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.

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