Environmental Engineering Reference
In-Depth Information
The basic mechanism for generating
ΟΗ involves the adsorption of
a photon, the generation of an electron-hole pair (
e
cb
-
+
h
vb
+
) and subse-
quently the production of ΟΗ and superoxide radical anions.
When photocatalyst absorbs ultraviolet radiation from a light source,
it will produce pairs of electrons and holes. The electron of the valence
band of photocatalyst becomes excited when illuminated by light. The
excess energy of this excited electron promotes the electron to the con-
duction band of photocatalyst, thereby creating the negative-electron
(e-) and positive-hole (h+) pair. The energy difference between the
valence band and the conduction band is known as the “band gap.” The
positive-hole of photocatalyst breaks apart the water molecule to form
hydrogen gas and hydroxyl radical. The negative-electron reacts with
oxygen molecule to form superoxide anion. This cycle continues when
light is available [146].
The mechanism of the photocatalytic degradation of organic com-
pounds and dyes can be summarized as follows [147,148]:
Absorption of photons by photocatalyst and production of photoholes and
electron pairs
:
In this step, semiconductor photocatalysis is initiated by electron-hole
pairs after band gap excitation. When a photocatalyst is illuminated by
light with energy equal to or greater than band-gap energy, the valence
band electrons can be excited to the conduction band, leaving a positive
hole in the valence band:
(photocatalyts) +
hν
h
vb
+
+ e
cb
-
(3.15)
Formation of superoxide radical anion (oxygen ionosorption):
(O
2
)
ads
+ e
cb
-
O
2
—
(3.16)
Neutralization of OH
-
by photoholes:
(H
2
O
H
+
+ OH—)
ads
+ h
vb
+
H
+
+ OH
(3.17)
It has been suggested that the hydroxyl radical ( OH) and superox-
ide radical anions (O
2
—) are the primary oxidizing species in the pho-
tocatalytic oxidation processes. Both species are strongly oxidizing and
capable of degrading organic pollutants. These oxidative reactions would
results in the degradation of the pollutants as shown in the following
Equations 3.18-3.19;
oxidation of the organic pollutants via successive attack by OH radicals
R + OH
R + H
2
O
(3.18)
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