Catalytic .OH. The decomposition rate of O3 is

ozonation can be divided into two categories:       
Homogeneous catalytic ozonation

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metals ions in water can catalyze decomposition of O3, generating O2.-
which transfer an electron to another molecule of O3 to form O3.-
and subsequently leads to the formation of  .OH. The decomposition rate of O3
is dependent on the pH and reagent concentrations. The other way of
decomposition is the formation of complex between metal catalyst and organic
molecule which further reacts with O3 leading to oxidation of
organic molecule (Gracia et al., 1996; Pines and Reckhow, 2002).       
Heterogeneous catalytic ozonation

AOP is catalyzed by solid catalyst such as iron oxides, titanium oxide, alumina
and their combination. The mechanism and efficiency of ozonation are dependent
on several factors such as type of catalyst, pH of the water and its surface
properties. As this catalytic system is heterogeneous so while selecting
catalyst, its surface area, density, pore volume, porosity pore size must be
considered because these factors affect catalyst efficiency. Metal oxide,
supported metal oxide and supported metal catalyze ozonation by different
mechanisms. In the supported metal oxide, first there is adsorption of ozone
followed by decomposition to .OH radical (Beltrán et al., 2000). When ozonation is catalyzed by metal on support, it
involves the transfer of electron from metal to O3 which leads to O3.-formation
and subsequent release of .OH radical (Kasprzyk-Hordern et al., 2004).

Ozone + hydrogen
peroxide (O3/H2O2)

The combination of O3 and H2O2 is known
as peroxone. This process involves the generation of conjugate base of H2O2
i.e., HO2- which reacts with O3 resulting
in formation .OH radical (Staehelin and Hoigne, 1985).




Fenton system (H2O2/Fe2+)

Fenton process was first applied for oxidation of maleic acid. In this
method, ferrous ion and H2O2 are reacted in acidic medium
and .OH radical is formed. The generated .OH radical
reacts with organic pollutants to transform them to less or non toxic products.



Photochemical methods

In this oxidation, 
ultra-violet light is used along with H2O2, O3,
Fe+2; to generate highly reactive .OH radicals. This
method is employed when conventional O3 and H2O2 are
not able to completely oxidize organic pollutants to CO2 and H2O.
In some cases, reaction resulted in the formation of toxic products compared to
initial organic pollutant. UV energy can be helpful in destruction of organic
compounds which are not transforming in absence of UV light. O3 also
undergoes photolysis at 254 nm wavelength (Munter, 2001).

Ozone– Ultra-violet radiation(O3/UV)

has a molar extinction coefficient 3300 M–1 cm–1and strongly
absorbs UV light of wavelength 254 nm. Once O3 absorbs UV light in
water, it produces H2O2 which further decomposes into .OH
(Peyton and Glaze, 1988). UV lamps such as mercury lamp is use to generate
energy at this wavelength. Due to high molar extinction coefficient, this
results in quick absorption of UV light by O3 and subsequently more
production of .OH. It has been observed that if water contains high
UV radiation absorbing compounds such as phenol, xylenols, 5-methylresorcinol etc
then UV light is absorbed by these compounds (Munter et al., 1995; Trapido and Kallas, 2000). However, phenolic compounds are easily oxidized by O3 but
complete conversion to CO2 and H2O is not achieved.
Organic compounds such as oxalic acid, glyoxal, glyoxylic acid and formic acid
are easily oxidize to CO2 and H2O (Gurol and Vatistas, 1987; Takahashi, 1990).




1.2.2     Hydrogen
Peroxide-Ultra-violet radiation(H2O2/UV)

Peroxide linkage undergoes homolytic cleavage in presence of UV energy
to form .OH radical which propagates chain reaction and finally leads
to formation O2 and H2O. The efficiency of photolysis of
H2O2 is dependent on absorption of UV energy which
ultimately depends on the molar extinction coefficient (19.6 M-1cm-1)
which is very less compared to organic pollutants present in water. However
quantum yield of photolysis of H2O2 is high.



1.2.3     Ozone-Hydrogen Peroxide-Ultra-violet

decomposition rate of O3 is increased by addition of H2O2
along with UV light, which results in increased rate of generation of .OH.

1.2.4     Photo-Fenton/Fenton-like

use of Fe+2/Fe+3 and H2O2 with UV/visible
radiation is known as Photo-Fenton/Fenton-like system, employed for the removal
and mineralization of organic pollutant from the drinking waterwith improve
rate of removal than fenton system without radiation.. The formation of .OH
radical takes place by photoreduction of Fe+3 to Fe+2.
The Fe+2 again reacts with H2O2 (Ruppert et al., 1993).


In acidic medium (at
pH=3), Fe(OH)2+ complex is formed which under UV radiation generates
Fe+2 and .OH (Pignatello et al., 2006).


1.2.5     Photocatalytic
oxidation (UV/TiO2)

In Photocatalytic
oxidation, a metal oxide
semiconductor, titanium dioxide (TiO2) is used with UV.  TiO2 has been found the most
effective catalyst and can be developed either in slurry form or immobilized on
some support (Belgiorno et al., 2007). The TiO2 absorbs UV
radiation that leads to its excitation and produces conduction band electrons
and valence band holes. Holes are able to oxidize almost all chemicals due to their
extremely positive oxidation potential and reacts with absorbed species on TiO2
surface, an outline in below equations (Matthews, 1986). Figure 1 represents various AOPs which
can be employed for drinking water treatment.


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