Environmental Engineering Reference
In-Depth Information
compared the phenol degradation rate at a wide pH range (pH = 39), and found that the
phenol destruction rate increases slightly when solution pH increases till 5, and almost
no change when solution pH is above 5 (Figure 3.14). This can be rationalized for
several reasons. First, the TiO
2
surface acidity is a function of pH according to the
following reactions (Huang and Stumm, 1973; Hoffmann et al., 1995):
>Ti
IV
OH + H
+
>Ti
IV
OH
2
+
(Eq. 3.25)
>Ti
IV
OH + OH
-
>Ti
IV
O
-
+ H
2
O
(Eq. 3.26)
10
pH 3
pH 5
pH 6.6
pH 7
pH 9
8
6
4
2
0
0
1
2
3
4
5
6
Time, h
Figure 3.14.
Phenol degradation rate as a function of solution pH (Replotted from Tseng
and Huang, 1990)
.
The point of zero charge (pzc) of TiO
2
surface is reported to be pH 3.8, below
which, the surface is positively charged and above which, the surface is negatively
charged. It is also worthy of mention that the pollutant substrate ionization is also
closely related to the media pH value, which has significant influence on the reaction
mechanism(s) and pathway in most cases. Furthermore, the states of surface charge and
substrate ionization states is critical in terms of surface adsorption, which is an important
step for the photocatalytic oxidation to take place (Fox and Dulay, 1993; Hoffmann et al.,
1995).
Second the ROS generation, as one of major oxidation mechanisms, can be
influenced by change of pH as discussed above. Many groups have reported that
alkaline condition favors OH• generation, and the reaction rate is enhanced (Fox and
Dulay 1993; Dong and Huang, 1995; So et al., 2002). Tang and Huang (1995) have
concluded that at high pH level, indirect oxidation is the dominated oxidation
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