Mechanisms of Aqueous Ozone

The lifetime of ozone in aqueous systems depends principally on the pH. Ozone decomposition is catalyzed by HO in a bimolecular process whose rate is dependent on both [O3] and [HO ]. However, the kinetics are complicated by the existence of a radical chain process involving HO and other intermediate odd-electron species. Forni et al. (1982) indicated that the initiation step for the decomposition reaction appeared to be an oxygen atom transfer to form molecular oxygen and the anion of H2O2  

However, other evidence (Biihler et al., 1984) suggested that the products of the above reaction were HOO and O2 . In any event, subsequent reactions of either HOO or HOO would result in the formation of HO-  

This radical takes part in further chain decomposition reactions leading to ozone loss  

In pure water, therefore, the operation of the above mechanisms would lead smoothly to conversion of ozone to molecular oxygen. However, actual waters almost always contain substrates which can compete efficiently for either ozone or the intermediate free radicals such as OH that are formed during its decomposition. In particular, in seawater molecular ozone reacts most rapidly with dissolved T, despite its low concentration of about 10 A/(Haag and Hoigne, 1983) in moderately alkaline freshwater with a typical degree of hardness (roughly millimolar in carbonate alkalinity), it is usually the carbonate or (with lower effectiveness) bicarbonate anion that reacts with the ozone-derived HO  

This process is described more fully below. Moreover, OH also rapidly attacks dissolved organic compounds (see also Section 4.B.2), with rate constants for most 

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TM Lesko, AJ Colussi, and MR Hoffman. Hydrogen isotope effects and mechanism of aqueous ozone and peroxone decompositions. J. Am. Chem. Soc. 126 2004. 

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