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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 161

the presence of H 2 O 2 is harmful to many of the microorganisms and will
affect the overall degradation efficiency significantly where Fenton oxida-
tion is used as a pretreatment to biological oxidation. One more negative
effect of H 2 O 2 , if present in large quantities, is that it acts as a scavenger
for the generated hydroxyl radicals. Thus, the loading of H 2 O 2 should be
adjusted in such a way that the entire amount is utilized; this can be decided
based on laboratory-scale studies with the effluent in question.

3.4 PHOTOCATALYTIC OXIDATION

Photocatalytic or photochemical degradation processes are gaining impor-
tance in the area of wastewater treatment, since these processes result in
complete mineralization with an operation at milder temperature and pres-
sure conditions. The photo-activated chemical reactions are characterized
by a free radical mechanism initiated by the interaction of photons of suffi-
cient energy levels with the molecules of chemical species present in the
solution, with or without the presence of the catalyst. The radicals can be
easily produced using UV radiation by the homogeneous photochemical
degradation of oxidizing compounds such as H 2 O 2 and ozone. An alterna-
tive way to obtain free radicals is using the photocatalytic mechanism occur-
ring at the surface of semiconductors (such as titanium dioxide), which
substantially enhances the rate of free-radical generation and hence the rates
of degradation (Bhatkhande et al., 2002; Mazzarino and Piccinini, 1999). A
major advantage of the photocatalytic oxidation–based processes is the pos-
sibility of effectively using sunlight or near UV light for irradiation, which
should result in considerable cost savings, especially for large-scale opera-
tions (Han et al., 2012; Konstantinou and Albanis, 2004). Various chalco-
genides (oxides such as TiO 2 , ZnO, ZrO 2 , and CeO 2 , or sulfides such as
CdS and ZnS) have been used as photocatalysts so far in different studies
reported in the literature (Bhatkhande et al., 2002).The surface area and
the number of active sites offered by the catalyst (thus nature of the catalyst,
i.e., crystalline or amorphous) is important because the adsorption of
pollutants plays an important role in deciding the overall rates of degradation
(Xu et al., 1999).
Photocatalytic reactions occur when charge separations are induced in a
large bandgap semiconductor by excitation with ultra bandgap radiations
(Daneshvar et al., 2003). In this way, the absorption of light by the photo-
catalyst greater than its bandgap energy excites movement of an electron
from the valence band of the irradiated particle to its conduction band,