Mitigation of Fluoride Contamination in Drinking Water Supply Sources by Adsorption Using Bone Char: Effects of Mineral and Organic Matrix

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 Overall comments

This manuscript describes analysis and treatment of fluoride impacted waters in Cameroon.  The topic is relevant and of great interest to fluoride impacted communities.  The paper does extensive analysis of bone chars prepared under various conditions, then conducts a treatment study for the best performing bone char.  The authors analyze four natural waters containing fluoride above the WHO standard, as well as deionized water that was spiked with fluoride.  I believe that the paper could be streamlined in some places—for example, the authors don’t need to show SEM images for all chars since they discuss how there are no differences between them in terms of morphology.  Also, I think the authors should be more careful with significant digits and discuss QA/QC procedures (i.e., did they analyze duplicates and blanks?  What was the relative standard deviation between duplicate samples).  In addition, I think the detailed mechanistic interpretation of the adsorption processes in some parts of the paper is not justified based on the data presented.  Finally, the authors should discuss if the treatment of the fluoride impacted waters achieved the WHO standard, and, if not, show (maybe through experiments?) what conditions would be required for this.  

Specific comments  
Throughout: Provide uncertainties associated with all measured parameters, and/or use fewer significant digits.  I think that, for example, the following values may contain too many digits (these examples are from the abstract, but this comment applies to all reported values):  
·   concentrations in fluoride ions (2.29 to 4.46 mg/L)
·  specific surface area (112.27 m2/g)
· adsorption performance range of 72.48 %to 80.27 %  
Line 33: Provide justification for the claim that bone char is “eco-friendly”, or at least some specifics about why you believe it is more eco-friendly than other sorbents.  Charring has significant environmental impacts.  I would not consider it particularly eco-friendly.  

General comment: Please leave a space between the word and the brackets enclosing a citation number that follow the word.  

Line 50: “within the western boarders of Africa”.  When you say “boarders”, do you mean “residents”?  Please clarify this.  

Line 68: I don’t think it is correct to say that bone char has “no significant environmental impact [11-13],” nor do the cited refs seem to address environmental impacts.   

More evidence would be required to make claims about environmental impacts.  See, for example:  
·       Sankhya Ramanan, Godfrey Mkongo, Qian Zhang, Haibao Wen, Felix Mtalo, Junjie Shen, Assessing the environmental impact of bone char production by in-situ emission monitoring and life cycle assessment, Journal of Cleaner Production, Volume 367, 2022, 132974, ISSN 0959-6526, https://doi.org/10.1016/j.jclepro.2022.132974.  
·       Yami, T. L., Du, J., Brunson, L. R., Chamberlain, J. F., Sabatini, D. A., Butler, E. C. (2015), Life cycle assessment of adsorbents for fluoride removal from drinking water in east Africa, International Journal of Life Cycle Assessment 20 (9), 1277-1286.  
Line 99: There is an extra symbol at the end of this sentence.
 
Lines 118-120: Please provide a reference accessible to readers for the DOC analysis, or else describe the analysis.  I don’t know what “LCK 385” is.

  Line 170: “The continual intake of water having high F- is consequent to maligned health effects such as fluorosis, ligaments calcification, nevernous weakness, liver and kidney dysfunction [22].”  I think the illnesses are consequent to the intake of water, not the other way around.  Also, check the spelling of “nervous” (I’m not sure what “nevernous” means).  

Figure 2: Suggest you add the XRD reference lines to the figure.  

Figure 5 and associated discussion: It would be helpful to report fluoride adsorption densities in addition to percent removal values.  It might be informative to plot before and after plots as well (i.e., concentration before and after treatment, with the WHO standard shown on the plot.)  

Line 287: “The results indicate that Ca2+ and Mg2+ were absorbed into the surface of BC.”  This is not consistent with the preceding discussion which described the surface of hydroxyapatite/bone char as positive.  

Line 313: “the amount of Na+ released into the treated water was almost equivalent with the amount of Ca2+ and Mg2+ adsorbed on the BC. This result indicated an ion exchange between Na+ in BC and Ca2+/Mg2+ (from the aqueous solution.) adsorbed on BC.”  The amount of sodium and potassium released should be two times the amount of calcium plus magnesium taken up if the process were ion exchange.  Is this what you observed?  Please clarify this in the text.

  Also, please provide a measure of uncertainty (e.g., a standard deviation) for the values reported in Table 5 and elsewhere, or else use fewer digits.  

Line 349: “Overall, the treated water samples had DOC levels exceeding the USEPA guideline (2 mg/L).”  Please specify the guideline you are referring to.  There is no USEPA maximum contaminant level for DOC in drinking water.   

Line 351: I suggest you replace “justified” with “explained”.  

Line 355: “However, it was convenient to note that the treated water does not exhibit a yellowish color using BC prepared at low temperature (< 550 °C)[20, 26]. Thus, the presence of yellowish color cannot be justified only by the preparation conditions of BC at low temperature.”  This is confusing.  Did you observe a yellow color?  There is no mention of this.  If not, then why say “the presence of yellowish color cannot be justified…”?  

Line 353: “Thus, it is necessary to perform a pretreatment of the prepared material (acid treatment for example) to remove any organic matter before applying it for defluoridation.”  It would be better to show through an experiment or citations that this pretreatment removes organic matter.  

Line 363: “The schematic diagram in Figure 8 reveals the adsorption process mechanism involved in the removal of the fluoride ion from the sampled water using the bone char.”  There is no evidence provided for some of the phenomena shown in Figure 8.  For example, calcium phosphate complexes are shown.  What evidence is there for these?  Also, the blue ovals with “Mg2+” written in them next to the SEM image of BC are at odds with the blue circles with the “+” signs to the left.  (If the surface is positively charged, why does it attract Mg+2?)  Also, the figure suggests there is some kind of complexation or reaction between fluoride and a calcium phosphate complex, but there isn’t any evidence presented for this.  In summary, I don’t think that evidence is presented to support some of the processes in this figure.  I recommend omitting it.   

Line 372: “The ionic exchange occurs via the replacement of the anions especially the OH- by the fluoride ion present in the sampled water. Also, during the process of ion diffusion in  the adsorption, fluoride ions can form inner and outer-sphere complexes with calcium metal cations present in the hydroxyapatite inner-sphere or outer-sphere complexes [44].”  First, you said “inner-sphere or outer-sphere” two times.  Also, an outer sphere complex would not involve ion exchange.  Unless you have evidence for the specific kind of adsorption process taking place, I recommend limiting this speculation.  

Line 376: “In addition to this process mechanism, the fluoride ions diffuse into these pore structure of the bone char (BC) and interact with the surface functional groups forming various forms of precipitates thus the ion is trapped onto the surface of the active sites of the adsorbent (BC) [41, 45].”  Please provide evidence for formation of a precipitate if you wish to make this claim. 

  Line 382: “The instrument analysis of the prepared adsorbent (BC) shows no significant change with respect to various calcination temperature and residence time.”  This is not clear.  There were quite a few differences reported with respect to calcination and residence time.  What do you mean by “instrument analysis”?  

Final comment: It would be helpful to report the final concentration of fluoride achieved in the samples, specifically in comparison to the WHO standard and potential health effects.  Was the treatment goal (WHO standard) achieved?  If not, what recommendations do you have?  Since this is a somewhat applied study using natural waters, whether the treatment was adequate to achieve WHO standards is very relevant, and in my opinion a better focus of discussion than speculation about the reaction mechanism.  Also, treatment of this water using bone char to WHO standards for fluoride did not mitigate the potential serious health impacts from nitrate.  As a practical matter, what would you recommend?  A dual treatment—first for fluoride, then for nitrate?  Choice of another treatment process that would address both nitrate and fluoride?  Any discussion of the high nitrate concentrations and the treatment options in the waters under study is missing.  This is relevant because just treating these waters with bone char would not mitigate the hazards from excess nitrate consumption.

Comments on the Quality of English Language

There were some awkward phrases/words, such as use of 

"justifies" for "explains", "boarders" for "residents".  Check carefully for English usage.

Author Response

Editorial system Water.

Article Type: Research Paper

Title:  Mitigation of fluoride contamination in drinking water supply sources by adsorption using bone char: Effects of mineral and organic matrix.

Corresponding Author’s: Sadou Dalhatou and Askar Hilonga

Authors: Mohamed Raoul Ibrahim, Joshua Akinropo Oyetade, Sadou Dalhatou, Anton Nikiforov, Christophe Leys, Askwar Hilonga

Responses to the reviewer comments

Dear Editor

The authors would like to thank the referees and the managing Editor for their comments and suggestions regarding our paper.

The points raised by the Reviewers are clarified below and the manuscript has been amended accordingly.

 

Reviewer 1# :

This manuscript describes analysis and treatment of fluoride impacted waters in Cameroon.  The topicis relevant and of great interest to fluoride impacted communities.  The paper does extensive analysis of bone chars prepared under various conditions, then conducts a treatment study for the best performing bone char.  The authors analyze four natural waters containing fluoride above the WHO standard, as well as deionized water that was spiked with fluoride.  I believe that the paper could best reamlined in some places—for example, the authors don’t need to show SEM images for all chars since they discuss how there are no differences between them in terms of morphology.  Also, I think the authors should be more careful with significant digits and discuss QA/QC procedures (i.e., did they analyze duplicates and blanks?  What was the relative standard deviation between duplicate samples).  In addition, I think the detailed mechanistic interpretation of the adsorption processes in some parts of the paperis not justified based on the data presented.  Finally, the authors should discussif the treatment of the fluoride impacted waters achieved the WHO standard, and, if not, show (maybe through experiments?) what conditions would be required for this.

Authors response : The Author’s are grateful for the prolific and constructive comments and recommendation give to re-shape the manuscript into better look. All experiments in this study were performed in triplicate and the corresponding standard deviation was included. The mechanism of fluoride adsorption was revised and confirmed based on the characterization of bone char after fluoride adsorption. Authors indicated in the abstract and conclusion the treatment of the fluoride impacted waters achieved the WHO standard. All raised comments have been addressed and the manuscript has been reviewed in all to troubleshoot syntax errors, and grammatical spellings.

Specific comments :

1. Throughout: Provide uncertainties associated with all measured parameters, and/or use fewer significant digits.  I think that, for example, the following values may contain too many digits (these examples are from the abstract, but this comment applies to all reported values):  

• concentrations in fluoride ions (2.29 to 4.46 mg/L)
• specific surface area (112.27 m2/g)
• adsorption performance range of 72.48 %to 80.27 %.

Authors response : The uncertainties associated with all measured parameters have been added to the document.

2. Line 33: Provide justification for the claim that bone char is “eco-friendly”, or at least some specifics about why you believe it is more eco-friendly than other sorbents.  Charring has significant environmental impacts.  I would not consider it particularly eco-friendly.

Authors response : Thanks for reporting this. The authors believe that charred bone is an environmentally friendly material because it is derived from animal bones, making it a natural and biodegradable material. It can also be found free in slaughterhouses and households as solid waste.

General comment:

1. Please leave a space between the word and the brackets enclosing a citation number that follow the word.
2. Line 50: “within the western boarders of Africa”.  When you say “boarders”, do you mean “residents”?  Please clarify this.

Authors response: The terms borders refer to the borders between West Africa and Central Africa. This sentence has been edited. The change will be found in the manuscript. 

3. Line 68: I don’t think it is correct to say that bone char has “no significant environmental impact [11-13],” nor do the cited refs seem to address environmental impacts.

More evidence would be required to make claims about environmental impacts.  See, for example:

• Sankhya Ramanan, GodfreyMkongo, Qian Zhang, HaibaoWen, Felix Mtalo, JunjieShen, Assessing the environmental impact of bone char production by in-situ emission monitoring and life cycle assessment, Journal of Cleaner Production, Volume 367, 2022, 132974, ISSN 0959-6526, https://doi.org/10.1016/j.jclepro.2022.132974.
• Yami, T. L., Du, J., Brunson, L. R., Chamberlain, J. F., Sabatini, D. A., Butler, E. C. (2015), Life cycle assessment of adsorbents for fluoride removal from drinking water in east Africa, International Journal of Life Cycle Assessment 20 (9), 1277-1286.

Author's response: We agreed with this point of view. However, Alkurdi et al. (2019) [14] examined that the environmental impact of BC was very low compared to other adsorbents. This reference has been added to support this agreement. 

4. Line 99: There is an extra symbol at the end of this sentence.

Authors response : This symbol has been removed from the manuscript.

5. Lines 118-120: Please provide a reference accessible to readers for the DOC analysis, or else describe the analysis.  I don’t know what “LCK 385” is.

Authors response : The detailed description of this method for DOC analysis has been added in the document.

6. Line 170: “The continual intake of water having high F^- is consequent to maligned health effects such as fluorosis, ligaments calcification, nevernous weakness, liver and kidney dysfunction [22].”  I think the illnesses are consequent to the intake of water, not the other way around.  Also, check the spelling of “nervous” (I’m not sure what “nevernous” means).

Authors response : This sentence has been change edited.

7. Figure 2: Suggest you add the XRD reference lines to the figure.

Authors response : The XRD reference lines was added to the Fig. 2 as suggested.

8. Figure 5 and associated discussion: It would be helpful to report fluoride adsorption densities in addition to percent removal values.  It might be informative to plot before and after plots as well (i.e., concentration before and after treatment, with the WHO standard shown on the plot.).

Authors response :   The concentration before and after treatment, with the WHO standard shown on the graph has been added in Fig. 5.

9. Line 287: “The results indicate that Ca²⁺ and Mg²⁺ were absorbed into the surface of BC.”  This is not consistent with the preceding discussion which described the surface of hydroxyapatite/bone char as positive.

Authors response : From the results in Table 5, it was found that the initial amount of Ca²⁺ and Mg²⁺ in the raw water samples was reduced after the adsorption process. Authors assumed that this reduction may suggests an adsorption of these ions on the BC surface.

10. Line 313: “the amount of Na⁺ released into the treated water was almost equivalent with the amount of Ca²⁺ and Mg²⁺adsorbed on the BC. This result indicated an ion exchange between Na⁺ in BC and Ca²⁺/Mg²⁺ (from the aqueous solution.) adsorbed on BC.”  The amount of sodium and potassium released should be two times the amount of calcium plus magnesium taken up if the process were ion exchange.  Is this what you observed?  Please clarify this in the text.

Authors response : We observed that for the Wouro Dow 2 water sample for example, the quantity of Na⁺ before and after adsorption is 2.23 and 39.37 mg/L respectively. The quantity of Na⁺ released is equal to the quantity after adsorption subtracted from the initial quantity found in the raw water. On the other hand, the amount of Ca²⁺ and Mg²⁺ adsorbed in the BC was calculated by subtracting the initial amount of Ca²⁺ and Mg²⁺ before adsorption with the amount found in the treated water. It was found that the amount of Na⁺ released into the treated water corresponded almost to the amount of Ca²⁺ and Mg²⁺ adsorbed on the BC. Same observation from  Hina water sample. Hence the absorption of Ca²⁺ and Mg²⁺ on the surface of the BC following an ion exchange with the Na⁺ present in the BC. However, the increase in the concentration of K⁺ in the treated water can probably be due to the dissolution of this ion from BC.

11. Also, please provide a measure of uncertainty (e.g., a standard deviation) for the values reported in Table 5 and elsewhere, or else use fewer digits.

Authors response : The standard deviation for the values ​​reported in Table 5 has been added.

12. Line 349: “Overall, the treated water sampleshad DOC levelsexceeding the USEPA guideline (2 mg/L).”  Please specify the guideline you are referring to.  There is no USEPA maximum contaminant level for DOC in drinking water.

Author's response: We agreed. There is no USEPA maximum contamination level for DOC in drinking water. The guideline of 2 mg/L was for total organic carbon (TOC) in treated water. We modified it in the manuscript.

13. Line 351: I suggest you replace “justified” with “explained”.

Authors response : This was done. The change will be found in the document.

14. Line 355: “However, it was convenient to note that the treated water does not exhibit a yellowish color using BC prepared at low temperature (< 550 °C) [20, 26]. Thus, the presence of yellowish color cannot be justified only by the preparation conditions of BC at low temperature.”  This is confusing.  Did you observe a yellow color ?  There is no mention of this.  If not, then why say “the presence of yellowish color cannot be justified…”?

Authors response : No, we did not observe a yellowish color in the treated water. We only made a comparison with previous studies. This sentence was revised. The change can be found in the document.

15. Line 353: “Thus, it is necessary to perform a pretreatment of the prepared material (acid treatment for example) to remove any organic matter before applying it for defluoridation.”  It would be better to show through an experiment or citations that this pretreatment removes organic matter.

Authors response : Thank you for this remark. Previous work indicated that the interaction between the acid and the BC constituent (calcium carbonate and/or carbon) results in the release of CO₂, suggesting conversion of carbon to gas.

Iriarte-Velasco, U., I. Sierra, L. Zudaire, J. L. J. F. Ayastuy and B. Processing (2016). "Preparation of a porous biochar from the acid activation of pork bones."  98: 341-353.

However, more research needs to be done on this line. This sentence was removed until experimental results were obtained on the effect of acid treatment of DOC release on BC. This will be followed up in subsequent research.

16. Line 363: “The schematic diagram in Figure 8 reveals the adsorption process mechanism involved in the removal of the fluoride ion from the sampled water using the bone char.”  There is no evidence provided for some of the phenomen as shown in Figure 8.  For example, calcium phosphate complexes are shown.  What evidence is there for these?  Also, the blue ovals with “Mg²⁺” written in them next to the SEM image of BC are atodds with the blue circles with the “+” signs to the left.  (If the surface is positively charged, why does it attract Mg²⁺?)  Also, the figure suggests there is some kind of complexation or reaction between fluoride and a calcium phosphate complex, but there isn’t any evidence presented for this.  In summary, I don’t think that evidence is presented to support some of the processes in this figure.  I recommend omitting it.

Authors response : The fluoride adsorption mechanism was revised in the document.

17. Line 372: “The ionic exchange occurs via the replacement of the anions especially the OH^- by the fluoride ion present in the sampled water. Also, during the process of ion diffusion in  the adsorption, fluoride ions canform inner and outer-sphere complexes with calcium metal cations present in the hydroxyapatite inner-sphere or outer-sphere complexes [44].”  First, you said “inner-sphere or outer-sphere” two times.  Also, an outer sphere complex would not involve ion exchange.  Unless you have evidence for the specific kind of adsorption process taking place, I recommend limiting this speculation.

Authors response : The ion exchange between fluoride ions and OH groups on BC was confirmed by performing FTIR before and after fluoride adsorption.

18. Line 376: “In addition to this process mechanism, the fluoride ions diffuse into these pore structure of the bone char (BC) and interact with the surface functional groups forming various forms of precipitates thus the ion is trapped onto the surface of the active sites of the adsorbent (BC) [41, 45].”  Please provide evidence for formation of a precipitate if you wish to make this claim.

Authors response : From the experimental results, there was no evidence of expected precipitates. In summary, the mechanism process has been revised based on all your comments and suggestions.

19. Line 382: “The instrument analysis of the prepared adsorbent (BC) shows no significant change with respect to various calcination temperature and residence time.”  This is not clear.  There were quite a few differences reported with respect to calcination and residence time.  What do you mean by “instrument analysis”?

Authors response : Instrument analysis refers to FTIR and SEM results. However, this sentence has been revised to make it clearer.

Final comment: It would be helpful to report the final concentration of fluoride achieved in the samples, specifically in comparison to the WHO standard and potential health effects.  Was the treatment goal (WHO standard) achieved?  If not, what recommendations do you have?  Since this is asome what applied study using natural waters, whether the treatment was adequate to achieve WHO standards isvery relevant, and in my opinion a better focus of discussion than speculation about the reaction mechanism.  Also, treatment of this water using bone char to WHO standards for fluoride did not mitigate the potential serious health impacts from nitrate.  As a practical matter, what would you recommend?  A dual treatment—first for fluoride, then for nitrate?  Choice of another treatment process that would address both nitrate and fluoride?  Any discussion of the high nitrate concentrations and the treatment options in the waters under study is missing.  This is relevant because just treating these waters with bone char would not mitigate the hazards from excess nitrate consumption.

Authors response : The authors accept the recommendations. The conclusion section has been revised to address these comments and suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The introduction should contain the originality of this work.

Thorough literature must be provided with respect to the present work. Use the following paper for reference. https://doi.org/10.1021/acs.langmuir.2c00702

An increase in peak intensity was observed as the calcination temperature increased must be discussed in detail in the XRD section.

FTIR after the adsorption of Fluoride must be provided for validation.

SEM images for change in morphology after adsorption and EDX be provided for elemental analysis must be provided.

BET plot must be given to identify the type of isotherm and its effect on adsorption.

The mechanism is superficial and must be authenticated by different characterizations.

 

The reusability of the BC be checked for its practicality.

Comments on the Quality of English Language

Moderate editing of English language required.

Author Response

Reviewer2# :

Comments and Suggestions for Authors :

1. The introduction should contain the originality of this work.

Authors response : The novelty of this paper is highlighted by a holistic evaluation of the performance of bone char in the removal of fluoride ions in raw natural water compared to simulated water. The study investigated for the first time fluoride contamination of water supplies in the village of Bahimi, in the northern region of Cameroon. The paper also systematically evaluated the effect of cations, anions, and dissolved organic carbon (present in the true aqueous matrix) on the process of fluoride adsorption on bone char. All this has been included in the introduction section.

2. Thorough literature must be provided with respect to the present work. Use the following paper for reference. https://doi.org/10.1021/acs.langmuir.2c00702

Authors response : The document has been improved and the suggested reference taken into account.

3. An increase in peak intensity was observed as the calcination temperature increased must be discussed in detail in the XRD section.

Authors response : Thank you for this observation. This is not discussed in detail because a previous study indicated that fluoride adsorption on BC was mainly affected by textural properties, particularly BET surface area, rather than hydroxyapatite content (reference 20).

Sawangjang, B.; Induvesa, P.; Wongrueng, A.; Pumas, C.; Wattanachira, S.; Rakruam, P.; Punyapalakul, P.; Takizawa, S.; Khan, E. Evaluation of fluoride adsorption mechanism and capacity of different types of bone char.Int. J. Environ. Res. Public Health2021, 18, 6878.

 

4. FTIR after the adsorption of Fluoride must be provided for validation.

Authors response : This was added in the adsorption mechanism part.

5. SEM images for change in morphology after adsorption and EDX be provided for elemental analysis must be provided.

Authors response : The authors were grateful for the recommendation. Although, efforts were made on to analyze bone char by SEM/EDX after fluoride adsorption. The breakdown of the instrument at the moment limited its accessibility as it is currently under maintenance. However, the main adsorption mechanism of fluoride on BC was confirmed using FTIR results after adsorption.

6. BET plot must be given to identify the type of isotherm and its effect on adsorption.

Authors response : The BET isotherm plot of the BC sample used for adsorption has been added to the document (Fig.5).

7. The mechanism is superficial and must be authenticated by different characterizations.

Authors response : The adsorption mechanism was elucidated using FTIR analyses (Fig. 5).

8. The reusability of the BC be checked for its practicality.

Authors response: Thank you for this suggestion. The reusability of theBC was not considered in this work because the present work focuses on the effect of aqueous matrix on the adsorption of fluoride onto BC. This recommendation is received and will be duly followed up in subsequent research.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Comments on the revised manuscript:

Line 50: “Boarders” is misspelled. Suggest you replace “borders” with “region” or “countries”.   Also, “Seldomly” is not correct.

Lines 66-67: some words are grouped together without spaces, e.g., “fordefluoridationof”.

Line 83: “Although, the set limit in treated water is 2 mg/L for total organic carbon (TOC) by the United States Environmental Protection Agency 84 (USEPA) [15]”  There is no drinking water standard for TOC set by the US EPA.  There is no effluent limit on TOC in drinking water set by the US EPA.  Also, this is not what the cited reference says.

Line 457: “Present” is misspelled.

 

Other comments:

Regarding Comment 10: Line 313: “the amount of Na+ released into the treated water was almost equivalent with the amount of Ca2+ and Mg2+adsorbed on the BC. This result indicated an ion exchange between Na+ in BC and Ca2+/Mg2+ (from the aqueous solution.) adsorbed on BC.” The amount of sodium and potassium released should be two times the amount of calcium plus magnesium taken up if the process were ion exchange. Is this what you observed? Please clarify this in the text.

The point of this comment was that the “amounts” of divalent ions and monovalent ions must be compared on an equivalent basis (using units of equivalents per liter) if you want to argue that they are replacing one another, or that the release of one corresponds to the uptake of another.  It is not clear from the author’s response if this was done (it doesn’t sound like it).  For example, the author’s response says “It was found that the amount of Na+ released into the treated water corresponded almost to the amount of Ca2+ and Mg2+ adsorbed on the BC.”  Instead of discussing amounts in mg/L, discuss amounts in equivalents per liter (where one mole of Ca+2 has two equivalents, one mole of Mg+2 has two equivalents, and one mole of Na+ has one equivalent).  Or, omit this discussion.

 

Comments on the Quality of English Language

Careful editing and proofreading is required.

Author Response

Responses to the reviewer 1 comments: ROUND 2#

1. Line 50: “Boarders” is misspelled. Suggest you replace “borders” with “region” or “countries”.   Also, “Seldomly” is not correct.

Response : This sentence has been change edited.  

2. Lines 66-67: somewords are grouped together without spaces, e.g., “for defluoridation of”.

Response : This sentence has been edited. The change will be found in the manuscript. 

3. Line 83: “Although, the set limit in treated water is 2 mg/L for total organic carbon (TOC) by the United States Environmental Protection Agency 84 (USEPA) [15]”  There is no drinking water standard for TOC set by the US EPA.  There is no effluent limit on TOC in drinking water set by the US EPA.  Also, this is not what the cited reference says.

Response : Yes we agreed. There are no specific guidelines for maximum TOC levels in drinking water. We removed it in the manuscript.

4. Line 457: “Present” is misspelled.

 Response : It is made

 

Othercomments:

Regarding Comment 10: Line 313: “the amount of Na⁺ released into the treated water was almost equivalent with the amount of Ca²⁺ and Mg²⁺ adsorbed on the BC. This result indicated an ion exchange between Na⁺ in BC and Ca²⁺/Mg²⁺ (from the aqueous solution.) adsorbed on BC.” The amount of sodium and potassium released should be two times the amount of calcium plus magnesium taken up if the process were ion exchange. Is this what you observed ? Please clarify this in the text.

 

The point of this comment was that the “amounts” of divalent ions and monovalent ions must be compared on an equivalent basis (using units of equivalents per liter) if you want to argue that they are replacing one another, or that the release of one corresponds to the uptake of another.  It is not clear from the author’s response if this was done (it doesn’t sound like it).  For example, the author’s response says “It was found that the amount of Na⁺ released into the treated water corresponded almost to the amount of Ca²⁺ and Mg²⁺ adsorbed on the BC.”  Instead of discussing amounts in mg/L, discuss amounts in equivalents per liter (where one mole of Ca²⁺ has two equivalents, one mole of Mg²⁺ has two equivalents, and one mole of Na⁺ has one equivalent).  Or, omit this discussion.

Response : Thank you for this observation. After verification, it was found that the the moles of Na⁺ released into the treated water were nearly equal to twice of the moles of Ca²⁺ adsorbed on the BC. Hence, this suggests an ion exchange only between Na⁺ from BC and Ca²⁺ from water solution. Changes have been made throughout the document. 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

I am satisfied with the revisions.

Author Response

Thank you for your valuable comments, which helped improve the quality of this document