Bromate oxidized from bromide during sonolytic ozonation

• We evaluate the effect of ultrasound on formation of bromate (BrO3−) in ozonation.
• The oxidation of bromide (Br−) to form BrO3− is enhanced by sonolysis.
• Formation of BrO3− was increased with increasing of pH.
• Free bromine (HOBr/OBr−) is decayed majorly to form Br− under ultrasound irradiation.

Sonolytic ozonation (US/O3) is an effective way to degrade many pollutants in drinking water as the elevated mass transfer rate of ozone gas and the enhanced forming of hydroxyl radicals (OH). This work investigated the formation of bromate (BrO3−) from bromide (Br−) in sonolytic ozonation. At neutral pH, the bromate conversion rate ([BrO3−]/[Br−]0) was increased to 60% by ultrasound at continuous ozone flow (0–0.2 L min−1), much higher than that without ultrasound or without bubbling. This indicates that the promoting effect of sonolysis on BrO3− formation is mainly due to the sonolytic decomposition of ozone and the enhancement of gas–liquid transfer. The [BrO3−]/[Br−]0 was increased with increasing pH. In addition, the reduction of HOBr/OBr− with ultrasound demonstrates that bromate may be inhibited as the bromide was formed with the H2O2 generation under ultrasound. This suggests the competition between bromate and bromide during the US/O3 led to the inhibition of bromate formation at high ozone flow. Therefore, our result reveals that the bromate formation under ultrasound is improved remarkably in US/O3 in quick treatment with proper ozone flow (<0.2 L min−1).