263
Such particle agglomeration is highly detrimental in terms of particle size preservation, surface-area reduction, and its reusable life span. Other technical challenges
include the catalyst development with broader photoactivity range and its integration with feasible photocatalytic reactor system. In addition, the understanding of
the theory behind the common reactor operational parameters and their interactions
is also inadequate and presents a difficult task for process optimization. A number
of commonly made mistakes in studying kinetic modeling on either the photomineralization or the photo-disinfection have also been seen over the years.
This review chapter aims to give an overview of the understanding and development of photocatalytic water treatment technology, from fundamentals of catalyst
and photoreactor development to process optimization and kinetics modeling, and
eventually the water parameters that affect the process efficiency. A short outline of
the feasible application of photocatalytic water technology via life cycle interpretation and the possible future challenges are also given.
Fundamentals and Mechanism of TiO 2 Photocatalysis
Heterogeneous TiO 2 Photocatalysis
The fundamentals of photo-physics and photochemistry underlying the heterogeneous photocatalysis employing the semiconductor TiO 2 catalyst have been intensively reported in many literatures [98, 103]. The semiconductor TiO 2 has been
widely utilized as a photocatalyst for inducing a series of reductive and oxidative
reactions on its surface. This is solely contributed by the distinct lone electron characteristic in its outer orbital. When photon energy (hv) of greater than or equal to the
bandgap energy of TiO 2 is illuminated onto its surface, usually 3.2 eV (anatase) or
3.0 eV (rutile), the lone electron will be photoexcited to the empty conduction band
in femtoseconds. Figure 13.3 depicts the mechanism of the electron/hole pair formation when the TiO 2 particle is irradiated with adequate hv. The light wavelength
for such photon energy usually corresponds to l < 400 nm. The photonic excitation
leaves behind an empty unfilled valence band, and thus creates the electron/hole
pair (e
−
/h
+
). The series of chain oxidative-reductive reactions (Eqs. (13.1)–(13.11))
that occur at the photon-activated surface was widely postulated as follows:
Photoexcitation TiO
e h
:
2 + → +
−
+
hv
(13.1)
Charge-carrier trapping of e- e
e
CB
TR
:
−
−
→
(13.2)
Charge-carrier trapping of h+ h
h
VB
TR
:
+
+
→
(13.3)
Electron hole recombination e
h h
e
heat
TR
VB
TR
CB
/
:
−
+
+
−
+
( ) → +
(13.4)
Recent Developments in Photocatalytic Water Treatment Technology
Such particle agglomeration is highly detrimental in terms of particle size preservation, surface-area reduction, and its reusable life span. Other technical challenges
include the catalyst development with broader photoactivity range and its integration with feasible photocatalytic reactor system. In addition, the understanding of
the theory behind the common reactor operational parameters and their interactions
is also inadequate and presents a difficult task for process optimization. A number
of commonly made mistakes in studying kinetic modeling on either the photomineralization or the photo-disinfection have also been seen over the years.
This review chapter aims to give an overview of the understanding and development of photocatalytic water treatment technology, from fundamentals of catalyst
and photoreactor development to process optimization and kinetics modeling, and
eventually the water parameters that affect the process efficiency. A short outline of
the feasible application of photocatalytic water technology via life cycle interpretation and the possible future challenges are also given.
Fundamentals and Mechanism of TiO 2 Photocatalysis
Heterogeneous TiO 2 Photocatalysis
The fundamentals of photo-physics and photochemistry underlying the heterogeneous photocatalysis employing the semiconductor TiO 2 catalyst have been intensively reported in many literatures [98, 103]. The semiconductor TiO 2 has been
widely utilized as a photocatalyst for inducing a series of reductive and oxidative
reactions on its surface. This is solely contributed by the distinct lone electron characteristic in its outer orbital. When photon energy (hv) of greater than or equal to the
bandgap energy of TiO 2 is illuminated onto its surface, usually 3.2 eV (anatase) or
3.0 eV (rutile), the lone electron will be photoexcited to the empty conduction band
in femtoseconds. Figure 13.3 depicts the mechanism of the electron/hole pair formation when the TiO 2 particle is irradiated with adequate hv. The light wavelength
for such photon energy usually corresponds to l < 400 nm. The photonic excitation
leaves behind an empty unfilled valence band, and thus creates the electron/hole
pair (e
−
/h
+
). The series of chain oxidative-reductive reactions (Eqs. (13.1)–(13.11))
that occur at the photon-activated surface was widely postulated as follows:
Photoexcitation TiO
e h
:
2 + → +
−
+
hv
(13.1)
Charge-carrier trapping of e- e
e
CB
TR
:
−
−
→
(13.2)
Charge-carrier trapping of h+ h
h
VB
TR
:
+
+
→
(13.3)
Electron hole recombination e
h h
e
heat
TR
VB
TR
CB
/
:
−
+
+
−
+
( ) → +
(13.4)
Recent Developments in Photocatalytic Water Treatment Technology