Page 194 - Industrial Wastewater Treatment, Recycling and Reuse

168 Industrial Wastewater Treatment, Recycling, and Reuse

•
H 2 O 2 + ÞÞÞ ! 2 OH (3.26)
• •
H 2 O+ ÞÞÞ ! H + OH (3.27)
• •
OH + OH ! H 2 O 2 (3.28)
• •
OH + H 2 O 2 ! HO +H 2 O (3.29)
2
• •
OH + HO ! H 2 O+O 2 (3.30)
2
• •
HO +H 2 O 2 ! OH + H 2 O+O 2 (3.31)
2
•
Pollutant molecules + OH ! CO 2 +H 2 O + other intermediates (3.32)
Pollutant molecules + H 2 O 2 ! CO 2 +H 2 O + other intermediates (3.33)
The efficiency of the combined process is very much dependent on the
•
rate at which OH radicals are consumed by the pollutant molecules. It has
been observed that the synergistic effects of the combined process of cavi-
tation and H 2 O 2 are obtained up to an optimum concentration of H 2 O 2 .
•
Afterwards, the effects are reduced because of scavenging of the OH radicals
•
•
by H 2 O 2 itself and the recombination of OH radicals with other OH
radicals present in the solution. Operating pH, the intensity of turbulence
existing in the reactor, state (whether molecular or ionic) and nature (hydro-
phobic or hydrophilic) of the pollutant, and sometimes concentration of the
pollutant and composition of the effluent stream, are the crucial factors to be
analyzed before selecting the combination of ultrasound with H 2 O 2 as the
oxidation treatment scheme. It can be said that the use of H 2 O 2 in conjunc-
tion with ultrasound is beneficial only till an optimum loading of H 2 O 2 and
where the free radical attack is the controlling mechanism of destruction.
The optimum loading of H 2 O 2 will be dependent on the nature of the efflu-
ent stream and the operating conditions, as mentioned above. Laboratory-
scale studies are essential to establish this optimum for the effluent stream in
question. HC coupled with H 2 O 2 can be used on a larger scale due to its
potential in generating cavitational conditions similar to those generated
in acoustic cavitation and that too on a larger scale. HC also gives higher
cavitational yield as compared to acoustic cavitation and is more energy effi-
cient than acoustic cavitation.
Merouani et al. (2010) have reported that the degradation rate of rhoda-
mine B increased by the addition of H 2 O 2 to the ultrasound. They have
observed that an optimum concentration of H 2 O 2 existed to maximize
the degradation rate of rhodamine B, using a combination of ultrasound
and H 2 O 2 ; excessive amounts of H 2 O 2 could reduce its degradation rate.