287
adducts on the cell DNA. However, not all microorganisms are susceptible to the
UV-C radiation and some highly resistant microorganisms can survive through disinfection process. These include Legionella pneumophila and Cryptosporidium
parvum oocysts [205].
The natural UV radiation that reaches the surface of the earth consists of both
UV-A and UV-B spectrums. The photolysis mechanism for both UV irradiations on
cell inactivation is dissimilar to the discussed UV-C mechanism. Both UV-A and
UV-B irradiations can be absorbed by cellular components called intracellular chromophores. l-Tryptophan is the best known intracellular chromophore and is thought
to contain unsaturated bonds such as flavins, steroids, and quinines [311]. Among
these UV irradiations, the UV-A irradiation is toxic only in the presence of oxygen.
The ROS or oxidative stress generated from the chromophore light absorption can
damage cells and cell components, leading to lipid peroxidation, pyrimidine dimer
formation, and eventually DNA lesions. The contact between the ROS and DNA
results in single-strand breaks and nucleic acid modifications. Such damages on the
DNA are usually lethal or mutagenic irreversible. With the presence of TiO 2 catalyst
as the light sensitizers, a high degree of cell damage is seen as the amount of ROS
generated increases accordingly. A few microorganisms that are resistant to UV-A
photolysis have been inactivated successfully by TiO 2 photocatalysis, namely
E. cloacae, E. coli, P. aeruginosa, and S. typhimurium [90].
A longer wavelength of solar irradiation (l > 400 nm) has also been used in solar
disinfection (SODIS) study [26, 151, 196, 220, 291]. However, the photo-killing
mechanism is as yet unclear as it involves a variety of microbial and a larger mixed
spectrum of UV-A and solar irradiation. A similar cell destruction mechanism to the
one proposed for UV-A irradiation is thought to take place in this mixed light spectrum. In the SODIS, the pathogens in the drinking water contained in PET bottles
were found to be inactivated within 6 h of sunlight exposure. However, significant
research and developments on the disinfection using photocatalytic mediated process need to be conducted to broaden the photoactivity of current TiO 2 catalysts used.
Light Intensity
The photonic nature of the photocatalysis reaction has outlined the dependency of
the overall photocatalytic rate on the light source used. Light intensity is one of the
few parameters that affect the degree of photocatalytic reaction on organic substrates. Fujishima et al. [98] indicated that the initiation of TiO 2 photocatalysis reaction rates is not highly dependent on light intensity, where a few photons of energy
(i.e., as low as 1 mW/cm) can sufficiently induce the surface reaction. To achieve a
high photocatalytic reaction rate, particularly in water treatment, a relatively high
light intensity is required to adequately provide each TiO 2 surface-active site with
sufficient photon energy required. However, when using the nominal TiO 2 particles
without modifications, the surface reaction is restricted to photons with wavelengths
shorter than the absorption edge of approximately 400 nm. The organic conversion
Recent Developments in Photocatalytic Water Treatment Technology
adducts on the cell DNA. However, not all microorganisms are susceptible to the
UV-C radiation and some highly resistant microorganisms can survive through disinfection process. These include Legionella pneumophila and Cryptosporidium
parvum oocysts [205].
The natural UV radiation that reaches the surface of the earth consists of both
UV-A and UV-B spectrums. The photolysis mechanism for both UV irradiations on
cell inactivation is dissimilar to the discussed UV-C mechanism. Both UV-A and
UV-B irradiations can be absorbed by cellular components called intracellular chromophores. l-Tryptophan is the best known intracellular chromophore and is thought
to contain unsaturated bonds such as flavins, steroids, and quinines [311]. Among
these UV irradiations, the UV-A irradiation is toxic only in the presence of oxygen.
The ROS or oxidative stress generated from the chromophore light absorption can
damage cells and cell components, leading to lipid peroxidation, pyrimidine dimer
formation, and eventually DNA lesions. The contact between the ROS and DNA
results in single-strand breaks and nucleic acid modifications. Such damages on the
DNA are usually lethal or mutagenic irreversible. With the presence of TiO 2 catalyst
as the light sensitizers, a high degree of cell damage is seen as the amount of ROS
generated increases accordingly. A few microorganisms that are resistant to UV-A
photolysis have been inactivated successfully by TiO 2 photocatalysis, namely
E. cloacae, E. coli, P. aeruginosa, and S. typhimurium [90].
A longer wavelength of solar irradiation (l > 400 nm) has also been used in solar
disinfection (SODIS) study [26, 151, 196, 220, 291]. However, the photo-killing
mechanism is as yet unclear as it involves a variety of microbial and a larger mixed
spectrum of UV-A and solar irradiation. A similar cell destruction mechanism to the
one proposed for UV-A irradiation is thought to take place in this mixed light spectrum. In the SODIS, the pathogens in the drinking water contained in PET bottles
were found to be inactivated within 6 h of sunlight exposure. However, significant
research and developments on the disinfection using photocatalytic mediated process need to be conducted to broaden the photoactivity of current TiO 2 catalysts used.
Light Intensity
The photonic nature of the photocatalysis reaction has outlined the dependency of
the overall photocatalytic rate on the light source used. Light intensity is one of the
few parameters that affect the degree of photocatalytic reaction on organic substrates. Fujishima et al. [98] indicated that the initiation of TiO 2 photocatalysis reaction rates is not highly dependent on light intensity, where a few photons of energy
(i.e., as low as 1 mW/cm) can sufficiently induce the surface reaction. To achieve a
high photocatalytic reaction rate, particularly in water treatment, a relatively high
light intensity is required to adequately provide each TiO 2 surface-active site with
sufficient photon energy required. However, when using the nominal TiO 2 particles
without modifications, the surface reaction is restricted to photons with wavelengths
shorter than the absorption edge of approximately 400 nm. The organic conversion
Recent Developments in Photocatalytic Water Treatment Technology