Environmental Engineering Reference
In-Depth Information
than that by sonolysis alone, particularly for mineralization. The synergy
is due to: (i) increased mass transfer, dispersion and solubility of ozone
and the formation of excess radical species via its thermal decomposition
in the gas phase; and (ii) the additional pathways of oxidation upon the
formation of secondary radicals as O 2 - and HO 2 . The depiction of these
phenomena by chemical reactions is as follows [72-75]:
O 3 + ))) → O 2 (g) + O ( 3 P)(g)
(7.16)
O ( 3 P) (g) + H 2 O (g) + ))) → 2HO
(7.17)
HO + O 3 HO 2 + O 2 (7.18)
The solution temperature is a crucial parameter in US/O 3 hybrid pro-
cesses, as the rate of bleaching/mineralization gradually increases by
heating the solution and rapidly declines when the temperature exceeds
65 C  [74]. The acceleration above the ambient temperature is due to the
reduction in viscosity and/or surface tension, which in turn lowers the
threshold intensity required for the formation of cavitation bubbles [74].
On the other hand, in some cases temperatures higher than 40 C were
found to decelerate the degradation of some dyes due to the reduction in
the solubility and rate of decomposition of ozone [22,76-77].
7.3.2 Sonophotolysis (US/UV) and Sonophoto-Ozonolysis
(US/UV/O 3 )
Integration of ultrasonic reactors with UV-irradiation results in the forma-
tion of excess HO (upon photolysis of US-generated H 2 O 2 ) that is respon-
sible for the observed enhancement in the rate of oxidation. The addition
of moderate doses of peroxide may further improve the efficiency provided
that the light source emits at around 200 nm, where H 2 O 2 absorbs strongly.
A more effective additive than peroxide is persulfate, which in the pres-
ence of UV and ultrasonic irradiation is converted to HO and SO 4 - , the
latter being highly reactive with the intermediate oxidation byproducts of
synthetic dyes [65]. The most commonly used additive to enhance miner-
alization is gaseous ozone, the catalytic effect of which was discussed in the
previous subsection. In the combined presence of US/UV, ozone under-
goes sonolytic and photolytic decomposition at the same time to generate
singlet state oxygen and additional radical species such as HO and HO 2 :
O 3 + hυ
O 2 + O( 1 D)
(7.19)
O( 1 D) + H 2 O
HO + HO
H 2 O 2
(7.20)
H 2 O 2 + hυ
2HO
(7.21)
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