Environmental Engineering Reference
In-Depth Information
and Haung
1991
; Fox
1993
; Chen et al.
2001
; Han et al.
2009
) (Eqs. 3.13-3.18,
ISC
−−→
(3.39)
1
DOM
∗
3
DOM
∗
DOM
+
h
υ →
→
T
I
O
2
→
T
I
O
2
(
E
−
+
H
+
)
T
I
O
2
+
H
υ
(3.40)
3
DOM
∗
+
O
2
→
DOM
∗+
+
O
2
•−
(3.41)
TIO
2
(
E
−
+
H
+
) +
O
2
→
TIO
2
(
H
+
) +
O
2
•−
(3.42)
(3.43)
O
•−
2
+
H
+
→
HO
2
•
PK
A
=
4. 8
2O
2
•−
→
O
2
−
= <
0. 35 M
−
1
S
−
1
(3.44)
+
O
2
PK
A
2
HO
2
•
+
HO
2
•
→
H
2
O
2
+
O
2
k
=
8. 6
×
10
5
M
−
1
s
−
1
(3.45)
HO
2
•
+
O
2
•−
+
H
2
O
→
H
2
O
2
+
O
2
+
OH
−
k
=
1. 0
×
10
8
M
−
1
s
−
1
(3.46)
(3.47)
H
2
O
2
+
h
υ →
2HO
•
H
2
O
2
+
HO
•
→
HO
2
•
+
HO
2
k
=
3. 0
×
10
7
M
−
1
s
−
1
(3.48)
(3.49)
DOM
•+
+
HO
•
→
the photo degraded products
Major oxidants arising upon UV irradiation of TiO
2
are the valence-band holes
(h
+
), which can be trapped by surface Ti-OH
-
groups to yield Ti-HO
•
radical spe-
or
HO
•
ADS
). In contrast, the conduction-band electrons
(e
-
) can be trapped to form surface and sub-surface Ti
III
species: The latter can
react with dissolved oxygen to give hydrogen peroxide radical (HO
2
•
cies (surface-bound HO
•
), which dis-
•
proportionates to H
2
O
2
that is photolyzed to give HO
in solution (Murov et al.
1993
; Tseng and Haung
1991
; Fox
1993
).
As already reported, a semiconductor photocatalyst generates electron/hole
pairs upon irradiation, with free electrons produced in a nearly empty conduction
band and positive holes remaining in the valence band (Bard
1979
; Schiavello
1987
). The lifetime of an electron/hole pair can vary from a few nano-seconds to
a few hours depending on the ambient conditions (Pleskov and Gurevich
1986
).
Once formed the HO
•
radical (surface-bound or in solution) reacts with organic
compounds, which results into the heterogeneous photocatalytic degradation
of organic contaminants in solution (Sun and Bolton
1996
; Ullah et al.
1998
;
Konstantinou and Albanis
2004
; Tseng and Haung
1991
; Fox
1993
; Han et al.
2009
; Prousek
1996
; Vione et al.
2001
; Muñoz et al.
2006
; Saquib et al.
2008
).
