Kinetic insights into removal of indole in aqueous solutions by ozonation: operating parameters, modeling and degradation pathways

Abstract

Nitrogenous heterocyclic compounds (NHCs) pose a serious threat to human health and ecosystems, and ozonation has been widely applied in the efficient removal of NHCs in water environments. In the present study, the degradation performance of indole in aqueous solutions was investigated. The second-order rate constants of indole reacting with molecular ozone at pH 2 and ·OH at pH 9 were determined to be 223.24 M−1 s−1 and 12.3 × 109 M−1 s−1, respectively. The effect of different operating parameters both on the mass transfer and chemical kinetics was evaluated during the degradation of indole by ozonation. To attain a better removal performance, a pH value around 9 and a temperature of 35.6 °C were preferred. Additionally, the higher initial ozone concentrations were proved to be beneficial for the enhancement of the removal rate. Compared with synthetic solutions, the removal of indole appeared to be more efficient in real bio-treated coal gasification wastewater. A chemical kinetic model was built based on the rate constants and exposures of O3 and ·OH, and experimental results demonstrated that the removal rates can be satisfactorily predicted. The relative contributions of O3 and ·OH measured by the modeling results indicated that the molecular ozone played a dominant role in the reaction system. By further analysis of intermediates with gas chromatography–mass spectrometry (GC–MS), a number of products were detected, and three degradation pathways were tentatively proposed. This work provides a reference for kinetic studies on the treatment of other NHCs using ozone technologies.