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Reactions of Pyrrole, Imidazole, and Pyrazole with Ozone

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Reactions of Pyrrole, Imidazole, and Pyrazole with Ozone Environmental Science Water Research & Technology View Article Online PAPER View Journal | View Issue Reactions of pyrrole, imidazole, and pyrazole with † Cite this: Environ. Sci.: Water Res. ozone: kinetics and mechanisms Technol., 2020, 6,976 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. 1 Analyses of reactive oxygen species ( O2, ˙OH, H2O2), quantum chemical computations (Gibbs energies), and kinetic simulations were used to further support the proposed reaction mechanisms. The species-specific 6 −1 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. second-order rate constants for the reactions of ozone with pyrrole and imidazole were (1.4 ± 1.1) × 10 M −1 5 −1 −1 s and (2.3 ± 0.1) × 10 M s , respectively. Pyrazole reacted more slowly with ozone at pH 7 (kapp =(5.6± 0.9) × 101 M−1 s−1). Maleimide was an identified product of pyrrole with a 34% yield. Together with other prod- ucts,formate,formamide,andglyoxal,CandNmassbalancesof∼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 Received 4th December 2019, that the transformation products contributed to the over-stoichiometric consumption of ozone (e.g., Accepted 22nd January 2020 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 This article is licensed under a DOI: 10.1039/c9ew01078e abated by a biological post-treatment typically following ozonation. However, as substructures of more com- rsc.li/es-water plex compounds (e.g., micropollutants), they might be more persistent during biological post-treatment. Open Access Article. Published on 31 January 2020. Downloaded 4/22/2020 7:32:57 PM. 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. Introduction ful to the aquatic ecosystem and/or pose a potential risk to human health through contamination of drinking water.3 Micropollutants are ubiquitously present in the aquatic envi- Ozonation has been proven to successfully abate micro- – ronment.1,2 Although micropollutants are present in surface pollutants in drinking waters and wastewater effluents.4 7 − − waters only in the μgL1 to ng L 1 range, they may be harm- During ozonation, micropollutants are not mineralized but a Westphalian University of Applied Sciences, Department of Environmental f Centre for Water and Environmental Research (ZWU), University of Duisburg- Engineering, Neidenburgerstraße 10, 45897 Gelsenkirchen, Germany Essen, Universitätsstraße 2, 45141 Essen, Germany b Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse g IWW Water Centre, Moritzstraße 26, 45476 Mülheim an der Ruhr, Germany 133, 8600 Dübendorf, Switzerland. E-mail: [email protected] h Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg East, c School of Architecture, Civil and Environmental Engineering (ENAC), École Denmark Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland i EMPA, Swiss Federal Laboratories for Materials Science and Technology, 8600 d University “Politehnica” of Timişoara, Faculty of Industrial Chemistry and Dübendorf, Switzerland Environmental Engineering, Bulevardul Vasile Pârvan Nr. 6, 300233 Timişoara, j Technical University Darmstadt, Franziska-Braun-Straße 3, 64287 Darmstadt, Germany Romania † Electronic supplementary information (ESI) available. See DOI: 10.1039/ e University of Duisburg-Essen, Faculty of Chemistry, Instrumental Analytical Chemistry, c9ew01078e Universitätsstraße 5, 45141 Essen, Germany. E-mail: [email protected] ‡ Co-primary authors equally contributed to this work. 976 | Environ. Sci.: Water Res. Technol., 2020, 6,976–992 This journal is © The Royal Society of Chemistry 2020 View Article Online Environmental Science: Water Research & Technology Paper converted to transformation products. Often, biologically ac- fied to be industrial processes related to acrylonitrile synthe- tive micropollutants lose their effects upon transformation sis which produces pyrazole as a by-product. – during ozonation,8 11 however in some cases, transformation Recently, haloazoles, azoles substituted with one or more products with higher toxicity than their parent compounds halogen atoms, have been identified as a new class of disinfec- – were formed.12 16 To predict the transformation products tion by-products.30 Tribromopyrroles were detected during and assess their toxicities upon ozonation, a detailed knowl- chlorine disinfection of drinking water and wastewater edge of ozone reaction mechanisms is needed. To this end, containing high bromide levels.32,33 2,3,5-Tribromopyrrole was several compilations of known reaction pathways are avail- found to be a strong cytotoxin and genotoxin in mammalian able,17,18 which recently resulted in an ozone pathway predic- cells.32 Tetrapyrroles, metabolites of the above-mentioned nat- tion tool.19 Many well-known ozone-reactive groups, such as ural pigments, were suggested to be precursors.33 Tri- and di- phenols, olefins and nitrogen-containing moieties are com- bromo-1-methylimidazoles were detected in swimming pool monly found as structural units of micropollutants. Several and spa waters treated by bromine.34 Histidine introduced by studies were carried out to understand ozone–nitrogen inter- urine, sweat, and personal care products containing an imidaz- actions for primary, secondary and tertiary amines and ani- ole moiety was proposed as a possible precursor. lines.18,20,21 Information is also available for some heterocy- Despite the importance of understanding the fate of clic amines such as N-heterocyclic aromatics, piperidine, azoles in water and wastewater treatment, only limited infor- – piperazine, and morpholine.22 24 Nevertheless, there is still mation is available regarding the kinetics and mechanisms lack of information on the reactivity and/or transformation of their reactions with oxidants and in particular with ozone. reactions of five-membered nitrogen-containing heterocyclic Early investigations on the oxidation of azoles, which date compounds (or azoles) such as pyrrole, imidazole, and back to the 1960s, were mostly carried out in organic – pyrazole (Fig. 1), which are commonly present in natural and solvents.35 38 More recently, six-membered heterocyclic synthetic organic compounds. Essential amino acids, histi- amines, such as pyridine, pyridazine, pyrimidine and pyr- Creative Commons Attribution-NonCommercial 3.0 Unported Licence. dine and tryptophan, contain an imidazole or a fused pyrrole azine were thoroughly investigated in terms of their reactions moiety, respectively.25 The nucleobases, adenine and gua- with ozone in aqueous solution.23 They were found to react − nine, contain a fused imidazole moiety, as part of the DNA.25 slowly with ozone with second-order rate constants <102 M 1 − Naturally occurring pigments, chlorophyll a (green pigments s 1.23 On the contrary, imidazole and histidine showed very in plant) and heme (red pigments in blood) contain four pyr- high reactivity towards ozone with second-order rate con- − − role units which jointly form a larger ring system that com- stants >105 M 1 s 1.39 This implies that different reaction plexes metal ions in the center.25 Azoles are also of great sig- mechanisms are involved in the reactions of ozone with five- nificance as synthetic compounds. They have been widely membered heterocyclic amines in comparison to the six- used as core units of pharmaceuticals, pesticides, dyes, plas- membered ones. This article is licensed under a – tics, etc.26 28 In this study the kinetics and mechanisms of the reactions Because of their widespread occurrences and numerous of five-membered heterocycles, i.e.,pyrrole,imidazole,and applications in medical and material sciences, azoles are pyrazole with ozone were investigated (Fig. 1). The second- Open Access Article. Published on 31 January 2020. Downloaded 4/22/2020 7:32:57 PM. likely to be present in municipal and industrial wastewaters. order rate constants of the reactions of azoles with ozone were Among more than 500 environmentally relevant micro- determined by competition kinetics with 3-buten-2-ol as a pollutants for the aquatic environment,7 approximately 50% competitor or by direct monitoring of the ozone decrease. are heterocyclic compounds and about 20%
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