|When ozone is produced it will decay rapidly, because ozone is an instable compound with a relatively short half-life. The half-life of ozone in water is a lot shorter than in air (see table 1). Ozone decays in water under drinking water conditions (pH: 6-8,5), partly in reactive OH-radicals. Therefor, the assessment of an ozone process always involves the reactions of two species: ozone and OH-radicals. When these OH-radicals are in the dominant particles in the solution, it is called an advanced oxidation process (AOP). The decay of ozone in OH-radicals in natural waters is characterized by a fast initial decrease of ozone, followed by a second phase in which ozone decreases by first order kinetics . Dependent on the quality of the water, the half-life of ozone is in the range of seconds to hours. Factors influencing the decomposition of ozone in water are temperature, pH, environment and concentrations of dissolved matter and UV light. Here, the main influence factors for ozone decomposition will be discussed. |
Table 1: half-life of ozone in gas and water at different temperatures
1 O3 + OH- → HO2- + O2
2 O3 + HO2- → •OH + O2 •- + O2
The radicals that are produced during reaction 2 can introduce other reactions with ozone, causing more OH-radicals to be formed.
Figure 1: effect of the pH on the decay of ozone (T = 15 °C)
3. Dissolved solids concentration
Figure 2: Ozone decomposition in different types of water at 20 °C. 1 = double-distilled water; 2 = distilled water; 3 = tap water; 4 = groundwater of low hardness; 5 = filtered water from Lake Zurich (Switzerland); 6 = filtered water from the Bodensee (Switzerland)
Ozone decomposes in water in OH-radicals. Dependent on the nature of the dissolved matter, these can accelerate (chain-reaction) or slow down the decay of ozone. Substances that accelerate this reaction are called promoters. Inhibitors are substances that slow down the reaction.
Figure 3: equilibrium carbonate, bicarbonate and carbon dioxide
5. Natural Organic Material