Biosynthesis of Zinc Oxide Nanoparticles Using Leaf Extract of Prosopis juliflora as Potential Photocatalyst for the Treatment of Paper Mill Effluent

Round 1

Reviewer 1 Report

In this manuscript, the author successfully synthesized zinc oxide nanoparticles using leaf extract of Prosopis julifora (P. juliflora), which can be used as photocatalyst to treat wastewater. The authors preformed comprehensive characterizations to study the as-prepared ZnO NPs, including XRD, FT-IR, SEM, and TEM. Furthermore, the ZnO NPs showed high catalytic activity for effective degradation of pollutants in paper mill effluents. Overall, this paper is logically written, very well organized, and detailed experimental works have done in a systematic manner to support the overall influence of ZnO NPs on catalytic performance. Since this paper provides valuable insights and contains important findings on the biosynthesis of ZnO NPs, it could provide certain and important findings to readers as an alternative source for environmentally friendly feed stock for biosynthesis. The topic will well be suitable for the audiences of Applied Sciences, I would recommend accepting this manuscript after a minor revision. The following comments/clarifications should be addressed:

Figure 7, it seems like the photocatalytic activity is not directly proportional to the catalyst dose. One would expect an incremental activity increase with the increasing of dose. Also, the difference between 500 mg/L and 900 mg/L is very minor (84% vs 85%), which could be due to the experimental error. It is hard to say that 500 mg/L is the optimum dose.

Figure 7 and Figure 8, how many replicas were used in the tests? Please include error bars.

Figure 8, why the deterioration of the paper mill peaked at 5 hours? Was it due to the loss activity of catalyst? Can you further deteriorate the remaining paper mill by adding more catalyst?

Such small NPs could be difficult for recovery and reuse. How did you recover the ZnO NPs? Do the authors have a practical approach to recovery the NPs that can be scale-up?

Figure 9, it there any way to restore the catalytic activity of ZnO NPs?

The used ZnO NPs should be characterized by SEM and FTIR to determine the reason of activity loss.

Author Response

Response to Reviewer 1 Comments

Comments and Suggestions for Authors

In this manuscript, the author successfully synthesized zinc oxide nanoparticles using leaf extract of Prosopis julifora (P. juliflora), which can be used as photocatalyst to treat wastewater. The authors preformed comprehensive characterizations to study the as-prepared ZnO NPs, including XRD, FT-IR, SEM, and TEM. Furthermore, the ZnO NPs showed high catalytic activity for effective degradation of pollutants in paper mill effluents. Overall, this paper is logically written, very well organized, and detailed experimental works have done in a systematic manner to support the overall influence of ZnO NPs on catalytic performance. Since this paper provides valuable insights and contains important findings on the biosynthesis of ZnO NPs, it could provide certain and important findings to readers as an alternative source for environmentally friendly feed stock for biosynthesis. The topic will well be suitable for the audiences of Applied Sciences, I would recommend accepting this manuscript after a minor revision. The following comments/clarifications should be addressed:

• Figure 7, it seems like the photocatalytic activity is not directly proportional to the catalyst dose. One would expect an incremental activity increase with the increasing of dose. Also, the difference between 500 mg/L and 900 mg/L is very minor (84% vs 85%), which could be due to the experimental error. It is hard to say that 500 mg/L is the optimum dose.

Response:  at first the results are precise because they are an average value of three replicates, and If we suppose that the difference between the treatment efficiencies of the two weights 500 mg/L and 900 mg/L is very minor but for economic aspects, we try to add the least amount of catalyst to obtain maximum efficiency.

 Figure 7 and Figure 8, how many replicas were used in the tests? Please include error bars.

Response: Three replicated were used in this study. The error bars were included in the figures

• Figure 8, why the deterioration of the paper mill peaked at 5 hours? Was it due to the loss activity of catalyst? Can you further deteriorate the remaining paper mill by adding more catalyst?

Response: As shown in Figure 8 the deterioration of the paper mill effluent increased with reflux time and peaked at 5 hours. This may be due to preventing more of light penetrating power, when the ZnO crystal complete interaction is enhanced, which saturated the rate of photodegradation. Furthermore, this behavior is sometimes explained as the reach by degradation to some complicated organic compounds which are more difficult to be oxidized by photocatalysis especially in the case of industrial wastes.

• Such small NPs could be difficult for recovery and reuse. How did you recover the ZnO NPs? Do the authors have a practical approach to recovery the NPs that can be scale-up?

Response: In order to handle the reusability issue of the catalysts, after separating it via centrifugation, the recovered catalyst was used with fresh paper mill water solutions. All the experimental parameters were kept constant and the experiments were repeated for 4 sets of fresh of paper mill water solutions. It was noticed that the recovered weight of catalyst is decreased gradually in each set due to the adherence of part of catalyst on the container wall thus the author settled for only four reusable sets. Therefore, after centrifugation, washing, and drying the catalyst, the weight is decreased slightly but still could be collected and reused.

• Figure 9, it there any way to restore the catalytic activity of ZnO NPs?

 Response: The efforts will be made to introduce a solve for this problem in future work.

The used ZnO NPs should be characterized by SEM and FTIR to determine the reason of activity loss.

Response: I agree with you on that, but the problem appears in the small amount that is extracted from the plant, as there is not enough of it left at the present time to conduct these analyzes, especially after consuming the rest of it in the study of reuse. We will try to remedy this matter and conduct another study on this subject in the future.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript shows an interesting alternative for water waste processing based on photocatalyst using ZnO nanoparticles. However, I do not think the manuscript is in conditions to be published in its current state. The introduction and the materials and methods section are clear, but the results and discussions need to improve. I recommend increasing the discussions, specially in the photocatalytic studies section, and if possible, compare the results obtained with conventional water purification methods.

Author Response

Response to Reviewer 2 Comments

Comments and Suggestions for Authors

This manuscript shows an interesting alternative for water waste processing based on photocatalyst using ZnO nanoparticles. However, I do not think the manuscript is in conditions to be published in its current state. The introduction and the materials and methods section are clear, but the results and discussions need to improve. I recommend increasing the discussions, especially in the photocatalytic studies section, and if possible, compare the results obtained with conventional water purification methods.

Response:  the result and discussions were improved especially in the photocatalytic studies section.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I think the revised manuscript is good enough for publication.

Author Response

Comments and Suggestions for Authors

I think the revised manuscript is good enough for publication.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript has improved considerably. I recommend the authors include a final section with the most important conclusions of this work. After that, I recommend accepting this manuscript for publication.

Author Response

Comments and Suggestions for Authors

The manuscript has improved considerably. I recommend the authors include a final section with the most important conclusions of this work. After that, I recommend accepting this manuscript for publication.

Response:  The conclusion was added.

Author Response File: Author Response.docx