Environmental Engineering Reference
In-Depth Information
Depending on the pH, the oxidation may follow different pathways,
which are direct (molecular ozone attack) and indirect (free radical mech-
anism) radical oxidation, resulting in different oxidation products, degrees
of mineralization and color removal efficiencies [28].
Direct ozonation involves degradation of organics by ozone molecule
under acidic conditions [29]. In this method, an ozone molecule attacks
an unsaturated bond and its dipolar structure breaks up the bond [30].
However, in direct oxidation only some parts of the organic compound can
be degraded, byproducts are produced and generally additional treatment
may be required to remove them.
Indirect ozonation considers the degradation mechanism of organics
throughout hydroxyl radicals under basic conditions [29]. The hydroxyl
radicals which are generated in radical chain reactions react with organic
compound according to the following equations:
O
3
+ OH
−
O
3
+ OH
(3.1)
O
3
−
O
−
+ O
2
(3.2)
O
−
+ H +
OH
(3.3)
The hydroxyl radical decomposes most organic compounds into CO
2
and H
2
O [29,30].
The ozonation of reactive dyes is known to produce mainly two
types of byproducts. These are carboxylic acids with hydroxyl func-
tional groups and aldehydes. Lopez
et al.
described the main ozonation
byproducts of azo dyes, as formaldehyde, acetaldehyde, glyoxal, acetone,
acetic, formic, oxalic and carbonic acids. Since these degradation prod-
ucts of azo dyes are known to have extremely complex characteristics,
their identification requires efficient analytical methods that combine
high separation efficiency and rich structural information. The tech-
niques such as MS, HPLC, GC, GC/MS, HPLC/MS are particularly rec-
ommended for analysis [31,32].
In the case where ozone cannot completely oxidize organic com-
pounds to CO
2
and H
2
O due to the formation of partial oxidation prod-
ucts relatively unreactive towards ozone, the combination of ozone
with homogeneous or heterogeneous catalysts, with or without metallic
phases, is recommended for the enhancement of degradation of organic
compounds [33].
Up till now, numerous studies have demonstrated that dyes can be effec-
tively degraded by ozone and some published studies are summarized in
Table 3.3.
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