Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation

Bicarbonate scavenges OH to form CO₃^- that enhances the bromate formation by ozonation. However, the role of CO₃^- in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO₃^- based on the detection of CO₃^--involved bromate and CO₃^- exposure. Experiments demonstrated that the CO₃^--involved bromate was responsible for 33.7-69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO₃^- exposure was two orders of magnitude higher than the corresponding OH exposure during ozonation. These results demonstrate that CO₃^- plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than OH. A model was developed based on the ratio of bromine oxidized by CO₃^-, which could predict the CO₃^--involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO₃^--involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of humic acid led to an increase in this contribution during ozonation. The results of this study provide a more in-depth understanding of the mechanism of bromate formation during ozonation.