Environmental Engineering Reference
In-Depth Information
3.4.7.2 Cation Effects
It is observed that the presence of Fe
3+
and Cu
2+
ions, even at low concentrations
(i.e., 0.05 mM), enhances the degradation rate of most pollutants (Chen and Liu, 2007).
Fe
3+
and Cu
2+
ions can favor the degradation in several aspects. First, the presence of
Fe
3+
generates Fenton's reaction as shown in Eqs. 3.29 and 3.30, which have synergetic
effects on the degradation of most organic compounds.
Fe
2+
+ H
2
O
2
+ H
+
Fe
3+
+ OH• + H
2
O
(Eq. 3.29)
Fe
3+
+ H
2
O
2
Fe
2+
+ H
+
+ OH
-
(Eq. 3.30)
Second, low concentration of Fe
3+
and Cu
2+
on the TiO
2
surface may delay the
electrons-hole recombination, and therefore, increase the electron holes lifetime, which
enhances the reaction rate. Third, some investigators have found that the presence of
Fe
3+
and Cu
2+
may alter the interfacial electron transfer pathway and, thus, can modify
the band gap of photocatalysts (Chen et al., 2002). Last, porous metal oxides can be
formed on the TiO
2
surface, on which, substrate can be adsorbed and transferred onto
photoactive sites for further reactions (Qu et al., 1998).
Most other metal cations are reported to have negative impact on the
photocatalytic reaction, although results of some investigators have indicated that these
metal ions have only negligible impacts. Therefore, systematic studies are required to
address the effects of cations on the photocatalytic degradation of specific organic
compounds.
3.4.7.3 Effects of Other Substrates
The existence of ethanol and humic acid, even at low concentrations, retards the
reaction significantly, attributed to the hydroxyl radical scavenge nature. Daneshvar et
al. (2003) have reported that, with 0.02 % (v/v) of ethanol, the photodegradation
efficiency of AR14 dye decreases from 80% without ethanol to 12.4% in the presence of
ethanol with 2 hr irradiation. The presence of chelating agents, such as EDTA, is
known to shift the flat-band to a more negative value due to the adsorption and chelation
or complexation with the surface metal ions (Uchihara et al., 1990).
3.5
Photocatalytic Degradation of Specific Waterborne Pollutants
The photocatalytic reactions already have been proven successful in treating a
wide range of waterborne pollutants, including halogenated compounds, aromatic
compounds, aliphatic compounds, natural organic matter, surfactants dyes, pesticides
and microorganisms. There are good literature reviews on the applications of
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