Development of TiO2-Based Photocatalyst Supported on Ceramic Materials for Oxidation of Organic Pollutants in Liquid Phase
, Cátia Oliveira 1, Regina Santos 2, Rui C. Martins 1, Margarida M. J. Quina 1 and João Gomes 1,*
Round 1
Reviewer 1 Report
In this study the development of TiO2-based photocatalyst supported on ceramic materials has been presented while its efficiency has been evaluated on the oxidation of sulfamethoxazole. This work is interesting, however the novelty of the manuscript is questioned since immobilization of TiO2 on ceramic supports has been extensively studied, Leca has already been evaluated as a support for TiO2, the immobilization method is not new and Sulfamethoxazole degradation has been also studied in numerous publications.
Moreover there are also some points that need to be taken into account.
More specifically:
1) The manuscript should be corrected for english
2) In the abstract the authors don’t mention that the evaluation of the catalyst was conducted on the degradation of Sulfamethoxazole. I believe they should include this information (and maybe in the title and keywords, too)
3) In figure 3 the authors should explain that the negative values of time represent the adsorption that has been taken place before photodegradation.
4) The efficiency of the immobilized photocatalyst is also questioned since there is a big percentage of TiO2 leaching after application. So if immobilization was conducted in order to avoid post separation of the catalyst, how this material can be considered a good solution if the problem remains?
5) Finally, the immobilized catalyst was not tested for reuse in consecutive cycles
Author Response
Reviewer #1
Comments and Suggestions for Authors
In this study the development of TiO2-based photocatalyst supported on ceramic materials has been presented while its efficiency has been evaluated on the oxidation of sulfamethoxazole. This work is interesting; however, the novelty of the manuscript is questioned since immobilization of TiO2 on ceramic supports has been extensively studied, Leca has already been evaluated as a support for TiO2, the immobilization method is not new and Sulfamethoxazole degradation has been also studied in numerous publications.
Moreover, there are also some points that need to be taken into account.
More specifically:
- The manuscript should be corrected for English
R: The English language will be revised along the manuscript.
2) In the abstract the authors don’t mention that the evaluation of the catalyst was conducted on the degradation of Sulfamethoxazole. I believe they should include this information (and maybe in the title and keywords, too)
R: Thank you. It was included in this new version of the manuscript in accordance to the reviewer's suggestions
3) In figure 3 the authors should explain that the negative values of time represent the adsorption that has been taken place before photodegradation.
R: It was included in accordance with the reviewer's suggestions. The following paragraph was inserted to explain this.
“An adsorption test of 15 min to the catalyst at the dark conditions was performed before the photocatalytic oxidation test. During this period was possible to see an increase of the adsorption with the catalyst load, but not exceed the 20 % of SMX removal for the highest TiO2 load. After this period the removal of SMX due to oxidation by photocatalysis.”
4) The efficiency of the immobilized photocatalyst is also questioned since there is a big percentage of TiO2 leaching after application. So if immobilization was conducted in order to avoid post separation of the catalyst, how this material can be considered a good solution if the problem remains?
R: Thank you for your comment. In fact, in this work, the aim was to establish the best synthesis process of supported photocatalysts and select the most promising ones. However, in a preliminary way, it is evident that the presence of huge amount of TiO2 powder in suspension will result in a great erosion of TiO2 in water. Moreover, considering the results in terms of SMX removal and TiO2 loss in aqueous media, the most promising photocatalysts were those incorporated at 3.6 and 5%w/w. In this cases the as the ersoin is almost negligible the degradation is related with supported photocatalyst and no by the TiO2 erosion as it happens for 10%w/w.Subsequently, in next work, both will be optimized based on the evaluation of their performance in the degradation of CECs.The following sentence was inserted.
“Moreover, considering the results in terms of SMX removal and TiO2 loss in aqueous media, the most promising photocatalysts were those incorporated at 3.6 and 5%w/w. In this cases the as the ersoin is almost negligible the degradation is related with supported photocatalyst and no by the TiO2 erosion as it happens for the 10%w/w load.”
5) Finally, the immobilized catalyst was not tested for reuse in consecutive cycles
R: Thank you for your consideration. In fact, the photocatalyst reuse will be considered for the most promising photocatalysts, as well as the operation conditions (pH, reaction time, catalyst load, type of radiation) of the reactor will be optimized in the next work. This work encompasses all the details of photocatalyst immobilized preparation considering different parameters that can affect the photoactivity and stability of photocatalyst.
Reviewer 2 Report
The authors describe TiO2-based photocatalysts supported on the Leca surface. This manuscript is publishable with the following edits:
1) Show the reusability of TiO2-based photocatalysts (preferably for 3-5 circles).
2) Effect of drying temperature: Did the authors perform calcination and wash after drying (at 105 C and 65 C) as described under Materials and Methods. When was the weight percent ratio of TiO2 calculated? Was this done post drying or calcination-wash?
3) Image 6 diagrams need to be labeled (a, b).
4) Lines 391-428: Authors have discussed a series of observations here. Each of these observations need to be written in a separate paragraph for proper enunciation.
Author Response
Reviewer #2
Comments and Suggestions for Authors
The authors describe TiO2-based photocatalysts supported on the Leca surface. This manuscript is publishable with the following edits:
- Show the reusability of TiO2-based photocatalysts (preferably for 3-5 circles).
R: Thank you for your consideration. In fact, the photocatalyst reuse will be considered for the most promising photocatalysts, as well as the operation conditions (pH, reaction time, catalyst load, type of radiation) of the reactor will be optimized in the next work. This work encompasses all the details of photocatalyst immobilized preparation considering different parameters that can affect the photoactivity and stability of photocatalyst.
- Effect of drying temperature: Did the authors perform calcination and wash after drying (at 105 C and 65 C) as described under Materials and Methods. When was the weight percent ratio of TiO2calculated? Was this done post drying or calcination-wash?
R: Thank you for your comment. The weight percent ratio of TiO2 calculation was made after the calcination-wash stage. This information was added to the revised version of manuscript.
“The weight percent ratio considered for TiO2 calculation was made after the calcination-wash stage”.
- Image 6diagrams need to be labeled (a, b).
R: Thank you for your consideration. We apologize by the mistake. It was corrected in this new version of the manuscript.
- Lines 391-428: Authors have discussed a series of observations here. Each of these observations need to be written in a separate paragraph for proper enunciation.
R: Thank you for your suggestion. We will revise it in the new manuscript version.
Reviewer 3 Report
The authors explored the effect of different parameters on the amount of TiO2 deposited on support and photocatalytic properties of the prepared materials. The procedure could be interesting although many papers on TiO2 immobilization already exist.
The paper could be further improved by improving the English language used since parts of the paper are not completely understandable.
Additionally, some questions arise when reading the research:
In part 3.1.2. Effect of the amount of Leca in contact with the TiO2 suspension, it is said that for higher loads of Leca, not all the support material is submerged in the coating solution. Why design an experiment like that and not limit the amount to what could still be fully submerged?
In part 3.1.5. Effect of drying temperature, it is not clearly mentioned that the solution was evaporated with the supports still submerged rather than removing the support and then drying it. This should be pointed out.
For photocatalytic testing: you said only the Leca particles that floated on water were used. How did this affect the reactor setup? Did mixing distribute the particles throughout the volume or did they just float on top?
On line 488 you say the thickness of the deposited layer is about 8.1 µm but the measurements on the figure are slightly higher.
In figure 10 you show that TiO2 is also deposited in the pores of Leca, but the question remains if light can even reach those places and therefore if that TiO2 is photocatalytically active at all.
Author Response
Reviewer #3
Comments and Suggestions for Authors
The authors explored the effect of different parameters on the amount of TiO2 deposited on support and photocatalytic properties of the prepared materials. The procedure could be interesting although many papers on TiO2 immobilization already exist.
R: We agree with the reviewer's opinion. However, regarding immobilization manuscripts, there is a lack of description and characterization of the supporting methodology considering the impact of different parameters on the immobilization yield as well as its impact on the degradation of microcontaminants such as sulfamethoxazole.
The paper could be further improved by improving the English language used since parts of the paper are not completely understandable.
R: Thank you for your suggestion. English language was revised throughout the manuscript.
Additionally, some questions arise when reading the research:
In part 3.1.2. Effect of the amount of Leca in contact with the TiO2 suspension, it is said that for higher loads of Leca, not all the support material is submerged in the coating solution. Why design an experiment like that and not limit the amount to what could still be fully submerged?
R: The aim of work at this stage was keeping constant the suspension volume and increase the Leca amount in order to understand the optimal dose of Leca to achieve a good immobilization. In fact, for 30 g of Leca, a small part of the particles is outside of the suspension solution but as the drying temperature was 105ºC it is expected that the turbulence generated promotes the contact between suspension and Leca particles. Moreover, the 15 g and 20 g of Leca particles are completely submerged in the suspension but the immobilization yield was lower compared to 10 g. Therefore, we can conclude that the increase in Leca amount decreases the TiO2 immobilization yield.
In part 3.1.5. Effect of drying temperature, it is not clearly mentioned that the solution was evaporated with the supports still submerged rather than removing the support and then drying it. This should be pointed out.
R: Considering the reviewer's suggestions, the following sentence was inserted.
“The Leca particles submerged in the TiO2 suspension solution were submitted to the drying stage for both temperatures.”
For photocatalytic testing: you said only the Leca particles that floated on water were used. How did this affect the reactor setup? Did mixing distribute the particles throughout the volume or did they just float on top?
R: The selection of Leca particles floating in water promoted the direct contact of the photocatalyst with the light source and synthetic solution. This fact, associated with the magnetic stirring and the injection of oxygen in the reactor allows simultaneous flow in the distribution of particles throughout the volume of the effluent. The following sentence was inserted into the manuscript.
“The Leca particles selected for the immobilization procedure floated on the water. In order, to promote the mixing and the contact between all the volume with particles a magnetic stirring and oxygen flow stream were used.”
On line 488 you say the thickness of the deposited layer is about 8.1 µm but the measurements on the figure are slightly higher.
R: The reviewer is right, and thus the following sentence was introduced in the manuscript.
“…the different points selected for 5%w/w show a more uniform layer thickness between 8.03 and 8.25 µm (Figure 9(b)).”
In figure 10 you show that TiO2 is also deposited in the pores of Leca, but the question remains if light can even reach those places and therefore if that TiO2 is photocatalytically active at all.
R: Thank you for your comment. The light only can enter the superficial pores. In this sense, at the superficial pores where the TiO2 is available the semiconductor is able to be photocatalytically active. However, for deeper layers, since some pores are interconnected as can be seen in Figure 10, it will be difficult to TiO2 receive light thus it cannot be photoactivated.
