Function of Nanomaterials in Removing Heavy Metals for Water and Wastewater Remediation: A Review
Round 1
Reviewer 1 Report
The paper by Ethaib et al. is a good review on the methods for water and wastewater remediation based on the use of nanomaterials. The article is rich of interesting literature and worth publishing after these minor revisions:
11) the main problem is the definition of Adsorption and the understanding of which is the main factor affecting the remediation: adsorption or absorption? For example, I think that in the case of carbonaceous materials, absorption in the mesoporous structures takes place. But, as also evidenced by Authors, it is not simple to verify and to be sure of what is the sorption mechanism. For this reason, I suggest the Authors to talk of SORPTION, in all the text and to distinguish in absorption and adsorption only when the cited papers are sure about the mechanism involved;
22) moreover, I think that at the beginning of the paper, the Authors should give a definition (taken by IUPAC definitions) of sorption, adsorption and absorption, to better clarify the matter;
33) I suggest a slight change in the paper title: “Function of nanomaterials in removing heavy metals for water and wastewater remediation: a review”;
44) I would change the keyword “removal” in “remediation”;
55) I suggest an English revision of the text.
Author Response
Dear Environments editor,
The authors highly appreciated the editorial team and reviewer efforts and thanked them for their valuable comments. The authors strictly tracked these comments and answered all of them point by point as below:
Reviewer 1
Comments
- The main problem is the definition of Adsorption and the understanding of which is the main factor affecting the remediation: adsorption or absorption? For example, I think that in the case of carbonaceous materials, absorption in the mesoporous structures takes place. But, as also evidenced by Authors, it is not simple to verify and to be sure of what is the sorption mechanism. For this reason, I suggest the Authors to talk of SORPTION, in all the text and to distinguish in absorption and adsorption only when the cited papers are sure about the mechanism involved;
Answer.
The authors followed the reviewer suggestion and used the specific word of “sorption” in all places that can be proper.
- Moreover, I think that at the beginning of the paper, the Authors should give a definition (taken by IUPAC definitions) of sorption, adsorption and absorption, to better clarify the matter;
Answer
As suggested the authors added the IUPAC definitions at the line 104.
- Isuggest a slight change in the paper title: “Function of nanomaterials in removing heavy metals for water and wastewater remediation: a review”;
Answer
The title was amended as suggested.
- I would change the keyword “removal” in “remediation”;
Answer
The keyword changed as suggested.
- I suggest an English revision of the text.
Answer
The English language was revised by proofreading and editing English service.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The submitted manuscript by Ethaib et al. describes current developments of nanomaterials for heavy metal remediation. The topic is interesting and fits with the scope of the chosen journal. However, the manuscript is very long and does not always deliver sufficient information. Tables should summarize the content of manuscript in a compact form and should not be written in full sentences. Another important issue is the missing link to remediation. The headline implies that nanomaterials for wastewater and water remediation are discussed. Therefore, I accepted to read more detailed about applications on remediation and field studies. The manuscript has a lot of tipping and grammar mistakes. Therefore intensive revision is urgently required.
General remarks:
1. The adsorption capacity is strongly depending on the dosage, pH, temperature, contact time and concentration of the contaminate to be adsorbed. Therefore, it is not sufficient reporting only adsorption capacities without any relation on these parameters. It seems to be more useful to summarize them in Tables with all important parameters influencing their efficiency instead of reporting some capacities in the main text. To the reader there is often no relation whether the reported capacity is high or low. Therefore, use a table and compare different adsorption capacities for the same heavy metal. There are some good reviews, for example look at Appl. Nano 2022, 3, 54–90. https://doi.org/10.3390/ applnano3010005 and Water 2020, 12, 803; https://doi.org/10.3390/w12030803.
2. About the mechanism of heavy metal ion removal several mechanisms are well known up to now. Very often different interactions depend on pH, functional groups, protonation/deprotonation of the active site of the adsorbents, selectivity of the adsorbents, electrostatic interaction, metal coordination and complexation, and others to name some. Thus, your statement in line 90/91 is not true. Please revise.
3. Please use consistent style for reporting heavy metals electronic state, i.e. Pb2+ instead of Pb(II). Please revise in the whole manuscript.
4. Chapter 4 should be moved before chapter 3. It might be better understandable why the adsorption mechanisms of different heavy metals are so very complex if the complete chapter 4 will follow chapter 2. And maybe you can shorten chapter 4 a little, because the whole manuscript is very long, which is quite exhausting to read and follow. Please revise.
5. There are several tipping mistakes and mistakes in right citing. The reference list is not according to the guideline of MDPI. The manuscript looks like to was previously submitted to another journal due to inconsistent style in layout and writing.
6. A table summarizing advantages and disadvantages of all nanomaterials discussed in the manuscript could be advantageous. This can be done either by comparing material by material or by application for heavy metal adsorption.
Author Response
Cover letter
Dear Environments editor,
The authors highly appreciated the editorial team and reviewer efforts and thanked them
for their valuable comments. The authors strictly tracked these comments and answered
all of them point by point as below:
Reviewer 2
Comments
1) The adsorption capacity is strongly depending on the dosage, pH, temperature,
contact time and concentration of the contaminate to be adsorbed. Therefore, it is
not sufficient reporting only adsorption capacities without any relation on these
parameters. It seems to be more useful to summarize them in Tables with all
important parameters influencing their efficiency instead of reporting some
capacities in the main text. To the reader there is often no relation whether the
reported capacity is high or low. Therefore, use a table and compare different
adsorption capacities for the same heavy metal. There are some good reviews, for
example look at Appl. Nano 2022, 3, 54–90. https://doi.org/10.3390/
applnano3010005 and Water 2020, 12, 803; https://doi.org/10.3390/w12030803.
Answer
The authors follow the reviewer instructions and added extra Table (Table 3) and figure
(Figure 4) in lines 763 and 896, respectively.
2) About the mechanism of heavy metal ion removal several mechanisms are well
known up to now. Very often different interactions depend on pH, functional
groups, protonation/deprotonation of the active site of the adsorbents, selectivity of
the adsorbents, electrostatic interaction, metal coordination and complexation, and
others to name some. Thus, your statement in line 90/91 is not true. Please revise.
Answer
Extra the paragraph was revised and extra clarification was added (line 103/105)
The authors stated at beginning of the section of mechanism that “The interaction
mechanisms that allow heavy metal ion removal from aqueous solutions are
currently not well known, and they are still a matter of debate”. Therefore, they
reported the most simplest mechanisms. They stated that in lines 100-110.” One of
the simplest mechanisms for heavy metal ion elimination from solutions is sorption
owing to chemical interactions between functional groups (especially those with
various oxygen-containing groups, hydroxyl groups, or carbonyl groups) and metal
ions [27]. It is important to mention that the International Union of Pure and
Applied Chemistry(IUPAC) defines sorption as the process by which a substance
(sorbate) is sorbed (adsorbed or absorbed) on or in another substance (sorbent)
[29]”
This statement in general implicitly refers to the general adsorption mechanism
(physical adsorption, electrostatic interaction, and surface interaction between the
heavy metal ions and surface functional groups) and close to the reviewer point of
view. For nano matrials the oxidation and reduction play significant roles in the
interactions between heavy metal ions and functionalized carbon-based
nanomaterials with redox capacity, especially during the removal of Cr(VI) and
As(III) as reported by Xu et al., (2018) . Also there is another openion about the
carbohydrate based mtrials like Cellulose Based Nanomaterials The adsorption of
heavy metals by these materials might be attributed to their proteins, carbohydrates,
and phenolic compounds which have carboxyl, hydroxyl, sulfate, phosphate, and
amino groups that can bind metal ions as reported by Bulut, (2007).
Xu, J., Cao, Z., Zhang, Y., Yuan, Z., Lou, Z., Xu, X., & Wang, X. (2018). A review
of functionalized carbon nanotubes and graphene for heavy metal adsorption from
water: Preparation, application, and mechanism. Chemosphere, 195, 351-364.
Bulut, Y. (2007). Removal of heavy metals from aqueous solution by sawdust
adsorption. Journal of environmental sciences, 19(2), 160-166.
3) Please use consistent style for reporting heavy metals electronic state, i.e.
Pb2+ instead of Pb(II). Please revise in the whole manuscript.
Answer
As suggested the authors revised and used the heavy metals electronic state form ,
i.e. Pb2+ in the whole manuscript.
4) Chapter 4 should be moved before chapter 3. It might be better understandable
why the adsorption mechanisms of different heavy metals are so very complex if
the complete chapter 4 will follow chapter 2. And maybe you can shorten chapter
4 a little, because the whole manuscript is very long, which is quite exhausting to
read and follow. Please revise.
Answer.
The authors understand the view point of the reviewer, but the found the it is
difficult to change the sequences of sections because the reader need to know at
first what is the process and or phenomena (the nanomaterials process) and then
can know what the parameters of this process. Therefore they provided
comprehensive piece reflecting the current state of the field.
5) There are several tipping mistakes and mistakes in right citing. The reference list is
not according to the guideline of MDPI. The manuscript looks like to was
previously submitted to another journal due to inconsistent style in layout and
writing.
Answer
As suggested. The authors correct all typo mistakes and strictly follow MDPI
Reference guideline.
6) A table summarizing advantages and disadvantages of all nanomaterials discussed
in the manuscript could be advantageous. This can be done either by comparing
material by material or by application for heavy metal adsorption.
Answer
The author build Table 3 line 763 based on the reviewer's instructions.
Author Response File: Author Response.pdf
Reviewer 3 Report
This review article by Ethaib et al is a great overview of nanomaterials in their application for removing heavy metals from water. Having a comprehensive piece of work on the current status of the field will be useful for researchers moving forward. I have minor suggestions before acceptance as follows:
1. In the introduction, page 2 line 59, mention well-established procedures are unsuitable or inefficient. I suggest giving specific examples and expanding on this so readers can fully understand how nanomaterials can go beyond what is currently used. Table 1 somewhat achieves this but the limitations listed are still quite vague. Either some rewording of the information in the table or including in introduction is suggested.
2. Page 6 line156, the sentence on purity of of CNTs seems very abrupt. Why is purity essential? This sentence may work better by combining with the previous sentence.
3. Page 7 line 197/198 the metals being removed is listed twice and in a different order so it is unclear which adsorption capacity goes with what metal.
4. As mentioned in review report there are several grammatical and formatting errors throughout the text.
5. The tables in the text are very useful to organize a lot of information, however, this reviewer suggests more visualizations/schemes/figures to provide more context to the vast amount of text in a review article.
6. Page 10 section 3.3.2 it is suggested to remove the statement in quotes and reword with acknowledgement of the research group.
7. I really enjoyed section 4 tying in all the factors together!
Author Response
Cover letter
Dear Environments editor,
The authors highly appreciated the editorial team and reviewer efforts and thanked them for their valuable comments. The authors strictly tracked these comments and answered all of them point by point as below:
Reviewer 3
Comments
- In the introduction, page 2 line 59, mention well-established procedures are unsuitable or inefficient. I suggest giving specific examples and expanding on this so readers can fully understand how nanomaterials can go beyond what is currently used. Table 1 somewhat achieves this but the limitations listed are still quite vague. Either some rewording of the information in the table or including in introduction is suggested.
Answer
As suggested the authors added extra explanation about this statement in introduction section line 58/62.
- Page 6 line156, the sentence on purity of of CNTs seems very abrupt. Why is purity essential? This sentence may work better by combining with the previous sentence.
Answer
As suggested the authors added a clarification about this statement in line 163/165 and merged with the proposed sentence.
- Page 7 line 197/198 the metals being removed is listed twice and in a different order so it is unclear which adsorption capacity goes with what metal.
Answer
As suggested the authors added a clarified this statement in line 211.
- As mentioned in review report there are several grammatical and formatting errors throughout the text.
Answer
All grammatical and formatting errors were corrected.
- The tables in the text are very useful to organize a lot of information, however, this reviewer suggests more visualizations/schemes/figures to provide more context to the vast amount of text in a review article.
Answer
An extra figure was added (Figure 4) line 895.
- Page 10 section 3.3.2 it is suggested to remove the statement in quotes and reword with acknowledgement of the research group.
Answer
The statement is reworded as suggested in line 325.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript has been improved according to my recommendations and can be accepted for publication.
