Topics in Current Chemistry (2020) 378:29
1 3
nanocubes were effectively applied for the visible light photodegradation of methyl
orange, rhodamine B and methylene blue. An interesting middle-sagged CaSn(OH) 6
microcube morphology, which could not be synthesized by conventional methods,
was reported to be uniformly synthesized sonochemically [45]. The obtained catalyst sonochemically demonstrated higher photodegradation activity and stability as
compared to the conventionally prepared catalyst with different morphology. Other
interesting sonochemically synthesized photocatalysts that have been found to demonstrate superior performance in photocatalysis or easier synthesis process for the
catalyst as compared to conventional methods are Nano BiPO 4 and silver-doped
BiPO 4 nanostructures [46], CdWO 4 nanoparticles [47] and Ag-PbMoO 4 [48].
4 Sonophotocatalytic Degradation
In sonophotocatalytic processes, the photocatalytic degradation of contaminants is
supplemented with the physiochemical effect of ultrasonic cavitation. The hydroxyl
radicals generated during the collapse of cavities provide a synergistic effect with
the electron–hole generation in the photocatalyst for inducing oxidation reactions.
Sonophotocatalytic (SPC) degradation has been studied widely to provide synergistic effects of sonolysis and photocatalysis in the combined process. The expected
benefit of using ultrasound with photocatalysis is that ultrasound promotes the
mechanical disaggregation of catalysts, enhancing the surface area for photocatalytic
degradation. Also, it enhances the formation of reactive hydroxyl radicals by sonolysis and thereby abatement of the effect of electron–hole recombination in photodegradation [49]. In addition to these beneficial effects on photocatalysis, the presence
of solid catalysts also supports the cavitation phenomena. The heterogeneity due
to the presence of solids favours the generation of cavities based on the nucleation
events and, consequently, the overall cavitational activity increases, further contributing to the synergy.
4.1 SPC Reactors
Studies on SPC degradation have generally involved batch reactors and three commonly used approaches [50–54]. One approach involves the use of a bath-type
sonicator (with ultrasonic transducers at the bottom of the reactor) with a UV/light
source at either the top of the vessel or immersed in the solution using quartz tubes,
as shown in Fig. 8. A second approach involves the use of a horn-type sonicator in
a cylindrical reactor, with UV light provided from the opposite direction, as shown
in Fig. 9. The third configuration is based on sequential sonolysis (with horn) and a
UV reactor (separate and not combined operation of ultrasound/UV), with a pump
to drive the solution through the system, as shown in Fig. 10. It is important to
understand that though these options are commonly used, most studies have been at
the laboratory scale and these may not be scaled up effectively to large-scale operation. A major factor in deciding the commercial application is the need for continuous reactors. Also, an optimized design needs to be looked at based on the use of
82
Reprinted from the journal
1 3
nanocubes were effectively applied for the visible light photodegradation of methyl
orange, rhodamine B and methylene blue. An interesting middle-sagged CaSn(OH) 6
microcube morphology, which could not be synthesized by conventional methods,
was reported to be uniformly synthesized sonochemically [45]. The obtained catalyst sonochemically demonstrated higher photodegradation activity and stability as
compared to the conventionally prepared catalyst with different morphology. Other
interesting sonochemically synthesized photocatalysts that have been found to demonstrate superior performance in photocatalysis or easier synthesis process for the
catalyst as compared to conventional methods are Nano BiPO 4 and silver-doped
BiPO 4 nanostructures [46], CdWO 4 nanoparticles [47] and Ag-PbMoO 4 [48].
4 Sonophotocatalytic Degradation
In sonophotocatalytic processes, the photocatalytic degradation of contaminants is
supplemented with the physiochemical effect of ultrasonic cavitation. The hydroxyl
radicals generated during the collapse of cavities provide a synergistic effect with
the electron–hole generation in the photocatalyst for inducing oxidation reactions.
Sonophotocatalytic (SPC) degradation has been studied widely to provide synergistic effects of sonolysis and photocatalysis in the combined process. The expected
benefit of using ultrasound with photocatalysis is that ultrasound promotes the
mechanical disaggregation of catalysts, enhancing the surface area for photocatalytic
degradation. Also, it enhances the formation of reactive hydroxyl radicals by sonolysis and thereby abatement of the effect of electron–hole recombination in photodegradation [49]. In addition to these beneficial effects on photocatalysis, the presence
of solid catalysts also supports the cavitation phenomena. The heterogeneity due
to the presence of solids favours the generation of cavities based on the nucleation
events and, consequently, the overall cavitational activity increases, further contributing to the synergy.
4.1 SPC Reactors
Studies on SPC degradation have generally involved batch reactors and three commonly used approaches [50–54]. One approach involves the use of a bath-type
sonicator (with ultrasonic transducers at the bottom of the reactor) with a UV/light
source at either the top of the vessel or immersed in the solution using quartz tubes,
as shown in Fig. 8. A second approach involves the use of a horn-type sonicator in
a cylindrical reactor, with UV light provided from the opposite direction, as shown
in Fig. 9. The third configuration is based on sequential sonolysis (with horn) and a
UV reactor (separate and not combined operation of ultrasound/UV), with a pump
to drive the solution through the system, as shown in Fig. 10. It is important to
understand that though these options are commonly used, most studies have been at
the laboratory scale and these may not be scaled up effectively to large-scale operation. A major factor in deciding the commercial application is the need for continuous reactors. Also, an optimized design needs to be looked at based on the use of
82
Reprinted from the journal