Environmental Engineering Reference
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was very low in alkaline pH because of the formation of anionic species on
negatively charged TiO 2 surface [40]. Ramirez et al. [32] reported the solar-
assisted degradation of acid orange 7 (anionic dye) in aqueous solution by
cerium-doped TiO 2 . Photocatalytic degradation was highest at pH 7 and
lowest at alkaline pH. Sakthivel et al. [35] compared solar photodegrada-
tion of an azo dye (acid brown 14) with ZnO and TiO 2 photocatalyst. The
ZnO has a zero point charge (zpc) of 9.0 ± 0.3 and the dye degradation was
higher at alkaline pH.
4.6.4
Effect of Light Intensity
Normally the rate of photo-oxidation of different model compounds
increases with increasing light intensity. At a high light intensity, elec-
tron-hole formation rate exceeds electron-hole recombination rate and
the photo-oxidation rate is found to be proportional to the square root of
light intensity. However, at low light intensity, electron-hole formation and
recombination rates compete with each other and the photo-oxidation rate
is directly proportional to the light intensity [34,35].
Park et al. [34] studied the degradation of Red 120 (reactive dye) under
natural sunlight between 10 am and 4 pm. Their results showed that
during a clear day, the UV light intensity changed from 0.9 mW cm -2 to
0.7  mW  cm -2 , whereas under thick cloudy skies the UV light intensity
changed from 0.11 mW cm -2 to 0.3 mW cm -2 . The dye degradation rate
under clear skies was 13 times higher than that of thick cloudy skies.
Zhou and Ray [40] performed experiments for Eosin B degradation with
immobilized photocatalysts at varying light intensities and established a
relationship among reaction rate constant (k r ), photocatalyst loading
w (kg/m 2 ), and light intensity I (W/m 2 ). Their experimental data followed
an empirical equation previously proposed by Ray and Beenackers [43] as
shown below:
n
kaw I
aw I
(4.3)
k
s
r
n
1
where k r is rate constant, w is photocartalyst mass, I is light intensity, and
n and
are constants.
4.6.5
Effect of Electron Scavenger
Photocatalytic oxidation reactions require the use of electron scavenger to
subdue its recombination with hole. Usually dissolved oxygen/air is used
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