Environmental Engineering Reference
In-Depth Information
metal oxide surfaces can have considerable implications upon their pho-
tocatalytic activity [163]. For example, in the case of using TiO
2
as photo-
catalyst the point of zero charge of the TiO
2
is at pH 6.8 [164]. Thus, the
TiO
2
catalyst surface will be positively charged in acidic media (pH < 6.8),
whereas it is negatively charged under alkaline conditions (pH > 6.8)
[149]. Consequently, pH changes can thus influence the adsorption of dye
molecules onto the catalyst surfaces [165]. In addition to this, the catalyst
particles (especially TiO
2
) might tend to agglomerate under acidic condi-
tions and the surface area available for dye adsorption and photon absorp-
tion would be less [165]. Another obvious point is that the formation of
hydroxyl radicals is more responsible for photocatalytic degradation and
that hydroxyl radicals can be formed by the reaction between hydroxide
ions and positive holes. The positive holes are considered as the major oxi-
dation species at low pH, whereas hydroxyl radicals are considered as the
predominant species at neutral or high pH levels [162]. An alkaline condi-
tion would thus favor OH formation and enhance degradation.
From the existing literature (Table 3.6), it can be seen that generally:
t the photocatalytic oxidation process is suitable for a wide
range of dyes,
t a high degree of decolorization can be achieved,
t pH may afect photocatalytic oxidation eiciency, and
t the efect of temperature is not considered that much.
3.2.2.2 Ultraviolet Irradiation and Hydrogen Peroxide (UV/H
2
O
2
)
Currently gaining more attention is the UV/H
2
O
2
system, which is an
advanced oxidation process in which hydrogen peroxide (H
2
O
2
) is added in
the presence of ultraviolet (UV) light to generate hydroxyl radicals ( OH).
As reported, hydrogen peroxide is a strong oxidant for reducing low lev-
els of pollutants present in wastewaters [68]. However, the individual use
of H
2
O
2
is not always efficient in oxidizing more complex pollutants. The
use of H
2
O
2
becomes more effective when it acts in conjunction with other
reagents or energy sources capable of dissociating it to generate hydroxyl
radicals.
With UV irradiation in wavelengths shorter than 300 nm, H
2
O
2
can
decompose and generate hydroxyl radicals. In the combined ultraviolet
irradiation and hydrogen peroxide system, because the rate of dye removal
is mainly dependent on the concentration of H
2
O
2
, it is necessary to deter-
mine the optimum H
2
O
2
concentration to avoid an excess that could retard
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