Ozone decomposition
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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 [15]. 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.
Influence
factors
1. Temperature Temperature has an important influence on the half-life of ozone. Table 1 shows the half-life of ozone in air and water. In water the half-life of ozone is much shorter than in air, in other words ozone decomposes faster in water [1]. The solubility of ozone decreases at higher temperatures and is less stable. On the other hand, the reaction speed increases with a factor 2 or 3 per 10 °C [5,6]. Principally, ozone dissolved in water cannot be applied when temperatures are above 40 °C, because at this temperature the half-life of ozone is very short.
Table
1: half-life of ozone in gas and water at different temperatures
2.
pH
As mentioned above, ozone decomposes partly in OH-radicals. When the pH value increases, the formation of OH-radicals increases. In a solution with a high pH value, there are more hydroxide ions present, see formulas below. These hydroxide ions act as an initiator for the decay of ozone:
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.
In addition the pH influences acid/base equilibriums of some compounds and also the reaction speed of ozone. This applies also to the reaction with scavenger CO32-, which is also pH dependant (Pka HCO32-/CO32- = 10,3). Figure 1 shows that the decay of ozone in a basic environment is much faster than in an acid environment. 
Figure
1: effect of the pH on the decay of ozone (T = 15 °C)
3.
Dissolved solids concentration
Dissolved ozone can react with a variety of matter, such as organic compounds, viruses, bacteria, etc. As a result, ozone decomposes to other matter; see figure 2. This figure illustrates that the half-life of ozone in distilled water is much shorter, compared to tap-water. 
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.
When water is ozonized, one often uses the term 'scavenging capacity'. Scavengers are entities that react with OH-radicals and slow down the chain-reaction. The scavenging capacity can be defined as follows [16]: kOH-DOC[DOC] + kOH-HCO3-[HCO3-] + kOH-CO32-[CO32-] 4. Carbonate and bicarbonate Scavengers slow down the chain-reaction. This is because after the reaction of scavengers with OH-radicals, the reaction products do not react with ozone any further. Carbonate is a scavenger with a strong effect. The addition of carbonate (CO32-) can increase the half-life of ozone [5,6]. The effect on the reaction speed is highest at low concentrations. Above 2 mmol-1 for ozonisation and 3 mmol l-1 for advanced oxidation process (AOP), the decrease in the reaction speed is negligible [6]. When a solution mainly undergoes indirect reactions (with OH-radicals), for instance in a solution with a high pH value or an AOP-process, the presence of scavengers is undesired. The scavengers react very fast with OH-radicals and lower the oxidation capacity. For this kind of processes a low scavenging capacity is required. Carbonate (CO32-) ions are a much stronger scavengers than bicarbonate (HCO32-) ions (reaction speed CO32-: k = 4,2 * 108 M-1s-1 and reaction speed HCO3-: k = 1.5 * 107 M-1s-1). That is why in an ozone process under drinking water conditions, the bicarbonate concentration is less important [6]. Figure 3 illustrates the relation of the carbonate ratio, bicarbonate ratio and pH. 
Figure
3: equilibrium carbonate, bicarbonate and carbon
dioxide
5.
Natural Organic Material
Natural organic material (NOM) exists in every kind of natural water and is often measured as dissolved organic carbon (DOC). NOM reduces the quality of the water with regard to color and odor. Ozone can be used in water treatment, for the reduction of the concentration of NOM. The concentration of NOM in natural waters can vary from 0,2 – 10 mg l-1 [6]. The influence of NOM on ozone is twofold. Dependent on the type of NOM, it can be oxidized directly by NOM. This is the case for compounds which easily react with ozone, such as double bonds, activated aromatic compounds, deprotonated amines and sulphide [15]. On the other hand, OH-radicals can react with NOM (indirect reaction) and act as a promoter or as a scavenger. In natural waters, it is difficult to determine the stability of ozone as a result of the indefinite effect of NOM. That means it is not possible to estimate the fraction that accelerates or slows down the reaction [15]. |
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