Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have submitted a well-presented work regarding chromium immobilisation. Given the hazard originating from Cr-contaminated sites and the magnitude of the problem, every effort towards mitigation is most welcome.
Thus, in my opinion, the paper can be published in Minerals after some minor revisions.
title: please consider shortening; e.g. Chromium Immobilisation as Cr-Spinel by Regulating Fe(II) and Fe(III) Concentrations
line 78: LB liquid medium; is it "Luria-Bertani (LB) broth"? Please give a reference for typical composition
line 83: are the studied ratios stated correctly? how did you set the range of each factor? a factorial design would help I believe
line 98: was it an ICP-MS? otherwise, the result was rather total iron and total chromium
line 103: did you also apply a determination method for Cr(VI)?
line 303: Please consider rephrasing, e.g. Remediation Strategies of Cr-contaminated Sites through Cr-spinel Precipitation. The presentation/structure of this section should be improved. Which are the different scenaria discussed? Also a schematic representation would add value.
Also, figure legends splitting between pages need re-formatting.
Comments on the Quality of English LanguageMinor editing of the English language.
line 16: are employed -> were employed; please use past tense for all experimental work described throughout the manuscript
line 26: ratios -> ratio
line 143: "Over two weeks"; is it "after two weeks"?
line 175: underscore -> underline
lines 223-224: duplicate sentence
lines 319-322: "In cases where remediation has already undergone chemical remediation and introduced excess Fe(III), only additional Fe(II) is necessary to drive the formation of stable Cr-spinel". Please rephrase.
line 322: Experimental observations -> Experimental results
lines 362/372: ion stoichiometries -> ion concentration ratios
line 364: reveals -> reveal
Author Response
The authors have submitted a well-presented work regarding chromium immobilisation. Given the hazard originating from Cr-contaminated sites and the magnitude of the problem, every effort towards mitigation is most welcome.
Thus, in my opinion, the paper can be published in Minerals after some minor revisions.
Response: We sincerely appreciate your detailed and constructive feedback. We have addressed each suggestion individually and documented the changes for your review and further discussion. All modifications in the manuscript are highlighted for clarity.
Detailed comments:
Comments 1: title: please consider shortening; e.g. Chromium Immobilisation as Cr-Spinel by Regulating Fe(II) and Fe(III) Concentrations
Response 1: We appreciate your feedback regarding the title. The revised title eliminates unnecessary adjectives and qualifiers to enhance conciseness and emphasize the key points. We are pleased to adopt your suggested title: Chromium Immobilisation as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations
Comments 2: line 78: LB liquid medium; is it "Luria-Bertani (LB) broth"? Please give a reference for typical composition.
Response 2: Yes, the LB medium refers to Luria-Bertani broth. We have added a reference for the typical composition and included the detailed formulation in the revised manuscript: Luria-Bertani medium (LB medium, containing tryptone 10 g/L, yeast extract 5 g/L, NaCl 10 g/L) [28]
Comments 3: line 83: are the studied ratios stated correctly? how did you set the range of each factor? a factorial design would help I believe
Response 3: Thank you for your question. The studied ratios were based on preliminary experiments to cover a broad range of Fe(II), Fe(III), and Cr(III) concentrations. We have clarified the basis for setting the ratios in the revised manuscript and will consider incorporating a factorial design in future studies: To cover a broad range of Fe(II), Fe(III), and Cr(III) concentrations, in the experiment investigating the role of Fe(II) in Cr-spinel formation, the molar ratios of ion concentrations were Fe(II):Fe(III):Cr(III) = 1:0:0, 0:1.5:0.5, 0:5:0.5, 2:1.5:0.5, and 0.5:1.5:0.5.
Comments 4: line 98: was it an ICP-MS? otherwise, the result was rather total iron and total chromium
Response 4: We used an ICP-OES for measuring total iron and total chromium. We have clarified this in the revised manuscript: An inductively coupled plasma optical emission spectrometer (ICP-OES) was employed to determine the concentrations of total Fe and Cr in solution, with accuracy ensured through quality control measures, including duplicate and blank sample analyses. The detection limit for Fe and Cr was 0.05 ppm. Prior to measurement, the samples underwent a secondary filtration. The Fe(III) concentration was calculated by subtracting the Fe(II) concentration from the total Fe concentration. The absence of Cr(VI) in the solution was confirmed using the diphenylcarbazide spectrophotometric method, allowing the Cr(III) concentration to be equated to the total Cr concentration.
Comments 5: line 103: did you also apply a determination method for Cr(VI)?
Response 5: As noted, Cr(VI) was measured but remained undetected. The experiment was conducted under anaerobic conditions in the presence of a high concentration of Fe(II), which facilitated the formation of stable products and inhibited the oxidation of Cr(III) to Cr(VI).
Comments 6: line 303: Please consider rephrasing, e.g. Remediation Strategies of Cr-contaminated Sites through Cr-spinel Precipitation. The presentation/structure of this section should be improved. Which are the different scenaria discussed? Also a schematic representation would add value.
Response 6: We agree that this section can be improved with clearer organization. We have restructured the section to discuss various scenarios in more detail: Prolonged exposure of Cr-contaminated sites to air typically results in the pre-dominant presence of Fe as Fe(III) and Cr as Cr(VI) [39]. In untreated sites, the native Fe(III) content is often insufficient to facilitate effective Cr stabilization. Thus, the first step in remediation involves adding Fe(II) to reduce Cr(VI) according to the following reaction:
3Fe(II) + Cr(VI) → 3Fe(III) + Cr(III)
After complete reduction of Cr(VI), additional Fe(II) is introduced to achieve the desired stoichiometric ratio, followed by the addition of Fe(III) in proportion to the to-tal Cr concentration to promote Cr-spinel formation. In contrast, for sites that have al-ready undergone chemical remediation and possess excess Fe(III), only the addition of Fe(II) is required to form stable Cr-spinel.
Comments 7: Also, figure legends splitting between pages need re-formatting.
Response 7: We appreciate your reminder. Subsequent modifications may cause the image name to extend across pages. We will ensure that this formatting issue is addressed in the final version.
Comments 8: line 16: are employed -> were employed; please use past tense for all experimental work described throughout the manuscript
Response 8: Corrected as suggested. We have revised the manuscript to ensure consistency in tense, using past tense for all experimental descriptions.
Comments 9: line 26: ratios -> ratio
Response 9: Corrected as suggested.
Comments 10: line 143: "Over two weeks"; is it "after two weeks"?
Response 10: Corrected to "after two weeks".
Comments 11: line 175: underscore -> underline
Response 11: Corrected as suggested.
Comments 12: lines 223-224: duplicate sentence
Response 12: We have deleted the duplicate sentence.
Comments 13: lines 319-322: "In cases where remediation has already undergone chemical remediation and introduced excess Fe(III), only additional Fe(II) is necessary to drive the formation of stable Cr-spinel". Please rephrase.
Response 13: We have rephrased it to: "In contrast, for sites that have already undergone chemical remediation and possess excess Fe(III), only the addition of Fe(II) is required to form stable Cr-spinel."
Comments 14: line 322: Experimental observations -> Experimental results
Response 14: Corrected as suggested.
Comments 15: lines 362/372: ion stoichiometries -> ion concentration ratios
Response 15: Corrected as suggested.
Comments 16: line 364: reveals -> reveal
Response 16: Corrected as suggested.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper under review prepared by Tianci Hua and co-authors is devoted to the investigation of Cr-spinel crystallization under natural-like conditions at the Cr-contaminated sites. The work seems to be quite interesting and promising from the point of view of environmental safety. I think the paper can be published after minor revision of moments pointed below.
- line 75 and further: it’s better use regular dots instead of large bold in formulae.
- line 77: “LB liquid medium” – abbreviation should be expanded.
- Chapter 2.1 – I recommend adding a few phrases on how your experimental protocols relates to potential natural conditions.
- Chapter 2.1 – the yield of synthetic experiments should be given.
- line 97: The chapter should be renamed. It’s not identification. Those are methods used for identification.
- Fig. 5: according to the Figure, SEM-EDS measurements were conducted not at polished surface, which means that chemical analyses are not so accurate, and can be considered only as approximate. Thus, at least several points should be measured to figure out mean composition.
It would be also useful to sketch a graph or ternary diagram with marked zones of various complexes formation.
Author Response
We would like to thank the reviewer for their constructive feedback and insightful suggestions. We have carefully considered each point and made the corresponding modifications in the manuscript as outlined below. All modifications in the manuscript are highlighted for clarity:
Comments 1: line 75 and further: it’s better use regular dots instead of large bold in formulae.
Response 1: Thank you for highlighting this formatting issue. We have adjusted all formulae in the manuscript to use regular dots, ensuring consistency in presentation.
Comments 2: line 77: “LB liquid medium” – abbreviation should be expanded.
Response 2: We agree that the abbreviation should be defined upon first use: Luria-Bertani medium (LB medium, containing tryptone 10 g/L, yeast extract 5 g/L, NaCl 10 g/L) [28]
Comments 3: Chapter 2.1 – I recommend adding a few phrases on how your experimental protocols relates to potential natural conditions.
Response 3: We appreciate this suggestion. We have added a brief discussion in Chapter 2.1 to explain how the experimental conditions:
All experiments were performed using deionized water (Millipore) at ambient temperature to replicate the natural conditions of the site. Reactions were conducted un-der an argon atmosphere to prevent premature oxidation of ions, and the resulting products were subsequently exposed to air to simulate the oxidative environment pre-sent at the site.
To cover a broad range of Fe(II), Fe(III), and Cr(III) concentrations,
Comments 4: Chapter 2.1 – the yield of synthetic experiments should be given.
Response 4: We have added the approximate yield of by-products in the Results section: During this process, a portion of the oxidized Fe(III) interacts with Fe(II) to form magnetite, while the remaining Fe(III) is catalyzed by Fe(II) to form goethite (The yield is approximately 5%, calculated based on the amount of substance).
The study primarily focuses on the behavior of Cr(III), while all by-products are Fe-containing minerals; and the by-products cannot be physically separated from the spinel phase, and XRD is limited to semiquantitative analysis. Consequently, the quantification of by-products was not included in previous analyses.
Comments 5: line 97: The chapter should be renamed. It’s not identification. Those are methods used for identification.
Response 5: Thank you for pointing this out. We have renamed the chapter to “Methods for Sample Characterization” to more accurately reflect its content.
Comments 6: Fig. 5: according to the Figure, SEM-EDS measurements were conducted not at polished surface, which means that chemical analyses are not so accurate, and can be considered only as approximate. Thus, at least several points should be measured to figure out mean composition.
Response 6: We acknowledge this limitation. In the revised manuscript, we have clarified that the SEM-EDS results are semi-quantitative: The microstructure and semi-quantitative elemental analysis of the precipitates are presented in Figure 5.
It should be noted that the results presented in the article are those closest to the average among multiple measurements. We did not use the results shown in Figure 5 as the proportion of Cr-spinel constituent elements in the main text but provided an approximate value, as the spinel structure inherently contains defects, making it challenging to accurately quantify the ion ratios.
Comments 7: It would be also useful to sketch a graph or ternary diagram with marked zones of various complexes formation.
Response 7: Thank you for your suggestion. However, the Fe(II)-Fe(III)-Cr(III) ternary diagram has been used by our other journals. As this study mainly focuses on the formation mechanism of Cr-spinel, a more comprehensive analysis will be conducted in an upcoming article. Therefore, it was not included in this manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors1. Section 3.1. The article mentions that by-products such as magnetite and goethite may occur during the formation of Cr-spinel. What are the relative quantities of those by-products, and how do they affect the immobilization of Cr? Could they have negative impacts?
2. Section 3.4. The article proposes a remediation model for Cr-contaminated sites based on laboratory conditions, but actual sites may have complex chemical and physical factors. The authors should discuss how the results from these studies could be adjusted and optimized for practical applications.
3. Although the article claims that Cr-spinel has strong environmental stability, it lacks data from long-term environmental simulation experiments. The author should discuss on the long-term stability of Cr-spinel under various conditions. For instance, results from leaching experiments would be helpful in assessing its stability.
4. This study primarily uses FeCl2 and FeCl3 as sources of Fe. The authors should consider other forms of Fe that may be present in actual contaminated sites, such as Fe(II) and Fe(III) in natural minerals. Are there experimental data that support similar behavior in Cr-spinel formation from these different Fe sources?
Author Response
We sincerely appreciate the reviewer’s constructive feedback and insightful comments. We have carefully addressed each point and made the necessary revisions in the manuscript, as detailed below. All modifications in the manuscript are highlighted for clarity:
Comments 1: Section 3.1. The article mentions that by-products such as magnetite and goethite may occur during the formation of Cr-spinel. What are the relative quantities of those by-products, and how do they affect the immobilization of Cr? Could they have negative impacts?
Response 1: Thank you for highlighting this aspect. In the revised manuscript, we have added semi-quantitative data on the relative proportions of goethite formed during Cr-spinel synthesis. Goethite and Cr-spinel are formed competitively. However, our previous research indicates that once spinel crystallizes, Cr preferentially occupies the octahedral sites within the spinel lattice and does not incorporate into other mineral structures. Thus, the presence of goethite has a limited impact on Cr incorporation into spinel. Magnetite serves as an intermediate phase during spinel formation. A detailed discussion on the effects of by-products is provided in the manuscript.
Comments 2: Section 3.4. The article proposes a remediation model for Cr-contaminated sites based on laboratory conditions, but actual sites may have complex chemical and physical factors. The authors should discuss how the results from these studies could be adjusted and optimized for practical applications.
Response 2: We agree that translating laboratory results to field applications can be challenging due to complex site-specific conditions. This article primarily investigates the formation mechanism and mineralization range of Cr-spinel under ideal conditions at ambient temperature and pressure, while also examining the mechanisms for remediating Cr-contaminated sites. Discussions regarding the adjustment and optimization of these findings for practical applications are detailed in other publications. Additionally, we have developed a free, publicly accessible online platform that functions as a calculator, providing real-time, intelligent strategies for the remediation of Cr-pollution (http://cr.ai4mineral.com/).
Comments 3: Although the article claims that Cr-spinel has strong environmental stability, it lacks data from long-term environmental simulation experiments. The author should discuss on the long-term stability of Cr-spinel under various conditions. For instance, results from leaching experiments would be helpful in assessing its stability.
Response 3: We appreciate this suggestion. As previously noted, this study primarily focuses on the mechanistic aspects of Cr immobilization. Cr-spinel exhibits greater stability compared to other Cr remediation products, such as CrxFe1-x(OH)3 and CrxFe1-xOOH, owing to its robust metal-oxygen covalent bonds. A comprehensive analysis of the leaching experiment results has been published in a separate journal.
Comments 4: This study primarily uses FeCl2 and FeCl3 as sources of Fe. The authors should consider other forms of Fe that may be present in actual contaminated sites, such as Fe(II) and Fe(III) in natural minerals. Are there experimental data that support similar behavior in Cr-spinel formation from these different Fe sources?
Response 4: We recognize the significance of exploring alternative sources of Fe. Our current research focuses on FeCl2 and FeCl3 to ensure consistency and control over experimental variables. These references present examples of Cr remediation utilizing various Fe-containing minerals. These data indicate that the primary role of these minerals is to supply Fe, thereby demonstrating the applicability of our research findings to a broader range of Fe-bearing minerals.
- Jianzhong, Z.; Mei, S.; Juan, L. Reductive immobilization of hexavalent chromium in contaminated soil and groundwater systems: A review. Chin. J. Environ. Eng. 2015, 9, 3077-3085.
- Palma, L.; Gueye, M.T.; Petrucci, E. Hexavalent chromium reduction in contaminated soil: a comparison between ferrous sulphate and nanoscale zero-valent iron. J. Hazard. Mater. 2015, 281, 70-76.
- Wang, T.; Zhao, D.; Liu, J. Effects of abiotic mineral transformation of FeS on the dynamic immobilization of Cr(VI) in oxic aquatic environments. Sci. Total Environ. 2023, 894, 164991.
- Erdem, M.; Tumen, F. Chromium removal from aqueous solution by the ferrite process. J. Hazard. Mater. 2004, 109, 71-77.
Author Response File: Author Response.pdf
