Page 193 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Oxidation Technologies for Wastewater Treatment: An Overview 167

if the photocatalytic oxidation technique is used in combination with
ultrasonic irradiation, not only the rate of generation of hydroxyl radicals
is increased (due to increased energy dissipation and generation of
extreme conditions of temperature and pressure due to the cavitation
phenomena), but, also, because of the acoustic streaming and turbulence
created by ultrasonic irradiation, mass transfer resistance will be elimi-
nated. Turbulence also helps in cleaning the catalyst, which increases
the efficiency of the photocatalytic oxidation process. Many other com-
bination techniques have been reported extensively in the literature for a
variety of contaminants.

3.5.1 Cavitation Coupled with H O 2
2
In the case of cavitation (US/HC), the main mechanism for the destruction
•
of organic pollutant is the reaction of OH radicals with the pollutant
molecules. Thermal pyrolysis of molecules present near or inside the collaps-
•
ing cavities is also possible. Hence, additional supplements of OH radicals
should enhance the rate of degradation of pollutants. H 2 O 2 is a commonly
available oxidizing agent that can be used for the treatment of wastewater
due to its high oxidation potential (1.78 V). The efficiency of H 2 O 2 in oxi-
•
dizing organic pollutants depends very much on the generation of OH rad-
icals through the dissociation of H 2 O 2 . It is a well-known phenomenon that
•
H 2 O 2 molecules can readily dissociate into OH radicals under the extreme
conditions of high temperature and pressure developed as a result of cavity
collapse (Abbasi and Asl, 2008; Pang et al., 2011). If used individually, the
efficiency of H 2 O 2 in oxidizing organic pollutants is low because of poor
•
dissociation of H 2 O 2 into OH radicals under the conventional stirred con-
ditions. But the efficiency of H 2 O 2 in degrading organic pollutant can be
enhanced significantly if used in combination with the cavitation process
•
because of the formation of significant quantum of OH radicals as a result
of dissociation of H 2 O 2 under cavitational conditions and its enhanced
microdiffusion into the bulk solution due to cavitation. The dissociation
energy for the O-O bond in H 2 O 2 is only 213 kJ/mol, which is significantly
less than that of the O-H bond in H 2 O, which is 418 kJ/mol (Pang et al.,
•
2011). Thus, more OH radicals will be generated if H 2 O 2 is used in com-
bination with a cavitational process because the energy required for the dis-
sociation of H 2 O 2 will be available through the cavitation. The following
reactions take place during the degradation of pollutant molecules using
the combined cavitation and H 2 O 2 process.