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Advanced Oxidation Technologies for Wastewater Treatment: An Overview 169

Similar trends have been reported by Saharan et al. (2011) for the degrada-
tion of reactive red 120 while using the combined processes of HC and
H 2 O 2 . They have reported the optimal molar ratio of dye to H 2 O 2 as
1:60. Wang et al. (2011a) have also demonstrated the existence of the opti-
mal loading of H 2 O 2 for the degradation of reactive brilliant red K-2BP
using a combination of HC and H 2 O 2 . The observed results for the com-
bined process of cavitation and H 2 O 2 illustrate that the scavenging of free
radicals by H 2 O 2 becomes a dominant process at a high concentration of
H 2 O 2 , thereby lowering the extent of degradation. Also, operating at such
high ratios of H 2 O 2 may not be economically feasible and could pose safety
hazards. Moreover, the retention of H 2 O 2 in the effluent stream needs to be
tested because it also presents as a possible contaminant and cannot be dis-
charged as such. Hence optimum loading of H 2 O 2 should be used for better
efficiency in the combined process of cavitation and H 2 O 2 .

3.5.2 Cavitation Coupled with Ozone

Ozone (O 3 ) has an excellent potential for degrading organic pollutants due
to its high oxidation potential (2.08 V). The degradation of organic pollut-
ants through ozonation takes place via two routes: (1) At basic pH, ozone
rapidly decomposes to yield hydroxyl and other radical species in solution
which oxidize the pollutants and (2) under acidic conditions, ozone is stable
and can react directly with organic substrates as an electrophile. Although
ozone is a strong oxidant, there are certain limitations of using ozonation
as an effective tool for the degradation of organic pollutants on an industrial
scale. These limitations are (a) a high energy/intensity requirement to gen-
erate O 3 , (b) pH sensitivity, and (c) selectivity for organic substrates; for
example, O 3 preferentially reacts with alkenes and sites with high electronic
density (Pang et al., 2011; Weavers et al., 1998). Therefore, many
researchers have tried ozonation in combination with other process to over-
come these drawbacks, including ozone combined with H 2 O 2 and ozone
combined with ultraviolet irradiation and ultrasound (Ince and Tezcanlı ´,
2001; Kang and Hoffmann, 1998; Song et al., 2007; Vecitis et al., 2010;
•
Wu et al., 2008). These combined processes generate OH radicals that have
higher oxidation potential than ozone and are nonselective toward organic
substances. Cavitation can be used in combination with ozone for efficiency
improvement because of its ability to create hot spots at ambient conditions.
It has been reported that ozone can readily be decomposed under the cavi-
tational conditions of high temperature and pressure generated as a result of