Ozonation has been successfully applied in water treatment as a disinfection and/or oxidation process. Nevertheless, its application to bromide-containing waters requires a careful assessment, due to the formation of bromate, a probable human carcinogen, which is difficult to quench during post-filtration steps. Previous studies delineated the bromate formation pathway, suggesting hypobromous acid (HOBr) as a key reaction intermediate, and developed bromate mitigation strategies that remove HOBr. Besides HOBr, there are many other reactive bromine species, potentially involved in the bromate formation, but it is less clear to what extent they participate in the reactions. An example is bromine atom (Br●), formed by oxidation of bromide by hydroxyl radical (a secondary oxidant during ozonation). Once formed, Br● can be oxidized directly by ozone to form BrO● and subsequently bromate or reduced to bromide by the reaction with dissolved organic matter (DOM). To assess the likelihood of the pathways, knowing the reaction kinetics of Br● is important. This study aims to understand the role of Br● in the bromate formation during ozonation, by determining the second-order rate constants of the reactions of Br● (kBr●) with aromatic and olefinic model compounds to represent DOM moieties. A method generating Br● has been developed by using the reaction of HOBr with superoxide radical anion (O2●–), which forms BrOH●– that is quickly dissociated into Br● at neutral pH (BrOH●– ↔ Br● + HO– with Keq = 3.2 × 10-4 M). An aliquot of O2●– stock (prepared by dissolving KO2 powder in alkaline solution) has been mixed with solution containing HOBr (prepared by oxidizing bromide with ozone) and radical probe compounds with known kBr● (nitrobenzene, benzoic acid, ibuprofen) to confirm the in-situ formation of Br●. The kBr● of the DOM model compounds have been determined by competition kinetic methods with the abovementioned probes as competitors. Based on the determined kBr● and other kinetic information in literature, the bromate formation pathway during ozonation has been re-evaluated by kinetic simulations.

Role of reactive bromine species in bromate formation during ozonation of bromide-containing waters

ACS Fall 2021

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