Enhanced Adsorption of Cu²⁺ from Aqueous Solution by Sludge Biochar Compounded with Attapulgite-Modified Fe

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the proposed manuscript, the authors examined modified sludge biochar as a potential sorbent of Cu²⁺ ions from an aqueous solution. Although the authors provide some modification techniques, the sorption capacity is low compared to carbonaceous materials, and experimental studies lack novelties. Certain corrections are necessary before publication.

1. The abstract needs to be supplemented with additional findings from this study. Also add how much the capacity of calcium-based magnetic biochar is better than others and unmodified biochar, and what material was used to produce the biochar.

2. Bach adsorption study - why did the authors use such a low mass of sorbent and low concentration of Cu²⁺? Suggestion to the authors to raise both parameters, because they are quite low. Why was the influence of pH investigated at values from 2 to 8 when it is known that Cu at values over 6 precipitated in the form of hydroxide? How do the authors guarantee that the reduced Cu concentration in the solution is a result of adsorption and not precipitation (this is also applicable to results from section 3.2?

3. Add references for used equations.

4. In the figures, increase the intensities of the lines, the spectra are quite difficult to see.

5. Are the standard deviations available for the results shown in Figures 5 and 6?

6. Compare the obtained capacities with the available literature.

7. Did the authors examine the regeneration properties of the material during multiple adsorption cycles?

 

Author Response

Please see the attachment.

 

Dear Reviewer

   On behalf of all the contributing authors, I would like to express our sincere appreciation of your letter and reviewers’ constructive comments concerning our article entitled “Enhanced Adsorption of Cu²⁺ from Aqueous Solution by Sludge Biochar Compounded with Attapulgite Modified Fe”(Manuscript No: water-2721625). These comments are all valuable and helpful for improving our article. According to the nice editors and reviewers’ comments, we have made extensive modifications to our manuscript and supplemented extra data to make our results more convincing. In this revised version, changes to our manuscript were all highlighted within the document by using red-colored text. Point-by-point responses to the nice editors and reviewers are listed below this letter.

We sincerely thank the editors and all reviewers for their valuable feedback that helped us to improve the quality of our manuscript. The reviewer comments are listed in bold below and specific concerns have been numbered. Our response is given in normal font and changes/additions to the manuscript given in red text.

We tried our best to improve the manuscript and made some changes to the manuscript. These changes will not influence the content and we do invite an expert of us who is a native English speaker from the USA to help polish our article. Here we did not list the changes but marked in the revised paper.

1. The abstract needs to be supplemented with additional findings from this study. Also add how much the capacity of calcium-based magnetic biochar is better than others and unmodified biochar, and what material was used to produce the biochar.

We feel great thanks for your professional review work on our article. According to your nice suggestions, we have made extensive corrections to our previous draft。

“The results suggested that calcium-based magnetic attapulgite/sludge biochar (CA–BC) have greater adsorption capacities for Cu²⁺ than three other composite materials (nZVI–BC, FA–BC and BC), which contained a variety of ferrite-containing and hydroxide - functional groups, as well as a more developed pore structure” has been corrected to “The results suggested that the adsorption capacity for Cu²⁺ of calcium-based magnetic attapulgite/sludge biochar (CA–BC) prepared from CaCO₃, FeCl₃ and FeSO₄ is 38.01% than the unmodified biochar, and 6.41% to 17.5% than the other biochar. CA-BC contained a variety of ferrite-containing and hydroxide-functional groups, as well as a more developed pore structure.” in L13.

2. Bach adsorption study - why did the authors use such a low mass of sorbent and low concentration of Cu²⁺? Suggestion to the authors to raise both parameters, because they are quite low. Why was the influence of pH investigated at values from 2 to 8 when it is known that Cu at values over 6 precipitated in the form of hydroxide? How do the authors guarantee that the reduced Cu concentration in the solution is a result of adsorption and not precipitation (this is also applicable to results from section 3.2?

We sincerely thank you for your valuable comments. We have carefully examined the literature and we have chosen this concentration parameter because the objective of this experimental study was the adsorption of Cu²⁺ from domestic types of composite pollution discharged from urban and rural areas. The concentration parameter of Cu²⁺ in this type of pollution is not as high as that of industrial pollution.

Thank you very much for your valuable suggestions, according to your suggestions we have searched a lot of references and the results indicate that Cu²⁺ starts to precipitate as hydroxide in a small amount of pH values of 6-8, and starts to precipitate as hydroxide in a large amount of p h values above 8. The manuscript focuses on the adsorption effect of modified biochar at pH 2-6, the experiment showed that the adsorption rate of modified biochar at pH 7-8 was almost the same when compared to 6. Thus, a pH range of 2-8 was chosen. Also, the reduction of Cu²⁺ concentration in the solution with the addition of modified biochar under neutral conditions is a combination of adsorption and precipitation.

We feel great thanks for your professional review work on our article. According to your nice suggestions, we have made corrections to our previous draft。

“In scenarios where the pH exceeds 8.0, the ion Cu²⁺ tends to form Cu(OH)₂, manifesting as a blue precipitate.” has been corrected to “In scenarios where the pH exceeds 8.0, the Cu²⁺ tends to form a large amount of Cu(OH)₂, manifesting as a blue precipitate. Consequently, the main consideration is the adsorption effect under acidic and neutral conditions, taking different pH values were considered, i.e., 2, 3, 4, 5, 6, 7, and 8.” in L233–L236

3. Add references for used equations.

We sincerely appreciate the valuable comments. We have checked the literature carefully and added more references on the equation to the manuscript.

Added references 24 to Eqs. (1),(2); 25 to Eqs. (3),(4); 26 to Eqs. (5),(6) at L128, L138, L150

4. In the figures, increase the intensities of the lines, the spectra are quite difficult to see.

We are very grateful for your professional review work on our article. Based on your suggestions, we have made detailed changes to all figures

5. Are the standard deviations available for the results shown in Figures 5 and 6?

We think this is a very good suggestion, and based on your suggestion, we have made detailed changes to Figures 5, and 6

6. Compare the obtained capacities with the available literature.

Thank you very much for your suggestion, we think this is a great suggestion and we have changed the manuscript according to the reviewers' comments, which have been given in red text in the manuscript.

Added in L170 “FTIR of sewage sludge prepared by Wang et al. also showed -C=C- and -C=O [30].”

Added in L194–198 “The SEM of sludge and calcium sulfate co-pyrolysis biochar prepared by Liu et al. also showed the same surface characteristics. Their adsorption of heavy metals is also effective, and the above studies have fully demonstrated that the load-modified biochar with different elements has excellent adsorption performance for heavy metals [33].”

Added in L223-227 “The magnetic biochar prepared by Wu et al. also showed the FeO, Fe₂O₃, Fe₃O₄ and that study pointed out that positively charged surface functional groups and iron oxides in magnetic biochar can both serve as sorption sites for ions in soil solution [36].”

7. Did the authors examine the regeneration properties of the material during multiple adsorption cycles?

We sincerely thank you for your professional opinion. We examined the regeneration characteristics of the material over multiple adsorption cycles, and since this regeneration characteristic has a long periodicity, the relevant experiments have not yet been completed. The results of the experiments will be synthesized in the next manuscript, thank you again for your advice!

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Unified font and size for abstract, such as “Cu²⁺”, “calcium-based magnetic”.

 

“In particular, XRD, FTIR, and SEM are employed to analyze the surface characteristics of the materials.” XRD, FTIR, and SEM use their full names when they first appear in the paper.

 

The color selection needs to be reconsidered, such as the fluorescent green color of Figure 1 representing nZVI-BC, which makes it difficult to see the details. In addition, it is more intuitive to label some important chemical bonds directly in Fig. 1 instead of just numbers. CA-CB needs to be corrected to CA-BC.

 

SEM does not reveal the microstructure of the different materials and the details of the composites; finer photographs are needed.

 

Line 179 “The absorption peak of FA–BC at 558 cm⁻¹ indicates considerable Fe–O accumulation in the biochar during pyrolysis”, Explain why Fe-O favors the immobilization of Cu²⁺.

 

Figure 4, there is no spacing symbol between the vertical coordinate name and the unit. Partial missing legend in Figures 5 and 6.

 

There are numerous formatting errors throughout the text, so please recheck carefully for corrections. For example, the position of the horizontal and vertical coordinate names in each data graph in the text should be centered. The spacing symbol between coordinate names and units should be unified. For example, the labeling form of the horizontal coordinate units in Figure 1 and Figure 6 is different.

 

Table 2 should not cross pages, and the font thickness of the row headings at the top of Table 2 should be consistent with Table 1.

The fact that there are differences in the adsorption properties of different crystalline iron minerals (e.g., α-Fe2O3 vs. β-Fe2O3), does the same apply to Ca minerals?

 

Is the performance of CA-BC material stable? Does desorption occur? What is the cycling performance of the material?

Comments on the Quality of English Language

Extensive editing of English language required

Author Response

Dear Reviewer

On behalf of all the contributing authors, I would like to express our sincere appreciation of your letter and reviewers’ constructive comments concerning our article entitled “Enhanced Adsorption of Cu²⁺ from Aqueous Solution by Sludge Biochar Compounded with Attapulgite Modified Fe”(Manuscript No: water-2721625). These comments are all valuable and helpful for improving our article. According to the nice editors and reviewers’ comments, we have made extensive modifications to our manuscript and supplemented extra data to make our results more convincing. In this revised version, changes to our manuscript were all highlighted within the document by using red-colored text. Point-by-point responses to the nice editors and reviewers are listed below this letter.

We sincerely thank the editors and all reviewers for their valuable feedback that helped us to improve the quality of our manuscript. The reviewer comments are listed in bold below and specific concerns have been numbered. Our response is given in normal font and changes/additions to the manuscript given in red text.

We tried our best to improve the manuscript and made some changes to the manuscript. These changes will not influence the content and we do invite an expert of us who is a native English speaker from the USA to help polish our article. Here we did not list the changes but marked in the revised paper.

1. Unified font and size for abstract, such as “Cu²⁺”, “calcium-based magnetic”.

We apologize profusely for our careless error and thank you for the reminder. The changes to the manuscript have been given in red text in the abstract at Line 10-23. We've corrected all the "Cu²⁺", "calcium-based magnetic"

2.“In particular, XRD, FTIR, and SEM are employed to analyze the surface characteristics of the materials.” XRD, FTIR, and SEM use their full names when they first appear in the paper.

We apologize profusely for our careless error and thank you for the reminder. The changes to the manuscript have been given in red text in the manuscript.

 “This study examines the effect of three modified attapulgite composite sludge biochar material on Cu²⁺ elimination. In particular, XRD, FTIR, and SEM are employed to analyze the surface characteristics of the materials” has been corrected as “This study examines the effect of three modified attapulgite composite sludge biochar materials on Cu²⁺ elimination. In particular, X-ray diffraction （XRD）, Fourier transform infrared spectroscopy （FTIR）, and scanning electron microscopy (SEM) are employed to analyze the surface characteristics of the materials.” at Line 6-67.

3. The color selection needs to be reconsidered, such as the fluorescent green color of Figure 1 representing nZVI-BC, which makes it difficult to see the details. In addition, it is more intuitive to label some important chemical bonds directly in Fig. 1 instead of just numbers. CA-CB needs to be corrected to CA-BC.

We are very grateful for your professional review work on our article. We also apologize for our carelessness and thank you for your suggestions, based on your suggestions, we have made detailed changes to Fig. 1, Fig. 3, and Fig. 4 Fig.

4. SEM does not reveal the microstructure of the different materials and the details of the composites; finer photographs are needed.

We are very grateful for your professional review work on our article. We also apologize for our carelessness, thank you for your suggestion, according to your suggestion, we have made a detailed modification of the Fig. 2 graphs, and have changed all the SEM graphs to the highest display magnification of 300 nanometers

5. Line 179 “The absorption peak of FA–BC at 558 cm⁻¹ indicates considerable Fe–O accumulation in the biochar during pyrolysis”, Explain why Fe-O favors the immobilization of Cu²⁺.

We are very grateful for your professional review work on our article. Thanks to the reviewer's professional advice, we found that the formation of Fe-O had a positive effect on the adsorption of Cu²⁺ during the pre-experiment. According to your suggestion, we reviewed a large amount of literature. In order to ensure the preciseness of the manuscript, it has been revised here as “The absorption peak of FA–BC at 558 cm⁻¹ indicates Fe–O accumulation in the biochar during pyrolysis, thus confirming that iron oxide precipitation occurred on the biochar surface..” in Line 179

6. Figure 4, there is no spacing symbol between the vertical coordinate name and the unit. Partial missing legend in Figures 5 and 6.

We are very grateful for your professional review work on our article. We also apologize for our carelessness and thank you for your suggestions, according to your suggestions, we have made detailed modifications to Fig. 4, Fig. 5 and Fig. 6 Fig.

7. There are numerous formatting errors throughout the text, so please recheck carefully for corrections. For example, the position of the horizontal and vertical coordinate names in each data graph in the text should be centered. The spacing symbol between coordinate names and units should be unified. For example, the labeling form of the horizontal coordinate units in Figure 1 and Figure 6 is different.

We are very grateful for your professional review work on our article. We also apologize for our carelessness and thank you for your suggestion, according to your suggestion, we have made a detailed revision of the diagrams of Fig. 1 and Fig. 6 to harmonize the horizontal coordinate units of Fig. 1 with Fig. 6

8. Table 2 should not cross pages, and the font thickness of the row headings at the top of Table 2 should be consistent with Table 1.

We are very grateful for your review work on our article. We also apologize for our carelessness and thank you for your suggestions, based on which we have made detailed changes to Table 2

9. The fact that there are differences in the adsorption properties of different crystalline iron minerals (e.g., α-Fe2O3 vs. β-Fe2O3), does the same apply to Ca minerals?

Thank you very much for your suggestion, we refer to a large number of literature, According to the reference (Synergistic role of inherent calcium and iron minerals in paper mill sludge biochar for phosphate adsorption by Yu J, Li X, Wu M, et al.), α-Fe₂O₃ and β-Fe₂O₃ are equally suitable for Ca minerals. It has been shown that novel Ca/Fe-rich biochar was prepared via a one-step process of pyrolyzing paper mill sludge (PMS) at various temperatures (300, 500, 700, and 800 °C). Ca/Fe-rich biochar obtained at 800 °C, which could be easily separated magnetically, exhibited the best phosphate adsorption capacity in a wide range of solution pH (5–11). The calcium carbonate and ferric salts in the sludge were converted into CaO and Fe₃O₄ through pyrolysis at 800 °C. The CaO inherent in Ca/Fe-rich biochar was proved to serve as active sites for the chemical precipitation, showing its synergistic effect with iron oxide compounds (i.e., Fe₃O_4, α-Fe₂O₃) on phosphate removal through chemical precipitation, ligand exchange, and complexation. The study suggested that Ca/Fe-rich magnetic biochar can be used as an effective phosphate adsorbent but also offers new insights into the synergistic effect of calcium and iron species for the adsorption of phosphate using biochar.

10. Is the performance of CA-BC material stable? Does desorption occur? What is the cycling performance of the material?

Thank you very much for your suggestion, we have referred to a large number of references, and relevant experiments are conducted. CA-BC material is very stable. The characterization of morphology and phase composition did not change, and no desorption phenomenon was found in the year after the preparation of the material. We examined the regeneration characteristics of the material in several adsorption cycles. Since this regeneration characteristics of the cycle is relatively long, the relevant experiments have not yet been completed. The experimental results will be synthesized in the next manuscript. Thank you again for your suggestions.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article entitled “Enhanced Adsorption of Cu2+ from Aqueous Solution by Sludge Biochar Compounded with Attapulgite Modified Fe” by Ruoan Wang and colleagues were completely revised. The article discussed uses of three types of modified biochar for Cu(II) removal. Authors investigate sorption at different pH values, sorption kinetics and sorption isotherms. The manuscript is well organized. The topic of the article is of general interest for readers working in the precious metal recovery and waste water treatment and environmental interest. I therefore recommend the publication of the article with the following revisions

 Comments.

1.        English Language should be revised throughout the whole manuscript.

2.        Support the conclusion at the end of the introduction

3.        Introduction has defected of references it should be supported by update references

4.        L 73-L77. Concentration of chemicals should be provided

5.        L 74: the authors mentioned the salt of copper as nitrate but what written is it as sulfate form.

6.        Does authors optimize the synthesis procedures, on other words the optimization of the condition used for: (BC), CA–BC, FA–BC, and nZVI–BC synthesis.

7.        The concentration expression should be mol M L−1 instead of mol L−1 M.

8.        FTIR and XRD discussion should be supported by references

9.        Figure 1 and Figure 3 should be replaced by another high resolute one

10.    Authors should investigate the pH of zero charge point to illustrate what is the pH that the surface will be positively or negatively charge (L223).

11.    Authors should provide Cu²⁺ species under different pH values.

12.    Authors should repeat sorption experiments twice or three time for investigate the reproducibility

Comments on the Quality of English Language

Moderate editing of English language required

Author Response

Dear Reviewer

 

On behalf of all the contributing authors, I would like to express our sincere appreciation of your letter and reviewers’ constructive comments concerning our article entitled “Enhanced Adsorption of Cu²⁺ from Aqueous Solution by Sludge Biochar Compounded with Attapulgite Modified Fe”(Manuscript No: water-2721625). These comments are all valuable and helpful for improving our article. According to the nice editors and reviewers’ comments, we have made extensive modifications to our manuscript and supplemented extra data to make our results more convincing. In this revised version, changes to our manuscript were all highlighted within the document by using red-colored text. Point-by-point responses to the nice editors and reviewers are listed below this letter.

We sincerely thank the editors and all reviewers for their valuable feedback that helped us to improve the quality of our manuscript. The reviewer comments are listed in bold below and specific concerns have been numbered. Our response is given in normal font and changes/additions to the manuscript given in red text.

1. English Language should be revised throughout the whole manuscript.

We tried our best to improve the manuscript and made some changes to the manuscript. These changes will not influence the content and we do invite an expert of us who is a native English speaker from the USA to help polish our article. Here we did not list the changes but marked in the revised paper.

2. Support the conclusion at the end of the introduction

We feel great thanks for your professional review work on our article. As you are concerned, several problems need to be addressed. According to your nice suggestions, we have made extensive corrections to our previous draft.

Based on your suggestion, we add the concentration of chemicals in L71-L76 as “The result suggests that CA–BC demonstrated the highest removal efficiency, reaching approximately 92.644% with a maximum theoretical adsorption of 3.98 at pH 6, a Cu²⁺ concentration of 2.813 mg/L, and a reaction time of 157 min. The CA–BC has great potential for removing Cu²⁺ from aqueous solutions.”

3. Introduction has defected of references it should be supported by update references

Thank you very much for your suggestion, we referenced a lot of literature and have added some new citations. We revised and added references [1], [2], [3], [4], [36].

4. L 73-L77. Concentration of chemicals should be provided

We are very grateful for your professional review work on our article. We also apologize for our carelessness and thank you for your suggestion. Based on your suggestion, we add the concentration of chemicals in L78-L81 as “Ethanol, FeCl₃ (purity: ≥99%), FeSO₄(purity: ≥99%), CaCO₃(purity: ≥99%), Cu(NO₃)₂(purity: ≥99%), NaOH(purity: ≥99%), HNO₃(purity: ≥99%), and HCl(purity: ≥99%) were purchased from Gansu HXEPT Co. Ltd. (Lanzhou, China). in L 99 as NaBH₄(purity: ≥98%)

5. L 74: the authors mentioned the salt of copper as nitrate but what written is it as sulfate form.

We are very grateful for your professional review work on our article. We also apologize for our carelessness and thank you for your suggestion, according to your suggestion, we have made a detailed revision of L 78 with the following corrections (Cu(NO₃)₂·3H₂O purity: ≥99%)

6. Does authors optimize the synthesis procedures, on other words the optimization of the condition used for: (BC), CA–BC, FA–BC, and nZVI–BC synthesis.

We are very grateful to you for your professional review work on our article, all the conditions for the synthesis of (BC), CA-BC, FA-BC and nZVI-BC. have all been optimized in the pre-experimental stage

7. The concentration expression should be mol M L−1 instead of mol L−1 M.

We sincerely thank you for your valuable comments, and we have rewritten this section based on the reviewer's comments to correct all concentrations in the manuscript to mg/L or g/L uniformly.

8. FTIR and XRD discussion should be supported by references

Thank you very much for your suggestion, we think this is a great suggestion and we have changed the manuscript according to the reviewers' comments, which have been given in red text in the manuscript.

Added in L170 “FTIR of sewage sludge prepared by Wang et al. also showed -C=C- and -C=O [30].”

Added in L194–198 “The SEM of sludge and calcium sulfate co-pyrolysis biochar prepared by Liu et al. also showed the same surface characteristics. Their adsorption of heavy metals is also effective, and the above studies have fully demonstrated that the load-modified biochar with different elements has excellent adsorption performance for heavy metals [33].”

Added in L223-227 “The magnetic biochar prepared by Wu et al. also showed the FeO, Fe₂O₃, Fe₃O₄ and that study pointed out that positively charged surface functional groups and iron oxides in magnetic biochar can both serve as sorption sites for ions in soil solution [36].”

9. Figure 1 and Figure 3 should be replaced by another high resolute one

Thank you for your professional advice. According to your suggestions, we have made detailed revisions to Figure 1, Figure 3. Figure 1, Figure 3 have been replaced with a clarity of 600 dpi.

10. Authors should investigate the pH of zero charge point to illustrate what is the pH that the surface will be positively or negatively charge (L223).

Thank you very much for your professional advice. This suggestion is very important to us，and based on your suggestions we roughly make the following explanations: We have determined the zeta potential of magnetic biochar. The point of zero charge was determined by interpolation of the pH value at zeta potential equal to zero. For non-magnetic biochar, the point of zero charge was 2.0, meaning that the biochar matrix has a permanent negative charge from pH 2.0 to pH 10. After surface precipitation of iron oxide, the point of zero charge was 6.9. Different values for the point of zero charge for composite (Biochar material), ranging from 4.9 to 6.8 This large variation in point of zero charge depends on the material preparation method (co-precipitation, solvothermal route etc.), and determination method (microelectrophoresis, pH drift). The surface charge of magnetic biochar is negative from pH 6.0 to more basic pH values, meaning that electrostatic interactions with cationic species from the solution should be favoured in this pH range. Since this experiment of the cycle is relatively long, the relevant experiments have not yet been completed. These experimental results will be synthesized in the next manuscript. Thank you again for your suggestions.

11. Authors should provide Cu²⁺ species under different pH values.

We sincerely thank you’re your valuable suggestions, and based on your suggestions we roughly make the following explanations: Under acidic conditions, most of the Cu²⁺ in solution is in the free state, and it combines with anions in solution to generate products such as Cu(NO₃)₂, CuCl₂ and CuSO4. In the process of adsorption and precipitation, Cu²⁺ is accompanied by dissolution-adsorption-re-precipitation during the reaction. The precipitation process produces Cu²⁺ complex precipitation and Ca(OH)₂ precipitation. Cu²⁺ also occupies the vacancies of ≡Ca+ in ≡Ca-OH on the surface of CA-BC minerals, and a very small amount of Cu²⁺ stays on the surface of biochar for a short period, resulting in the precipitation of Cu-OH by sharing -OH with ≡Ca+ on the surface.
However, due to the limited time at present, all the species of Cu²⁺ generated at different pH values are still being tested. I am very grateful for your suggestions, and this part will be the focus of my next research.

12. Authors should repeat sorption experiments twice or three times for investigate the reproducibility

We thank you very much for your professional review work on our article. We also apologize for our carelessness. All our experiments were set up with three replications in the text and the changes to the manuscript have been given in the abstract in red text in L123-L125. All relevant figures are changed.

“Each experiment was conducted thrice, ensuring the reproducibility of the findings. The data presented were derived from averaging the results of the three experiments, with error bars signifying the standard deviation from the mean.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors extensively corrected their manuscript, so now it can be considered for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript was improved and I suggest acceptance in the present form