• No increase of salinity.
• Easy process control.
• Effectiveness in batch reactions.
10.2 Fundamental Concept Behind the Photocatalyst UV
Irradiation Mediated Decontamination
With a specific end goal to emulate the nature, a noteworthy advancement has been
made by our researchers towards use of photochemical response to dispose of the
dangerous pollutants from contaminated water, which shall be discussed in the
ensuing sections. But it is important to understand what actually happens when
contaminants are exposed to UV radiation. A radical reaction is directly initiated
during a photocatalyst-mediated reaction due to the activation energy supplied by the
highly energetic UV radiation to the catalyst surface and the water molecules in the
effluent. Different types of semiconductor oxides have been used as catalysts for this
particular purpose. Upon illumination with radiation of energy greater than its
optical band gap, the semiconductor molecules transfer an electron to the conduction
band leaving behind a hole in the valence band. The electron transfer takes place
either from the conduction band to an acceptor in the solution or from a donor in
solution to the valence band at the solid–liquid interface. These processes compete
with the recombination of electron and hole to produce thermal energy, which is not
useful for decontamination. In the absence of suitable electron and hole scavengers,
the stored energy is dissipated within a few nanoseconds by recombination. The
valence band holes are powerful oxidants (1.0 to 3.5 V) while conduction band
electrons are good reductants (0.5 to À1.5 eV). This eventually leads to
photocatalytic redox processes ultimately leading to the destruction of the organic
molecules.
10.2.1 Reactive Oxidizing Species
The effectiveness of the photo-oxidation process, (i.e., the complete oxidation of
pollutants into carbon dioxide and water) depends on the oxidation potential of the
oxidizing agent employed for this purpose. The oxidation potentials of some important oxidants are given below in Table 10.1 (Barrera-Díaz 2014; Hassaan and El
Nemr 2017).
Legrini et al. (1993) reported that fluorine has higher oxidation potential than the
hydroxyl radical, but because of its adverse effects on humans and environment, it is
undesirable to use fluorine in water treatment. Therefore, hydroxyl radical stands out
to be the most promising reactive species for oxidation processes (Fig. 10.3)
(Buthiyappan et al. 2016).
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• Easy process control.
• Effectiveness in batch reactions.
10.2 Fundamental Concept Behind the Photocatalyst UV
Irradiation Mediated Decontamination
With a specific end goal to emulate the nature, a noteworthy advancement has been
made by our researchers towards use of photochemical response to dispose of the
dangerous pollutants from contaminated water, which shall be discussed in the
ensuing sections. But it is important to understand what actually happens when
contaminants are exposed to UV radiation. A radical reaction is directly initiated
during a photocatalyst-mediated reaction due to the activation energy supplied by the
highly energetic UV radiation to the catalyst surface and the water molecules in the
effluent. Different types of semiconductor oxides have been used as catalysts for this
particular purpose. Upon illumination with radiation of energy greater than its
optical band gap, the semiconductor molecules transfer an electron to the conduction
band leaving behind a hole in the valence band. The electron transfer takes place
either from the conduction band to an acceptor in the solution or from a donor in
solution to the valence band at the solid–liquid interface. These processes compete
with the recombination of electron and hole to produce thermal energy, which is not
useful for decontamination. In the absence of suitable electron and hole scavengers,
the stored energy is dissipated within a few nanoseconds by recombination. The
valence band holes are powerful oxidants (1.0 to 3.5 V) while conduction band
electrons are good reductants (0.5 to À1.5 eV). This eventually leads to
photocatalytic redox processes ultimately leading to the destruction of the organic
molecules.
10.2.1 Reactive Oxidizing Species
The effectiveness of the photo-oxidation process, (i.e., the complete oxidation of
pollutants into carbon dioxide and water) depends on the oxidation potential of the
oxidizing agent employed for this purpose. The oxidation potentials of some important oxidants are given below in Table 10.1 (Barrera-Díaz 2014; Hassaan and El
Nemr 2017).
Legrini et al. (1993) reported that fluorine has higher oxidation potential than the
hydroxyl radical, but because of its adverse effects on humans and environment, it is
undesirable to use fluorine in water treatment. Therefore, hydroxyl radical stands out
to be the most promising reactive species for oxidation processes (Fig. 10.3)
(Buthiyappan et al. 2016).
224
R. K. Sharma et al.