addition, traditional processes (physical and chemical) are found to be comparatively
expensive and may also lead to other secondary pollution due to the requirement of
additional chemicals (Oller et al. 2011). Conventional biological procedures in
association with physical/chemical processes to obtain improved decolorization of
effluents may generally be costly and limited applicable. The adsorption processes
using suitable adsorbents (activated carbons, charcoals, clays, diatomaceous earth,
microbial biomass, compost, unmodified and modified lignocellulose, etc.) for the
wastewater treatment have disadvantage of regeneration, which also increases the
cost of the process. At times, it demands a very long time, and it may be ineffective
for low concentrations of pollutants. Oxidation can be used to decolorize the dye
effluent using various oxidizing agents such as hydrogen peroxide and ozone
activated with Fe(II) salts. However, the pollutant removal mechanisms involve
flocculation, and contaminants are shifted from the wastewater to the sludge,
which further demands disposal in landfill (Boczkaj and Fernandes 2017; De Lima
et al. 2017). Hence, the abovementioned techniques may not provide sustained
solution with ecological point of view. Very recently, electrochemical treatment
methods have achieved a good position as effective and efficient treatment technologies for dye effluents. However, the application of electrochemical techniques
warrants a higher investment in equipment and energy requirement (Vaghela et al.
2005; Butler et al. 2011; Gautam et al. 2019).
In the recent years, researchers have used ionic liquids for various applications
such as biocatalytic transformation, chemical synthesis, electrochemistry, analytical
chemistry, and separation processes, because of the green characteristics of ionic
liquids (Wasserscheid and Keim 2000; van Rantwijk et al. 2003; Pandey 2006;
Hernández-Fernández et al. 2015). Main advantages of ionic liquids are the better
possibilities of reusing and relatively easy recovery of ionic liquids. Therefore,
application of ionic liquids effectively reduces the amount of waste generated during
treatment process. The photodegradation of dye molecules using ionic liquids (ionic
liquids), also regarded as green solvents, has advantageous over other processes with
the view of environment protection. However, ionic liquids are still quite expensive.
Hence, regeneration and recycling of ionic liquids are important to make a technology economically viable (Fernández et al. 2010). The scheme to reduce the amount
of pollutants from industrial effluents using ionic liquids and potential sources of
environmental release is presented in Fig. 12.1.
12.2 Ionic Liquids
An ionic liquid is a molten salt comprising of positive charged ions, cations and
negative charged ions, anions having melting point below 100
C. Combination of
bulky and asymmetrical cations such as imidazolium, pyrrolidinium, and pyridinium
and evenly shaped symmetrical anions such as halides, triflate, and nitrate lowers the
lattice energy which is accounted for low melting points. Ionic liquids are also
known as liquid electrolytes, room temperature molten salts, low temperature molten
12 Functionalized Ionic Liquids for the Photodegradation of Dyes
393
expensive and may also lead to other secondary pollution due to the requirement of
additional chemicals (Oller et al. 2011). Conventional biological procedures in
association with physical/chemical processes to obtain improved decolorization of
effluents may generally be costly and limited applicable. The adsorption processes
using suitable adsorbents (activated carbons, charcoals, clays, diatomaceous earth,
microbial biomass, compost, unmodified and modified lignocellulose, etc.) for the
wastewater treatment have disadvantage of regeneration, which also increases the
cost of the process. At times, it demands a very long time, and it may be ineffective
for low concentrations of pollutants. Oxidation can be used to decolorize the dye
effluent using various oxidizing agents such as hydrogen peroxide and ozone
activated with Fe(II) salts. However, the pollutant removal mechanisms involve
flocculation, and contaminants are shifted from the wastewater to the sludge,
which further demands disposal in landfill (Boczkaj and Fernandes 2017; De Lima
et al. 2017). Hence, the abovementioned techniques may not provide sustained
solution with ecological point of view. Very recently, electrochemical treatment
methods have achieved a good position as effective and efficient treatment technologies for dye effluents. However, the application of electrochemical techniques
warrants a higher investment in equipment and energy requirement (Vaghela et al.
2005; Butler et al. 2011; Gautam et al. 2019).
In the recent years, researchers have used ionic liquids for various applications
such as biocatalytic transformation, chemical synthesis, electrochemistry, analytical
chemistry, and separation processes, because of the green characteristics of ionic
liquids (Wasserscheid and Keim 2000; van Rantwijk et al. 2003; Pandey 2006;
Hernández-Fernández et al. 2015). Main advantages of ionic liquids are the better
possibilities of reusing and relatively easy recovery of ionic liquids. Therefore,
application of ionic liquids effectively reduces the amount of waste generated during
treatment process. The photodegradation of dye molecules using ionic liquids (ionic
liquids), also regarded as green solvents, has advantageous over other processes with
the view of environment protection. However, ionic liquids are still quite expensive.
Hence, regeneration and recycling of ionic liquids are important to make a technology economically viable (Fernández et al. 2010). The scheme to reduce the amount
of pollutants from industrial effluents using ionic liquids and potential sources of
environmental release is presented in Fig. 12.1.
12.2 Ionic Liquids
An ionic liquid is a molten salt comprising of positive charged ions, cations and
negative charged ions, anions having melting point below 100
C. Combination of
bulky and asymmetrical cations such as imidazolium, pyrrolidinium, and pyridinium
and evenly shaped symmetrical anions such as halides, triflate, and nitrate lowers the
lattice energy which is accounted for low melting points. Ionic liquids are also
known as liquid electrolytes, room temperature molten salts, low temperature molten
12 Functionalized Ionic Liquids for the Photodegradation of Dyes
393