Review Reports - Coating on Steel Discs with a Photocatalytic System CuO/SiO2 for the Degradation of the Ubiquitous Contaminants Methylene Blue and Amoxicillin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper reports "Coating on steel discs with the photocatalytic system CuO/SiO2 for the degradation of emerging contaminants (methylene blue and amoxicillin)". I would like to suggest a revision on this manuscript.

1. The current photocatalytic results should be compared with the reported studies.

2. More explanation on the obtained photocatalytic results should be provided.

3. The photocatalytic mechanism of current CuO/SiO2 system in degrading MB and AMX should be clearly elaborated.

4. Error bar should be added in the degradation curve to ensure the reproducibility of the results.

Author Response

Reviewer 1

The current photocatalytic results should be compared with the reported studies.

Thank you very much for the observation. The current photocatalytic results are compared with reported studies (Refs.19 and 29). In the case of Ref. 19 (”A new hybrid process for Amoxicillin elimination by combination of adsorption and photocatalysis on (CuO/AC) under solar irradiation”), the photocatalytic results of non-impregnated CuO only archived 18.18% of AMX elimination; with impregnated CuO/AC, the best results occurred at pH 9. In contrast, in our research work, the experiments were carried out at neutral pH. In the other work (Ref. 29) dealing with the photocatalytic treatment of wastewater using Bi₂O₃ to degrade the anionic dye indigo carmine (IC) and bisphenol-A (BPA), 30% and 70% of mineralization of the IC and BPA moieties were achieved.

More explanation on the obtained photocatalytic results should be provided.

Thank you very much for the suggestion. To this end, the following paragraph was added:

“Every part of the photocatalytic system plays an important role in the degradation process of the mentioned organic structures. The incorporation of mechanical stirring and upward bubbling make possible the improvement of the photocatalyst reaction conditions employing the model molecules (MB and AMX). The photocatalyst embedded in the matrix was deposited on perforated steel discs to allow its introduction into the reactor and avoid subsequent separation stages. Finally, a photodegradation study is carried out with pure CuO powders, varying the CuO load on SiO₂ films, and with the whole CuO/SiO2 system in the reactor. Four 4.5-W lamps are used in both processes; for the reactor, a 100-W lamp is added, approximating the increase in power with the increase in volume. The equivalence in a factor of 10 is 180 W for the 18 W supplied by the four 4.5-W lamps. Then, the overall sum of the reactor power is 118 W, which implies a reduction of 35%. Therefore, the reduction in efficiency is also linked to the lower supplied power. A series of stages is necessary, which involves obtaining the photocatalyst powders, making the films, building the reactor and finally performing the photocatalytic evaluation.”

The photocatalytic mechanism of current CuO/SiO2 system in degrading MB and AMX should be clearly elaborated.

Figure 13. AMX degradation by synergistic action.

“The synergistic action occurring between the CuO/SiO₂ /water and UV irradiation forms the OH radicals, biradicals and O₂^-ion-radicals, where every set contributes to the AMX degradation (Figure 11). Initially, the complex AMX structure suffers a gradual rupture, where the penicilloic and penilloic acids have been identified by different authors [51]. Finally, the sulfate and ammonium ions result from the complete fragmentation of AMX. The thorough mineralization process yields CO₂ and H₂O as final products [52], achieving the efficient photocatalytic degradation.”

Error bar should be added in the degradation curve to ensure the reproducibility of the results.

Thank you very much for your recommendation. The error bar was added to the degradation curve.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript Number: Coatings-2955791

The manuscript “Coating on steel discs with the photocatalytic system CuO/SiO₂ for the degradation of emerging contaminants (methylene blue and amoxicillin)”

The topic is interesting and it is suitable for this journal.

However, current manuscript has deficiencies and can be recommended for publishing in Coatings only after major revisions.

The main problem is poor description of experimental procedures which makes difficult to review obtained results. In addition, section Results and Discussion should be carefully rewritten following accepted scientific methodology.

Some specific remarks are given bellow:

Abstract: size of microcrystals 1.262±0.07 mm – what are the significant figures?

1. Introduction:

-Line 47: part of the sentence is unclear “no implementation of stages after the separation of residues” – please rewrite

-Line 70: part of the sentence is unclear “antifungal features have been profited in construction materials” – please rewrite

-At the end of this part specify characterization methods used in this work and described for what purposes

2. Materials and methods

I suggest to Authors to first describe materials and later characterization methods

For each characterization method used explain which samples were analyzed by that method and specify experimental parameters (not only type of instrument as it is now). Also, UV/Vis experiments were performed on powders as well as on liquid samples. Please specify and describe.

How long were CuO sampled treated at 400C?

For photocatalytic experiments please specify type of lamp and distance of lamp from the photocatalytic mixture.

Line 128: where “4 visible-light lamps (4.5 watts and 400 lm)” were installed?

Experiments need to be precisely described. Also, what is the difference in photocatalytic experiments in beaker and tank, and why are experiments preformed in such way. Why amount of MB was decreased in experiments where thin films were tested compared to experiments where powders were tested for photocatalytic activity? Describe what is depicted in Fig. 1.

3. Results and discussion

Fig 2: UV/Vis diffuse reflectance spectrum?

Line 180: cited paper states “Copper (II) oxide, or CuO, is one of these p-type semiconductors and is typically recognized as an indirect bandgap oxide material with bandgap ranging from 1.0 to 1.9 eV” not 1.2 to 1.9 as Authors state in the manuscript.

Figure 3 – describe how particle size distribution has been obtained

Please provide XRD pattern of CuO obtained by precipitation method.

Lines 228-233: this part should be moved in discussion after photocatalytic results

Fig 5: add explanations how FTIR spectra have been recorded for each shown spectrum.

FTIR spectrum of SiO2 shows signal at about 900 cm-1- please comment its origin.

SEM micrographs are marked as Fig 4 instead Fig 6.

Lines 270-271: when and how was thermal treatment at 600C performed?

Describe how EDX measurements were performed (point measurements or mapping; how many areas were averaged…). Were EDX experiments performed for all samples?

Correct figure numbers – Fig 5 should be Fig 7, Fig 6 should be Fig 8….

Lines 312-313: photocatalytic results should be discussed after they are shown.

Figure8 caption: precisely describe what is shown in Fig 8b)

Figure 9a): It is important to explain that effect of photocatalysts loading was investigated either in experimental part or as subtitle.

Specify at what results following comment refers to: “The assay performed with the CuO/SiO2 system films achieved 25% of efficiency degrading MB at 10 ppm.”

Lines 370-373: if one considers experimental error difference of 2 % means the same results or almost the same.

Line 374: what are “small substrates”?

Comments on the Quality of English Language

General comment: English language needs improvement throughout the text

Author Response

Reviewer 2

Some specific remarks are given bellow:

Abstract: size of microcrystals 1.262±0.07 mm – what are the significant figures?

We appreciate your observation. This part of the text and numbers were corrected as follows:

“…where the particle size distribution ranges from 0.5 to 2.5 μm (with average of 1.26 ± 0.06 μm).”

Introduction:

-Line 47: part of the sentence is unclear “no implementation of stages after the separation of residues” – please rewrite

We do appreciate the Reviewer’s accurate observation. We rewrote part of the sentence as follows:

“…and the implementation of subsequent stages of waste separation, increasing the number of unit operations in the process…”

-Line 70: part of the sentence is unclear “antifungal features have been profited in construction materials” – please rewrite

We do appreciate the Reviewer’s accurate observation. We rewrote part of the sentence as follows:

“…the antimicrobial and antifungal features of CuO and SiO₂...”

-At the end of this part specify characterization methods used in this work and described for what purposes

We do thank the Reviewer’s accurate observation. The following paragraph was added at the end of the Introduction:

“In the present research work, a synthesis route was proposed to obtain the photocatalytic system CuO/SiO₂. The CuO bandgap was calculated and the compositional characterization of CuO and CuO/SiO₂ was carried out by FT-IR; the structural characterizations were carried out by X-ray diffraction (XRD) and the morphological characterizations were performed by scanning electron microscopy (SEM). Finally, the use of a CuO photocatalyst embedded in a SiO₂matrix was proposed to evaluate the degradation efficiency of methylene blue (MB) and amoxicillin (AMX) in an aerated and stirred tank.”

Materials and methods

I suggest to Authors to first describe materials and later characterization methods

Thank you very much. The suggested change was taken into account so that in the revised version of the manuscript the materials and synthesis are described first and then the characterization methods.

The authors are grateful for this contribution; consequently, the characterization section was corrected featuring the suggested changes and specifications.

How long were CuO sampled treated at 400C?

The treatment time of CuO at 400°C was 4 h, and this specification was added to the methodology as suggested.

For photocatalytic experiments please specify type of lamp and distance of lamp from the photocatalytic mixture.

Thank you very much for the suggestion. Figure 1 was added to specify these characteristics. In addition, the lamp model was specified too.

Line 128: where “4 visible-light lamps (4.5 watts and 400 lm)” were installed?

In order to clarify this point, Figure 1 was changed and Figure 2 was added to specify the arrangement of lamps for the experiments at 180 mL and 1.8 L.

Figure 2. Description of the lamp arrangement used in the aerated and stirred tank reactor. The distance and irradiance level are specified. The irradiance was tested in a straight line to the lamp in the distance between the tank and the light source.

In response to these timely observations, the purpose of the small scale experiments was to find the best molar ratio to be evaluated in the reactor, in addition to economize on material resources. The evaluation of powders was carried out in order to compare their limited practicality with respect to the advantage of using coatings. The change in concentration was due to the tracking times, which were reduced to better observe the degradation curves. In addition, a test was added with the films in the evaluation of powders under the same conditions. These points were added to the methodology as suggested. Finally, Figure 1 was changed.

Figure 1. Arrangement of the substrates inside the dark box; the distance of the lamps and irradiance levels are specified. The irradiance was tested in a straight line to the lamp in the distance between the tank and the light source.

Results and discussion

Fig 2: UV/Vis diffuse reflectance spectrum?

This is a UV/Vis absorbance spectrum, which is specified in Figure 3 (left).

In response to the appropriate observation, the aforementioned data were modified: “The common Eg value for CuO ranges from 1.0 to 1.9 eV” [32]…”

Figure 3 – describe how particle size distribution has been obtained

The particle size analysis was obtained from counting 300 particles from different sections (now, Figure 4 (right)

Please provide XRD pattern of CuO obtained by precipitation method.

In response to the Reviewer’s comment, this specification was attached to Figure 5, where the standard is of the individual CuO powders obtained by the coprecipitation method.

Lines 228-233: this part should be moved in discussion after photocatalytic results

The authors are grateful for the observation, which is why the above mentioned paragraphs were moved to the appropriate section.

Fig 5: add explanations how FTIR spectra have been recorded for each shown spectrum.

This suggestion was taken into account, adding the corresponding description.

FTIR spectrum of SiO2 shows signal at about 900 cm-1- please comment its origin.

The observation is correct. The signal at 957 cm^-1 belongs to the Si-OH bond, derived from the surface functionalization of SiO₂, characteristic of SiO₂ synthesized by the sol-gel pathway. As it can be seen in the following reference: https://doi.org/10.3390/gels8110744

SEM micrographs are marked as Fig 4 instead Fig 6.

The figure numbers were corrected in response to the appropriate observation.

Lines 270-271: when and how was thermal treatment at 600C performed?

The authors appreciate this point, and a paragraph with the specifications was added to Section 2.3. Deposition and characterization of CuO/SiO2 films by Dip Coating.

Describe how EDX measurements were performed (point measurements or mapping; how many areas were averaged…). Were EDX experiments performed for all samples?

The technique was carried out by mapping two zones, which are framed in the figure in attention to the observation. The study sought to elucidate the composition of the films, and since the other molar ratios showed a similar trend, it was decided to show only the most efficient molar ratios to check the presence of Cu and Si in the deposits. In response to the comment, the description was added to the figure.

Correct figure numbers – Fig 5 should be Fig 7, Fig 6 should be Fig 8….

The figure numbers were corrected by taking into account the observations made.

Lines 312-313: photocatalytic results should be discussed after they are shown.

The authors accept the proposed change; consequently, the lines were placed at the end of Section 3.3.1. Methylene blue (MB).

Figure8 caption: precisely describe what is shown in Fig 8b)

The Reviewer's request was taken into account and adequately described in the above mentioned figure.

Figure 9a): It is important to explain that effect of photocatalysts loading was investigated either in experimental part or as subtitle.

In response to this relevant comment, the name of Section 2.5 was changed: “Evaluation of the photocatalytic efficiency of CuO powders at different loadings.”

Specify at what results following comment refers to: “The assay performed with the CuO/SiO2 system films achieved 25% of efficiency degrading MB at 10 ppm.”

The results were duly noted.

Lines 370-373: if one considers experimental error difference of 2 % means the same results or almost the same.

In response to this observation, the line wording was changed to read as follows: “For the molar ratio of 1:15, the percentage of degradation was similar to that achieved with the 1:10 ratio.”

Line 374: what are “small substrates”?

Considering this observation, a more detailed description of the substrates was added along these lines as follows: “1.5 x 5 cm 304 stainless steel substrates coated with the CuO/SiO₂ system…”

Comments on the Quality of English Language

General comment: English language needs improvement throughout the text

Thank you very much for the comment. English was revised through the manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this work, CuO/SiO₂ films were deposited on steel substrates with CuO:SiO₂ molar ratios of 1:5, 1:10 and 1:15. The photoactive CuO/SiO₂ coating degraded methylene blue (98%) in 1500 min and amoxicillin (55%) in 450 min. XRD confirmed the existence of SiO₂ amorphous phase and CuO monoclinic system in the coating. The grain size distribution is between 0.5~2.5 μm, and the microcrystalline arrangement is 1.262+0.07 μm. Scanning electron microscopy shows that the surface of the coating is uniform without cracks. Energy dispersive X-ray analysis showed the presence of Si and Cu in the coating. Nevertheless, I do have some suggestions and comments on this manuscript that I think would help to increase the impact of this manuscript. A minor revision is required before it can be accepted for publication.

1. For the sake of the beauty of the picture, please unify the font size in the picture.

2. Other samples in Figure 4 are 10-80^o, please make CuO consistent with other samples.

3. "Figure 1 (on the left) shows the UV-vis spectrum of CuO." This sentence does not match the text. Please check and correct it.

4. For the discussion part, some important papers should be cited like“Materials & Design 233 (2023) 112291. Materials Characterization 208 (2024) 113666”.

Comments on the Quality of English Language

The language should be checked carefully. The full name of "UV-vis" should only be used for the first time. Please check the full text for similar errors and correct them.

Author Response

Reviewer 3

I do have some suggestions and comments on this manuscript that I think would help to increase the impact of this manuscript. A minor revision is required before it can be accepted for publication.

For the sake of the beauty of the picture, please unify the font size in the picture.

Thank you very much for your suggestion. The font size in the picture was unified.

Other samples in Figure 4 are 10-80o, please make CuO consistent with other samples.

In response to your comment, this specification was featured in Figure 5 (before Figure 4), where the standard is of the individual CuO powders obtained by the coprecipitation method. We made CuO consistent.

"Figure 1 (on the left) shows the UV-vis spectrum of CuO." This sentence does not match the text. Please check and correct it.

Thank you very much for your observation. We corrected the text and figure number, which is Figure 3 in the corrected manuscript version.

For the discussion part, some important papers should be cited like“Materials & Design 233 (2023) 112291. Materials Characterization 208 (2024) 113666”.

Thank you very much for the observation. The recommended paper was discussed and referenced.

At the end of Section 3.2.1 (X-ray diffraction), the following lines were added:

“In addition, Guoqin Cao et al. elucidated the formation of a nanostructured coating system with Si, diversifying the requirement for a “structure-performance relationship”, where the depressed Si inner-diffusion led to the formation of an amorphous bilayer structure (36, 37).”

Comments on the Quality of English Language

The language should be checked carefully. The full name of "UV-vis" should only be used for the first time. Please check the full text for similar errors and correct them.

We do thank the Reviewer’s observation. English was checked throughout the manuscript ant the mentioned errors were corrected accordingly.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors revised it suitably.

Author Response

We really appreciate all the Reviewer’s observations and suggestions. Thank you very much for your important revision.

Reviewer 2 Report

Comments and Suggestions for Authors

The Authors have performed corrections which, in my opinion, improved manuscript. Still, SEM/EDX results need more clarification.

Hence, manuscript can be accepted for publication after following minor changes:

- Line 94: add” deposited on SS” at the end of the first sentence

- Line 95: add “FTIR spectroscopy”

- Line 124: “thickness” instead of “thick”

- Line 151: what is AM?

- Lines 151-152: “A test was added featuring 151 films at the 1:10 molar ratio of CuO:SiO2.” Needs to be better explained

- Figure 1: All 4 substrate are the same? Please comment why 4 substrate and 4 lamps were used instead usual experimental set-ups with one lamp

- Line 163: explain AM:CuO

- Section 2.7.: explain location of the fifth lamp and here explain why it is used.

- Lines 202-203: Based on what evidence did Authors conclude that the same molar ratio of CuO and SiO2 that showed the best degradation of MB will show the best results for AMX?

- Line 221: 1.86 ± 0.08 hos did authors determined given error?

- Section 3.1.2.: add information about software used for particle size analysis

- At how many areas were EDX mapping experiments performed? What was standard deviation?

- Line 354: “The weight percent increase from zone 2 to zone 1 evidences the concentration growth of the CuO/SiO2 system deposited in zone 2.” wt% of what are Authors referring to in this sentence?

- Lines 356-357: “The decrease in the Cu percentage from zone 2 to zone 1….” 1wt% Cu in zone 2 and 1.56 wt% Cu in zone 1 (values at Fig. 8) – how is that decrease?

- Lines 390-399: comment shown results in comparison to available literature

- Lines 430-432: show results that support this claim

- Line 435: based on what results authors claim that there is uniformity in SiO2 coating

Comments on the Quality of English Language

Author Response

We really appreciate all the Reviewer’s observations and suggestions. Thank you very much for your important revision.

The Authors have performed corrections which, in my opinion, improved manuscript. Still, SEM/EDX results need more clarification.

Hence, manuscript can be accepted for publication after following minor changes:

- Line 94: add” deposited on SS” at the end of the first sentence

We acknowledge the Reviewer’s suggestion. These words were added at the end of the sentence.

- Line 95: add “FTIR spectroscopy”

We do thank the Reviewer’s accurate observation. “FTIR spectroscopy” was added at the end of this sentence.

Line 124: “thickness” instead of “thick”

We do appreciate the Reviewer’s accurate observation. We replaced the word accordingly.

- Line 151: what is AM?

Thank you very much for the question. The acronym "AM" stands for the compound methylene blue (MB). The acronyms in the manuscript have been corrected.

- Lines 151-152: “A test was added featuring 151 films at the 1:10 molar ratio of CuO:SiO2.” Needs to be better explained

In response to the Reviewer's suggestion and to improve the test explanation, this statement was relocated next to the final explanation of the 4-substrate arrangement of the CuO/SiO₂ system.

Figure 1: All 4 substrate are the same? Please comment why 4 substrate and 4 lamps were used instead usual experimental set-ups with one lamp

Thank you very much for the questions. We hope that the following answer is satisfactory: Indeed, the four 304 stainless steel substrates (5 cm x 1.5 cm) were used in all the tests. The 4 substrates were placed in a radial arrangement of the photocatalyst. In the same way, the lamps were installed to provide illumination to the entire circumference of the vessel, as shown in the figure below:

An extra statement has been added to the description of Figure 1 to further clarify the arrangement.

Line 163: explain AM:CuO

In response to the Reviewer's timely remark, we can say that "AM:CuO" refers to the methylene blue and CuO powder (MB:CuO) molar ratio. The incorrect acronym in the manuscript has been replaced.

- Section 2.7.: explain location of the fifth lamp and here explain why it is used.

Thank you very much. What follows is the response to your thoughtful request: The fifth lamp was located in this way in order to prevent the lamp heat from exerting any influence on the degradation phenomenon of methylene blue; it was also derived from the ergonomics of the reactor tubular support. Its use was intended to increase the power of the 4 4.5-W lamps, as the volume increased from 180 mL to 1.8 L. In addition, the 304 stainless steel discs coated with the CuO/SiO2 system had an increase in the coated area. The corresponding statement was modified to improve the explanation in the section mentioned by the reviewer.

- Lines 202-203: Based on what evidence did Authors conclude that the same molar ratio of CuO and SiO₂ that showed the best degradation of MB will show the best results for AMX?

The authors are grateful for the Reviewer's question. We hope that the following answer is satisfactory:

The statement tried to explain that the best results of MB degradation would be considered to use the same molar ratio in the reactor. One of the crucial factors is the amount of AMX required for the tests in the reactor. In order to avoid the overuse of AMX, the best molar ratio was chosen from the small MB evaluations to assess the degradation of AMX. The wording has been changed to be more concise.

- Line 221: 1.86 ± 0.08 hos did authors determined given error?

Thank you very much for the question. The standard error was derived from performing the calculation of Eg 3 times with the same Tauc graphical method. A clarifying comment was added after the Eg value.

- Section 3.1.2.: add information about software used for particle size analysis

Thank you very much for the suggestion. The software information has been added to the description in Figure 4.

- At how many areas were EDX mapping experiments performed? What was standard deviation?

Thank you very much for the questions. Only 2 mappings of a micrograph at 2000 x were performed for the 1:10 ratio, which are marked with a yellow circle in zones 1 and 2. No standard deviation was obtained because only one mapping was carried out per area. The aim was to elucidate the presence of Si and Cu on the substrate surface.

- Line 354: “The weight percent increase from zone 2 to zone 1 evidences the concentration growth of the CuO/SiO₂ system deposited in zone 2.” wt% of what are Authors referring to in this sentence?

We do thank the Reviewer’s question. In this sense, the percentage was clarified in the manuscript.

- Lines 356-357: “The decrease in the Cu percentage from zone 2 to zone 1….” 1wt% Cu in zone 2 and 1.56 wt% Cu in zone 1 (values at Fig. 8) – how is that decrease?

The authors are grateful for the Reviewer’s timely observation. The correction of the areas was done accordingly. The decrease occurred from zone 1 to zone 2.

- Lines 390-399: comment shown results in comparison to available literature

We do thank the Reviewer’s suggestion. A comment has been added alluding to results reported in the literature.

- Lines 430-432: show results that support this claim

Thank you very much for the request. In this sense, the corresponding figure numbers associating the results with the statements were added.

- Line 435: based on what results authors claim that there is uniformity in SiO2 coating

Thank you very much for the observation. The figure number from which the statement is derived was placed accordingly.

Author Response File: Author Response.pdf