Passage V

To treat water for drinking, ozone （O₃) gas can be bubbled through the water to kill bacteria and viruses. This process is called ozonation. When bromide ions, Br^-, are present in the water, O₃ reacts with the Br^- to produce bromate ions, BrO₃^-. In drinking water, a BrO₃^- concentration at or above 10 micrograms per liter (μg/L) is considered unsafe.

Three studies examined how initial Br^-concentration, contact time (the amount of time O₃ has been bubbled through the water) , pH, and the addition of ammonia affected BrO₃^- formation during ozonation of 25°C water from a particular source. Each water sample was buffered to maintain a constant pH during ozonation.

Study 1

Three 1 L water samples were prepared, all having the same pH but each having a different initial Br^- concentration: 50 μg/L, 200 μg/L, or 500 μg/L. Each sample was then placed in a separate 2 L container and ozonated at a rate of 10 L O₃/hr for 30 min. At various times during ozonation, a small volume of water was removed from each container and analyzed for BrO₃^- (see Figure 1).

Study 2

Three 1 L water samples were prepared, all having an initial Br^- concentration of 200 μg/L but each having a different pH: 8.4, 7.0, or 6.4. Each sample was then ozonated and analyzed as in Study 1 (see Figure 2).

Study 3

Two 1 L water samples were prepared, both having an initial Br^- concentration of 200 μg/L and a pH of 8.4. Ten mL of ammonia solution having a concentration of 0.2 mg/L was added to one sample. No ammonia was added to the other sample. Each sample was then ozonated and analyzed as in Study 1 (see Figure 3).