Environmental Engineering Reference
In-Depth Information
oxygen to produce superoxide (O
2
•-
). This superoxide is an effective oxygenation agent
that attacks neutral substrates as well as surface-adsorbed radicals and radical ions.
Hydroxyl radicals are formed through reactions of O
2
•-
and h
+
with adsorbed H
2
O,
hydroxide, surface titanol groups (>TiOH), and other species. Theoretically, the redox
potential of the electron
ದ
hole pair permits H
2
O
2
formation. The main pathway of H
2
O
2
formation is by two conduction band electron reduction of the adsorbed oxygen. H
2
O
2
contributes to the degradation pathway by acting as an electron acceptor or as a direct
source of hydroxyl radicals due to homolytic scission. Recently, the formation of singlet
oxygen (
1
O
2
) in TiO
2
photocatalysis was directly detected
.
Holes, HO
•
, O
2
•-
, H
2
O
2
, or
1
O
2
can play important roles in the photocatalytical reactions under different conditions.
The photodegradation processes of MMA and DMA are illustrated in Figure 5.16.
MMA is reduced from an initial concentration of 10 to 0.7 mg-As/L during 72 hours of
reaction (Figure 5.16-A). Meanwhile, the As(V) concentration increases from 0 to 9.0
mg/L. The sum of MMA and As(V) concentrations at different reaction times is
approximately the same as the initial MMA concentration of 10 mg-As/L, which
indicates that all degraded MMA molecules is converted to As(V) species. The results
also suggest that very little arsenic species is adsorbed by TiO
2
, due to the high As/TiO
2
ratio (10 mg/0.02 g) used in the suspension.
12
12
(B)
(A)
10
10
8
8
DMA
MMA
Arsenate
To t a l A s
MMA
Arsenate
To t a l A s
6
6
4
4
2
2
0
0
0 1020304050607080
0 1020304050607080
Time (hour)
Time (hour)
Figure 5.16
Photodegradation processes of (A) MMA and (B) DMA. Initial MMA =
DMA = 10 mg-As/L, TiO
2
= 0.02 g/L, ionic strength = 0.04 M NaCl.
During the degradation of DMA, both MMA and As(V) are formed in the
solution (Figure 5.16B). DMA is first transformed into MMA by losing a methyl group,
and then is degraded to As(V). The DMA concentration is reduced gradually from 10 to
3.5 mg-As/L after 72 hours. MMA, an intermediate of the DMA photodegradation
process, increases from zero to 1.8 mg-As/L in the first 48 hours and remains at a steady
state level thereafter. The As(V) concentration increases almost linearly with the
increase of UV-irradiation time. The mineralization of DMA to As(V) occurs in two
steps with MMA as an intermediate product. The sum of MMA, As(V) and DMA
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