Page 198 - Industrial Wastewater Treatment, Recycling and Reuse

172 Industrial Wastewater Treatment, Recycling, and Reuse

implementation of a photocatalyst on an industrial scale. Cavitation through
US or HC is one such technique that can be used simultaneously with UV/
solar irradiation. Moreover, the photocatalytic oxidation technique is also
affected by severe mass transfer limitations, especially in the immobilized
catalyst type of reactors, which are generally preferred over slurry reactors
to avoid solid catalyst separation problems. One further factor suggesting
that the two techniques will give better results when operated in combina-
tion is the fact that for both the techniques (UV and cavitation), the basic
reaction mechanism is the generation of free radicals and subsequent attack
by these on the pollutant species. If the two modes of irradiations are
operated in conjunction, more free radicals will be available for the reaction,
thereby increasing the rates of degradation. Thus, the expected synergism
between these two modes of irradiation can be possibly attributed to:
1. Cavitational effects leading to an increase in the temperatures and
pressure at the localized cavity implosion sites.
2. Cleaning and sweeping of the photocatalyst surface due to micro turbu-
lence and high velocity liquid jets created as a result of cavity oscillation
and collapse.
3. Mass transport of the reactants and products is increased at the catalyst
surface and in the solution, due to the facilitated transport by shockwave
propagation.
4. Surface area is increased by the fragmentation or pitting of the catalyst
and hence more actives sites will be available for the adsorption of the
pollutant molecule at the photocatalyst surface.
5. More OH radicals are generated through the dissociation of water mol-
ecules under cavitational conditions, in addition to those generated by
photocatalytic process, thereby increasing the concentration of OH
radicals in the solution for the oxidation of pollutant molecules.
6. The organic substrate reacts directly with the photo-generated surface
holes and electrons under cavitating conditions.
There have been many studies depicting the observed synergism and the
enhanced rates of degradation for the combinatorial operation of sonochem-
ical reactors and photocatalytic oxidation (Adewuyi, 2005; Cheng et al.,
2012; Madhavan et al., 2010; Ragaini et al., 2001; Saien et al., 2010). For bet-
ter efficiency or higher synergistic effect, these processes should operate
simultaneously rather than having sequential irradiation of ultrasound
followed by photocatalytic oxidation. The different reactor configurations
used so far for evaluating the synergism between ultrasound and ultraviolet
irradiation are depicted in Figure 3.8 (batch reactors) and Figure 3.9