Review Reports - Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods

Round 1

Reviewer 1 Report

This review article deals with ‘Uranium and fluoride removal from aqueous solution using biochar: a critical review for understanding the role of feedstock types’. The manuscript has been well written and authors have made a tremendous effort to gather a lot of information on this topic, however, the manuscript still has some shortcomings, which need to be addressed before accepting this article in this esteemed journal.

Comments:

#In most of the sections, a lot of studies have been compiled, however, the key highlight of each paragraph is missing. This needs to be properly addressed, so that the conclusion will be clearly reflected in the manuscript.

# In some cases, the information has been supported without any valid reference. This should be provided.

# Authors have provided only 31 studies on fluoride removal and 50 on uranium removal. I think more studies should be added to make this review very comprehensive.

# Please discuss the adsorption capacity with respect to raw and modified one and elaborate their adsorption capacity with respect to different environmental conditions (like pH, Temp, redox condition, etc.) and experimental conditions (feedstock type, dose, initial conc, particle size, real/synthetic water, etc). Evaluate each one with respect to the adsorption and which one is better for U and F removal and why? You can make a comparative analysis with respect to the experimental conditions.

# Please discuss the properties changes in modified biochar and how this is related to adsorption capacities. In similar manner make a comparative adsorption analysis with respect to feedstock type and modification.

# Mention the limitation and challenges of this work and whether it can be applicable to treat real surface/groundwater.

# I found the statistical treatment of data is highly missing. This makes this paper weak, although a lots of information has been compiled. I would suggest to apply any statistical/metadata analysis to strengthen this paper

# The real conclusion of this work is missing. It should be modified focusing the key highlights of the papers.

Author Response

Reviewer 1' comments:

Response: We are highly thankful to the reviewer for finding this work very interesting and highlighting the efforts made by the authors. As suggested, all valuable comments and suggestions were taken into account and incorporated into the revised manuscript accordingly.

Comments:

Response: As suggested, key highlights have been incorporated in each section, and the conclusion has been modified to focus on the important findings of the paper.

# In some cases, the information has been supported without any valid reference. This should be provided.

Response: The relevant missing references were added in the revised manuscript (see line numbers 71, 72, 73, 145, 147, 149, 161, 163, 164, 210, 309, 310, 314, 316, 317, 318, 367, 469, 474, 475, 476, 600, 641,773).

# Authors have provided only 31 studies on fluoride removal and 50 on uranium removal. I think more studies should be added to make this review very comprehensive.

Response: As suggested, additional new studies have been added on F^- and U removal (see pages 17, 15).

Response: Comparative analysis with respect to raw and modified biochar, along with the relevant environmental parameters and experimental conditions, has been incorporated in the revised manuscript (Please see section 3.6).

Response: As suggested, properties changes in modified biochar have been incorporated in the section 3.5 (page 11-20). Similarly, effect of modification on the feedstock has been described in section 3.5 (Page 11-20), 3.6 (Page 20-21), and 3.7.6 (Page 34).

# Mention the limitation and challenges of this work and whether it can be applicable to treat real surface/groundwater.

Response: Limitations and challenges of this work have been discussed in detail in section 3.6. Regarding the field application, yes, biochar can remove both U and F^- from real surface/groundwater, but limited studies have been done in this regard.

Response: We agree with the reviewer regarding statistical treatment, however, due to scatter data type it is very difficult to apply statistical/meta-analysis in this short span of time. However, we will try to incorporate this in our future work.

# The real conclusion of this work is missing. It should be modified focusing the key highlights of the papers.

Response: As suggested, conclusion has been modified reflecting the key highlights of the paper in section 7, pages 39-40.

Author Response File: Author Response.docx

Reviewer 2 Report

In this work, the authors review the U and F^- removal from aqueous solution by biochar. Feedstock type, pyrolysis temperature, modified methods, solution pH, surface area, and surface charge-influenced biochar adsorption capacities have been discussed in detail. This study reached a comprehensive conclusion by analyzing a large number of previous works, and presented current research gaps, and future perspectives. Nevertheless, there are some problems in the manuscript. I recommend that this paper could be accepted for publication in your journal after major revision.

1. The title is not very appropriate, because the authors summarize many factors for the removal of U and F^-, not just the feedstock types.

2. The abstract is too simple and lacks useful information.

3. Line 126 “temp”?

4. Line 147-152 There is a lack of references.

5. Section 3.2.1 and 3.2.2. The authors should briefly introduce that biochar can remove U and F^- instead of listing a lot of feedstock types made of biochar can remove U and F^-. The effect of feedstock types on U and F^- removal can be described in detail in Section 3.6.

6. Line 243 and 277. There is two Section 3.3, each section should be renumbered.

7. The units of full text should be unified, such as mg g^-1 and mg/g.

8. The writing types of uranium and fluoride in full text should be unified. 9. Section 3.5 Modification methods. The authors list the effect of different modification methods on improving the adsorption capacity, but most of the reasons are attributed to the increase of specific surface area, while more other reasons should be discussed in detail. In addition, there is a lack of discussion on the adsorption capacity of pristine biochar.

Author Response

Reviewers 2' comments:

Response: We are grateful to the reviewer for his/her valuable comments and suggestions. We have carefully taken into account all comments and modified the manuscript accordingly.

Comments:

The title is not very appropriate, because the authors summarize many factors for the removal of U and F^-, not just the feedstock types.

Response: As suggested, title has been modified. Modified title of the paper is ‘Uranium and fluoride removal from aqueous solution using biochar: a critical review for understanding the role of feedstock types, mechanisms, and modification methods’.

The abstract is too simple and lacks useful information.

Response: As suggested, abstract has been modified and useful information has been added (See line 12, 17-22, page 1)

Line 126 “temp”?

Response: This is corrected in line 135, page 5. It is temperature.

Line 147-152 There is a lack of references.

Response: As suggested, references have been added in section 3.2 from line 161-164 on page number 6 in the revised version.

Section 3.2.1 and 3.2.2. The authors should briefly introduce that biochar can remove U and F^-instead of listing a lot of feedstock types made of biochar can remove U and F^-. The effect of feedstock types on U and F^- removal can be described in detail in Section 3.6.

Response: As suggested, influence of different feedstocks on U and F^- adsorption has been discussed separately in section 3.7.6 (page 34, line 737-754), and a brief introduction of U and F^- removal through biochar has been described in section 3.2 (page 6, line 151-240).

Line 243 and 277. There is two Section 3.3, each section should be renumbered.

Response: The is corrected as section 3.5 in line 344 on page number 10.

The units of full text should be unified, such as mg g^-1and mg/g.

Response: This has been corrected in the entire manuscript (line 514, 651, 667, 669, 688, 689, 691, 695, 696).

The writing types of uranium and fluoride in full text should be unified.

Response: This has been unified throughout the text in the revised version.

Section 3.5 Modification methods. The authors list the effect of different modification methods on improving the adsorption capacity, but most of the reasons are attributed to the increase of specific surface area, while more other reasons should be discussed in detail. In addition, there is a lack of discussion on the adsorption capacity of pristine biochar.

Response: As suggested, the effect of different modification methods on biochar properties and adsorption capacity have been discussed under section 3.5, page 11-20. Apart from surface area, other properties of biochar are discussed, such as porosity, adsorption sites, functional groups, and porous structure. In addition, adsorption capacity of pristine biochar has been discussed in section 3.6, page 20-21.

Author Response File: Author Response.docx