Water Treatment Using Metallic Iron: A Tutorial Review
Round 1
Reviewer 1 Report
The authors introduced the metallic iron (Fe0) for water treatment. They summarized the state-of-the-art knowledge on the remediation Fe0/H2O system, discussed relevant contaminant removal mechanisms, and provided solutions for practical engineering application of Fe0-based systems for water treatment. The challenges for the design of the next generation Fe0/H2O systems were discussed. The paper needs a very significant revision before acceptance.
Please also avoid "lump sum references", such as XXXXX [1-5]; all references should be cited with detailed and specific description. Please revise the English writing in the whole text. Some of the latest studies about to other advanced oxidation methods can be summarized in “introduction”, such as Water Research, 151 (2019) 243-251, etc. Authors should introduce the application of metallic iron (Fe0) in the actual engineering applications. The impact of pollutant types should be introduced.To illustrate the extent of confusion, why did you choose these four examples? Please add the schematics of these processes. In Figure 1, there are two Figure 1a. Please check them carefully. In Section 3, why did you only present the electrochemical corrosion? What are the main impact factors on the corrosion? You may need to present the results and changing trends. The structure of the paper is confused. (The Chemistry of the Fe0/H2O system--Investigating the electrochemical corrosion in Fe0/H2O systems--The dynamic nature of the Fe0/H2O system--Investigating the Fe0/H2O system) I think some sections should be merged. In Table 1, I think these results were only cited from one work. You should add more data from other works. Please improve the conclusions and provide the future directions of the area.
Author Response
We are very thankful for these very useful comments!
Reviewer 1
The authors introduced the metallic iron (Fe0) for water treatment. They summarized the state-of-the-art knowledge on the remediation Fe0/H2O system, discussed relevant contaminant removal mechanisms, and provided solutions for practical engineering application of Fe0-based systems for water treatment. The challenges for the design of the next generation Fe0/H2O systems were discussed. The paper needs a very significant revision before acceptance.
Many thanks for this appreciation!
Please also avoid "lump sum references", such as XXXXX [1-5]; all references should be cited with detailed and specific description.
We think this cannot be said in this way. In the very first case, we are referencing a review article (2019) and a textbook (2012). Then, the second time one recent textbook (2012) and two ancient ones (1887, 1898) are referenced to show that the knowledge is old and textbook level! It is our feeling that the selection of references in appropriate and where applicable the description is specified.
Action: We have checked on a case-by-case basis and add precisions where necessary. In one case the reviewer is fully right. We have specified which refs. are supporting individual additives (pumice, sand...).
Please revise the English writing in the whole text.
Done, thanks!
Some of the latest studies about to other advanced oxidation methods can be summarized in “introduction”, such as Water Research, 151 (2019) 243-251, etc.
Many thanks, the introduction is modified and Chen et al. considered together with two other excellent papers: Shannon et al. (2008) and Gonzalez-Perez et al. (2018). Ali 2012 and Ali 2014 are also added.
Chen W.， Mo J.， Du X.， Zhang Z.， Zhang W. (2019): Biomimetic dynamic membrane for aquatic dye removal. Water Res. 151, 243-251.
Authors should introduce the application of metallic iron (Fe0) in the actual engineering applications.
This is consider in revising the introduction. It is now clear that it is the one of several technologies, but with a long history!
The impact of pollutant types should be introduced.
Exactly this is the problem! Our view is that the type of contaminant should be categorized by the size and the affinity to the oxide scale. This has been considered in the original manuscript, no action!
To illustrate the extent of confusion, why did you choose these four examples?
The blue-marked text is added for clarifications
“The long history of engineered Fe0-based systems for water treatment is not a continuous one [15]. Related systems have been abandoned and (partly) independently rediscovered several times [11,16-23]. In fact, after the first large scale applications in Antwerp and elsewhere [10-13], the Fe0 technology was abandoned after World War I and there was no trace of it in the Western scientific literature until 1951 [4,18]. On the other hand, the Harza Process (1986) [20] and all subsequent ones including reactive barriers (1994) have not considered available knowledge from Western scientific journals [21-23]. Four examples will be given in a chronological order to illustrate the extent of confusion in the literature.”
Please add the schematics of these processes.
We think that adding the schematics is not bringing any new information but just lenghten the presentation. If the reviewer insists, we can add! No action!
In Figure 1, there are two Figure 1a. Please check them carefully.
“a” and “b” are replaced by “top” and “bottom”. Can also turn to “left” and “right” in the final version.
In Section 3, why did you only present the electrochemical corrosion?
This is the key of the manuscript. There is no other corrosion mechanism. At pH > 4.0 water is the corroding agent, the contaminants just accelerated corrosion. They are transformed by a chemical path. No action!
What are the main impact factors on the corrosion?
Our view as expressed within the text is that the pH value is the most important influencing factor, but all are interdependent, and are yet to be addressed in a systematic (non pragmatic) approach! No action!
You may need to present the results and changing trends.
The point is not the results, but their interpretation, or better approach to achieve better results! No action!
The structure of the paper is confused. (The Chemistry of the Fe0/H2O system--Investigating the electrochemical corrosion in Fe0/H2O systems--The dynamic nature of the Fe0/H2O system--Investigating the Fe0/H2O system) I think some sections should be merged.
The structure is selected to demonstrate the discrepancy between the reported results within the Fe0 remediation community and the nature of iron corrosion according to Mainstream Science! No action!
In Table 1, I think these results were only cited from one work. You should add more data from other works.
No, there are no results in Tab. 1 but rather a suggestion for synthetic solutions to simulate natural waters. There are other, for instance from Lui and colleagues segregating between soft, normal and saline, the one of Heffron (Table 1) was the most realistic we have seen in the literature (easy to mimic and representative for natural water at many sites).
Please improve the conclusions and provide the future directions of the area.
“Summarizing, thousands of papers are available on water treatment by Fe0-based systems using batch systems. Some few of them apply column systems at laboratory, pilot and field scales, including commercial-scale applications. Unfortunately, the whole effort was based on a pragmatic, experience-based approach which cannot enable any reliable prediction of the long-term performance of any system under actual environments. Therefore, it is time to move towards long-term, well-designed experiments which could enable knowledge-based Fe0 selection for the design of sustainable systems. There is a great need to explore more granular Fe0 materials for developing commercial-scale decentralized water treatment systems.
To the best of the author’s knowledge and experience, the future of “Fe0 in water treatment’ is bright. Collaborative efforts of research and industry are needed to materialize a dream of economical and feasible decentralized water treatment technology. Only by working together, it will be possible to achieve universal safe drinking water provision and global clean environment. The present tutorial review has revealed that a major obstacle on this path is of educational nature. There is practically no formalized corrosion education of scientists and professionals working on “Fe0 in water treatment”. Thus, this article presents an opportunity for universities, educational institutions and professional associations to play a lighthouse role in this field.”
Reviewer 2 Report
This review presents an insufficient level of novelty, conceptual or technological advances for publication in Processes. There are many previous, extremely similar to the current manuscript, published works/reviews of the same authors. For example:
Hu, R., Cui, X., Gwenzi, W., Wu, S., Noubactep, C., Fe0/H2O systems for environmental remediation: The scientific history and future research directions, Water (Switzerland), Volume 10, Issue 12, 27 November 2018, Article number 1739.
Hu, R., Noubactep, C. Iron Corrosion: Scientific heritage in Jeopardy, Sustainability (Switzerland), Volume 10, Issue 11, 10 November 2018, Article number 4138.
Gatcha-Bandjun, N., Noubactep, C., Loura, B.B, Mitigation of contamination in effluents by metallic iron: The role of iron corrosion products, Environmental Technology and Innovation, Volume 8, November 2017, Pages 71-83.
It is noteworthy that the article refers to more than 30 of them (unethical excessive self-citation). Therefore, I would suggest rejecting this manuscript.
Author Response
Our impression is that Reviewer 2 is reviewing the authorship, not the manuscript. In essence there is no argument against a new review article.
Reviewer 2
This review presents an insufficient level of novelty, conceptual or technological advances for publication in Processes. There are many previous, extremely similar to the current manuscript, published works/reviews of the same authors. For example:
Hu, R., Cui, X., Gwenzi, W., Wu, S., Noubactep, C., Fe0/H2O systems for environmental remediation: The scientific history and future research directions, Water (Switzerland), Volume 10, Issue 12, 27 November 2018, Article number 1739.
Is this a tutorial review? It is even not a review!
Hu, R., Noubactep, C. Iron Corrosion: Scientific heritage in Jeopardy, Sustainability (Switzerland), Volume 10, Issue 11, 10 November 2018, Article number 4138.
This is an Opinion!
Gatcha-Bandjun, N., Noubactep, C., Loura, B.B, Mitigation of contamination in effluents by metallic iron: The role of iron corrosion products, Environmental Technology and Innovation, Volume 8, November 2017, Pages 71-83.
This is a research article!
It is noteworthy that the article refers to more than 30 of them (unethical excessive self-citation).
It would be fair to state the it is 30 from more that 120 on ZVI, all or almost all published “Against the Tide” since 2007 in the peer-reviewed literature. We are used to this type of evaluation but would have preferred scientific argumentation.
Therefore, I would suggest rejecting this manuscript.
Rejected for no reason?
Reviewer 3 Report
The paper needs a deep revision that should include the following points:
Lines 38-39. Excessive number of references. Example: 6 references are provided just to say that a water filter was patented. The same applies for the following sentence in 39-40.
Lines 129-132. Some expressions can be "excessive": "unethical behaviour";
line 129: "science ... be falsified", etc.
Line 166: some spelling/grammar errors must be corrected: "cannot quantitatively reached", I guess "be" should added.
Line 218: "present" should be "presents".
Lines 168-169: This sentence is confusing, please clarify the difference between chemical and electro-chemical reaction since in both cases there is an electron interchange.
Line 182: why "certainly" in parenthesis?
Line 185: the reaction is not the reduction of water but that of protons. Please complete the lacking reactions.
Page 6: reactions should have their Eo values. Lines 216-217. The sentence starting by: "This shows that fixing..." is not clear. Please re-write to clarify.
Lines 294-297: I cannot understand the opportunity/value of including the so-called Leffmann view here.
Line 303. Do you mean "bottle-neck" instead of "bottle-point"? Line 340: "water (H+)????. I guess you should indicate protons in water or similar.
Lines 410-415: if contaminant removal is quantitative a pH > 4.5, how can it be that the pH of natural waters must be fixed at pH 4 to optimize removal?
Line 449: What do you mean with "corresponding concepts"?
Author Response
We are very thankful to this reviewer with have considered all his comments and those of Reviewer 1 in detail.
Reviewer 3
The paper needs a deep revision that should include the following points:
Lines 38-39. Excessive number of references. Example: 6 references are provided just to say that a water filter was patented. The same applies for the following sentence in 39-40.
These and others are revision also in responses to comments by Reviewer 1. Thanks!
Lines 129-132. Some expressions can be "excessive": "unethical behaviour";
line 129: "science ... be falsified", etc.
The text is revised and both expressions are avoided, thanks!
Line 166: some spelling/grammar errors must be corrected: "cannot quantitatively reached", I guess "be" should added.
It is “cannot quantitatively reach”, thanks!
Line 218: "present" should be "presents".
Corrected, thanks!
Lines 168-169: This sentence is confusing, please clarify the difference between chemical and electro-chemical reaction since in both cases there is an electron interchange.
It is clear from the text, all depends on the origin of electrons. For the electrochemical reaction, electrons come from the metal body (the oxide scale must be conductive) (reduction is simultaneous to iron oxidative dissolution). The chemical reaction implies electrons from FeII or H2 (parallel reaction to iron oxidative dissolution).
Line 182: why "certainly" in parenthesis?
The parenthesis are deleted, thanks!
Line 185: the reaction is not the reduction of water but that of protons. Please complete the lacking reactions.
Protons comes from water: H2O  H+ + HO-
We have added: 2 H2O + 2 e-  H2 + 2OH- (2a)
But Eq. 2 is the most currently used form for the reduction of water.
Page 6: reactions should have their Eo values.
We have deliberately preferred the approach Eq. and E0 value in the discussion/presentation.
Lines 216-217. The sentence starting by: "This shows that fixing..." is not clear. Please re-write to clarify.
The sentence is revised to:
“ Accordingly, rationalizing the efficiency of Fe0/H2O systems for water decontamination by any electrochemical process involving the pollutants has been a huge mistake [69-71]. The electrochemistry-based reasoning implies the electrode potential of the couple FeII/Fe0 (E0 = -0.44 V), representing the anodic half-reaction in the electrochemical cell. This thermodynamically possible (E0 values) but physically impossible because of the omnipresence of a non-conductive oxide scale. ”
Lines 294-297: I cannot understand the opportunity/value of including the so-called Leffmann view here.
The Leffmann view is corroborated herein as iron corrosion generates contaminant scavengers, like in electrocoagulation (and like generating O3 for desinfection).
Line 303. Do you mean "bottle-neck" instead of "bottle-point"?
Is revised to difficulty, thanks!
Line 340: "water (H+)????. I guess you should indicate protons in water or similar.
2 H2O + 2 e-  H2 + 2OH-. This is always an equilibrium, regardless from the pH value.
Lines 410-415: if contaminant removal is quantitative a pH > 4.5, how can it be that the pH of natural waters must be fixed at pH 4 to optimize removal?
Fixing the pH value to about 4.0 enables the production/generation of more contaminant scavengers and the final pH values will be larger than six,
Line 449: What do you mean with "corresponding concepts"?
It is now revised to:
“While sand (inert) alone certainly increases the efficiency of Fe0/H2O systems, other more or less reactive aggregates have been introduced for the same purpose, but without systematic investigations demonstrating the corresponding concepts. For example, Huang et al. [145] reported that the co-presence of Fe0, Fe3O4, and dissolved FeII creates a highly reactive system for molybdate removal. This is a tangible experimental observation. However, given that addition of both Fe3O4 and FeII enhances the Fe0 efficiency, it is difficult to assess the specificity of this ternary system. Moreover, the named system should have been compared to the Fe0/sand/FeII system and the affordability of using Fe3O4 discussed. ”
Additionnaly, the following is added to the conclusions:
“Summarizing, thousands of papers are available on water treatment by Fe0-based systems using batch systems. Some few of them apply column systems at laboratory, pilot and field scales, including commercial-scale applications. Unfortunately, the whole effort was based on a pragmatic, experience-based approach which cannot enable any reliable prediction of the long-term performance of any system under actual environments. Therefore, it is time to move towards long-term, well-designed experiments which could enable knowledge-based Fe0 selection for the design of sustainable systems. There is a great need to explore more granular Fe0 materials for developing commercial-scale decentralized water treatment systems.
To the best of the author’s knowledge and experience, the future of “Fe0 in water treatment’ is bright. Collaborative efforts of research and industry are needed to materialize a dream of economical and feasible decentralized water treatment technology. Only by working together, it will be possible to achieve universal safe drinking water provision and global clean environment. The present tutorial review has revealed that a major obstacle on this path is of educational nature. There is practically no formalized corrosion education of scientists and professionals working on “Fe0 in water treatment”. Thus, this article presents an opportunity for universities, educational institutions and professional associations to play a lighthouse role in this field.”
Round 2
Reviewer 1 Report
It can be accepted if the authors could not provide more figures and tables.
Reviewer 2 Report
The current review is a summary of the authors works published since 2007 in the peer-reviewed literature. This work has no scientific problems, only the lack of novelty and the dubious ethics of such a "personal" review.
Reviewer 3 Report
The authors have improved the paper significantly and responded satisfactorily to all the questions made by this referee.
