284
an electron sink but also provides sufficient buoyant force for complete suspension
of TiO 2 particles. Photoreactor sparging with pure oxygen in TiO 2 slurry reactor is
usually a cost-ineffective solution, as the amount of DO being held up is a function
of the photoreactor geometry. The difference between the two sparging media of air
and oxygen is usually not very drastic as the mass transfer of oxygen to the close
vicinity of the surface is the rate-dependent step [118]. Generally Henry’s law can
be assumed to give a good approximation of the amount of oxygen dissolved under
the experimental conditions, provided that the oxygen sparging rate and the photoreactor gas holdup are known [61]. In this equilibrium law, it is also necessary to
account for the decrease in oxygen solubility with increasing reaction temperature.
As discussed, it is preferential to operate the photoreactor under ambient conditions
to prevent elevation of cost of air or oxygen sparging for enhanced electron sink.
The presence of dissolved oxygen is also suggested to induce the cleavage mechanism for aromatic rings in organic pollutants that are present in water matrices.
Wang and Hong [322] proposed that during aromatic ring cleavage in the degradation of a dioxyl compound, the molecular oxygen will follow the cleaving attack of
a second hydroxyl radical. The analogous partial pressure of oxygen applied during
the reaction for a closed reactor system is also important. A significantly higher
partial pressure of oxygen will result in a higher initial organics photomineralization rate than its structural transformation [45, 322]. It was reported that at low
oxygen pressure of 0.5 kPa, 75% of the original 2-CB compounds were transformed,
but only 1% of them was mineralized to CO 2 after 5 h of UV irradiation [322].
Though elevated partial pressure of oxygen is imperative, it is difficult to quantify
their specific influence on the surface activity of TiO 2 , owing to the polyphasic
nature of the photocatalytic liquid-phase reaction [121].
The detrimental effect of DO on the photocatalysis reaction was reported by
Shirayama et al. [290]. They observed an elevated photodegradation rate of chlorinated hydrocarbons in the absence of DO. This could be explained by the strong
absorption characteristics for UV photons at intensities of 185 nm and 254 nm,
respectively. The DO molecules act as an inner filter in this case and cause a sharp
attenuation in UV light intensity mainly at UV-C germicidal region. There is no
report found to determine the DO impact on a photocatalysis process using UV-A or
UV-B light sources. To date, the effect of DO on the efficiency of photo-disinfection
rate has been paid little attention. The formation of various ROS under the series of
redox reactions on the catalyst surface was assumed to be similar in both photomineralization and photo-disinfection reactions. If sufficient nutrients are available, the
constant sparging of DO will generally promote microbial growth and offset the
photo-disinfection rate. This indirectly prolongs the irradiation time necessary to
achieve the desired inactivation level. It is thus recommended that the effect of DO
on the microbial inactivation should be investigated thoroughly in a particular photoreactor system before improvising the DO sparging strategy.
13 Wastewater
an electron sink but also provides sufficient buoyant force for complete suspension
of TiO 2 particles. Photoreactor sparging with pure oxygen in TiO 2 slurry reactor is
usually a cost-ineffective solution, as the amount of DO being held up is a function
of the photoreactor geometry. The difference between the two sparging media of air
and oxygen is usually not very drastic as the mass transfer of oxygen to the close
vicinity of the surface is the rate-dependent step [118]. Generally Henry’s law can
be assumed to give a good approximation of the amount of oxygen dissolved under
the experimental conditions, provided that the oxygen sparging rate and the photoreactor gas holdup are known [61]. In this equilibrium law, it is also necessary to
account for the decrease in oxygen solubility with increasing reaction temperature.
As discussed, it is preferential to operate the photoreactor under ambient conditions
to prevent elevation of cost of air or oxygen sparging for enhanced electron sink.
The presence of dissolved oxygen is also suggested to induce the cleavage mechanism for aromatic rings in organic pollutants that are present in water matrices.
Wang and Hong [322] proposed that during aromatic ring cleavage in the degradation of a dioxyl compound, the molecular oxygen will follow the cleaving attack of
a second hydroxyl radical. The analogous partial pressure of oxygen applied during
the reaction for a closed reactor system is also important. A significantly higher
partial pressure of oxygen will result in a higher initial organics photomineralization rate than its structural transformation [45, 322]. It was reported that at low
oxygen pressure of 0.5 kPa, 75% of the original 2-CB compounds were transformed,
but only 1% of them was mineralized to CO 2 after 5 h of UV irradiation [322].
Though elevated partial pressure of oxygen is imperative, it is difficult to quantify
their specific influence on the surface activity of TiO 2 , owing to the polyphasic
nature of the photocatalytic liquid-phase reaction [121].
The detrimental effect of DO on the photocatalysis reaction was reported by
Shirayama et al. [290]. They observed an elevated photodegradation rate of chlorinated hydrocarbons in the absence of DO. This could be explained by the strong
absorption characteristics for UV photons at intensities of 185 nm and 254 nm,
respectively. The DO molecules act as an inner filter in this case and cause a sharp
attenuation in UV light intensity mainly at UV-C germicidal region. There is no
report found to determine the DO impact on a photocatalysis process using UV-A or
UV-B light sources. To date, the effect of DO on the efficiency of photo-disinfection
rate has been paid little attention. The formation of various ROS under the series of
redox reactions on the catalyst surface was assumed to be similar in both photomineralization and photo-disinfection reactions. If sufficient nutrients are available, the
constant sparging of DO will generally promote microbial growth and offset the
photo-disinfection rate. This indirectly prolongs the irradiation time necessary to
achieve the desired inactivation level. It is thus recommended that the effect of DO
on the microbial inactivation should be investigated thoroughly in a particular photoreactor system before improvising the DO sparging strategy.
13 Wastewater