Topics in Current Chemistry (2020) 378:7
1 3
deionized water solution by ultrasound. During the process, the precipitates are
separated by centrifugation, then washed and vacuum-dried overnight until the final
product is obtained. In addition, the application of high-intensity ultrasound irradiation allows a mesoporous TiO 2 to be obtained without the addition of surfactant
compounds [75]. According to the scientific literature, this method has the capacity
to (1) increase crystal growth rate, (2) decrease induction periods and metastable
zone width, (3) improve particle size distribution and morphology, and (4) achieve
higher control of the nucleation process [76]. Moreover, the synthesis of photocatalysts by ultrasound is considered a “green technology”, as it is a fast, clean, and
efficient option that enables the formation of reaction conditions at ambient temperatures in cases that would otherwise require high temperature and pressure to yield
the same results. An additional benefit is that it is an extremely versatile synthetic
procedure that cab be used for the development of various photocatalysts. The materials synthesized by this method show clear improvements in terms of photocatalytic
performance over photocatalysts synthesized using traditional methods.
2.2.6 Comparison of Photocatalyst Preparation Methods
Table 1 summarizes the main advantages, disadvantages, and feasibility of largescale application of the photocatalyst preparation methods discussed in the previous
paragraphs.
Although all of the methods described can generate effective photocatalysts, from
a large-scale application point of view, it appears that SCS and hydrothermal synthesis are less strongly recommended methods, as they require high thermal energy
and high pressure [83]. On the other hand, sol–gel and precipitation methods may be
feasible at a large scale if no expensive metal precursors are required.
3 Wastewater Treatment by HPC: Great Potential but Some
Limitations and Drawbacks
The efficiency of photocatalytic processes in water and wastewater treatment can
be negatively affected by different factors, including low photoconversion efficiency and the occurrence of reactive oxygen species (ROS)-scavenging substances
in the target water/wastewater matrix. Moreover, technological limitations such as
the selection of a more suitable photocatalyst and preparation method or the choice
between slurry (need to recovery the powder photocatalyst after treatment) and supported photocatalyst systems have thus far discouraged the application of heterogeneous photocatalysis for water and wastewater treatment at full scale.
3.1 Photoconversion Efficiency
Low photoconversion efficiency is a major limiting factor in the application of
photocatalytic processes, even when compared with other AOPs. Quantum yield
can vary widely depending on the photocatalyst, experimental conditions, and
234
Reprinted from the journal
1 3
deionized water solution by ultrasound. During the process, the precipitates are
separated by centrifugation, then washed and vacuum-dried overnight until the final
product is obtained. In addition, the application of high-intensity ultrasound irradiation allows a mesoporous TiO 2 to be obtained without the addition of surfactant
compounds [75]. According to the scientific literature, this method has the capacity
to (1) increase crystal growth rate, (2) decrease induction periods and metastable
zone width, (3) improve particle size distribution and morphology, and (4) achieve
higher control of the nucleation process [76]. Moreover, the synthesis of photocatalysts by ultrasound is considered a “green technology”, as it is a fast, clean, and
efficient option that enables the formation of reaction conditions at ambient temperatures in cases that would otherwise require high temperature and pressure to yield
the same results. An additional benefit is that it is an extremely versatile synthetic
procedure that cab be used for the development of various photocatalysts. The materials synthesized by this method show clear improvements in terms of photocatalytic
performance over photocatalysts synthesized using traditional methods.
2.2.6 Comparison of Photocatalyst Preparation Methods
Table 1 summarizes the main advantages, disadvantages, and feasibility of largescale application of the photocatalyst preparation methods discussed in the previous
paragraphs.
Although all of the methods described can generate effective photocatalysts, from
a large-scale application point of view, it appears that SCS and hydrothermal synthesis are less strongly recommended methods, as they require high thermal energy
and high pressure [83]. On the other hand, sol–gel and precipitation methods may be
feasible at a large scale if no expensive metal precursors are required.
3 Wastewater Treatment by HPC: Great Potential but Some
Limitations and Drawbacks
The efficiency of photocatalytic processes in water and wastewater treatment can
be negatively affected by different factors, including low photoconversion efficiency and the occurrence of reactive oxygen species (ROS)-scavenging substances
in the target water/wastewater matrix. Moreover, technological limitations such as
the selection of a more suitable photocatalyst and preparation method or the choice
between slurry (need to recovery the powder photocatalyst after treatment) and supported photocatalyst systems have thus far discouraged the application of heterogeneous photocatalysis for water and wastewater treatment at full scale.
3.1 Photoconversion Efficiency
Low photoconversion efficiency is a major limiting factor in the application of
photocatalytic processes, even when compared with other AOPs. Quantum yield
can vary widely depending on the photocatalyst, experimental conditions, and
234
Reprinted from the journal