269
Other combined processes of photo-Fenton and oxidative processes have also been
proposed in the literature, such as ozone and ultrasound [23, 308]. However, their
significance as compared to the TiO 2 /photo-Fenton will not be discussed in detail.
Advancements in Photocatalyst Immobilization and Supports
Since the discovery of photocatalytic effect on water splitting by Fujishima and
Honda (1972) using TiO 2 electrode, numerous researches have evolved to synthesize TiO 2 catalyst of different scales, characterize its physical properties, and determine its photooxidation performances to the surface-oriented nature of photocatalysis
reaction [97, 127, 144, 160, 321]. The TiO 2 catalyst in nanodimensions allows having a large surface area-to-volume ratio and can further promote the efficient charge
separation and trapping at the physical surface [238, 239]. The light opaqueness of
this nanoscale TiO 2 catalysts was reported to have an enhanced oxidation capability
compared to the bulk TiO 2 catalysts [292]. Although the nanoscale TiO 2 catalysts
show considerable improvement in terms of their physical and chemical properties,
their particle size and morphology remain the main problems in a large- scale water
treatment process [38, 348]. In this section, the current technical challenges that
prevent the application of slurry TiO 2 photocatalytic system are discussed together
with the possible engineering solutions to resolve the problem. We will have a brief
discussion on the modified TiO 2 catalyst with dopants for enhanced photoactivity
under solar irradiation.
Challenges in the Development of Photocatalytic Water
Treatment Process
To date, the most widely applied photocatalyst in the research of water treatment is
the Degussa P-25 TiO 2 catalyst. This catalyst is used as a standard reference for
comparisons of photoactivity under different treatment conditions [284]. The fine
particles of the Degussa P-25 TiO 2 have always been applied in a slurry form. This
is usually associated with a high volumetric generation rate of ROS as proportional
to the amount of surface-active sites when the TiO 2 catalyst is in suspension [263].
On the contrary, the fixation of catalysts into a large inert substrate reduces the
amount of catalyst-active sites and also enlarges the mass transfer limitations.
Immobilization of the catalysts results in increasing the operation difficulty as the
photon penetration might not reach every single surface site for photonic activation
[263]. Thus, the slurry type of TiO 2 catalyst application is usually preferred.
With the slurry TiO 2 system, an additional process step would need to be entailed
for post-separation of the catalysts. This separation process is crucial to avoid the
loss of catalyst particles and introduction of the new pollutant of contamination of
Recent Developments in Photocatalytic Water Treatment Technology
Other combined processes of photo-Fenton and oxidative processes have also been
proposed in the literature, such as ozone and ultrasound [23, 308]. However, their
significance as compared to the TiO 2 /photo-Fenton will not be discussed in detail.
Advancements in Photocatalyst Immobilization and Supports
Since the discovery of photocatalytic effect on water splitting by Fujishima and
Honda (1972) using TiO 2 electrode, numerous researches have evolved to synthesize TiO 2 catalyst of different scales, characterize its physical properties, and determine its photooxidation performances to the surface-oriented nature of photocatalysis
reaction [97, 127, 144, 160, 321]. The TiO 2 catalyst in nanodimensions allows having a large surface area-to-volume ratio and can further promote the efficient charge
separation and trapping at the physical surface [238, 239]. The light opaqueness of
this nanoscale TiO 2 catalysts was reported to have an enhanced oxidation capability
compared to the bulk TiO 2 catalysts [292]. Although the nanoscale TiO 2 catalysts
show considerable improvement in terms of their physical and chemical properties,
their particle size and morphology remain the main problems in a large- scale water
treatment process [38, 348]. In this section, the current technical challenges that
prevent the application of slurry TiO 2 photocatalytic system are discussed together
with the possible engineering solutions to resolve the problem. We will have a brief
discussion on the modified TiO 2 catalyst with dopants for enhanced photoactivity
under solar irradiation.
Challenges in the Development of Photocatalytic Water
Treatment Process
To date, the most widely applied photocatalyst in the research of water treatment is
the Degussa P-25 TiO 2 catalyst. This catalyst is used as a standard reference for
comparisons of photoactivity under different treatment conditions [284]. The fine
particles of the Degussa P-25 TiO 2 have always been applied in a slurry form. This
is usually associated with a high volumetric generation rate of ROS as proportional
to the amount of surface-active sites when the TiO 2 catalyst is in suspension [263].
On the contrary, the fixation of catalysts into a large inert substrate reduces the
amount of catalyst-active sites and also enlarges the mass transfer limitations.
Immobilization of the catalysts results in increasing the operation difficulty as the
photon penetration might not reach every single surface site for photonic activation
[263]. Thus, the slurry type of TiO 2 catalyst application is usually preferred.
With the slurry TiO 2 system, an additional process step would need to be entailed
for post-separation of the catalysts. This separation process is crucial to avoid the
loss of catalyst particles and introduction of the new pollutant of contamination of
Recent Developments in Photocatalytic Water Treatment Technology