According to the phenomenon of boiling, as the temperature increases, the vapor
bubble also grows. This limits the bubble’s acoustic cycles, and the bubble attains
the critical radius early which causes lesser cavitational effect. Therefore, to achieve
the maximum cavitation effect, it is suggested to perform the experiments at lower
temperature. The detailed discussion on this topic may be found at Chakma and
Moholkar (2013b).
6.3 Sono-photolysis System
In the sonolysis process, the entrapped water vapor molecules in the cavitation
bubble experience thermal dissociation and produce several radicals (i.e.,
•
H,
• OH,
•
O, and HO
•
2 ) and other oxidizing species such as H 2 O 2 and O 3 . During the transient
collapse, these radicals are unconfined into the medium with the fragmentation of
cavitation bubble and attack the molecules of the recalcitrant pollutants at a whopping rate constant of 10
6
–10
9 mole
À1 s
À1 (Andreozzi et al. 1999). However, there is
also a chance of recombination due to nonuniform interaction when the concentration of pollutant molecules is comparatively less. In that case, the generated
•
OH
radicals combine and produce H 2 O 2 which has less oxidation potential (1.77 eV) as
compared to that of
•
OH radicals (2.8 eV). Therefore, the application of UV light
irradiation during the process can regenerate the
•
OH radicals through re-splitting the
in situ-generated H 2 O 2 as follows:
H 2 O 2 þ hv ! 2
• OH
The concentration of H 2 O 2 is a crucial parameter for H 2 O 2 -based photolysis
process. In this process, higher concentration of H 2 O 2 may result in scavenging of
oxidizing
• OH radicals before it can react with the organic pollutants (Mishra and
Gogate 2011; Chakma and Moholkar 2016b):
• OH þ H 2 O 2 ! H 2 O þ HO
•
2
• H þ H 2 O 2 ! HO
•
2 þ H 2
HO
•
2 þ
• OH ! O 2 þ H 2 O
When the conventional photolysis is employed with sonolysis process, the
mineralization of recalcitrant pollutants can be enhanced as shown in Fig. 6.2
(Chakma and Moholkar 2016b). During the sonolysis process, some amount of the
externally added H 2 O 2 is decomposed and produces
• OH radicals, while in situgenerated H 2 O 2 during sonolysis process can also undergo photodecomposition
under ultraviolet light and produce additional
•
OH radicals. This process is stronger
when the liquid medium contains dissolved oxygen as discussed earlier. This leads
to generation of HO
•
2 and
•
O radicals as follows:
198
S. Chakma et al.
bubble also grows. This limits the bubble’s acoustic cycles, and the bubble attains
the critical radius early which causes lesser cavitational effect. Therefore, to achieve
the maximum cavitation effect, it is suggested to perform the experiments at lower
temperature. The detailed discussion on this topic may be found at Chakma and
Moholkar (2013b).
6.3 Sono-photolysis System
In the sonolysis process, the entrapped water vapor molecules in the cavitation
bubble experience thermal dissociation and produce several radicals (i.e.,
•
H,
• OH,
•
O, and HO
•
2 ) and other oxidizing species such as H 2 O 2 and O 3 . During the transient
collapse, these radicals are unconfined into the medium with the fragmentation of
cavitation bubble and attack the molecules of the recalcitrant pollutants at a whopping rate constant of 10
6
–10
9 mole
À1 s
À1 (Andreozzi et al. 1999). However, there is
also a chance of recombination due to nonuniform interaction when the concentration of pollutant molecules is comparatively less. In that case, the generated
•
OH
radicals combine and produce H 2 O 2 which has less oxidation potential (1.77 eV) as
compared to that of
•
OH radicals (2.8 eV). Therefore, the application of UV light
irradiation during the process can regenerate the
•
OH radicals through re-splitting the
in situ-generated H 2 O 2 as follows:
H 2 O 2 þ hv ! 2
• OH
The concentration of H 2 O 2 is a crucial parameter for H 2 O 2 -based photolysis
process. In this process, higher concentration of H 2 O 2 may result in scavenging of
oxidizing
• OH radicals before it can react with the organic pollutants (Mishra and
Gogate 2011; Chakma and Moholkar 2016b):
• OH þ H 2 O 2 ! H 2 O þ HO
•
2
• H þ H 2 O 2 ! HO
•
2 þ H 2
HO
•
2 þ
• OH ! O 2 þ H 2 O
When the conventional photolysis is employed with sonolysis process, the
mineralization of recalcitrant pollutants can be enhanced as shown in Fig. 6.2
(Chakma and Moholkar 2016b). During the sonolysis process, some amount of the
externally added H 2 O 2 is decomposed and produces
• OH radicals, while in situgenerated H 2 O 2 during sonolysis process can also undergo photodecomposition
under ultraviolet light and produce additional
•
OH radicals. This process is stronger
when the liquid medium contains dissolved oxygen as discussed earlier. This leads
to generation of HO
•
2 and
•
O radicals as follows:
198
S. Chakma et al.