Conceptualizing the Fe⁰/H₂O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe⁰-Based Permeable Reactive Barrier Technology

Round 1

Reviewer 1 Report (New Reviewer)

dear authors:

1. Multiuse of abbreviations without mentioning the total phrase.
2. The title should be totally understandable without any ambiguous words (PRB!). Please don’t use abbreviation here.
3. Your abstract should clearly state the essence of the problem you are addressing, what you did and what you found and recommend. That would help a prospective reader of the abstract to decide if they wish to read the entire article.
4. Tables’ format should be edited.
5. The use of words like Aids (!) in the beginning of abstract is misleading. The first paragraph of abstract should clearly represent the main concern of the paper.
6. Simplicity of the language (mainly in addressing the concerns and discussion novelty) should be considered in the whole manuscript. (Polishing the language is highly recommended!)
7. Keywords should be reconsidered.
8. Table 1 caption: …in Fe0/H2O system” or …using Fe0/H2O system”? please check all similar items in the text too.
9. Fifth column in Table 1 (for example!): .. is pay” or ..was paid”? please polish the whole content accordingly.
10. A brief discussion of the remediation technologies (in general!) is needed in the beginning of introduction. Some useful references can be:

-          https://www.sciencedirect.com/science/article/abs/pii/S0263876219304149

-          https://www.sciencedirect.com/science/article/abs/pii/S0926860X22003179

-          https://patents.google.com/patent/US11014082B2/en

11. Please make sure your conclusions' section underscores the scientific highlights of the opinion, and/or the applicability of your results. Highlight the novelty of your statements and talk remarkably about the Research Gaps/Contributions!
12. Challenges/restrictions should be also considered in the introduction (or conclusion) section.

good luck

Author Response

(x) Extensive editing of English language and style required, 

Considered, thanks!

1. Multiuse of abbreviations without mentioning the total phrase. Considered, thanks!
2. The title should be totally understandable without any ambiguous words (PRB!). Please don’t use abbreviation here. Corrected, thanks!
3. Your abstract should clearly state the essence of the problem you are addressing, what you did and what you found and recommend. That would help a prospective reader of the abstract to decide if they wish to read the entire article.

This is actually what we did! It is not a resarch paper, however the abstract is written according the rules of academic writing.

1. Tables’ format should be edited. Corrected, thanks!
2. The use of words like Aids (!) in the beginning of abstract is misleading. The first paragraph of abstract should clearly represent the main concern of the paper.

We have removed AIDs/VIH, thanks! However, it is a block of four sentences that presents the context.

1. Simplicity of the language (mainly in addressing the concerns and discussion novelty) should be considered in the whole manuscript. (Polishing the language is highly recommended!)

We have tried to address this concern. However, we find that the language is appropriate and is even very polite compared to what is available in the literature, particularly concerning concerns on adsorption studies during the past five years (since Tran et al. 2017) or even the past 15 years (since Tien 2007, 2008).

Tien C. (2007): Remarks on adsorption manuscripts revised and declined: An editorial. Sep. Purif. Technol. 54, 277–278.

Tien C. (2008): Remarks on adsorption manuscripts revised and declined: An editorial. J. Hazard. Mater. 150, 2–3.

Tran H.N., You S.-J., Hosseini-Bandegharaei A., Chao H.-P. (2017): Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review. Water Res. 120, 88–116.

1. Keywords should be reconsidered.

We disagree with the Reviewer, our keywords correspond to the content of the submission.

1. Table 1 caption: …in Fe⁰/H₂O system” or …using Fe⁰/H₂O system”? please check all similar items in the text too.

Thanks for this remarks! We think that both expressions are equivalent since it is about aqueous processes in the presence of metallic iron. The most important feature is that there is no Fe⁰/water interface. This is reflected in both expressions, which are therefore randomly interchangeable.

1. Fifth column in Table 1 (for example!): .. is pay” or ..was paid”? please polish the whole content accordingly. Corrected, thanks!
2. A brief discussion of the remediation technologies (in general!) is needed in the beginning of introduction. Some useful references can be:

The two publications are considered, thanks!

We have already considerd Antia (2020) which is a mega-review of the patent literature. The references are considered toward the end of the manuscript. We have purposefuly introduced our Opinion with the evidence that the remediation Fe⁰/H₂O system is not new.

1. Please make sure your conclusions' section underscores the scientific highlights of the opinion, and/or the applicability of your results. Highlight the novelty of your statements and talk remarkably about the Research Gaps/Contributions!

Many thanks for these important remarks, we have reworded the conclusions. The key changes reads as:

According to the viewpoints discussed in this article, the following key conclusions and perspectives are put forward:

(i) operational reference Fe⁰ materials are needed to enable at least a semi-quantitative comparison of results achieved under independent conditions.

(ii) experiments regarding the operating mode of the Fe⁰/H₂O systems should be performed under diffusion controlled conditions.

(iii) pure Fe⁰ filters (100 % Fe⁰) are not sustainable. Fe⁰ should always be mixed to non-expansive aggregates like pumice or sand.

(iv) results based on the reductive transformation concept have been the cornerstone for the development of the adsorption/co-precipitation concept. Accordingly, made observations, recommendations/suggestions should be acknowledged as valuable contributions in transferring scientific knowledge.

(v) the entire environmental research community should question the validity of the view that Fe⁰ is a (strong) reducing agent under environmental conditions.

(vi) the MB method is a powerful tool for characterizing the dynamics of Fe⁰/H₂O systems.

1. Challenges/restrictions should be also considered in the introduction (or conclusion) section.

Which chanllenges? The message is 're-direct your efforts', just remove direct reduction from the equation, We see no restrictions nor any needs to address such because the whole presentation (opinion) suggests that adopting or at least testing the adsorption/ci-precipitation view ist he way forward.

Reviewer 2 Report (New Reviewer)

According to the authors, in this opinion paper, the end result is that given environmental circumstances, no electron transfer from Fe0 to pollutants is conceivable. Therefore, their conclusion challenges the reductive transformation concept's veracity and argues for new alternatives. Their concept is quite elegant and deserves attention from the scientific community. The arguments against the reductive transformation concept, the adsorption/co-precipitation concept, and their final development of a new research tool for Fe0/H2O systems are well constructed and scientific-based and have my respect as a reviewer. Breaking scientific skepticism in any area is important for the development of world science, so I endorse the author's opinion. Moreover, they are of the opinion that developing Fe0-based water treatment systems that are affordable, reliable, and long-lasting will be made possible by basing future research on the adsorption co-precipitation concept, which can be a new paradigm. Congratulations to the authors and go for it for new achievements.

Author Response

Many thanks for this evaluation and for the congratulations as well!

Dr. Noubactep

Reviewer 3 Report (New Reviewer)

The subject of the proposed manuscript is somewhat controversial. The reviewer believes that this kind of subject should not be avoided and that it is important to promote the presentation of opposing viewpoints as long as they are supported by sound scientific evidence. The key ideas in the current manuscript are presented in a straightforward and concise manner. But there are certain things that can be improved. The following are some suggestions for the manuscript's improvement:

·         In the introduction section, the author presents the background for further discussion. However, the text's tone is sometimes inappropriate for a scientific paper, where the author complains about possible unfair treatment in the publication process. Moreover, further, in the manuscript, similar claims are made again. In the reviewer's opinion, this kind of remark should not be part of a scientific paper.

·         In the reviewer's opinion, the more conceptual parts of the manuscript (sections 3 to 4) should be presented in more detail, with a better description of the mechanisms involved and enriched with figures, schemes, and equations.

·         In section 5, the author talks about the MB method and how it is the key to supporting the newly proposed paradigm. However, in the reviewer's opinion, the MB method should be better described in this section. Moreover, the authors should present the main results obtained by the MB method and how they support the adsorption/co-precipitation concept.

Author Response

But there are certain things that can be improved. The following are some suggestions for the manuscript's improvement:

• In the introduction section, the author presents the background for further discussion. However, the text's tone is sometimes inappropriate for a scientific paper, where the author complains about possible unfair treatment in the publication process. Moreover, further, in the manuscript, similar claims are made again. In the reviewer's opinion, this kind of remark should not be part of a scientific paper.

Responses: We appreciate these remarks but point out that: (i) our submission is an opinion, and (ii) the further claims (including a table) were added in response to reviewer comments of the previous version. We would like to add, that as active researchers, we see no problem in pointing out things that are hardly addressable in review papers. We have even published a review of reviews (2015), but the message seems not to be clear enough. Even the current submission was initially on 11 pages, now 10 pages are added… However, we strongly disagree that our remarks are not part of a scientific paper. Researchers can no more afford to let other people reporting on their daily life and even newspapers are recruiting real scientists (or inviting contributions) to fill this gap.

• In the reviewer's opinion, the more conceptual parts of the manuscript (sections 3 to 4) should be presented in more detail, with a better description of the mechanisms involved and enriched with figures, schemes, and equations.

Responses:

We have constantly referred to very recent papers. It is not fair to have to demonstrate the same this again and again for individual reviewer or in individual papers in the same journal. The core of the information cited herein are available in papers in Water and Processes (MDPI). Coming back to sections 3 to 4, the difficulty in following the recommendation of the Reviewer is that there are hardly comparative studies in the literature. Even the used Fe⁰ specimens are not characterized and when fortunately, the Fe⁰ is from the same source (e.g. Connelly) the experimental conditions vary so largely that no real comparison is possible. In an Opinion, we think it is enough to point out this aspect.

• In section 5, the author talks about the MB method and how it is the key to supporting the newly proposed paradigm. However, in the reviewer's opinion, the MB method should be better described in this section. Moreover, the authors should present the main results obtained by the MB method and how they support the adsorption/co-precipitation concept.

Responses:

Many thanks for these comments, we have rather added a negative example (Song et al. 2005) to better evidence the importance of the MB method. Other results (if positive) will make a stand-alone article in this SI (experiments are running). The reader is really invited to read the referenced papers, Summarizing the results here would just lengthen the presentation.

The addition reads as:

Results on Cr^VI removal in Fe⁰/H₂O systems have long falsified the reductive transformation concept [159].

In 2005, Song et al. [159] presented a mathematical model to explain why the presence of sand could enhance Cr^VI "reduction" by Fe⁰. The standard redox potential of the couple Cr^VI/Cr^III is 1.52 V, making Fe⁰ (E⁰ = −0,44 V) a relevant reducing agent. However, reduction of Cr^VI by Fe^II is also possible (E⁰ = 0,77 V for the couple Fe^II/Fe^III) and even well-documented [160, 161]. In other words, Cr^VI reduction is possible both by Fe⁰ and Fe^II. However, the question remains open, why sand addition, which was then largely considered as "Fe⁰ dilution" [143,144,162], rather enhanced the "reduction" efficiency. The answer is given when considering that, in the Fe⁰/sand system, the negatively charged sand surface is progressively coated with a positively charged oxide scale, which is good scavenger for negatively charged Cr^VI (chromate CrO₄²⁻) [163]. In other words, regardless whether Cr^VI is reduced or not, its quantitative removal in Fe⁰/sand systems is justified by a larger adsorptive surface, made available by coated sand.

Round 2

Reviewer 1 Report (New Reviewer)

The authors have well considered the comments to improve the manuscript. It can be now accepted for publication in Water.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This manuscript is a great attempt to invite researchers across the planet  working on ZVI to join efforts and work together to address and more to confirm that reduction process is not any more the only mechanism to consider while cleaning water from organic and inorganic contaminants. The efforts lead by the corresponding author across almost two decades inspired young researcher to think outside the box and to consider publishing scientific data based on experiments relevant to natural conditions. 

The topic of ZVI is not over, there still lot of work to do. The manuscript submitted here for publication is shedding the light on the necessity of informing the large public on the actual situation with regard to contradictory outcomes. The selected literature cited is very relevant and present an outstanding work of highly appreciated researches working independently. 

The manuscript is well written and clear. It deserves publication without the need of any revision.    

Author Response

Many thanks for this evaluation1

Reviewer 2 Report

Reply to the paper “Conceptualizing the Fe0/H2O system: A call for collaboration to mark the 30th anniversary of the Fe0 PRB technology”

I am amazed at the structure of the paper. I was expecting to read a summary of the concept of the FeO-H2O system developed over the past 30 years. However, it seems more like a complaint from a specific working group. I gather from the manuscript that the author(s) are disappointed because they don't see their point of view reflected in the scientific community in the way they would like. Such an emotionally charged text could at best be part of a comment paper and should not be included in a scientific work in this form.

     The introduction to the work is intended to give an overview of the various statements that will be discussed later. However, it appears to be a list of articles by one of the co-authors that were published or rejected. Approaches or statements are not presented in a way that picks up the reader.   I consider the tables 1 and 2 to be redundant for this work. Instead, the subject of the study should be better presented.

The scientific discourse is not properly conducted because the systems under consideration and their subsystems are not properly defined. As a result, various aspects related to the two subsystems are mixed up, which in sum creates more confusion than clarity and leads to an incorrect conclusion

Fe°-H2O system

The description of the system is correct in principle. But in chapter 2.2. the authors claim that FeII species, FeII/FeIII species, and H2 are stand-alone reducing agents and that Fe° plays no role. The authors ignore that without Fe° (or some other element or component with low electronegativity) the system will drift to oxide conditions. Any reaction of contaminants with Fe2+ inclusing electron transfer leads to the oxidation of Fe2+ to Fe3+.

Thermodynamically, Fe° is not stable in an aqueous environment and will rust. Fe° is not only necessary to keep the system at a low Eh condition, but also to promote the formation of Fe2+.

In their argument, I miss the relevance of the pH condition as it is mandatory for the Fe°-H2O system.

Formation of Fe-hydooxides, called here FCP.

With reaction progress, two subsystems are established: a) Fe° + Fe-Hydrooxides and b) Fe-Hydrooxides + reaction media in the aqueous solution. The formation of these phases requires a source for iron and that cannot be ignored. (Correspondingly, the authors reflect it when is seems them as necessary). So, the potential gradient is the controling factor and therefore reaction 1 remains the main driver of the whole process.

By the way, this is not in contraction to the importance of adsorption processes to remove contaminants and there is no reason to deny the importance of FCPs as reaction partners

Kinetics referred to in chap. 2.4

It is known that kinetics play a major role. However, the formation of reaction rims depends on the pH-Eh condition in the system. This step controls the overall reaction speed. An oxide rim around Fe° is sufficient to inhibit reactivity, while a pH shift to lower or higher pH values is sufficient to remove the oxide layer and thus speed up the reaction.

The authors write that materials such as carbonates are overestimated in the reaction system. Why is not explained. Carbonates control the pH of a system. Therefore, they must be taken into account.

In general, all these aspects are well known and thermodynamically well founded. The authors' statement that the reductive concept of Fe°-H2O is a consensual approach is not supported by their discourse. Likewise, the arguments in Chap. 3 and 4 are not objections to the concept and therefore the procedure is splitting hairs and a waste of time.

Author Response

We feel, Reviewer 2 has not evaluated our submission.

We have commented some of the views of his impressive but mostly non target comments; see the attached documents.

Sincerely,

Dr. Noubactep

Author Response File: Author Response.pdf

Reviewer 3 Report

The article, overall, reads well. Some interesting aspects are discussed, such as the fact that, on occasion, an affirmation is referred repeatedly and becomes a dogma that is difficult to question. According to the authors, the fact of proposing another approach in the investigation of FeO as an agent for water remediation has been a challenge in order to obtain funding, sorry funding obtaintion is not relevant for an article. As the title of the manuscript indicates, the authors intend to make a call for collaboration, however, the article is pretentious and the authors question the work of other researchers using expressions such as "pseudo-scientific argument against the concept of co-precipitation-adsorption", not contributing no argument for the use of that adjective. The approach of the article is not friendly and does not contribute to the end it pursues. The authors should limit themselves to duly arguing the advantages of their proposal. They definitely need to review section 4.

On the other hand, the authors have not shown that reductive removal is not an efficient way to remove some contaminants. There are numerous articles describing the effectiveness of this method and examining the byproducts of this decomposition. As an example, although there are more, San Román, I.; Alonso, M.L.; Bartholomew, L.; Galdames, A.; Goiti, E.; Eye, M.; Moragües, M.; Alonso, R.M.; Vilas, JL; Roman, IS; et al. Relevance study of bare and coated zero valence iron nanoparticles for the degradation of lindane from the monitoring of its by-products. Chemosphere 2013, 93, 1324-1332, doi:10.1016/j.chemosphere.2013.07.050, in which it has been determined that the reductive elimination mechanism has been effective for the total elimination of lindane in aqueous solution by means of nanoiron.

Author Response

Reviewer 3

Comments 1: The article, overall, reads well. Some interesting aspects are discussed, such as the fact that, on occasion, an affirmation is referred repeatedly and becomes a dogma that is difficult to question.

Responses 1: Many thanks for this evaluation!

Comments 2: According to the authors, the fact of proposing another approach in the investigation of Fe⁰ as an agent for water remediation has been a challenge in order to obtain funding, sorry funding obtaintion is not relevant for an article.

Responses 2: Please consider that our submission is an opinion.

Comments 3: As the title of the manuscript indicates, the authors intend to make a call for collaboration, however, the article is pretentious and the authors question the work of other researchers using expressions such as "pseudo-scientific argument against the concept of co-precipitation-adsorption", not contributing no argument for the use of that adjective. The approach of the article is not friendly and does not contribute to the end it pursues. The authors should limit themselves to duly arguing the advantages of their proposal. They definitely need to review section 4.

Responses 3: Please consider that the submission is an opinion. It is not about being polite; but to address a urgent issue.

Comments 4: On the other hand, the authors have not shown that reductive removal is not an efficient way to remove some contaminants. (We are saying that introducing “reductive removal” was a mistake as the knowledge that no single electron can be transferred a dissolved contaminant is century old) There are numerous articles describing the effectiveness of this method and examining the byproducts of this decomposition. As an example, although there are more, San Román, I.; Alonso, M.L.; Bartholomew, L.; Galdames, A.; Goiti, E.; Eye, M.; Moragües, M.; Alonso, R.M.; Vilas, JL; Roman, IS; et al. Relevance study of bare and coated zero valence iron nanoparticles for the degradation of lindane from the monitoring of its by-products. Chemosphere 2013, 93, 1324-1332, doi:10.1016/j.chemosphere.2013.07.050, in which it has been determined that the reductive elimination mechanism has been effective for the total elimination of lindane in aqueous solution by means of nano-iron.

Responses 3: Román et al. (2013) has just repeated the mistakes because using nano-scale Fe⁰ does not change anything.

Based on our responses to your comments; we see no reason to revise the manuscruipt.

Sincerely,

Dr. Noubactep