1 3
Topics in Current Chemistry (2020) 378:29
performed at the laboratory scale, before the actual application at the commercial
level depending on the cost–benefit analysis.
4.3 Overview of Reported Studies on SPC Degradation and Guidelines
for Operating Conditions
A number of recent studies on SPC degradation [60–93] are summarized in Table 1
in terms of the operating conditions applied for sonolysis and photocatalysis, the
pollutant studied and the main results obtained in the work. Some of the representative cases for the most commonly applied catalysts, such as TiO 2 and ZnO, are
discussed in the following subsections to provide a better insight into the expected
intensification and controlling mechanisms.
4.3.1 Use of TiO 2 Catalyst
Neppolian et al. [94] studied both the synthesis of the catalyst and subsequent degradation of 4-chlorophenol using ultrasound combined with photocatalysis. TiO 2 ,
GO (graphene oxide)–TiO 2 and Pt–GO–TiO 2 were sonochemically synthesized
and compared for their SPC degradation efficiency. The results clearly established
that the frequency of ultrasonic irradiation was an important factor in determining
the extent of degradation and that the effect was not synergistic, but additive. Platinum-doped GO–TiO 2 showed the best degradation performance for 4-chlorophenol at acidic pH [94]. A similar additive effect was seen for the SPC degradation
of diclofenac using Fe–TiO 2 [95]. The observed effect, i.e. additive or synergistic,
depends strongly on the pollutant in question. A synergistic effect was reported for
the use of gold-doped TiO 2 in the SPC degradation of simazine, a herbicide, with
1.38- to 1.68-fold enhanced mineralization for the combined approach compared to
the sonochemical and photocatalytic processes separately [96]. The reported trends
for the kinetic rate constant (K) obtained for different processes as a function of
catalyst loading are shown in Fig. 12. This graph shows that the rate constant of
degradation reached an optimum with respect to the catalyst loading and also that
the rate constant for the combination approach was higher than that for the individual processes separately. The results for the mineralization of simazine reported
in the study using the various approaches in the absence and presence of Au–TiO 2
(1.5 g/L as the optimum loading) confirmed the role of the catalyst in enhancing
the mineralization and also the better efficacy of the combination approach. Another
study on a low-cost Fe–TiO 2 photocatalyst demonstrated a total decolourization of
Orange II dye using sonophotocatalysis in the visible light spectrum [84]. The study
also confirmed the synergistic effects of the combination of ultrasound and UV irradiations. Studies in which different ultrasonic frequencies (250, 500 and 1000 kHz)
were used confirmed that the best results were obtained at 500 kHz. Test results also
established that total decolourization is achieved by the combined process based
on visible light, without any formation of hazardous byproducts. A recent study
reported that along with the increase in the final degradation percentage of oflaxcin,
the rate of degradation was also higher in SPC oxidation due to the faster formation
87
Reprinted from the journal
Topics in Current Chemistry (2020) 378:29
performed at the laboratory scale, before the actual application at the commercial
level depending on the cost–benefit analysis.
4.3 Overview of Reported Studies on SPC Degradation and Guidelines
for Operating Conditions
A number of recent studies on SPC degradation [60–93] are summarized in Table 1
in terms of the operating conditions applied for sonolysis and photocatalysis, the
pollutant studied and the main results obtained in the work. Some of the representative cases for the most commonly applied catalysts, such as TiO 2 and ZnO, are
discussed in the following subsections to provide a better insight into the expected
intensification and controlling mechanisms.
4.3.1 Use of TiO 2 Catalyst
Neppolian et al. [94] studied both the synthesis of the catalyst and subsequent degradation of 4-chlorophenol using ultrasound combined with photocatalysis. TiO 2 ,
GO (graphene oxide)–TiO 2 and Pt–GO–TiO 2 were sonochemically synthesized
and compared for their SPC degradation efficiency. The results clearly established
that the frequency of ultrasonic irradiation was an important factor in determining
the extent of degradation and that the effect was not synergistic, but additive. Platinum-doped GO–TiO 2 showed the best degradation performance for 4-chlorophenol at acidic pH [94]. A similar additive effect was seen for the SPC degradation
of diclofenac using Fe–TiO 2 [95]. The observed effect, i.e. additive or synergistic,
depends strongly on the pollutant in question. A synergistic effect was reported for
the use of gold-doped TiO 2 in the SPC degradation of simazine, a herbicide, with
1.38- to 1.68-fold enhanced mineralization for the combined approach compared to
the sonochemical and photocatalytic processes separately [96]. The reported trends
for the kinetic rate constant (K) obtained for different processes as a function of
catalyst loading are shown in Fig. 12. This graph shows that the rate constant of
degradation reached an optimum with respect to the catalyst loading and also that
the rate constant for the combination approach was higher than that for the individual processes separately. The results for the mineralization of simazine reported
in the study using the various approaches in the absence and presence of Au–TiO 2
(1.5 g/L as the optimum loading) confirmed the role of the catalyst in enhancing
the mineralization and also the better efficacy of the combination approach. Another
study on a low-cost Fe–TiO 2 photocatalyst demonstrated a total decolourization of
Orange II dye using sonophotocatalysis in the visible light spectrum [84]. The study
also confirmed the synergistic effects of the combination of ultrasound and UV irradiations. Studies in which different ultrasonic frequencies (250, 500 and 1000 kHz)
were used confirmed that the best results were obtained at 500 kHz. Test results also
established that total decolourization is achieved by the combined process based
on visible light, without any formation of hazardous byproducts. A recent study
reported that along with the increase in the final degradation percentage of oflaxcin,
the rate of degradation was also higher in SPC oxidation due to the faster formation
87
Reprinted from the journal