Inhibition of bromate formation in the ozone/peroxymonosulfate process by ammonia, ammonia-chlorine and chlorine-ammonia pretreatment: Comparisons with ozone alone

Abstract The ozone/peroxymonosulfate (O3/PMS) process, generating hydroxyl radicals (HO•) and sulfate radicals (SO4•-), can effectively remove organic contaminants in water, while forming a remarkable amount of bromate (BrO3-) in the presence of bromide (Br-). This paper firstly investigated the ammonia (NH3), chlorine-ammonia (Cl2-NH3) and ammonia-chlorine (NH3-Cl2) pretreatment in inhibiting BrO3- generation in O3/PMS process at different conditions, and the inhibition degree was compared with O3 alone. All the three kinds of pretreatment at Cl2 and NH3 dosages of 50 and 100 mM reduced 90 % or more of the overall BrO3- formation, while the NH3-Cl2 pretreatment strategy is prior to that of the NH3 and Cl2-NH3. The inhibition degree of the pretreatment strategies under different pH were: 5.0 > 9.0 > 7.0 > 3.0. Meanwhile, the inhibition degree increased as the initial Br- concentration increased from 15 to 50 µM. After each pretreatment strategy, there was a higher BrO3- inhibition efficiency accompanied with a lower ozone utilization efficiency in O3 alone than that of O3/PMS process. Furthermore, the mechanism of BrO3- inhibition by the pretreatment strategies was also clarified. The main products produced by masking Br- in the NH3-Cl2 pretreatment were more difficult to regenerate Br-, as a result, the NH3-Cl2 pretreatment strategy produced the least BrO3- among the three kinds of pretreatment strategies. Finally, compared with the deionized water, the pretreatment strategies shown better inhibition performance in actual water because the water matrices would trap free radicals and HOBr to reduce BrO3- formation. At the same time, the NH3-Cl2 pretreatment strategy generated fewer disinfection by-products than that of Cl2-NH3 pretreatment in actual water. Therefore, the NH3-Cl2 pretreatment strategy is a practical way to control the BrO3- formation in the O3/PMS process.