280
TiO 2 Loading
Concentration of TiO 2 in the photocatalytic water treatment system affects the overall photocatalysis reaction rate in a true heterogeneous catalytic regime, where the
amount of TiO 2 is directly proportional to the overall photocatalytic reaction rate
[103]. A linear dependency holds until certain extent when the reaction rate starts to
aggravate and becomes independent of TiO 2 concentration. This is attributed to the
geometry and working conditions of the photoreactor where the surface reaction is
initiated upon light photon absorption [20]. When the amount of TiO 2 increases
above a saturation level (leading to a high turbidity state), the light photon absorption coefficient usually decreases radially. However, such a light attenuation over
the radial distance could not be well correlated with the Beer–Lambert law owing to
the strong absorption and scattering of light photons by the TiO 2 particles [44]. The
excess TiO 2 particles can create a light screening effect that reduces the surface area
of TiO 2 being exposed to light illumination and the photocatalytic efficiency.
Therefore, any chosen photoreactor should be operated below the saturation level of
TiO 2 photocatalyst used to avoid excess catalyst and ensure efficient photon absorption. In this sense, both catalyst loading and light scattering effect can be considered
as a function of optical path length in the reactor.
A large number of studies have reported the effect of TiO 2 loadings on the process efficiency [48, 61, 62, 103, 121, 247]. These results are mostly independent and
a direct comparison cannot be made, as the working geometry, radiation fluxes,
intensity, and wavelengths used were different. It was reported that the optimum
catalyst loadings for photomineralization and photo-disinfection are varied, and
mainly depend on the dimension of the photoreactor. In addition, the determination
of photoreactor diameter is crucial in not only the effective photon absorption but
also the water flow hydrodynamics [205]. Uniform flow region can ensure that a
steady-state residence time is obtained, while turbulence flow removes catalyst
deposition or reaction dead zone [204]. Reactor diameters smaller than 20–25 mm
were not feasible for turbulent flow while diameter larger than 50–60 mm is impractical. This is because large diameters usually have lower saturated catalyst loading
and efficiency. In this instance, the amount of catalyst should be considered. Usually
the TiO 2 catalysts can be mixed uniformly with the targeted water prior to the introduction into the reactor system. During the dark homogenization period of the catalyst, a lower initial concentration of organic pollutants is observed owing to the
strict adsorption of organics onto the catalyst surface [336]. Similarly, catalyst–bacterium interaction was reported in the photo-disinfection treatment of microorganisms [112].
pH
In heterogeneous photocatalytic water system, pH is one of the most important
operating parameters that affect the charge on the catalyst particles, size of catalyst
aggregates, and positions of conductance and valence bands. Due to the nature of
13 Wastewater
TiO 2 Loading
Concentration of TiO 2 in the photocatalytic water treatment system affects the overall photocatalysis reaction rate in a true heterogeneous catalytic regime, where the
amount of TiO 2 is directly proportional to the overall photocatalytic reaction rate
[103]. A linear dependency holds until certain extent when the reaction rate starts to
aggravate and becomes independent of TiO 2 concentration. This is attributed to the
geometry and working conditions of the photoreactor where the surface reaction is
initiated upon light photon absorption [20]. When the amount of TiO 2 increases
above a saturation level (leading to a high turbidity state), the light photon absorption coefficient usually decreases radially. However, such a light attenuation over
the radial distance could not be well correlated with the Beer–Lambert law owing to
the strong absorption and scattering of light photons by the TiO 2 particles [44]. The
excess TiO 2 particles can create a light screening effect that reduces the surface area
of TiO 2 being exposed to light illumination and the photocatalytic efficiency.
Therefore, any chosen photoreactor should be operated below the saturation level of
TiO 2 photocatalyst used to avoid excess catalyst and ensure efficient photon absorption. In this sense, both catalyst loading and light scattering effect can be considered
as a function of optical path length in the reactor.
A large number of studies have reported the effect of TiO 2 loadings on the process efficiency [48, 61, 62, 103, 121, 247]. These results are mostly independent and
a direct comparison cannot be made, as the working geometry, radiation fluxes,
intensity, and wavelengths used were different. It was reported that the optimum
catalyst loadings for photomineralization and photo-disinfection are varied, and
mainly depend on the dimension of the photoreactor. In addition, the determination
of photoreactor diameter is crucial in not only the effective photon absorption but
also the water flow hydrodynamics [205]. Uniform flow region can ensure that a
steady-state residence time is obtained, while turbulence flow removes catalyst
deposition or reaction dead zone [204]. Reactor diameters smaller than 20–25 mm
were not feasible for turbulent flow while diameter larger than 50–60 mm is impractical. This is because large diameters usually have lower saturated catalyst loading
and efficiency. In this instance, the amount of catalyst should be considered. Usually
the TiO 2 catalysts can be mixed uniformly with the targeted water prior to the introduction into the reactor system. During the dark homogenization period of the catalyst, a lower initial concentration of organic pollutants is observed owing to the
strict adsorption of organics onto the catalyst surface [336]. Similarly, catalyst–bacterium interaction was reported in the photo-disinfection treatment of microorganisms [112].
pH
In heterogeneous photocatalytic water system, pH is one of the most important
operating parameters that affect the charge on the catalyst particles, size of catalyst
aggregates, and positions of conductance and valence bands. Due to the nature of
13 Wastewater