Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management

Round 1

Reviewer 1 Report

In this manuscript, Xiao et.al. review the reasons for the different levels of iron in groundwater in different regions, propose the formation mechanism of high levels of iron in groundwater and the chemical reactions involved in iron in groundwater, and discuss the influencing factors of iron. At the same time, how to apply the biogeochemical interaction between iron in groundwater and aquifer medium to groundwater early warning is proposed, and the future research direction is proposed. Overall, some concerns need to be addressed before further consideration. Some comments are listed as follows:

1. The language should be checked carefully，some sentences can be shortened or deleted.
2. In lines 339-342,Ref.69 was cited, however, Ref.68 was cited in Fig.5 caption. Please check it.
3. Please provide a figure to describe the effect of pH on morphology change of Fe2+ and transformation process of Fe（Ⅱ）/ Fe（Ⅲ）.
4. In lines 280-281, “Fe-bearing minerals will be more negatively charged than other minerals, which will decrease the amounts of other heavy metals”, why?

 

Author Response

Comments and Suggestions for Authors

In this manuscript, Xiao et.al. review the reasons for the different levels of iron in groundwater in different regions, propose the formation mechanism of high levels of iron in groundwater and the chemical reactions involved in iron in groundwater, and discuss the influencing factors of iron. At the same time, how to apply the biogeochemical interaction between iron in groundwater and aquifer medium to groundwater early warning is proposed, and the future research direction is proposed. Overall, some concerns need to be addressed before further consideration. Some comments are listed as follows:

Response: Thank you very much for your approval of our work. Each section of the manuscript has been improved by us according to your requirements. The other details have also been revised according to the comments, and the specific responses are as follows.

1.The language should be checked carefully，some sentences can be shortened or deleted.

Response: Adopted. Thank you for your reminding. After completing the revision of the technical content, we further polish the language of the full text (please see the revised manuscript document).

2.In lines 339-342, Ref.69 was cited, however, Ref.68 was cited in Fig.5 caption. Please check it.

Response: Thank you for your reminding. We examine the citations in the manuscript and fully agree with you. Figure 5 is reused from reference. In order to prevent copyright problems, we delete Figure 5 from the manuscript, which does not affect the content to be expressed in the manuscript.

3.Please provide a figure to describe the effect of pH on morphology change of Fe²⁺ and transformation process of Fe(Ⅱ)/Fe(ⅡI).

Response: Adopted. Thank you for your reminding. The manuscript is supplemented with content related to pH on the speciation of Fe and its hydrolyzate trends. The new contents are as follows:

 

Figure 4.  The combined distribution of Fe at different pH (a)Fe(II) and (b)Fe(III).

ɑ₀=[Fe²⁺]/Fe(II)_T, ɑ₁=[FeOH²⁺]/Fe(II)_T, ɑ₂=[Fe(OH)0 2]/Fe(II)_T, ɑ₃=[Fe(OH)- 3]/Fe(II)_T;

ɑ₀'=[Fe³⁺]/Fe(III)_T, ɑ₁'=[FeOH²⁺]/Fe(III)_T, ɑ₂'=[Fe(OH)0 2]/Fe(III)_T, ɑ₃'=[Fe(OH)- 3]/Fe(III)_T, ɑ₄'=[Fe(OH)2-4]/Fe(III)_T.

4.In lines 280-281, “Fe-bearing minerals will be more negatively charged than other minerals, which will decrease the amounts of other heavy metals”, why?

Response: Thank you for your reminding. We combed the original content of in lines 280-281 and added references. Lower pH can destroy the lattice structure of iron-bearing minerals in sediments and release Fe(II) and other high-valent heavy metals adsorbed on the mineral surface. The new contents are as follows:

“Low pH (high concentration of H⁺) will destroy the lattice structure of Fe-containing minerals in sediments, and transform the morphology of Fe oxides. Therefore, it is possible to release Fe(II) and other high-valent heavy metals (such as As (V)) adsorbed on the mineral surface, creating body conditions for the enrichment of heavy metals in groundwater [61].”

Author Response File: Author Response.pdf

Reviewer 2 Report

This study (sustainability-1748437) deals with biogeochemical features of iron enrichment in groundwater as a potential indicator of environmental pollution. The study is timely and the findings reported by the authors have relevance which adds new information to the current state of the art and mitigation models of groundwater pollution. After a careful review, I have noticed that the current version of this manuscript needs several improvements in terms of language, fluency, and presentation. Thus, I suggest major revision. My specific comments are:

1. The title of the study needs some modification to become clearer: Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management.
2. Kindly rephrase the first sentence of the Abstract. Do not use abbreviated forms (e.g. Fe) at the beginning of sentences/sections. Define their full forms at their first use.
3. Line: Effects? I think it should be “affected”. Check the grammar issue also.
4. Avoid using impersonal forms of the sentences e.g., Line 17, 20, 69, 74, 460, etc.
5. Improve the keywords, they look very ordinary.
6. Introduction: well written, but correct the language issues.
7. Add the source of the data in the footnote of Table 1.
8. Provide more information about the name of locations within the table e.g., state/province names and type of localities i.e., rural, urban, or industrial….
9. Discuss the maximum permissible limits of Fe in drinking, irrigation, and other usages.
10. Figures 1-4: commendable. Check if they are original or taken from previously published papers? If yes, please provide sources.
11. More emphasis should be given to the role of industrial water supply disposal on groundwater iron enrichment. The information provided is too general despite having a big role in today’s era.
12. Kindly define all the terms used in equations. They are meaningless until you provide full explanations of each variable used in them. E.g., Eh, E, E⁰, RT, nF, a, etc.
13. Statements made in Line 426-437 strongly require valid references.
14. Conclusion and prospects: this heading should be split into two different headings: “5. Management and prospects of Fe pollution” and “6. Conclusion”.
15. I suggest the authors reduce the length of the conclusion section, maybe max 300 words, focusing on the major outcome of this study and efficient management of iron pollution in groundwater.

Author Response

Comments and Suggestions for Authors

This study (sustainability-1748437) deals with biogeochemical features of iron enrichment in groundwater as a potential indicator of environmental pollution. The study is timely and the findings reported by the authors have relevance which adds new information to the current state of the art and mitigation models of groundwater pollution. After a careful review, I have noticed that the current version of this manuscript needs several improvements in terms of language, fluency, and presentation. Thus, I suggest major revision. My specific comments are:

Response:

Thank you very much for your approval of our work. Our previous manuscript submitted does have many issues in writing. The manuscript has undergone a major revision according to your requirements. After this revision, we polished the language again, hoping to meet the publishing requirements.

1.The title of the study needs some modification to become clearer: Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management.

Response: Adopted.

2.Kindly rephrase the first sentence of the Abstract. Do not use abbreviated forms (e.g. Fe) at the beginning of sentences/sections. Define their full forms at their first use.

Response: Adopted. Thank you for your reminding. The new contents are as follows:

“Iron (Fe) is one of the most biochemically active and widely distributed elements and one of the most important elements for biota and human activities.”

3.Line: Effects? I think it should be “affected”. Check the grammar issue also.

Response: Adopted.

4.Avoid using impersonal forms of the sentences e.g., Line 17, 20, 69, 74, 460, etc.

Response: Adopted. The new contents are as follows:

“This study describes recent progress and breakthroughs and assess the biogeochemistry of Fe enrichment in groundwater, factors controlling Fe reactivity, and Fe biogeochemistry effects on the environment.”

“This study also describes the implications of Fe biogeochemistry for managing Fe in groundwater, including the importance of Fe to groundwater monitoring and evaluation and early groundwater pollution warnings.”

“Here, this study reviews anomalous increases in Fe concentrations in groundwater around the world. Also focus on Fe-mediated hydrogeochemical cycles and groundwater pollution, and analyze factors affecting redox reactions.”

“This study also discusses how biogeochemical interactions between Fe in groundwater and aquifer media can be used to warn of groundwater contamination and remediate contaminated materials, evaluating available remediation techniques. Suggestions for future research directions are given.”

“This study argues that the biogeochemistry of Fe in groundwater and aquifers will become an important research topic, and the response of Fe in groundwater and aquifers to environmental pollution and global change will also receive attention.”

“This study reviews recent research into Fe redox reactions and biogeochemical Fe enrichment processes, including reduction, biotic and abiotic oxidation, adsorption, and precipitation in groundwater.”

5.Improve the keywords, they look very ordinary.

Response: Adopted. Thank you for your reminding. The new contents are as follows:

“Fe; Biogeochemistry; Enrichment; Environmental pollution; Mnvironmental management.”

6.Introduction: well written, but correct the language issues.

Response: Adopted. Thank you very much for your affirmation. After this revision, we polished the language of introduction again, hoping to meet the publishing requirements (please see the revised manuscript document).

7.Add the source of the data in the footnote of Table 1.

Response: Adopted.

8.Provide more information about the name of locations within the table e.g., state/province names and type of localities i.e., rural, urban, or industrial….

Response: Adopted. Thank you for your reminding (please see the revised table document).

9.Discuss the maximum permissible limits of Fe in drinking, irrigation, and other usages.

Response: Adopted. Thank you for your reminding. The maximum permissible limits of Fe in drinking, irrigation and other usages are added to section 2.1 of the manuscript. The new contents are as follows:

“In China, the maximum allowable limit of Fe emission in industrial wastewater is 1 mg/L, and there is no limit of Fe in farmland irrigation water quality standards for the time being. The maximum allowable limits for surface water and groundwater were 1.5 mg/L, and the class III quality standards for surface water and groundwater and sanitary standards for drinking water were 0.3 mg/L.”

10.Figures 1-4: commendable. Check if they are original or taken from previously published papers? If yes, please provide sources.

Response: Thank you very much for your affirmation. Figures 1-3 is the original of the author, Figure 4 is reused from reference. In order to prevent copyright problems, we delete original Figure 4 from the manuscript, which does not affect the content to be expressed in the manuscript, and the manuscript is supplemented with content related to pH on the speciation of Fe and its hydrolyzate trends. The new contents are as follows:

 

Figure 4.  The combined distribution of Fe at different pH (a) Fe(II) and (b) Fe(III).

ɑ₀=[Fe²⁺]/Fe(II)_T, ɑ₁=[FeOH²⁺]/Fe(II)_T, ɑ₂=[Fe(OH)0 2]/Fe(II)_T, ɑ₃=[Fe(OH)- 3]/Fe(II)_T;

ɑ₀'=[Fe³⁺]/Fe(III)_T, ɑ₁'=[FeOH²⁺]/Fe(III)_T, ɑ₂'=[Fe(OH)0 2]/Fe(III)_T, ɑ₃'=[Fe(OH)- 3]/Fe(III)_T, ɑ₄'=[Fe(OH)2-4]/Fe(III)_T.

11.More emphasis should be given to the role of industrial water supply disposal on groundwater iron enrichment. The information provided is too general despite having a big role in today’s era.

Response: Adopted. Thank you for your reminding. More detailed discussion on Fe enrichment in groundwater by industrial water treatment was added to the manuscript (shown in Section 4.1). The new contents are as follows:

“Among them, the pollutants produced by industrial wastewater treatment have played a huge role in the iron enrichment of groundwater in the current era. First, industrial wastewater includes direct discharge of high-concentration heavy metal wastewater [75], significantly increase the Fe concentration in groundwater; second, industrial wastewater includes a large number of pollutants (usually nitrogen, acid-base substances, DOC, COD and BOD, etc.) [36], These anthropogenic pollutants further complicate hydrogeochemical processes [33], and the released soluble Fe can contaminate groundwater. Soluble Fe released from minerals can pollute groundwater and pose health risks to millions of people around the world.”

12.Kindly define all the terms used in equations. They are meaningless until you provide full explanations of each variable used in them. E.g., Eh, E, E0, RT, nF, a, etc.

Response: Adopted. Thank you for your reminding. The manuscript defines all the terms used in the equation. The new contents are as follows:

“where Eh is redox potential of the system, E⁰ is standard redox potential, R is the universal gas constant, T is the temperature in degrees Kelvin, n is the number of electrons transferred, F is the Faraday constant, a is oxidized ionic species.”

13.Statements made in Line 426-437 strongly require valid references.

Response: Adopted. Thank you for your reminding. Line 426-437 adds references in the manuscript. The new contents are as follows:

“As indicated in the information about the Fe cycle given in section 2.3, pollutants entering an aquifer in water can chemically interact with Fe in the aquifer–groundwater system and interrupt natural water–rock interactions make the hydrogeochemical processes more complicated [33]. Decreasing the risks posed by pollutants will require groundwater monitoring networks to be established and potential sources of pollutants to be identified [85]. In summary, decreasing the risks posed by pollutants requires potential sources of pollution to be identified, contaminated site monitoring and early warning systems to be improved, and groundwater pollution control and remediation measures to be implemented [43]. Protecting and improving groundwater quality will depend on preventing pollution, blocking the routes pollutants enter aquifers, remediating contaminated areas, monitoring groundwater, characterizing pollutant plumes, verifying remediation effects, and monitoring dynamic changes in groundwater quality.”

14.Conclusion and prospects: this heading should be split into two different headings: “5. Management and prospects of Fe pollution” and “6. Conclusion”.

Response: Adopted.

15.I suggest the authors reduce the length of the conclusion section, maybe max 300 words, focusing on the major outcome of this study and efficient management of iron pollution in groundwater.

Response: Adopted. Thank you for your reminding. We fully agree with you. Shorten the length of the conclusion in the manuscript (shown in Section 6).

Author Response File: Author Response.pdf

Reviewer 3 Report

• English should be revised.
• The title "Biogeochemistry of Iron Enrichment in Groundwater and it's Indicator for Environmental Management" should be corrected to "Biogeochemistry of Iron Enrichment in Groundwater and its Indicator for Environmental Management".
• In the abstract, the statement "Acidification, anoxia, and warming of the groundwater environment and inputs of reducing pollutants increase Fe migration into groundwater (Fe(III)→Fe(II)), and acidity and anoxia decreases and cooling increase migration to the aquifer (Fe(II)→Fe(III))" is not clear due to the language.
• Don't use "we" anywhere in the manuscript. You could replace it with "This study or this work".
• Line 60: What is the connection between the "degradable organic matter and ammonia" and the accumulation of Fe ions in the groundwater? and how will they react with Fe oxides? List the chemical reactions, please.
• The statement "Fe oxidation can cause water to become turbid and odorous ....." is repeated more than once. please check it and others.
• The present study is concerned with Fe ions but you have mentioned Mn ions in many places in the manuscript. You cited from your previous study here "https://doi.org/
  10.3390/agronomy11122392". Please focus on your main target here.
• Line 111: Is the statement "Fe concentrations in groundwater are generally <10 mg/L" general for everywhere? Because the data listed in Table 1 for some places located in the India zone are not agreed with your statement. Please revise.
• In Fig. 2: is there (Fe 4+)?
• Please cite the reference who proved the result: "In fact, Fe(III) is very unstable. Fe(III) is only stable in solution at 25 °C and at pH
  <2.19.".
• Line 260: The statement "Fe will mainly ..." should be "Fe ions will mainly ....".
• Line 284: What is the meaning of the term "the Eh"? All symbols used should be introduced. 

Author Response

Comments and Suggestions for Authors

1.English should be revised.

Response: Adopted. Thank you for your reminding. The manuscript has undergone a major revision according to your requirements. After this revision, we polished the language again, hoping to meet the publishing requirements.

2.The title "Biogeochemistry of Iron Enrichment in Groundwater and it's dicator for Environmental Management" should be corrected to "Biogeochemistry of Iron Enrichment in Groundwater and its Indicator for Environmental Management".

Response: Thank you for your reminding. We agree with you, the title should indeed be modified. After our careful consideration, the title is changed to “Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management”. We believe that the revised title is more in line with the content of the manuscript.

3.In the abstract, the statement "Acidification, anoxia, and warming of the groundwater environment and inputs of reducing pollutants increase Fe migration into groundwater (Fe(III)→Fe(II)), and acidity and anoxia decreases and cooling increase migration to the aquifer (Fe(II)→Fe(III))" is not clear due to the language.

Response: Adopted. Thank you for your reminding. We polished the language, hoping to meet the publishing requirements. The new contents are as follows:

“Generally speaking, acidification, anoxia and warming of groundwater environment as well as the inputs of reducing pollutants, are beneficial to the migration of Fe into groundwater (Fe(III)→Fe(II)); conversely, it is beneficial to the migration of it into the media (Fe(II)→Fe(III)).”

4.Don't use "we" anywhere in the manuscript. You could replace it with "This study or this work".

Response: Adopted. Thank you for your reminding. We modified all the sentences about impersonal forms in the manuscript. The new contents are as follows:

“This study describes recent progress and breakthroughs and assess the biogeochemistry of Fe enrichment in groundwater, factors controlling Fe reactivity, and Fe biogeochemistry effects on the environment.”

“This study also describes the implications of Fe biogeochemistry for managing Fe in groundwater, including the importance of Fe to groundwater monitoring and evaluation and early groundwater pollution warnings.”

“Here, this study reviews anomalous increases in Fe concentrations in groundwater around the world. Also focus on Fe-mediated hydrogeochemical cycles and groundwater pollution, and analyze factors affecting redox reactions.”

“This study also discusses how biogeochemical interactions between Fe in groundwater and aquifer media can be used to warn of groundwater contamination and remediate contaminated materials, evaluating available remediation techniques. Suggestions for future research directions are given.”

“This study argues that the biogeochemistry of Fe in groundwater and aquifers will become an important research topic, and the response of Fe in groundwater and aquifers to environmental pollution and global change will also receive attention.”

“This study reviews recent research into Fe redox reactions and biogeochemical Fe enrichment processes, including reduction, biotic and abiotic oxidation, adsorption, and precipitation in groundwater.”

5.Line 60: What is the connection between the "degradable organic matter and ammonia" and the accumulation of Fe ions in the groundwater? and how will they react with Fe oxides? List the chemical reactions, please.

Response: Adopted. Thank you for your reminding. The new contents are as follows:

“The oxidation process of organic matter in aquifer needs to consume dissolved oxygen in water, and Fe oxides with high valence in reduction environment replace O₂ as oxidant. The reaction is based on the continuous oxidation of organic matter [26].

Organic matter (e.g. CH₂O)+O₂→CO₂+H₂O

Fe₂O₃+Organic matter (e.g. CH₂O) +2H⁺→Fe²⁺+CO₂+2H₂O

Fe oxides can also be used as electron donors for autotrophic denitrification. NO₃^- is non-biologically reduced to release Fe (II) and generate excessive NH₄⁺ [27].

NH₄⁺+2O₂→NO₃^-+2H⁺

Fe₂O₃+NO₃^-+6H₂O→5Fe(Ⅱ)+N₂+12OH^-

NO₃^-+ Fe₂O₃+10H⁺→4Fe(Ⅱ)+NH₄⁺+3H₂O”

6.The statement "Fe oxidation can cause water to become turbid and odorous ....." is repeated more than once. please check it and others.

Response: Adopted. Thank you for your reminding. We deleted duplicate sentences in the manuscript (shown in Section 2.1).

7.The present study is concerned with Fe ions but you have mentioned Mn ions in many places in the manuscript. You cited from your previous study here "https://doi.org 10.3390/agronomy11122392". Please focus on your main target here.

Response: Adopted.

8.Line 111: Is the statement "Fe concentrations in groundwater are generally <10 mg/L" general for everywhere? Because the data listed in Table 1 for some places located in the India zone are not agreed with your statement. Please revise.

Response: Adopted. Thank you for your reminding. The sentence “Fe concentrations in groundwater are generally <10 mg/L” does not apply to all places and needs to be modified. We deleted this content in the manuscript.

9.In Fig. 2: is there (Fe⁴⁺)?

Response: Thank you for your reminding. After investigation, we found the existence of Fe⁴⁺, but not in the form of ions, so we modified Figure 2. The new contents are as follows:

 

Figure 2. The circulation of iron between groundwater and water-containing media and the formation mechanism of groundwater iron pollution.

10.Please cite the reference who proved the result: "In fact, Fe(III) is very unstable. Fe(III) is only stable in solution at 25 °C and at pH<2.19.".

Response: Adopted. The new references are as follows:

“Burgos, W.; Borch, T.; Troyer, L.; Luan, F.; Larson, L. Schwertmannite and Fe oxides formed by biological low-pH Fe(II) oxidation versus abiotic neutralization: Impact on trace metal sequestration. Geochim. Cosmochim. Ac. 2012, 76, 29-44.”

11.Line 260: The statement "Fe will mainly ..." should be "Fe ions will mainly ....".

Response: Adopted.

12.Line 284: What is the meaning of the term "the Eh"? All symbols used should be introduced.

Response: Adopted. Thank you for your reminding. The manuscript defines all the terms used in the equation. The new contents are as follows:

“where Eh is redox potential of the system, E⁰ is standard redox potential, R is the universal gas constant, T is the temperature in degrees Kelvin, nF is the Faraday constant, a is oxidized ionic species.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript can be accepted.

Reviewer 2 Report

My all queries/comments were adequately answered and implemented. So, I suggest to accept this manuscript in present form.