Environmental Engineering Reference
In-Depth Information
the degradation [55]. For high removal efficiencies, the H
2
O
2
concentra-
tion must be above the stoichiometric demand and sufficient reaction of
time under UV radiation must be applied [174].
The main advantage of using the combined ultraviolet irradiation and
hydrogen peroxide process is that H
2
O
2
is totally soluble in water, there is
no mass transfer limitation, and there is no need for a separation process
ater treatment [17,46,55].
Studies on the removal of dyes using UV/H
2
O
2
have shown that it is a
promising method to treat dilute aqueous solutions of azo dyes [175,176].
3.2.2.3 Photocatalytic Ozonation (UV/O
3
)
Ozonation combined with ultraviolet (UV) radiation is deemed as a
more effective process to remove organics; compared to sole ozonation, a
larger quantity of OH radicals can be formed. UV radiation is commonly
employed to enhance ozone decomposition, yielding more free radicals
and resulting in a higher ozonation rate [177].
The photodecomposition of ozone leads to two hydroxyl radicals per
ozone molecule [178]:
H
2
O + O
3
+ h
ν
2 OH + O
2
(3.20)
2 OH H
2
O
2
(3.21)
The key parameters for the success of ozone/UV system are ozone dos-
age, UV irradiation level and pH.
The main difference between combined photocatalytic ozone decom-
position and ozone decomposition alone in aqueous solution is the initia-
tion of the reaction. The starting radical is formed photochemically by an
electron transfer from photocatalyst to oxygen and not by the reaction of
OH
−
ion with ozone [40].
In the literature there are various promising studies using ozonation com-
bined with ultraviolet (UV) radiation in the treatment of dyes [179-181].
3.3 Concluding Remarks
The use of conventional wastewater treatment processes for dye removal
has become increasingly challenging because of the complex structure and
recalcitrant nature of dyes and the identification of more and more inter-
mediates. Consequently, advanced oxidation processes have emerged as
powerful technologies and are widely used in dye-containing wastewater.
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