27
Electrons react with dissolved oxygen producing superoxide radical O
2
• or its protonated form, perhydroxyl radical HO
2
•:
O
2
+ e
cb -
→ O
2
• 2.25
O
2
• + 2H
+
→ 2HO
2
• 2.26
In water, two HO
2
• can recombine if their concentration allow them to react significantly yielding hydrogen peroxide H
2
O
2
and oxygen O
2
. It follows photocatalytic reduction of hydrogen peroxide by scavenging an electron band where
hydroxyl radical generated:
H
2
O
2
+ e
cb -
→ HO• + HO
-
2.27
The efficiency of TiO
2
photocatalysis could be improved by addition of hydrogen peroxide, but the optimal dosage of addition should be taken an account.
When it is excess, it decrease process effectiveness.
Photocatalyst such semiconductor are comprised of microcrystalline or microcrystalline particles and they are used in a form of thin layer or as powder
dispersion. Like TiO
2
, alternative photocatalysts used are ZnO, CdS and SnO
2
.
2.3.3 Ozone-based Processes
Ozone is an inorganic molecule constituted by three atoms of oxygen. It is present in the atmospheric layer around the earth, and it is formed by the photolysis of
diatomic oxygen and further recombination of atomic and diatomic oxygen:
O
2
+ h υ → 2O•
2.28
O • + O
2
→ O
3
2.29
28
Under the term of ozone –based processes, ozone is the main component in
many oxidation processes. In this process, ozone is applied either alone or with combination of an oxidant such as hydrogen peroxide H
2
O
2
O
3
processes, UV radiation UV O
3
processes, catalyst, activated carbon, ultrasound, etc.
Ozone can be generated artificially in an ozone generator. There are two ways of generating ozone:
a. the cleavage of oxygen molecules under the influence of a strong electrical
field b.
the photolysis of oxygen by the same mechanism as in the nature, but induced artificially.
The first use of ozone is as a disinfectant in many water treatment processes and in hospitals. Ozone application as oxidant for water purification was retained
through 20
th
century, and its significant increase was noticed in 1970s when the production of trihalomethanes and other organohalogenated carbon were identified
during the water treatment by chlorine. Reactivity of ozone is very high with a redox potential 2.07 V, either in liquid or in gas. Its high reactivity owes to electronic
configuration; ozone can be presented as a hybrid in four different molecular resonance structures that give ozone characteristics of an electrophilic, dipolar or
even nucleophilic agent. Furthermore, ozone molecule could react with an organic compound under two mechanisms: direct or indirect. Direct mechanism involves
organic compound degradation by molecular ozone and occurs in acidic pH range. While the main reaction involving indirect mechanism are reactions of addition to
unsaturated part of hydrocarbon molecule and electron transfer. Rather high oxidationreduction potential enables ozone to react with many organics, and also
inorganic compounds.
O
3
+ 2H
+
+ 2e
-
→ O
2
+ H
2
O 2.30
29
Reaction of ozone and hydroxyl ions present in water by indirect ozone mechanism at basic condition generate hydroxyl radical, which further reacts with an
organic compound present in water.
O
3
+ H
2
O + OH
-
→ HO• + O
2
+ HO
2
• 2.31
Mechanism of ozone decomposition in water depends on the presence of chemical species that can initiate, promote or inhibit its decomposition. The most
accepted ozone decomposition mechanism is expressed in Figure 2.2.
According to the presented mechanism of ozone decomposition in water, it can be shown that ozonation can classified in AOPs when the degradation of organics
occur by indirect mechanism. While classified as classical chemical treatment method when direct mechanism method is dominant in the degradation of organics in water
Koprivanac and Kusic, 2007; Lenntech, 2008; Russel, 2006.
Among the new alternative O
3
processes, either direct or indirect, the one that has been widely investigated is O
3
H
2
O
2
. This process is named peroxone process or perozonation. In this process, hydroxyl radical is generated by reaction of ozone and
perhydroxyl ion HO
2 -
, which present in water by dissociation of H
2
O
2
. Overall mechanism of reaction expressed below:
H
2
O
2
+ O
3
→ 2 HO• + 3O
2
2.32
2.3.4 High Voltage Electrical Discharge Processes
