Effects of ammonia concentrations on bromate formation in seawater during ozonation and oxidation effects of ozone on ammonia, nitrite and nitrate

Abstract

During ozonation in seawater ammonia will react very quickly with free bromine (HOBr/BrO-) to produce bromamines (NH2Br/NHBr2). Bromamines are then oxidized to bromide and nitrate. The presence of ammonia in this cycle prevents bromate formation until all ammonia react with bromamines. However, the relationship between ammonia concentrations and bromate formation in sea water during ozonation is not cleared. Therefore this study was conducted to find out the effects of different ammonia concentrations on bromate formation. It has been known that nitrite and nitrate will come out whenever ammonia oxidized. Therefore, nitrite and nitrate with certain concentrations were tested to observed the effect of ozonation on nitrite and nitrate oxidation in seawater. During ozonation, ozone effects on DO and pH were also monitored. Ammonia was significantly (p < 0.05) suppressed bromate formation in seawater during ozonation. The linear relationship was found between ammonia concentrations and the suppression time of bromate formation, bromate was not formed for 6 minutes when 0.5 mg/L ammonia was treated. When 1, 2, 3, 4 and 5 mg/L of ammonia were treated bromate formation was suppressed for 15, 30, 45, 60 and 75 minutes, respectively. Therefore, ammonia concentrations and the time for suppressed bromate in seawater during ozonation was expressed as the equation of y = 15.205x ？ 0.7808 (r2=0.99). No bromate was formed when 1, 2, 3, 4 and 5 mg/L of ammonia were added continuously with the intervals of 15, 30, 45, 60 and 75 minutes, respectively. Nitrite and nitrate were significantly (p < 0.05) oxidized by 0.1 mg/L of ozonation. Average reduction rate of nitrite was y = -0.0008x + 0.1002 and that in nitrate was y = -0.0068x + 1.9846. During ozonation DO was increased from 7.87 to 14.60 mg/L within 24 hours and pH level was decreased from 8.20 to 7.81 within 6 hours.