Environmental Engineering Reference
In-Depth Information
Ti0
2
+
hv
4h
VB
+
+ e
CB
~
(Eq. 15.56)
An electron is exited from the valence band (VB) to the conduction band (CB) leaving
the hole (hvs
+
) behind in the VB. The created electron-hole pair then undergoes a
charge transfer to the adsorbate on the TiO2 surface. With a continuous charge transfer,
the decomposition of the adsorbate is possible. The trapped holes can directly oxidize
contaminants adsorbed (bounded) or in bulk solution (unbounded)
S - C + h
VB
+
Ki
"""""1 S - C
0
+
->-> end products
(Eq. 15.57)
C + h
VB
+
""""""""1 C
0
+
->-> end products
(Eq. 15.58)
where S = the adsorption site on the NPs; and S-C or C = bounded or unbounded
contaminants. The trapped holes can also react with the surface hydroxyl groups or
water to produce the hydroxyl radicals:
S-OH- + h
VB
+
«S-OH
(Eq. 15.59)
S-H
2
0 + h
VB
+
«S-OH + H
+
(Eq. 15.60)
These reactions can also occur in the bulk solution, leaving OH- in the bulk. Since the
hydroxyl radicals are strong oxidizing agents, four possible combinations can occur:
S-C+ S-OH«S+ S-C'° + M
+
OH- -»-» end products
(Eq. 15.61)
C+ S-OH«S + C'° + M
+
OH- -»-» end products
(Eq. 15.62)
S - C + OH « S - C'° + M
+
OH- -»-» end products
(Eq. 15.63)
C + OH « C'° + M
+
OH~ -»-» end products
(Eq. 15.64)
where C^ and C'° are the action radicals formed by direct hole attack and hydroxyl
radical attack, respectively, and M^ is the cation released from the contaminant
molecule. Kinetic studies showed that photodegradation of some contaminants by NMs
is in accordance to the Langmuir-Hinshelwood (L-H) model (see Table 15.6) (Matthews,
1988; Sivalingam et al, 2003; Nagaveni, et al., 2004).
As a light-induced redox reaction, photolysis has significant importance in
transformation of NMs. For photodegradation of NMs in the environment, two major
mechanisms may be involved. NMs in the upper layers of aquatic environments, on soil
surfaces, and in water droplets in the atmosphere are exposed to sunlight. Light-induced
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