Chain reactions ozone decomposition

In the schemes considered to this point, even the complex ones, the products form by a limited succession of steps. In these ordinary reaction sequences the overall process is completed when the products appear from the given quantity of reactants in accord with the stoichiometry of the net reaction. The only exception encountered to this point has been the ozone decomposition reaction presented in Chapter 5, which is a chain reaction. In this chapter we shall consider the special characteristics of elementary reactions that occur in a chain sequence. 

Chain reaction. A possible mechanism for the chlorine-catalyzed decomposition of ozone is... 

Due to high activity in reactions with free radicals, ozone undergoes the chain decomposition in solutions also. The chain reaction of ozone decomposition was evidenced in 1973 in the kinetic study of cyclohexane and butanone-2 oxidation by a mixture of 02 and 03 [146-151], It was observed that the rate of ozone consumption obeys the equation [112] ... 

The second term characterizes the chain decomposition of ozone. The mechanism of chain reaction was found to include the following reactions ... 

So, three different chain reactions of ozone decomposition were observed in solutions ... 

The catalytic decomposition of ozone by chlorine is believed to occur by the following chain reactions,... 

Chain reactions are a t3q)e of overall reactions, which require two or more steps to accomplish. They are also known as consecutive reactions or sequential reactions. Examples of chain reactions include nuclear hydrogen burning, nuclear decay chains, ozone production, and ozone decomposition. Some steps of a chain reaction may be rapid and some may be slow. The slowest step is the ratedetermining step. During a chain reaction, some intermediate and unstable species may be produced and consumed continuously. 

Staehelin J, Hoigne J (1985) Decomposition of Ozone in Water in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions, Environmental Science Technology 19 1206-1213. 

These reactions can account for the formation of water and the decomposition of ozone by a chain reaction. The relative importance of these three reactions would, of course, depend upon the relative concentration... 

Kusakabe et al. (1990) reported that the destruction rate coefficients increase as temperature increases. UV light intensity of 8.7 W/m2 yielded a slightly more than tenfold increase in the decomposition rate. The decomposition rate of ozone increases with UV intensity. These results imply that, under UV irradiation, radical chain reactions are predominant over molecular ozone reactions. When light intensity is greater than 3 W/m2, the degradation rate of TOC by UV/03 can be expressed as follows ... 

Staehelin, J. and Hoigne, J., Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions, Environ. Sci. Technol., 19, 1206-1213, 1985. 

Chlorofluorocarbons, such as CF3C1, catalyze this reaction and are responsible for the formation of the ozone hole. The decomposition is a chain reaction involving chlorine atoms as the chain-carrying species. Suggest a mechanism for this reaction. 

The role of oxygen derived short-lived radical species in AOPs is still a field of active research. However, extensive knowledge has been accumulated on the mechanism of ozone aqueous chemistry (Rice, 2001, Gottschalk, 2000, Hoigne, 1998). The decomposition of molecular ozone in water by a radical-type chain reaction involves species such as hydrogen trioxide (HO ) with a lifetime of about 10 ps and HO radicals. The latter equilibrate with their dissociation products in about 30 ps, i.e. O3 and OH (Hoigne, 1998) or they decompose into O2 and HO promoting the chain reaction. 

In connection with the decomposition of ozone with ultraviolet radiation, the energy chain (Reactions 9 and 12) explains the high quantum yield. A similar chain might be expected in the case of NOCl thus... 

On the one hand, part of the ozone (O3) dissolved in water reacts directly with the solutes M. Such direct reactions are highly selective and often rather slow (minutes). On the other hand, part of the ozone added decomposes before it reacts with solutes this leads to free radicals. Among these, the OH radicals belong to the most reactive oxidants known to occur in water. OH can easily oxidize all types of organic contaminants and many inorganic solutes (radical-type reactions). They are therefore consumed in fast reactions (microseconds) and exhibit little substrate selectivity. Only a few of their reactions are of specific interest in water treatment processes. Measured oxidations in model solutions indicate up to 0.5 mol OH formed per mole of ozone decomposed. The higher the pH, the faster the decomposition of ozone, which is catalyzed by hydroxide ions (OH ). The decomposition is additionally accelerated by an autocatalyzed sequence of reactions in which radicals formed from decomposed ozone act as chain carriers. Some types of solutes react with OH radicals and form secondary radicals (R ), which still act as chain carriers. Others, for instance, bicarbonate ions, transform primary radicals to inefficient species () and thereby act as inhibitors of the chain reaction. Therefore the rate of the decomposition of ozone depends on the pH of the water as well as on the solutes present. The overall effect is a superposition of the direct reaction and the radical-type reaction. For a review, see Hoigne (1988). 

The oxidation induced by ozone is often controlled by a preceding chain reaction that leads to the decomposition of ozone to a more reactive secondary oxidant, OH. This chain reaction, in which radicals act as chain carriers, is promoted by certain types of solutes but inhibited by others. Therefore, the overall oxidation rale often increases with the ratio of the concentration of the promoter relative to I hat of the inhibitor. However, a more generally useful treatment would involve I reating each reaction step separately and relating it to individual and known reaction steps of OH (Staehelin and Hoigne, 1985). 

The hydroxyl radical ( OH) generation from decomposition of ozone in alkaline pH mainly depends on the hydroperoxide free-radical initiating step (rate-determining step) in the chain reaction and the regeneration of the superoxide radical ion Oi ... 

This reaction was studied later also by Taube and Bray (44), who applied the chain reactions of the ozone decomposition but replaced reaction (01) by the reaction ... 

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 ... 

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