1 3
Topics in Current Chemistry (2020) 378:29
dye using 95 W of ultrasonic power for only 20 min [76], and ZnO-decorated CNTs
showed 49% rhodamine B degradation under SPC conditions [99]. The effect in the
latter study was reported to be pronounced due to the prevention of electron hole
recombination as a result of the presence of CNTs [99]. A synergistic effect was also
reported for the use of Pr-doped ZnO catalysis combined with sonochemistry, leading up to 89% degradation for the sonocatalytic degradation of AR17 dye [100].
4.3.3 Guidelines for the Operating Parameters
Based on the critical analysis of the literature and personal expertise in the research
field, the following useful guidelines for the selection of the operating parameters
can be given so as to maximize the degradation or mineralization efficacy:
1. Operating pH plays a key role in deciding the efficacy of treatment. In general,
acidic conditions are favoured although it also depends on the dissociation coefficient of the pollutants present in the system. It is also possible that the effects
are less pronounced in the combined approach, and exact intensification should
be quantified before deciding on the pH adjustment. This is even more important
in the case of commercial effluent treatment schemes as usually the discharge
conditions mandate neutral pH conditions.
2. Catalyst loading needs to be properly decided so that all of the catalyst is in
proper suspension in the reactor and that it is also not affecting the passage of the
incident irradiations in the reactor. An optimum catalyst loading usually exists,
which can be established using laboratory studies for the specific applications in
question.
3. Use of radical promoters, such as hydrogen peroxide or persulphate, can bestow
beneficial results, especially when the free radical attack is the controlling mechanism for oxidation and the rate of reaction between the radicals and pollutant
molecules is also substantially high. This is important as the generated hydroxyl
radicals must be utilized quickly so as to get any beneficial results. Use of persulphate offers simultaneously acting oxidation mechanism based on the sulphate
radicals.
4. The observed synergy for the combined approach as compared to individual
approaches is also dependent on the pollutants in question. Specifically, if the
sonochemical oxidation is based on the pyrolytic mechanism of oxidation, significantly beneficial results may not be obtained as the combination is typically
accelerating the free radical formation. This has also been confirmed based on
the reported data for the synergy in Table 1.
5. Use of doping and composites allow the utilization of the solar bandwidth which
can result in a cost-effective operation. Consequently, the utilization of such catalyst forms is recommended as compared to the traditionally used single oxide
forms. This is also important to obtain a catalyst that can be recycled back into
the operation, leading to favourable economics for the treatment system.
6. Use of gases has been reported to enhance the degradation efficacy, especially
in terms of enhanced sonochemical activity and also the production of a higher
quantum of radicals. Taking the operating costs into consideration, especially
99
Reprinted from the journal
Topics in Current Chemistry (2020) 378:29
dye using 95 W of ultrasonic power for only 20 min [76], and ZnO-decorated CNTs
showed 49% rhodamine B degradation under SPC conditions [99]. The effect in the
latter study was reported to be pronounced due to the prevention of electron hole
recombination as a result of the presence of CNTs [99]. A synergistic effect was also
reported for the use of Pr-doped ZnO catalysis combined with sonochemistry, leading up to 89% degradation for the sonocatalytic degradation of AR17 dye [100].
4.3.3 Guidelines for the Operating Parameters
Based on the critical analysis of the literature and personal expertise in the research
field, the following useful guidelines for the selection of the operating parameters
can be given so as to maximize the degradation or mineralization efficacy:
1. Operating pH plays a key role in deciding the efficacy of treatment. In general,
acidic conditions are favoured although it also depends on the dissociation coefficient of the pollutants present in the system. It is also possible that the effects
are less pronounced in the combined approach, and exact intensification should
be quantified before deciding on the pH adjustment. This is even more important
in the case of commercial effluent treatment schemes as usually the discharge
conditions mandate neutral pH conditions.
2. Catalyst loading needs to be properly decided so that all of the catalyst is in
proper suspension in the reactor and that it is also not affecting the passage of the
incident irradiations in the reactor. An optimum catalyst loading usually exists,
which can be established using laboratory studies for the specific applications in
question.
3. Use of radical promoters, such as hydrogen peroxide or persulphate, can bestow
beneficial results, especially when the free radical attack is the controlling mechanism for oxidation and the rate of reaction between the radicals and pollutant
molecules is also substantially high. This is important as the generated hydroxyl
radicals must be utilized quickly so as to get any beneficial results. Use of persulphate offers simultaneously acting oxidation mechanism based on the sulphate
radicals.
4. The observed synergy for the combined approach as compared to individual
approaches is also dependent on the pollutants in question. Specifically, if the
sonochemical oxidation is based on the pyrolytic mechanism of oxidation, significantly beneficial results may not be obtained as the combination is typically
accelerating the free radical formation. This has also been confirmed based on
the reported data for the synergy in Table 1.
5. Use of doping and composites allow the utilization of the solar bandwidth which
can result in a cost-effective operation. Consequently, the utilization of such catalyst forms is recommended as compared to the traditionally used single oxide
forms. This is also important to obtain a catalyst that can be recycled back into
the operation, leading to favourable economics for the treatment system.
6. Use of gases has been reported to enhance the degradation efficacy, especially
in terms of enhanced sonochemical activity and also the production of a higher
quantum of radicals. Taking the operating costs into consideration, especially
99
Reprinted from the journal