Heterocyclic compounds are important functional groups in anthropogenic substances such as pharmaceuticals and pesticides and widespread in nature as substructures of micropollutants. The structures and chemical properties of heterocyclic compounds are extremely diverse, which makes it challenging to predict their fate during oxidation processes. During ozonation, a viable oxidation process for micropollutant abatement, heterocyclic compounds exhibit a wide spectrum of reaction kinetics from < 0.1 M^-1s^-1 to 10⁸ M^-1s^-1 as second-order ozone rate constants (k_O3). However, it is not fully understood which properties of heterocycles are responsible for determining the ozone reactivity. Moreover, heterocyclic compounds are converted into transformation products upon ozonation but their structures and toxic effects in the receiving environment are largely unknown. This study investigated the kinetics and mechanisms of the reactions of five-membered heterocyclic compounds with ozone, by using pyrrole, imidazole, and pyrazole as model compounds to represent common ozone reactive moieties of micropollutants. The apparent k_O3, determined by a competition kinetic method, revealed that pyrrole and imidazole react very fast with ozone (k_O3 > 10⁵ M^-1s^-1 at pH ≥ 7) and pyrazole reacts moderately fast with ozone (k_O3 ~60 M^-1s^-1 at pH 7). Transformation products, identified by NMR, LC-HRMS/MS, HPLC-UV, and IC-MS, suggested that all studied heterocycles underwent the same initial reaction pathway, an ozone addition to a carbon-carbon double bond in the heterocyclic ring. Subsequent pathways after the initial ozone addition varied considerably among the heterocycles: e.g., a ring cleavage followed by further oxidation by ozone (pyrrole) or a complete fragmentation without consuming further ozone (imidazole). These results from kinetics and transformation product formation for five-membered heterocycles are remarkably different from previous studies on six-membered heterocycles, which reported very low ozone reactivity (k_O3 < 1 M^-1s^-1) and an ozone addition to nitrogen (rather than to a carbon-carbon double bond) as a major pathway.

Christa McArdell

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