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Ozone reaction mechanisms

An ozone process is always based on the effect of direct and indirect reaction mechanisms. This is consequential to the disintegration of ozone in water, into OH-radicals. These radicals are very short-living compounds that have an even stronger oxidation mechanism than that of ozone. This is because the radicals have a high oxidation potential [10,11], see table 1.

Table 1: redox potential of oxidizing agents *

Substance

Potential (V)

Fluorine (F)

2,87

Hydroxyradical (OH)

2,86

Oxygen atom (O)

2,42

Ozone molecule (O3)

2,07

Hydrogen peroxide (H2O2)

1,78

Chlorine (Cl)

1,36

Chlorine dioxide (ClO2)

1,27

Oxygen molecule (O2)

1,23

When the number of OH-radicals in a solution rises, one speaks of an Advanced Oxidation Process (AOP). This unique process causes dissolved solids to be oxidized by both ozone (direct) and OH-radicals (indirect). The ozone oxidation process is represented scematically in figure 1.

This figure shows that ozone oxidation consists of:

- Direct reactions of ozone
- Indirect reaction of secondary oxidators, such as free OH-radicals

In practise, both direct and indirect oxidation reactions will take place. One kind of reaction will dominate, depending on various factors, such as temperature, pH and chemical composition of the water. To determine the role OH-radicals play in ozone oxidation, the Rc-value is used. This value represents the rate of ozone versus the rate of OH-radicals.

Rc = [OH]/[O3]

When ozone is applied in water, this value varies between 10-6 and 10-9. During the ozone process, this value will be about 10-8 in the second phase [15].

Figure 1: reactions of ozone and dissolved solids *

Direct reactions

On the ozone introduction page, the structure of ozone was represented. Based on this structure it is known, that ozone can act as a 1,3-dipole, an electrophilic agent and a nucleophilic agent during reactions. These three types of reactions usually occur in solutions that contain organic pollutants. Here, we discuss these three types of reaction mechanisms.

Cyclo addition (Criegee mechanism)

Consequentially to is dipolar structure, an ozone molecule can undergo a 1-3 dipolar cyclo addition with saturized compounds (double or tripple bonds). This leads to the formation of a compound called ‘ozonide’ (I), see figure 2.

Figure 2: dipolar cyclo addition *

In a protonic solution, such as water, primary ozonide disintegrates into an aldehyde, a keton or a zwitter ion, see figure 3. The zwitter ion will eventually be disintegrated further into hydrogen peroxide and carboxyl compounds.

Figure 3: disintegration of ozonide *

Electrophilic reactions

Electrophilic reactions occur in molecular solutions that have a high electronic density and mainly in solutions that contain a high level of aromatic compounds. Aromatic compounds that are substituted by electron donors (such as OH and NH2), have a high electronic density on the carbon compounds in ortho and para position. Consequentially, in these positions aromatic compounds react actively with ozone.
Below, we can see an example of a reaction between ozone and Phenol. Phenol groups react with ozone relatively quickly.

Figure 3: reaction between phenol and ozone *

Nucleophilic reactions

Nucleophilic reactions mainly take place where there is a shortage of electrons and particularly at carbon compounds that contain electron-retreating groups, such as –COOH and -NO2. For electron-retreating groups, the reaction speed is much lower [4].

From the above-mentioned data, it appears that direct oxidation of organic matter by ozone is a quite selective reaction mechanism, during which ozone reacts quickly with organic matter that contains double bonds, activated aromatic groups or amines [6,15].
It is also stated that ozone reacts quicker with ionized and dissociated organic compounds than with the neutral (non-dissociated) type.

For most inorganic compounds in drinking water, the reaction speed is relatively high [15]. The main reaction mechanism for oxidation of inorganic compounds is determined by transfer of the extra oxygen atom of ozone to the inorganic compounds. For inorganic compounds, reaction speed is also higher for ionized and dissociated compounds.
Summarized, ozone oxidizes organic compounds selectively and partly. A large number of inorganic compounds are oxidized fast and completely.

Indirect reactions

Contrary to those of ozone, OH-radical reactions are largely a-selective. Indirect reactions in an ozone oxidation process can be very complex. Matter of factly, an indirect reaction takes place according to the following steps:

1. Initiation
2. Radical chain-reaction
3. Termination

1. Initiation

The first reaction that takes place is accelerated ozone decomposition by a type of initiator. This can be an OH-molecule, see reaction 1:

1: O3 + OH- -> O2• - + HO2

This radical has an acid/ base equilibrium of pKa = 4,8. Above this value, this radical no longer splits, because it forms a superoxide radical, see reaction 2:

2: HO2• -> O2- + H+ (pKa = 4,8)

Radical chain-reaction

Now, a radical chain-reaction takes place, during which OH-radicals are formed. The reaction mechanism is as follows:

3: O3 + O2- -> O3- + O2

4: O3- + H+ -> HO3• (PH < ≈ 8)

The OH-radicals that have formed react with ozone according to the following reaction mechanism:

5: OH• + O3 -> HO4

6: HO4• -> O2 + HO2

During the last reaction, HO2• radicals are formed, which can start the reaction all over again (see reaction 2). As a result, a chain-reaction develops, which is maintained by so-called promotors.
Promotors are substances that transform OH-radicals to superoxide radicals. Various substances can become promotors, including organic molecules (see table 2).

Table 2: examples of initiators, promotors and radical catchers *

Initiator

Promotor

Radical catcher (inhibitor)

OH-

Humic acid

HCO3-/CO32-

H2O2

Aryl-R

PO34-

Fe2+

Primary and secondary alcohols

Humic acids

Aryl-R

Tert-butyl alcohol (TBA)

Advanced oxidation processes

The Advanced Oxidation Process (AOP) is a type of chemical oxidation, which has evoked interest during the last couple of years. The benefit of this process is that there is no formation of concentrate or remnant sludge.
Harmful substances are decomposed to less harmful substances, or even completely mineralized in media such as water, carbon dioxide and nitrogen.

During the AOP, oxidation is largely brought about by OH-radicals. These radicals are very reactive compounds or atoms that have a very short half-life (10 μS at a 10-4 M concentration) [6]. This causes an OH-radical to react non-selective and directly with dissolved solids. These radicals can be initiated into the water by means of a certain type of substance (activator). For an AOP, one often uses ozone combined with hydrogen peroxide (H2O2), or ozone and UV-light, or hydrgen peroxide and UV-light. In the reaction mechanism shown below, ozone is combined with hydrogen peroxide.

Hydrogen peroxide splits in water according to the following reaction:

H2O2 -> HO2- + H+

The HO2- ion reacts with ozone, causing radical production. Eventually, two ozone molecules are produced for each two OH-radicals:

2 O3 + H2O2 -> 2 OH• + 3 O2

The OH• compounds are radicals that contain a very high electronic potential, which makes it one of the strongest oxidizers ever known by men. The activation of OH-radicals is a very complex process, which can take place according to a variety of different reaction mechanisms.

Direct oxidation or indirect oxidation (AOP)?

AOP can be a solution to the problem that ozone does not oxidize all compounds rapidly. An ozone-based AOP can be applied relatively simply in conventional ozone processes. This can be brought about by increasing the pH value and by hydrogen peroxide addition. Hydrogen peroxide addition is the most economic method [15].
Ozone-resistant compounds are for example pesticides, aromatic compounds and chlorinated solvents. Lenntech beholds tables that contains a comparison of reaction speed for a number of substances with ozone and OH-radicals. Oxidation reactions with ozone and OH-radicals can be considered second-order reactions [6,15]:

d[S] / dt = k [S] [O3]

k = reaction speed
S = dissolved solids concentration (solvent)
O3 = ozone concentration

The reaction speed of OH-radicals is much higher than that of ozone. Globally, ozone has a reaction speed of between 1 and 103 L mol-1 s-1. For OH-radicals, the reaction speed lies between 108 en 1010 L mol-1 s-1 [6]. However, this does not mean that OH-radical oxidation takes place more rapidly that ozone oxidation. OH-radicals are consumed by radical catchers in water much faster that ozone. This is illustrated in figure 5.

Figure 5: oxidation of pCBA by ozonization and AOP for groundwater (GW) and surface water (SW) (parameters GW: DOC: 1 mg/L, alkalinity 5,2 mM; SW: DOC 3,2 mg/L, alkalinity 3,8 mM. Experiment conditions: pH = 7, T = 11 oC, [O3]0 = 2.1 * 10-5 M, [H2O2]=1*10-5 M, [pCBA] = 0,25 μM) *

This shows various oxidation processes of an ozone-resistant compound (para-chlorobenzenic acid, pCBA): oxidation by means of a conventional ozone process and AOP in groundwater (GW) and surface water (SW). The oxidation rate of pCBA in groundwater is higher for AOP than for conventional oxidation, for surface water this is nearly equal. This is caused by a large capacity of radical catchers (scavenging capacity, see carbonate and bicarbonate) in the surface water, which react with OH-radicals. In these situations, AOP is an inefficient process.
For organic substances, ozone oxidation is efficient for aromatic compounds, and for substances that contain amino groups, or double bonds. Sulphide groups are also quickly oxidized by ozone. Electron-retreating groups (e.g. –Cl, -NO2, -COOH) cause a decrease in reaction speed, whereas electron-donating groups (e.g. -NH3, -OH, -O, OCH3) cause an increase in reaction speed. Most protein (amino) groups react with ozone very slowly.
For a number of relevant inorganic compounds in drinking water, such as Fe(II), Mn(II), H2S, NO2-, the reaction with ozone is fast and efficient.
Table 5 shows the reaction speed of ozone and OH-radicals with inorganic pollutants that are often found in drinking water [15]. The reaction speed of ozone usually strongly depends on the pH value. This is because the dissociation rate of many inorganic acids is dependent on the pH value, see reaction:

HB + H2O -> H3O+ + B-

The left-hand side of this reaction is non-dissociated and the right-hand side is dissociated. Acids, such as HOCl, HOBr, HCN, HNO2, H2SO3, but also NH3 and H2O2 are only reactive in dissociated form [21].

Table 3 shows a column with the half-life of ozone. When the half-life of ozone is short (t1/2 < 5 min), ozonization is efficient. With this half-life, substances are decomposed mainly by ozone. OH-radicals play a bigger part in slower processes [15].

Table 3: kinetics of inorganic substances with ozone and OH-radicals *

Compound

kO3 (M-1 s-1)

t1/2a

Ref.

kOH (M-1 s-1)

Ref.

Nitrite (NO2-)

3,7*105

0,1 s

21

6*109

51

Ammonia (NH3/NH4+)

20/0

96 h

21

9,7*107b

52

Cyanide (CN-)

103-105

~ 1 s

21

8*109

53

Arsenite (H2AsO3-)

> 7

82 min

47

8,5*109c

54

Bromide (Br-)

160

215 s

48

1,1*109

55

Sulphide

21

56

H2S

≈ 3*104

~ 1 s

1,5*1010

S2-

3*109

20 μs

9*109

Manganese (Mn(II))

1,5*103

~ 23 s

49

2,6*107

57

Iron (Fe(II))

8,2*105

0,07 s

50

3,5*108

58

The text above shows, that ozone is very selective when it comes to particle oxidation, whereas OH-radicals react with practically any compound. Selectivity is caused by the chemical structure of ozone (dipole structure).

In general, indirect oxidation can be applied on a large number of organic pollutants, whereas oxidation can be applied on a large number of inorganic (drinking water) pollutants [15]. This is clearly illustrated by figure 6. Compounds that are mainly oxidized by ozone (>50%) are below, these are the inorganic compounds. During ozone processes, the Rc-value lies between 10-9 and 10-7, for AOP processes this is Rc < 10-7.

Figure 6: fraction of compounds that react with OH-radicals for drinking water ratois Rc (Oh-ozone). Per: tetra chlorethene; MTBE: methyl tert-butyl ether; GEO: geosmin; ATRA: atrazine; tri: trichlorethene. *

Complete mineralization of compounds is too expensive in most cases and is therefor not achievable by both ozone and AOP. These processes are therefor better applicable for compounds that can be freed of unwanted effects by partial oxidation. This can be color oxidation, taste and scent oxidation, or pre-biodegradation of various organic compounds.

* Sources:
U. von Gunten (2003) Ozonation of drinking water. Part I. Oxidation kinetics and product formation, Water Res., in press.
U. von Gunten (2003) Ozonation of drinking water. Part II. Disinfection and by-product formation in presence of bromide, iodide and chlorine. Water Res., in press.
W. Gujer, U. von Gunten (2003) A stochastic model of an ozonation reactor. Water Res., in press.







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