Reducing bromate formation with H⁺-form high silica zeolites during ozonation of bromide-containing water: Effectiveness and mechanisms

This paper investigated the effect of H⁺-form high silica ZSM-5 (HZSM-5) zeolites on bromate formation. HZSM-5 zeolites with different Si/Al molar ratios (i.e., 25-300) were tested taking ozonation alone as control. The zeolites were more effective in reducing bromate formation for the filtered surface water than CeO₂, a former reported oxide that can reduce bromate formation at slightly acidic pH. The reduction efficiencies were not closely related to their Si/Al ratios. The HZSM-5 (Si/Al=300) selected for detailed studies effectively reduced bromate formation by 58% for the filtered water, and also enhanced the removal of dissolved organic carbon (DOC) during ozonation. The efficiency of the HZSM-5 in reducing bromate formation increased with ozone dose (0.38-1.16mg O₃ mg^-1 DOC) and pH (6.6-9.3). The HZSM-5 adsorbed OBr^- (one of the critical intermediates in bromate formation) quickly with an adsorption capacity of 54mgg^-1, but had no adsorption for ozone, Br^-, HOBr and BrO3-. It also significantly inhibited the formation of trace H₂O₂ which was generated from ozone decomposition and had been considered promoting bromate formation at low concentrations during ozonation. The reduction of bromate formation in O₃/HZSM-5 is possibly ascribed to the selective OBr^- adsorption in combination with the inhibited H₂O₂ formation.