Environmental Engineering Reference
In-Depth Information
reaction. However, the limited surface area of catalyst particles and the diffusion rate of
species involved in the reaction reduce the eficiency of photons in inducing the oxida-
tion reaction. At higher incident light intensity, the degradation rate is proportional to
the square root of the light intensity. The reason for the square root relationship has been
explained by bulk recombination of e
−
-h
+
pairs within the catalyst particles.
25.6.3 Photocatalyst Dosage
It is well documented that the photocatalytic reaction rate and eficiency would increase
with photocatalyst dosage. The increase in the photocatalytic eficiency seems to be due
to the increase in the total surface area available for the photocatalytic reaction as the dos-
age of the photocatalyst is increased. However, when the TiO
2
is overdosed, the number
of active sites on the TiO
2
surface may become almost constant because of the decreased
light penetration, increased light scattering, and the loss in surface area due to agglomera-
tion (particle-particle interactions) at high solid concentration. In any given application,
the optimum photocatalyst concentration has to be determined in order to avoid excess
photocatalyst usage and to ensure total adsorption of eficient photons. The optimal pho-
tocatalyst dosage or effective optical penetration length, under given conditions, is very
important in designing a slurry reactor for effective use of reactor space and photocatalyst.
25.6.4 Solution pH
The pH of an aqueous solution signiicantly affects all semiconductor oxides, including the
surface charge on the semiconductor particles, the size of the aggregates formed, and the
energy of the conduction and valence bands. At higher pH, valence band electrons become
more effective and the conduction band holes become less effective. Additionally, sus-
pended TiO
2
particles in water are known to be amphoteric. The effect of solution pH on
degradation rate depending on the acidity or basic property of the substrate should also
be considered while designing a photocatalytic reactor. In alkaline conditions, a high level
of hydroxide ions (OH
−
) induce the generation of hydroxyl free radicals (HO
∙
), which come
from the photooxidation of OH
−
by holes forming on the TiO
2
surface. Since the hydroxyl
free radical is the dominant oxidizing species in the photocatalytic process, the photode-
cay of organic compounds is therefore accelerated at higher solution pH.
25.6.5 Temperature
Like most photochemical reactions, a photocatalytic reaction is not signiicantly sensitive to
minor variations in temperature. Therefore, the potentially temperature-dependent steps
such as adsorption and desorption do not appear to be rate determining. However, higher
temperatures may have a negative effect on the concentration of dissolved oxygen (DO) in
the solution. DO concentration below a certain point may allow for e
−
-h
+
recombination at
the surface of the TiO
2
. In this respect, the photocatalytic system is appropriate for treating
contaminated water and wastewater at temperatures close to the ambient conditions.
25.6.6 Electron Scavenger
The photo-induced electrons must be removed from the TiO
2
to prevent the e
−
-h
+
pair
recombination and to enhance the eficiency of photocatalytic degradation. Molecular
oxygen has been employed as an effective electron acceptor in most of the photocatalysis
applications. Oxygen can be reduced to the superoxide,
O
2
, which may also participate
