Preparation of MnOx-Modified Biochar and Its Removal Mechanism for Cr(VI) in Aqueous Solution
Round 1
Reviewer 1 Report
In this study, the authors modified biochar derived from rice husk with Mn ions and tested the adsorbents to remove Cr(IV) from an aqueous solution.
In general, the study is scientifically sound. The experimental design is suitable for the adsorption studies, and the scientific interpretation and explanation are sufficient. The data also support the conclusion. However, the manuscript can be further improved.
The introduction can be improved to be more standalone by discussing and including other metal ions that are used for modifying biochar, and the methodology needs to include some experimental detail to ensure reproducibility. There are also some minor errors observed. Please refer to the specific comment for the details.
Specific comments
1. The abbreviations should be defined on their first appearance in the main text
2. In the introduction, the use of metal ions to modify biochar should be discussed in depth. Other than Mn, other metals such as iron and copper are often used and should be included to make the introduction more standalone. Here are a few recommendations the authors can use. (doi.org/10.1007/s11270-021-05333-7 doi.org/10.1016/j.enmm.2021.100562)
3. Line 117-118. The authors need to explain a bit more about the determination of Cr(IV). The concentration and volume of diphenylcabazide(DPCZ) added to the volume of treated water, and the duration allows for the DPCZ reaction prior to measurement. This information is essential for the reproducibility of the method.
4. Recheck the word in line 137 “filer”. Is the word supposed to be “fiber” or “filter”.
5. Line 147 typo error “apllied”.
6. Equations 5-6 and line 156. The expression of the symbol in the equation are kL and kf, respectively, however, in the text is KL and Kf. Authors need to be careful and ensure the consistency of the assigned symbols, as from the mathematical point of view, “k” is not the same as “K”.
7. Figure 2 is not necessary and can be moved to the supplementary section as the important data is already summarised in Table 1.
8. Fig 4A. Authors have to be careful when labeling the wavenumber as the number“1” from “1417” is missing.
9. Consistency of the unit. Throughout the manuscript, the author expressed the unit of qe in mg g-1. However, in the figures expressed as mg/g.
10. Clarify if there is any testing to verify if the Mn leached to the solution after the adsorption process?
Author Response
Dear reviewer,
We thank you very much for review the manuscript (water-1855771) titled as above. your comments were the most helpful and we hereby wish to submit a revised version of our manuscript. In the modified version of the manuscript, we have incorporated all remarks made by you. We sincerely hope that the modified manuscript is acceptable for publication and look forward to your decision.
Best regards,
Dr. Jianxin Fan
Author Response File: 
Author Response.docx
Reviewer 2 Report
Introduction:
1. You can also find some helpful information in the following works, that can help you improve your Introduction/Results and discussion section: https://doi.org/10.3390/w11071390; https://doi.org/10.1007/s11356-019-07373-3; https://doi.org/10.1007/s13399-020-00649-6.
2. Despite you have mentioned some biochar applications in heavy metals removal, I would recommend to add a table where you can show the latest findings in the field of Cr(VI) ions removal using biochar and compare them with your materials.
Materials and Methods:
1. P2 L89: What was the N2 flow? What was the furnace type? Who was the producer of chemicals: NaOH, HNO3, Mn(NO3)2? Why the solution concentrations of 0.05 and 0.10 M were selected? Is it based on your previous experience? Please provide these information.
2. P3 L108: I am missing the basic characteristics of biochar: ash, volatile matter. The materials were washed as I suppose to remove the mineral matter prior to treatment under acidic pH values in experiments, but after the modification the materials should be characterized by means of ash content and volatile matter. These information would provide a better understanding of the effect of modification.
3. P3 L108: “The specific surface area and pore space characterization were analyzed by multi-channel specific surface area and pore size analyzer (MicroActive for TriStar II Plus 2.02, USA)”. According to the latest research: https://doi.org/10.1016/j.cej.2021.129234, it is not accurate to calculate the specific surface area of biochars using N2 adsorption-desorption. Biochar characterization should be made by applying CO2 or Ar for analysis. Please treat this as a suggestion for your further research, only. There is no need to reply on this comment.
4. P3 L119: Who was the producer of K2Cr2O7, and HCl? Please provide this information.
5. P4 L156: The unit of Freundlich model constant KF is wrong. Should be: mg1−1/n L1/n g-1. Also please unify the size of letters: KL, KF or kL, kF.
Results and Discussion:
1. P4 L166: “ranged from 1 nm to 10 nm, suggesting that they are mesoporous materials”. Mesoporous materials are in range from 2 to 50 nm. Please change the range provided in the manuscript for example: “ranged from 2 to 10 nm”.
2. Fig 4b. Please indicate the peaks of chemical individuals on the figure, not only in the text.
3. P7 L223: What is the pHpzc of the investigated materials? Please show this value in the manuscript.
4. How much biochar is in the material after modification? TG analysis can provide this information or simple ashing of the material.
5. P8 L237: You have mentioned that the changes were not ‘significant’. What statistical test did you used to confirm this thesis? Please be very careful and accurate when using term ‘significant’ as it should be confirmed by statistical analysis of obtained data.
6. EDS of the material should be made together with the SEM image from which it origins.
General:
1. Please change term ‘sorption’ to ‘adsorption’ in entire work.
2. Please spell-check the manuscript to avoid grammar and typing errors.
Comments for author File: 
Comments.pdf
Author Response
Dear reviewer,
We thank you very much for review the manuscript (water-1855771) titled as above. your comments were the most helpful and we hereby wish to submit a revised version of our manuscript. In the modified version of the manuscript, we have incorporated all remarks made by you. We sincerely hope that the modified manuscript is acceptable for publication and look forward to your decision.
Best regards,
Dr. Jianxin Fan
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
After the revision, the quality of the manuscript has improved, and I recommend publication in this journal.
Reviewer 2 Report
The manuscript has been improved by the Authors. I recommend its publication in Water.
