The Chemistry of Ozone in Water
From lists of published rate constants for the direct reactions of aqueous ozone with substrates we can conclude: if, for example, 0.5 mg/l ozone is present, the following types of compounds react within less than 10 seconds: sulfite, nitrite, olefinic aliphatic hydrocarbons, phenols, polyaromatic hydrocarbons, organic amines and sulfides. Much slower to react are some chlorosubstituted olefins (chlorinated solvents). Benzene, saturated hydrocarbons, or tetrachloroethylene require days to be significantly oxidized by molecular ozone.
In bromide-containing waters (seawater, brackish waters, pool waters), the bromide is the primary reactant with ozone. It is oxidized to hypobromite or even bromate, and can destroy part of the ozone in a catalytic chain process. The hypobromite can produce bromoform.
Some of the aqueous ozone always decomposes. This decomposition is often initiated by increased pH (OH--ions) and accelerated by many types of organic compounds which may act as promoters of a radical type chain reaction. This chain reaction is, however, somewhat inhibited by carbonate. Therefore, the half-life of ozone varies according to the type of water treated. For example, in typical drinking water it can be anything from 1 to 20 minutes.
Ozone decomposition leads to the formation of the OH- radical. This is so reactive that, in drinking water, most of it is consumed by humics and other dissolved organic materials. The OH- radical can even be reduced by carbonate and bicarbonate ions. But a fraction of this oxidant is still available to oxidize a specific micropollutant, even when this is present in relatively low concentrations. Based on known kinetic constants, all these effects are easily predictable.
UV-irradiation of aqueous ozone produces primarily hydrogen peroxide which, under certain circumstances, may initiate decomposition of additional ozone to OH- radicals.
The chemical effects of ozonation processes must also be considered with respect to combined processes, where ozonation is, e.g. followed by treatment with chlorine or microbiological treatment steps, or even by membrane filtration in the case of industrial waste water treatment.
KeywordsDirect Reaction Secondary Oxidant Chlorine Dioxide Ozone Decomposition Ozonation Process
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