Stability of Functionally Modified Biochar: The Role of Surface Charges and Surface Homogeneity

Round 1

Reviewer 1 Report

The manuscript studied the stability of modified biochars. The adsorption characteristics of Cd by biochars after and before oxidation also were investigated. This study is interesting and significant. I suggest accepting this paper after a minor revision.

  1.      The format of some citings in manuscript is wrong, which should be revised.

 2.      Table 1 and Table 2. The lowercase letters in the same row indicate the significantly different of the data at <0.05 level? I also suggest showing it during the discussion.

 3.      Figure 1. How to oxidize the modified biochars? By K2Cr2O7?

 4.      Lin 392-399. I suggest describing the result of Figure 1 (b) during explaining the difference of adsorption capacity between modified biochars after and before oxidization.

 5.      It did not mention to the adsorption of Cd by modified biochars oxidized by K2Cr2O7 in the section of materials and methods.

Author Response

Dear Editor and Reviewer,

Thank you very much for processing our manuscript (sustainability-2309323) We highly appreciate all comments and revision suggestions. All the comments from the three reviewers are valuable. We have made the revision following the reviewers’s comments.Our response and modifications are indicated in blue font and highlighted in yellow in the text. We have improved the quality and clarity of our manuscript, and sincerely hope our revisions are satisfactory. Blue represents explanation and response, while yellow represents modification.

The manuscript studied the stability of modified biochars. The adsorption characteristics of Cd by biochars after and before oxidation also were investigated. This study is interesting and significant. I suggest accepting this paper after a minor revision.

 

1. The format of some citings in manuscript is wrong, which should be revised.

Response: We thank reviewers for their comment. The citations in the manuscript have been corrected accordingly. See the manuscript for details.

2. Table 1 and Table 2. The lowercase letters in the same row indicate the significantly different of the data at <0.05 level? I also suggest showing it during the discussion.

Response: We thank reviewers for their comment. The lowercase letters in the same row indicate the significantly different of the data at p<0.05 level. We have also showed and discussed it in the manuscript accordingly.

Line 607 “The lowercase letters in the same row indicate a significant difference in the data (p<0.05).”

Line 507-509 “Table 1 shows that the O/C ratio of BC0 gained by elemental analysis was 0.16, and it also did not change after 5% and 20% H₂O₂ modification, but the O/C ratio of BCA25, BCA65 and BCN increased from 0.16 to 0.33, 0.47, and 0.22, respectively (p<0.05).”

Line 683 “The different lowercase letters in the same column indicate a significant difference in the data (p<0.05).”

3. Figure 1. How to oxidize the modified biochars? By K2Cr2O7?

Response: We thank reviewers for their comment. It is potassium dichromate, which we described in Materials and Methods. In addition, we specified it in the title of Figure 2.

Line 716 “Figure 2. FTIR spectra of different modified biochars before (a) and after (b) oxidation by potassium dichromate.”

4. Lin 392-399. I suggest describing the result of Figure 1 (b) during explaining the difference of adsorption capacity between modified biochars after and before oxidization.

Response: We thank reviewers for their comment. The analysis of FTIR spectrum (Figure 2b) has been added during explaining the difference of adsorption capacity between modified biochars before and after oxidation.

Line 987-988 “Moreover, the carboxyl group shown in FTIR spectra (Figure 2b) can also correspond to the content analyzed by Boehm titrated and XPS spectra.”

Line 1049-1054 “In this study, the FTIR spectrum (Figure 2b) shows that when the -COOH on the surface was further oxidized, the C-C bond between the -COOH and the carbon matrix was broken, resulting in the fall of -COOH from the biochar surface, leading to an obvious decline in the adsorption capacity of Cd2+. Although a large amount of -COOH was shed by oxidation on the surface of BCA25 and BCA65, the content of -COOH remained higher than that of other modified biochar.”

5. It did not mention to the adsorption of Cd by modified biochars oxidized by K₂Cr₂O₇ in the section of materials and methods.

Response: We thank reviewers for their comment. Adsorption of Cd by modified biochars oxidized by K₂Cr₂O₇ has been added in Materials and Methods.

Line 409-410 “The remaining solid-liquid mixture filtered out the biochar. Biochar was washed and dried for future use.”

Line 416 “All the batch sorption isotherm experiments were carried out at room temperature (22 ± 2 °C) by adding 8 mg of modified biochar before and after oxidation by K₂Cr₂O₇ into 10 ml brown bottles, which contained 8 mL 1.0-20.0 mg·L^-1 of Cd²⁺ solution (0.01 mol·L^-1 of NaNO₃ as background solution).”

 

 

 

 

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

The content of the article is interesting and to the point. However, there need some minor modifications.

1. In the abstract define the abbreviations  

2. The justification of this study is not clear, please address the 5Ws & 1H  questions in the introduction section. Also, add the difference between the use of BC and mBC

3. Add the F value to the Tables. 1 & 2

4. In my opinion the results and discussion section should be seperate

5. Check the reference style of all the citations, and correct them, i.e., lines 236-237

6. Fig. 1; change the K2Cr2O7 with appropriate wording, both in the figure and title

7. The conclusion section is unnecessarily lengthy, consider this

All the best.

 

Author Response

Dear Editor and Reviewer,

Thank you very much for processing our manuscript (sustainability-2309323) We highly appreciate all comments and revision suggestions. All the comments from the three reviewers are valuable. We have made the revision following the reviewers’s comments.Our response and modifications are indicated in blue font and highlighted in yellow in the text. We have improved the quality and clarity of our manuscript, and sincerely hope our revisions are satisfactory. Blue represents explanation and response, while yellow represents modification.

 

The content of the article is interesting and to the point. However, there need some minor modifications.

Blue represents explanation and response, while yellow represents modification.

1. In the abstract define the abbreviations  

Response: We thank reviewers for their comment. Abbreviations have been defined in the abstract and corrected in the manuscript.

Abstract “three commonly used functional groups of modified biochars (5% and 10% H₂O₂-modified (BCH5 and BCH20); 25% and 65% HNO₃-modified (BCA25 and BCA65); and amino-modified (BCN)) were prepared in this study to explore their stability and the Cd(II) removal performance of aged functional-group-modified biochars.” “The results showed that the O/C ratio is not sensitive enough to predict the stability of functional-groups-modified biochars, which was commonly used to evaluate pristine biochar (BC0); “

2. The justification of this study is not clear, please address the 5Ws & 1H  questions in the introduction section. Also, add the difference between the use of BC and mBC

Response: We thank reviewers for their comment. The content about “5Ws & 1H questions” has been added. The content about “difference between the use of BC and mBC” has been added.

Line 93-95 “Biochars are a promising option for C-fixing and removal from the short-term carbon cycle, and can be stored in soils for long periods of time [1]. “

Line 122-123 “However, it is not clear whether O/C ratio can accurately evaluate the stability of functional modified biochar.”

Line 205-210 “ It is unclear whether the methods used to evaluate the functional and structural stability of functionally modified biochar are the same as those used in pristine biochar. Since functional-group-modified biochar is the most common type of modified biochar, it is important to understand the antioxidant properties of this kind of modified biochar. In addition, determining the parameters of the stability of modified biochar is the premise of its large-scale environmental application. ”

3. Add the F value to the Tables. 1 & 2

Response: We thank reviewers for their comment. We have added different lowercase letters in the table annotation to indicate significant differences at p<0.05 level. And conducted a single factor ANOVA test.

Line 608 “CHN H/C passed a single factor ANOVA test. F(5,6)=23.31 > F_0.05(5,6)=4.39. CHN O/C passed a single factor ANOVA test. F(5,6)=2251.16 > F_0.05(5,6)=4.39.”

Line 683 “-COOH ^B passed a single factor ANOVA test. F(5,6)=152.57 > F_0.05(5,6)=4.39.”

3. In my opinion the results and discussion section should be seperate

Response: We thank reviewers for their comment. We think that the results and analysis are better to integrated together to present the results and conclusion of our manuscript.

5. Check the reference style of all the citations, and correct them, i.e., lines 236-237

Response: We thank reviewers for their comment. The citations in the manuscript have been corrected accordingly. See the manuscript for details.

1. 1; change the K₂Cr₂O₇with appropriate wording, both in the figure and title

Response: We thank reviewers for their comment. The “K₂Cr₂O₇” in the figure and title of manuscript was all modified “potassium dichromate”. See the manuscript for details.

2. The conclusion section is unnecessarily lengthy, consider this

Response: We thank reviewers for their comment. The conclusion has been reduced as necessary.

Line 1086-1101 “In this study, by comparing the antioxidant capacity of the modified biochar treated with H₂O₂, HNO₃ and amino groups, it was clearly shown that the antioxidant capacity of functionally modified biochars decreased, which was due to the removal of ash. The -COOH content rather than the O/C ratio was crucial for modified biochars stability. Surface charges and homogeneous surface of modified biochars can reduce their contact with oxidizers such as dichromate, which protect them from further oxidation. In addition, -NH₂ was easily oxidized on the surface of biochars; thus, the type of functional groups also played an important role in the stability of modified biochars. Therefore, the order of stability of functionally modified biochars is HNO₃-modified > H₂O₂-modified > amino-modified. Moreover, the adsorption property of modified biochars was significantly reduced after oxidation and the adsorption mechanism changed from multilayer adsorption to monolayer adsorption. The maximum adsorption capacity of BCA65 decreased from 18.15 mg·g^-1 to 4.86 mg·g^-1. The above results revealed that the stability and adsorption performance of modified biochar is not as stable as we expected. Therefore, in the design and preparation of modified biochar in the future, we should comprehensively consider the long-term durability and stability of the structure and function of modified biochars in the environment.”

 

Author Response File: Author Response.docx

Reviewer 3 Report

The authors have studied the stability of the biochar modified with hydrogen peroxide and nitric acid with the aid of potassium dichromate oxidation. Thereafter, the authors have also explored the ability of these biochars to remove cadmium ions. The paper is fairly written and authors have well characterized the synthesized biochar. However, there are few concerns which need to be addressed before the work can be considered for publication.

1. The English language at many places need to be changes specifically the articles and grammar. I would suggest to either use a professional software or an editing service to revise the manuscript thoroughly.

2. As the authors have claimed that there is a collapse or modulation of the pore size upon modification, the authors should characterize the synthesized biochars with SEM which will be visually more appropriate to show the collapse of the pores.

3. The authors claim that “65% HNO₃ modified biochar (BCA65) displayed the highest antioxidant property”. This is a very interesting inference, however there are no experiments conducted to verify the antioxidant ability of the modified biochar. Was any field experiment performed to verify the antioxidant activity of the biochars? (10.1016/j.scienta.2016.12.002, 10.1038/s41598-022-26578-0, 10.1007/s12298-021-01062-7).

 

4. As the work is emphasized on the stability of the modified biochar, which is mainly based on the characterizations data, all the characterization data i.e. XPS, Raman, BET etc, should be included in the main manuscript. 

Author Response

Dear Editor and Reviewer,

Thank you very much for processing our manuscript (sustainability-2309323) We highly appreciate all comments and revision suggestions. All the comments from the three reviewers are valuable. We have made the revision following the reviewers’s comments.Our response and modifications are indicated in blue font and highlighted in yellow in the text. We have improved the quality and clarity of our manuscript, and sincerely hope our revisions are satisfactory. Blue represents explanation and response, while yellow represents modification.

 

The authors have studied the stability of the biochar modified with hydrogen peroxide and nitric acid with the aid of potassium dichromate oxidation. Thereafter, the authors have also explored the ability of these biochars to remove cadmium ions. The paper is fairly written and authors have well characterized the synthesized biochar. However, there are few concerns which need to be addressed before the work can be considered for publication.

Blue represents explanation and response, while yellow represents modification.

1. The English language at many places need to be changes specifically the articles and grammar. I would suggest to either use a professional software or an editing service to revise the manuscript thoroughly.

Response: We thank reviewers for their comment. We have asked the professional language editor of MDPI to modify according to our own requirements.

2. As the authors have claimed that there is a collapse or modulation of the pore size upon modification, the authors should characterize the synthesized biochars with SEM which will be visually more appropriate to show the collapse of the pores.

Response: We thank reviewers for their comment. We have characterized biochars by SEM. SEM results showed that the inner surface of pores of BC0, BCH5 , BCH20, and BCN are smooth, but the inner surface of pore structures of BCA25 and BCA65 are relatively rough and even have small new pores generated on the cross-section area. However, the collapsed pore structure after oxidation, which controlled adsorption performance, is in nanoscale, while the SEM observed structure is in microscale. Therefore, SEM images may not be relevant results to reflect the structure changes caused by oxidation, while BET data can directly reflect the changes.

Line547-551 “SEM (Figure 1) clearly showed that the changes of the porous structure for different functionally modified biochars. The inner surface of pores of BC0, BCH5 , BCH20, and BCN are smooth, but the inner pore structures of BCA25 and BCA65 are relatively rough (green circles) and even have some small new pores on the cross-section area (red circles).”

3. The authors claim that “65% HNO3 modified biochar (BCA65) displayed the highest antioxidant property”. This is a very interesting inference, however there are no experiments conducted to verify the antioxidant ability of the modified biochar. Was any field experiment performed to verify the antioxidant activity of the biochars? (10.1016/j.scienta.2016.12.002, 10.1038/s41598-022-26578-0, 10.1007/s12298-021-01062-7).

Response: We thank reviewers for their comment. I disagree with the conclusion of the reviewers. “there are no experiments conducted to verify the antioxidant ability of the modified biochar.” Our results showed that after the modified biochars were oxidized by potassium dichromate, the consumption of potassium dichromate by BCA65 was the smallest, which means that this type of biochar has the highest chemical stability. Therefore, we concluded that BCA65 has the highest antioxidant property. We did not further confirm this antioxidant capacity of BCA65 in the environment through field experiments because the preparation and use of modified biochar are still in the laboratory stage due to low yield. We will focus on the large-scale generation and application of modified biochars in future research, and further discuss the actual antioxidant capacity in real field experiments. Unfortunately, we couldn't find the paper (10.1038/s41598-022-26578-0) that reviewer suggested to us. Additionally, the other two articles are mainly focused on biochars application to enhance plant antioxidant property. It does not seem to be very relevant to the content of our manuscript. 

4. As the work is emphasized on the stability of the modified biochar, which is mainly based on the characterizations data, all the characterization data i.e. XPS, Raman, BET etc, should be included in the main manuscript. 

Response: We thank reviewers for their comment. This conclusion of this work is based on the analysis and compared of characterization data. We think it is important to showed the compared results in the main manuscript to support our conclusion. The original of the characterization results, such as Raman and XPS spectra, have added to the Supplementary Information.

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors have addressed all the concerns raised. However, instead of writing "antioxidant property" it should be replaced with "less prone to oxidation". The authors should revise the manuscript accordingly.

Author Response

Dear Editor and Reviewer,

Thank you very much for processing our manuscript (sustainability-2309323) We highly appreciate all comments and revision suggestions. All the comments are valuable. We have made the revision following the reviewer’s comments. We have improved the quality and clarity of our manuscript, and sincerely hope our revisions are satisfactory. 

 

The authors have addressed all the concerns raised. However, instead of writing "antioxidant property" it should be replaced with "less prone to oxidation". The authors should revise the manuscript accordingly.

 

Response: We thank reviewers for their comment. We have revised the manuscript accordingly. We have replaced "antioxidant property" with "less prone to oxidation".

Author Response File: Author Response.docx