Environmental Engineering Reference
In-Depth Information
physical modii cation approaches were developed to extend the absorption
band-edge of TiO
2
into visible region [1-5].
In the photocatalytic oxidation process, organic pollutants are destroyed
in the presence of semiconductor photocatalyts, an energetic light source,
and an oxidizing agent such as oxygen or air. Only photons with ener-
gies greater than the band gap energy (ΔE) can result in the excitation of
valence band (VB) electrons which then promote the possible reactions.
h e absorption of photons with energy lower than ΔE or longer wave-
length usually causes energy dissipation in the form of heat.
h e illumination of the photocatalytic surface with surface energy leads
to the formation of positive hole (h
+
) in the valence band and an electron
(e
-
) in the conduction band. h e positive hole oxidizes either pollutants
directly or water to produce
.
OH radicals, whereas the electron in the con-
duction band reduces the oxygen adsorbed on the photocatalyt (TiO
2
). h e
activation of TiO
2
by UV light can be:
Overall reaction:
TiO
2
+ hv
e
-
+h
+
e
-
+
O
2
O
2
-
(7.1)
Oxidative reaction:
h
+
+ organic moiety
CO
2
(7.2)
h
+
+H
2
O
.
OH + H
+
(7.3)
Reductive reaction:
OH + organic moiety
CO
2
(7.4)
In recent years, advanced oxidation processes (AOPs) using titanium
dioxide (TiO
2
) have been ef ectively used to detoxify recalcitrant pollut-
ants present in industrial wastewater. Titanium dioxide has singular char-
acteristics that have made it an extremely attractive photocatalyst: high
photochemical reactivity, high photocatalytic activity, low cost, stability
in aquatic systems and low environmental toxicity. h e general detailed
mechanism of dye degradation upon irradiation is described below.
Dye +
hν
Dye*
(7.5)
Dye* + TiO
2
Dye•+ + TiO
2
(e)
(7.6)
TiO
2
(e) + O
2
TiO
2
+ O
2
•−
(7.7)
O
2
•−
+ TiO
2
(e) + 2H
+
H
2
O
2
(7.8)
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