Ozonation of water: kinetics of oxidation of ammonia by ozone and hydroxyl radicals

Ozone initiated oxidation of ammonia in water can result from the direct reaction of O3 with NH3 as well as from the reactions of OH· radicals formed upon O3 decomposition. The direct reaction of O3 predominates at pH <9. In the presence of low concentrations of free NH3, it is a slow reaction; the rate constant for O3 depletion by reaction of O3 with NH3 is 20 ± 1 M-1 s-1. At pH values >9, OH· and radical-catalyzed decomposition of O3 to the very reactive intermediate, OH·, determines the kinetics of the NH3 oxidation. The reaction rate constant of OH· with NH3 is relatively small. Whenever the OH· mechanism is involved, NH3 is easily protected by other solutes which also consume OH· radicals. Even carbonate and bicarbonate ions may act as efficient OH· scavengers. Because carbonate and organic solutes affect the lifetime of O3 in water, they consequently shift the pH range in which either the direct O3 or the OH· mechanism predominates. A comparison with reactions initiated by gamma irradiation proves helpful in elucidating the reaction mechanisms.