformations during ozonation can occur. This study focused on the azole–ozone chemistry of pyrrole, imidazole, and pyrazole as model compounds. Reaction kinetics and ozonation products were determined by kinetic and analytical methods including NMR, LC-HRMS/MS, HPLC-UV, and IC-MS. Analyses of reactive oxygen species ( 1 O 2 , ˙OH, H 2 O 2 ), quantum chemical computations (Gibbs energies), and kinetic simulations were used to further support the proposed reaction mechanisms. The species-specific second-order rate constants for the reactions of ozone with pyrrole and imidazole were (1.4 ± 1.1) × 10 6 M -1 s -1 and (2.3 ± 0.1) × 10 5 M -1 s -1 , respectively. Pyrazole reacted more slowly with ozone at pH 7 (k app = (5.6 ± 0.9) × 10 1 M -1 s -1 ). Maleimide was an identified product of pyrrole with a 34% yield. Together with other prod- ucts, formate, formamide, and glyoxal, C and N mass balances of ∼50% were achieved. Imidazole reacted with ozone to cyanate, formamide, and formate (∼100% yields per transformed imidazole, respectively) with a closed mass balance. For pyrazole, a high ozone : pyrazole molar stoichiometry of 4.6 was found, suggesting that the transformation products contributed to the over-stoichiometric consumption of ozone (e.g., hydroxypyrazoles). Glyoxal and formate were the only identified transformation products (C mass balance of 65%). Overall, the identified major products are suspected to hydrolyze and/or be biodegraded and thereby abated by a biological post-treatment typically following ozonation. However, as substructures of more com- plex compounds (e.g., micropollutants), they might be more persistent during biological post-treatment. Introduction Micropollutants are ubiquitously present in the aquatic envi- ronment. 1,2 Although micropollutants are present in surface waters only in the μgL -1 to ng L -1 range, they may be harm- ful to the aquatic ecosystem and/or pose a potential risk to human health through contamination of drinking water. 3 Ozonation has been proven to successfully abate micro- pollutants in drinking waters and wastewater effluents. 4–7 During ozonation, micropollutants are not mineralized but Environ. Sci.: Water Res. Technol. This journal is © The Royal Society of Chemistry 2020 a Westphalian University of Applied Sciences, Department of Environmental Engineering, Neidenburgerstraße 10, 45897 Gelsenkirchen, Germany b Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland. E-mail: urs.vongunten@eawag.ch c School of Architecture, Civil and Environmental Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland d University “Politehnica” of Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, Bulevardul Vasile Pârvan Nr. 6, 300233 Timişoara, Romania e University of Duisburg-Essen, Faculty of Chemistry, Instrumental Analytical Chemistry, Universitätsstraße 5, 45141 Essen, Germany. E-mail: torsten.schmidt@uni-due.de f Centre for Water and Environmental Research (ZWU), University of Duisburg- Essen, Universitätsstraße 2, 45141 Essen, Germany g IWW Water Centre, Moritzstraße 26, 45476 Mülheim an der Ruhr, Germany h Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg East, Denmark i EMPA, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland j Technical University Darmstadt, Franziska-Braun-Straße 3, 64287 Darmstadt, Germany † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c9ew01078e ‡ Co-primary authors equally contributed to this work. Water impact Azoles are commonly found structures in biomolecules and micropollutants. However, their fate during ozonation is not well understood. This study provides information on the kinetics and mechanisms of ozone-reactions with pyrrole, imidazole, and pyrazole as model compounds. The findings of this study lead to a better understanding of the fate of micropollutants containing azole moieties during water treatment with ozone. Open Access Article. Published on 31 January 2020. Downloaded on 3/21/2020 8:28:57 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal