Efficient Chromium(VI) Removal Through In Situ Nano-Iron Sulfide Formation at the Cathode of Microbial Fuel Cells

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

Major Comments

1. authors proposed the use of in situ biosynthesized iron sulfide (FeS) nanoparticles in microbial fuel cell (MFC) biocathodes to enhance Cr(VI) removal efficiency. While the study demonstrates some novelty in hybrid biofilm formation and the integration of FeS nanoparticles to improve extracellular electron transfer (EET) and microbial activity, a few concerns regarding the justification and comparative effectiveness of the proposed approach should be addressed.

Firstly, previous studies have extensively explored the use of bioelectrodes integrated with specific catalysts such as redox-active enzymes, electroactive bacteria (e.g., Geobacter, Shewanella), and conductive materials (e.g., carbon nanotubes, polyaniline), which have achieved high Cr(VI) reduction efficiencies often exceeding the 1.08-fold increase reported here. Therefore, the authors should provide a clear rationale for choosing FeS nanoparticles over these well-established alternatives. It is important to clarify how the current approach offers significant advantages in terms of performance, cost, sustainability, or operational stability.

Secondly, while the application of a small external voltage (0.2 V) and glucose as an organic carbon source is mentioned as a strategy to enhance cathodic performance, this raises questions about energy input and long-term feasibility. If energy is externally supplied, how does this influence the energy balance and economic viability of the system, especially when MFCs are generally promoted for their low-energy or energy-neutral operation?

Lastly, the manuscript would benefit from a more detailed comparative analysis with previous MFC studies that employed different bioelectrodes or catalytic strategies for Cr(VI) reduction. This would help contextualize the contribution of this study and better justify the innovation claimed.

2. While the study presents an interesting approach using in situ biosynthesized FeS nanoparticles at the MFC biocathode for Cr(VI) removal, a critical aspect appears to be overlooked. Since Fe²⁺ in FeS is oxidized during the reduction of Cr(VI), the formation of Fe³⁺ in the treated effluent is expected. However, the manuscript does not report any post-treatment analysis of iron using techniques such as AAS or ICP to determine residual Fe concentrations. Moreover, no discussion is provided on potential color changes or secondary contamination due to Fe leaching. These omissions are significant, as they impact both the environmental safety and practical applicability of the proposed treatment process. The authors should address this gap by including Fe quantification data and a discussion on the fate of iron post-treatment.
3. The proposed technology demonstrates promising results at the laboratory scale for Cr(VI) removal using in situ biosynthesized FeS nanoparticles in MFC biocathodes. However, the manuscript lacks a discussion on the potential challenges and feasibility of scaling up this system. The authors are encouraged to provide their perspective on the applicability of this technology at the pilot or field scale, including factors such as operational stability, electrode fouling, cost-effectiveness, and long-term performance. Addressing scalability considerations would enhance the practical relevance and impact of the study.

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Good

Author Response

Major Comments

1. authors proposed the use of in situ biosynthesized iron sulfide (FeS) nanoparticles in microbial fuel cell (MFC) biocathodes to enhance Cr(VI) removal efficiency. While the study demonstrates some novelty in hybrid biofilm formation and the integration of FeS nanoparticles to improve extracellular electron transfer (EET) and microbial activity, a few concerns regarding the justification and comparative effectiveness of the proposed approach should be addressed.

Response: We sincerely appreciate you for your time in reviewing the manuscript and giving your valuable comments. We have thoroughly considered your precious comments and suggestions, and carefully addressed every one of them as best as we can. Revisions in the manuscript were highlighted in red. Here, we provide a point-to-point response to your major and specific comments below.

 

Firstly, previous studies have extensively explored the use of bioelectrodes integrated with specific catalysts such as redox-active enzymes, electroactive bacteria (e.g., Geobacter, Shewanella), and conductive materials (e.g., carbon nanotubes, polyaniline), which have achieved high Cr(VI) reduction efficiencies often exceeding the 1.08-fold increase reported here. Therefore, the authors should provide a clear rationale for choosing FeS nanoparticles over these well-established alternatives. It is important to clarify how the current approach offers significant advantages in terms of performance, cost, sustainability, or operational stability.

Response: Thank you very much for raising this important point. Based on your suggestion, we have added the rationale for selecting FeS nanoparticles as the catalyst, as well as the significant advantages of the current method in terms of performance, cost, sustainability, and operational stability. The revisions can be found in Section 3.2.1, highlighted in red.

 

Secondly, while the application of a small external voltage (0.2 V) and glucose as an organic carbon source is mentioned as a strategy to enhance cathodic performance, this raises questions about energy input and long-term feasibility. If energy is externally supplied, how does this influence the energy balance and economic viability of the system, especially when MFCs are generally promoted for their low-energy or energy-neutral operation?

Response: Thank you very much for raising this important point. Regarding the application of a small external voltage (0.2 V), which may raise concerns about energy input and long-term feasibility, we would like to clarify that the purpose of applying this external voltage in our study is primarily to enhance cathodic performance. It is important to emphasize that the external voltage (0.2 V) is relatively small and can be provided by the voltage generated by the MFC itself, along with external green and sustainable energy sources such as wind and solar power. As a result, the energy demand for the external voltage does not significantly affect the overall energy balance of the system, and the energy consumption remains minimal.

Moreover, MFC systems are generally capable of generating sufficient electrical energy through microbial bio-electrochemical reactions to drive the system’s operation. The external voltage is used only as an optimization measure to further improve the removal efficiency. Over the long term, the energy demand for the external voltage can be met by these green and sustainable energy sources, thereby maintaining the system’s energy-neutral or slightly energy-positive balance. Therefore, despite the application of an external voltage, this method remains feasible and sustainable in the long-term operation.

In response to your comments, we have added this clarification in Section 3.4 of the revised manuscript, highlighted in red, to further explain the impact of the external voltage on the energy balance and to emphasize the sustainability of the approach.

 

Lastly, the manuscript would benefit from a more detailed comparative analysis with previous MFC studies that employed different bioelectrodes or catalytic strategies for Cr(VI) reduction. This would help contextualize the contribution of this study and better justify the innovation claimed.

Response: Thank you for your valuable suggestion. In response to your comment, we have added a more detailed comparative analysis with previous studies in Section 3.2.1 of the revised manuscript, highlighted in red. This includes a comparison of different bioelectrode materials, catalysts, and operational conditions used for Cr(VI) reduction. We have compared these studies in terms of removal efficiency, reaction rate, system stability, and energy efficiency, and further discussed the innovations of our approach in terms of performance, cost, sustainability, and operational stability.

 

2. While the study presents an interesting approach using in situ biosynthesized FeS nanoparticles at the MFC biocathode for Cr(VI) removal, a critical aspect appears to be overlooked. Since Fe²⁺ in FeS is oxidized during the reduction of Cr(VI), the formation of Fe³⁺ in the treated effluent is expected. However, the manuscript does not report any post-treatment analysis of iron using techniques such as AAS or ICP to determine residual Fe concentrations. Moreover, no discussion is provided on potential color changes or secondary contamination due to Fe leaching. These omissions are significant, as they impact both the environmental safety and practical applicability of the proposed treatment process. The authors should address this gap by including Fe quantification data and a discussion on the fate of iron post-treatment.

Response: Thank you for your valuable comment. Regarding the issue of Fe(III) treatment, in our study, Fe(II) is oxidized to Fe(III) during the Cr(VI) reduction process. However, active microorganisms are capable of reducing Fe(III) back to Fe(II) by utilizing organic carbon sources from the cathode and electrons supplied by the MFC anode. This regeneration of Fe(II) allows it to continue participating in the in situ synthesis of nano-FeS, which not only prevents the formation of Fe(III) residuals in the treated effluent but also avoids secondary contamination caused by electrode passivation. Therefore, there is no significant Fe(III) residual issue in our system, and as such, we believe there is no need for post-treatment Fe analysis or concern about the potential environmental impact of residual Fe. In response to your comment, we have further elaborated on this point in Section 3.3.1 of the revised manuscript, highlighted in red.

3. The proposed technology demonstrates promising results at the laboratory scale for Cr(VI) removal using in situ biosynthesized FeS nanoparticles in MFC biocathodes. However, the manuscript lacks a discussion on the potential challenges and feasibility of scaling up this system. The authors are encouraged to provide their perspective on the applicability of this technology at the pilot or field scale, including factors such as operational stability, electrode fouling, cost-effectiveness, and long-term performance. Addressing scalability considerations would enhance the practical relevance and impact of the study.

 

Response: Thank you for your valuable comments. Regarding the scalability of the proposed technology, we believe that the strategy demonstrated in this study has shown promising Cr(VI) removal performance at the laboratory scale and exhibits good scalability. By utilizing organic wastewater as a carbon source, sulfur/iron-containing wastewater as a sulfur/iron source, renewable energy (such as wind and solar power), and MFC self-generated electricity to provide electrons for the in situ synthesis of nano-FeS, this approach offers high economic benefits and effectively reduces time and cost. Additionally, under continuous electron supply, microorganisms can stably regenerate nano-FeS in situ over the long term, preventing issues such as catalyst pore blockage that occur in traditional methods, thus enhancing the long-term stability of the system. Compared to traditional methods, our approach is simpler, more stable, and has better practicality and scalability. While electrode fouling and cost-effectiveness should still be considered in scaling up, we are confident that the proposed technology has strong scalability potential, especially in terms of removal efficiency, operational stability, and cost control. Based on your suggestions, we will further elaborate on the practical application potential and challenges of this technology in Section 3.2.1 of the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study presents a well-designed and innovative strategy for enhancing microbial fuel cell (MFC) performance in Cr(VI)-contaminated wastewater by promoting in situ nano-FeS biosynthesis through glucose regulation and applied voltage. The mechanistic insights are clearly articulated, and the findings convincingly demonstrate improved electrochemical activity and Cr(VI) removal efficiency.

1. The organization of some figures is suboptimal and fails to effectively present the information in a single panel; it is recommended to consider splitting them into two separate figures.
2. Should the manuscript include a speciation or equilibrium analysis of different chromium forms?
3. The overall discussion could be further strengthened by incorporating a more detailed comparison with similar studies.
4. The mechanistic diagram (Figure 8) could be further optimized to integrate both the mechanisms proposed in this study and those reported in the literature, thereby providing a more comprehensive mechanistic overview.

Author Response

Comments and Suggestions for Authors

This study presents a well-designed and innovative strategy for enhancing microbial fuel cell (MFC) performance in Cr(VI)-contaminated wastewater by promoting in situ nano-FeS biosynthesis through glucose regulation and applied voltage. The mechanistic insights are clearly articulated, and the findings convincingly demonstrate improved electrochemical activity and Cr(VI) removal efficiency.

Response: We sincerely appreciate you for your time in reviewing the manuscript and giving your valuable comments. We have thoroughly considered your precious comments and suggestions, and carefully addressed every one of them as best as we can. Revisions in the manuscript were highlighted in red. Here, we provide a point-to-point response to your major and specific comments below.

 

1. The organization of some figures is suboptimal and fails to effectively present the information in a single panel; it is recommended to consider splitting them into two separate figures.

Response: Thank you very much for your careful review of the manuscript. In response to your suggestion, we have split the relevant figure into two separate figures to improve the presentation and clarity of the information. The revised content can be found in Figure 3 of Section 3.2.1.

 

2. Should the manuscript include a speciation or equilibrium analysis of different chromium forms?

Response: Thank you very much for your valuable suggestions. Based on your suggestion, we have included an analysis of the different chromium species in the manuscript. Please refer to Section 3.3.2 for the updated content.

 

3. The overall discussion could be further strengthened by incorporating a more detailed comparison with similar studies.

 

Response: Thank you very much for raising this important point. Based on your suggestion, we have added a more detailed discussion comparing our findings with similar studies. Please refer to Section 3.3.1 for the updated content.

 

4. The mechanistic diagram (Figure 8) could be further optimized to integrate both the mechanisms proposed in this study and those reported in the literature, thereby providing a more comprehensive mechanistic overview.

Response: Thank you very much for your valuable suggestions. Based on your suggestion, we have included additional modifications to the mechanism diagram. Please refer to Section 3.4 for the revised content.

Author Response File: Author Response.pdf