gas bubbles (with equilibrium size of few microns), which is driven by the pressure
variation induced by the ultrasound. The implosive collapse of transient cavitation
bubbles induces enormous concentrated energy which has a very diminutive, spatial,
and temporal scale. During the transient collapse, a hot spot is generated, and the
temperatures and pressures inside the cavitation bubble are extremely high (~5000 K
and 500 bar) (Suslick 1990). The generated radicals at this extreme condition due to
the dissociation of gases and solvent vapor molecules either diffuse out of the
cavitation bubble (during the maximum compression or minimum radius) or get
released into the bulk liquid medium as the bubble undergoes fragmentation at the
point of maximum compression during radial motion. These generated radicals can
induce and accelerate numerous chemical reactions in the bulk liquid medium
including oxidative degradation of the organic pollutants. The coupling of sonolysis
technique with other conventional advanced oxidation processes is called the
sono-hybrid advanced oxidation processes. The widely used sono-hybrid advanced
oxidation processes are sono-Fenton, sono-photolysis, sono-photocatalysis, sonoferrioxalate, and sono-persulfate (Bagal and Gogate 2014a; Katsumata et al. 2010;
Chakma and Moholkar 2013a, 2014, 2015a, b, c; Chakma et al. 2013, 2017; Dinesh
and Chakma 2019a, b). These hybrid advanced oxidation processes are more
effective and efficient for mineralization of biorecalcitrant pollutants (Entezari and
Petrier 2005; Kim et al. 2015; Chakma and Moholkar 2016a; Malani et al. 2014),
aniline (Chen and Huang 2015), pharmaceutical drugs (Dinesh and Chakma
2019a, b), azo and non-azo textile dyes (Chakma and Moholkar 2015c; Malani
et al. 2014), plastic intermediate bisphenol A (Chakma and Moholkar 2014; Huang
et al. 2012), chlorinated aromatic compounds (Peller et al. 2003), distillery wastewater (Sangave and Pandit 2006), herbicide linuron and insecticide fenitrothion
(Katsumata et al. 2010, 2011), nonsteroidal anti-inflammatory drug diclofenac
(Bagal and Gogate 2014b), copolymer styrene–acrylic acid (Saien et al. 2010), and
cholesterol (Sun et al. 2011).
6.2 Sonolysis
Sonolysis is the application of ultrasound which induces physical and chemical
effects in the medium. During sonolysis degradation, the gas and vapor molecules
present in the bubbles of the liquid medium are subjected to extreme conditions
generated inside the microbubbles. This results in thermal dissociation of these
molecules and generation of numerous reactive chemical species including radicals
as mentioned earlier. This phenomenon is directly related to the transport of vapor
molecules across the bubbles’ interfaces. During the growth of cavitation
microbubble in radial motion, evaporation of the solvent at the interface of the
bubble causes diffusion of the vapor molecules in the direction of the center of the
bubble. But in the subsequent compression period, only a few vapor molecules can
diffuse back at the interface of the bubble and undergo condensation. Then the
“entrapped” vapor molecules undergo an extreme condition at the moment of
192
S. Chakma et al.
variation induced by the ultrasound. The implosive collapse of transient cavitation
bubbles induces enormous concentrated energy which has a very diminutive, spatial,
and temporal scale. During the transient collapse, a hot spot is generated, and the
temperatures and pressures inside the cavitation bubble are extremely high (~5000 K
and 500 bar) (Suslick 1990). The generated radicals at this extreme condition due to
the dissociation of gases and solvent vapor molecules either diffuse out of the
cavitation bubble (during the maximum compression or minimum radius) or get
released into the bulk liquid medium as the bubble undergoes fragmentation at the
point of maximum compression during radial motion. These generated radicals can
induce and accelerate numerous chemical reactions in the bulk liquid medium
including oxidative degradation of the organic pollutants. The coupling of sonolysis
technique with other conventional advanced oxidation processes is called the
sono-hybrid advanced oxidation processes. The widely used sono-hybrid advanced
oxidation processes are sono-Fenton, sono-photolysis, sono-photocatalysis, sonoferrioxalate, and sono-persulfate (Bagal and Gogate 2014a; Katsumata et al. 2010;
Chakma and Moholkar 2013a, 2014, 2015a, b, c; Chakma et al. 2013, 2017; Dinesh
and Chakma 2019a, b). These hybrid advanced oxidation processes are more
effective and efficient for mineralization of biorecalcitrant pollutants (Entezari and
Petrier 2005; Kim et al. 2015; Chakma and Moholkar 2016a; Malani et al. 2014),
aniline (Chen and Huang 2015), pharmaceutical drugs (Dinesh and Chakma
2019a, b), azo and non-azo textile dyes (Chakma and Moholkar 2015c; Malani
et al. 2014), plastic intermediate bisphenol A (Chakma and Moholkar 2014; Huang
et al. 2012), chlorinated aromatic compounds (Peller et al. 2003), distillery wastewater (Sangave and Pandit 2006), herbicide linuron and insecticide fenitrothion
(Katsumata et al. 2010, 2011), nonsteroidal anti-inflammatory drug diclofenac
(Bagal and Gogate 2014b), copolymer styrene–acrylic acid (Saien et al. 2010), and
cholesterol (Sun et al. 2011).
6.2 Sonolysis
Sonolysis is the application of ultrasound which induces physical and chemical
effects in the medium. During sonolysis degradation, the gas and vapor molecules
present in the bubbles of the liquid medium are subjected to extreme conditions
generated inside the microbubbles. This results in thermal dissociation of these
molecules and generation of numerous reactive chemical species including radicals
as mentioned earlier. This phenomenon is directly related to the transport of vapor
molecules across the bubbles’ interfaces. During the growth of cavitation
microbubble in radial motion, evaporation of the solvent at the interface of the
bubble causes diffusion of the vapor molecules in the direction of the center of the
bubble. But in the subsequent compression period, only a few vapor molecules can
diffuse back at the interface of the bubble and undergo condensation. Then the
“entrapped” vapor molecules undergo an extreme condition at the moment of
192
S. Chakma et al.