generation of secondary waste, and maximum pollutants get transformed to water,
carbon dioxide and salts during mineralization (Saxena et al. 2016).
The general objective of advanced oxidation processes to treat tannery wastewater is to lessen the contamination load to such a degree that they might be restored to
the water reservoirs or reused during the other operations. Table 11.4 highlights the
various advanced oxidation processes applied over tannery wastewater.
As shown in above table, there are few studies which implements solar energy as
photocatalysis to treat tannery wastewater, and in the context to current environmental scenario, the use of solar energy in terms of photocatalysis could play a key
role to treat groundwater, drinking water and industrial wastewater. However, solar
photocatalysis technique has been used for water splitting to produce hydrogen and
degradation of toxic elements, dyes and chemicals (Shimura and Yoshida 2011).
Hence, the treatment of tannery wastewater through solar photocatalysis could be an
economic and cost-effective approach.
11.7 Solar Photocatalysis Process
The solar photocatalysis process involves the use of solar energy to excite a
semiconductor catalyst also known as a photocatalyst, and the electronic structure
of a photocatalyst (consists of a valence band and conduction band) can act as a
sensitizer for the light-driven redox reaction. If a surface reaction has more positive
oxidation potential in comparison to valence band potential, then there will be no
oxidation; similarly if a surface reaction has more negative reduction potential
compared to conduction band, then reduction will not take place, which denotes
the absence of hydroxyl and superoxide radicals (Simonsen 2014). Hence, the
position of conduction and valence band potential plays an important role in solar
photocatalysis. If a photocatalyst is exposed through light having a wavelength equal
or larger than its band gap energy, an electron will be excited to the conduction band
leaving a positive hole in the valence band, and these electron and hole initiate
reduction and oxidation process. Furthermore, the electron and hole tend to recombine very rapidly; if an appropriate surface defect or scavenger restricts this recombination rate, then there will be an efficient photocatalytic effect to mineralize
organic impurities present in wastewater. In addition, the oxygen present in the
atmosphere leads to produce superoxide ions which are a dominant oxidant
(Spasiano et al. 2015).
The fundamentals of this technique are well established, and the characteristics of
solar photocatalysis which make the applicability to treat industrial effluent are
followed (Malato et al. 2009):
1. The process takes place at ambient condition.
2. Complete oxidation of polluting substance into carbon dioxide and other inorganic species.
11 Solar Photocatalytic Treatment of Tannery Effluents
371
carbon dioxide and salts during mineralization (Saxena et al. 2016).
The general objective of advanced oxidation processes to treat tannery wastewater is to lessen the contamination load to such a degree that they might be restored to
the water reservoirs or reused during the other operations. Table 11.4 highlights the
various advanced oxidation processes applied over tannery wastewater.
As shown in above table, there are few studies which implements solar energy as
photocatalysis to treat tannery wastewater, and in the context to current environmental scenario, the use of solar energy in terms of photocatalysis could play a key
role to treat groundwater, drinking water and industrial wastewater. However, solar
photocatalysis technique has been used for water splitting to produce hydrogen and
degradation of toxic elements, dyes and chemicals (Shimura and Yoshida 2011).
Hence, the treatment of tannery wastewater through solar photocatalysis could be an
economic and cost-effective approach.
11.7 Solar Photocatalysis Process
The solar photocatalysis process involves the use of solar energy to excite a
semiconductor catalyst also known as a photocatalyst, and the electronic structure
of a photocatalyst (consists of a valence band and conduction band) can act as a
sensitizer for the light-driven redox reaction. If a surface reaction has more positive
oxidation potential in comparison to valence band potential, then there will be no
oxidation; similarly if a surface reaction has more negative reduction potential
compared to conduction band, then reduction will not take place, which denotes
the absence of hydroxyl and superoxide radicals (Simonsen 2014). Hence, the
position of conduction and valence band potential plays an important role in solar
photocatalysis. If a photocatalyst is exposed through light having a wavelength equal
or larger than its band gap energy, an electron will be excited to the conduction band
leaving a positive hole in the valence band, and these electron and hole initiate
reduction and oxidation process. Furthermore, the electron and hole tend to recombine very rapidly; if an appropriate surface defect or scavenger restricts this recombination rate, then there will be an efficient photocatalytic effect to mineralize
organic impurities present in wastewater. In addition, the oxygen present in the
atmosphere leads to produce superoxide ions which are a dominant oxidant
(Spasiano et al. 2015).
The fundamentals of this technique are well established, and the characteristics of
solar photocatalysis which make the applicability to treat industrial effluent are
followed (Malato et al. 2009):
1. The process takes place at ambient condition.
2. Complete oxidation of polluting substance into carbon dioxide and other inorganic species.
11 Solar Photocatalytic Treatment of Tannery Effluents
371