Improvement of Power Density and COD Removal in a Sediment Microbial Fuel Cell with α-FeOOH Nanoparticles

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have red with interest the submitted manuscript. The field of applied microbial electrochemistry is an exciting topic considering the assumptions made by the scientific community: electricity producing biogases system. Such assumption has been  claimed since the early 2000s when the first batch of publications started to be available for researchers. Up to date, no available public study has confirmed that these systems are able to produce a current great enough to power tiny light bulbs. That the authors claim that their systems is capable of producing electricity is misleading taking into consideration the amount of electrical current reported. The authors have also failed in reporting enough replicates that could make results comparable among each other. There is an excess of figures since not all of the add value to the publication. It is not clear what the main goal of the study is or what the main goal of this type of bioelectrochemical system is. It is not clear why the authors have decided to use this specific type of bioelectrochemical system to study somehow the kinetics of the system along the 30 days that took the experiment to conduct.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

 

Dear Authors.

This manuscript presents the results of a study on the development of precipitation microbial fuel cells (PMFCs). The authors note that nanoparticles (NPs) are not commonly used in microbial fuel cells. However, based on the analysis of information presented in a recent review (https://doi.org/10.1016/j.biortech.2022.126844), it can be concluded that the use of metallic and semiconductor NPs for microbial current amplification is a common practice. However, we can agree with the authors that marine sediments and marine microorganisms are rarely used in MTE development studies, so the results presented are new and interesting. It should be noted that the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. Therefore, research on the application of NPs in the design of SMFC bioanodes is important, and the results presented in the manuscript will be of interest to the global scientific community.

 Overall, the authors have conducted an important study for the development of SMFC. But some observations and comments are given below.

1. It is recommended to edit the abstract, since some of the points that the authors make are not clear (lines 11-14). Some information provided in the Conclusion can be used for this purpose.

2. The goal needs to be redone. Now the authors give the goal, then the principal goal, and at the end the main conclusion of the study!

3. In the Conclusion, the authors once again summarize the results. It is necessary to evaluate the results, generalizations and recommendations.

4. Line 21. It is necessary to explain why a resistor with a nominal value of 1 kOhm was chosen.

5. Line 75: It is unclear whether the carbon felt was further treated to improve its hydrophilic properties.

6. Lines 81 and 93. The authors note that they used Ag/AgCl, 3M KOH! Probably a typo in the electrolyte used.

7. Line 95. The pH of the buffer solution must be indicated.

8. Lines 132-139. Fig.2 Either the average size of FeOOH nanoparticles should be indicated, or a diagram of the particle size distribution in the sample should be provided.

9. Lines 196-198. Based on the analysis of morphology using scanning electron microscopy data, the authors conclude that the observed bacteria are Bacillus subtilis. Such typing of bacteria based on morphological data obtained by SEM is invalid and requires other confirmation by molecular biology methods. In addition, the authors report that there is a bacterial biofilm on the electrode surface, although no evidence is given to support the formation of a bacterial biofilm.

10. Lines 231-234. It is necessary to provide evidence of an increase in the surface area of ​​the material modified with FeOOH nanoparticles.

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

In this report, the authors report the synthesis of α-FeOOH NPs and applied them in the bioanode for SMFCs. The SMFC with α-FeOOH show much superior performance than that without α-FeOOH. The topic is interesting and could attract interest from researchers working in the microbial fuel cells. Therefore, I recommend its publication after the following issues are addressed. 

1. α-FeOOH nanorods are used in the bioanodes. The advantages of using the nanorods rather than other shapes should be described.

2. There are some reports on using FeOOH for microbial fuel cells. The novelty of this work should be emphasized.

3. Why does the value of R1 decreases for the bioanode with FeOOH NPs. Both the bacteria and FeOOH are not quite conductive.

4. The polarization curves of the SMFCs are quite linear. Please provide the reasons for this.

5. It is better for the authors to add a SEM image of the bioanode after the stability test. The comparison with that of before the test could provide useful information.

6. The authors are recommended to cite relevant literatures such as ACS Appl. Energy Mater. 2020, 3, 3966 and RSC Adv. 2014, 4, 46265. These literatures could help the authors to analyze the data more thoroughly, especially in terms of nanotechnology and electrochemistry.

Comments on the Quality of English Language

N. A.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

This paper  titled" Improvement of Power Density, COD Removal and Electrical  Energy Production in a SMFC with α-FeOOH Nanoparticles" is looking interesting, but the presentation of the obtained results is very weak and is not well organized, and if the author wants to resubmit to the journal or other journal, they must note the following:

1- The title is some how long and needs to be shorten

2- The abstract needs to rewrite again as the first 4 lines should be removed or transfer to introduction part.

3- All given images and figures are very very poor in quality if compared to other published or similar figures.

4- The author in most of his results only given the technical methods and scientific explanation is not exist like for example the XRD analysis, the author have given indexing (h,k,L) without showing on the pattern, and that is also may be difficult because of double phase pattern. In addition, it is better to use fullproof or Gsas for refinement analysis. 

5- The given patterns of EIS and Power density curves are very very poor, the author must redraw using a reasonable software. 

6- The drawn conclusion form the authors are not clear, as the limitations and challenges of this work are not mentioned 

 

Comments on the Quality of English Language

The English writing needs minor editing as there are some typo errors  

Author Response

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Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Although the authors have replied to my comments, I still consider that the measurements presented in the manuscript were NOT conducted in a proper environment that allows comparison with other studies. Thus, I do not recommend the publication of the current manuscript.

In my opinion, a novel electrode material configuration like the one presented by the authors, should be tested in a more controlled, comparable experimental system. In the present manuscript, the authors draw conclusions from using their electrode material in a complex system like a sediment microbial fuel cell with several limitations:

 

-low mass transfer due to the electrode being placed at the bottom of the cell

-ambiguos analysis of the electrochemical interaction between microorganisms and electrodes due to the use of a mixed and complex microbial inoculum

-misleading identification of possible electroactive microorganisms due to the appearance of redox peaks in cyclic voltammetry

-inconsistent power production due to the mass transfer limitations of the system, which hampers the possibility of a proper, comparable performance amongst experimental conditions

Author Response

Comment 1: 

Although the authors have replied to my comments, I still consider that the measurements presented in the manuscript were NOT conducted in a proper environment that allows comparison with other studies. Thus, I do not recommend the publication of the current manuscrip
In my opinion, a novel electrode material configuration like the one presented by the authors, should be tested in a more controlled, comparable experimental system. In the present manuscript, the authors draw conclusions from using their electrode material in a complex system like a sediment microbial fuel cell with several limitations:

Response 1: Thank you for your time and your observations. The electrode material was analyzed by electrochemical techniques before and after biofilm growth under controlled conditions and is described in methodology Line 308.

The anode with α-FeOOH NPs was characterized before and after the biofilm formation with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) techniques. The electrochemical characterization was performed with the Solartron Potentiostat in a cell with three electrodes with the bioanode as the working electrode, Ag/AgCl (3 M KCl) as the reference electrode and graphite rod as counter electrode. The CV was recorded from −0.7 V to 0.7 V at the scan rate of 1 mV/s. The electrolyte was 100 mM sodium acetate with a carbonate buffer of pH 9. The EIS analysis was carried out with alternating current and voltage amplitude of 50 mV in the frequency range 100 kHz to 10 mHz.

Comment 2: low mass transfer due to the electrode being placed at the bottom of the cell

Response 2: Thanks for the observation. Yes, there is work to do in many areas to improve the efficiency of the system. Specifically, that one can be improved by the design of the cell, there is research that deals with this topic (you can see: https://doi.org/10.1371/journal.pone.0145430 ). However, this was not the main objective of the present manuscript.

Comment 3: ambiguos analysis of the electrochemical interaction between microorganisms and electrodes due to the use of a mixed and complex microbial inoculum

Response 3: The techniques used for the analysis and characterizations of the electrode and the system are the usual ones for researchers in this field. Other wise, the advances on this device would not have been achieved. However, the technique used for the biofilm formation is already proved and reported to be selective to a single kind of bacteria as it is mentioned in the methodology Line 296.

For the biofilm formation the material was enriched in a sealed electrochemical cell of 100 mL with 70 mL of sea water and 10 g of fresh marine sediment (the sediment used was coarse with a sandy consistency) with bacteria. This was followed by applying -0.45 V vs Ag/AgCl (3 M KCl) reference electrode for 5 h at 37 °C as reported by M. Mejía-López et al. [55].

Comment 4: misleading identification of possible electroactive microorganisms due to the appearance of redox peaks in cyclic voltammetry.

Yes, that is the technique usually used for that and it is mentioned in the methodology, the electrochemical analysis were carried out before and after the formation of the biofilm line 308.

The anode with α-FeOOH NPs was characterized before and after the biofilm formation with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) techniques. The electrochemical characterization was performed with the Solartron Potentiostat in a cell with three electrodes with the bioanode as the working electrode, Ag/AgCl (3 M KCl) as the reference electrode and graphite rod as counter electrode. The CV was recorded from −0.7 V to 0.7 V at the scan rate of 1 mV/s. The electrolyte was 100 mM sodium acetate with a carbonate buffer of pH 9. The EIS analysis was carried out with alternating current and voltage amplitude of 50 mV in the frequency range 100 kHz to 10 mHz.

Comment 5: inconsistent power production due to the mass transfer limitations of the system, which hampers the possibility of a proper, comparable performance amongst experimental conditions.

Response 5: The limitations of the system due to mass transfer could be improved in new cell designs for example https://doi.org/10.1371/journal.pone.0145430 , however it was not the main objective of the present manuscript.

Reviewer 4 Report

Comments and Suggestions for Authors

The author have main corrections and now it can be suitable for publication in the journal 

Author Response

The author have main corrections and now it can be suitable for publication in the journal.

Thank you for your time and comments to improve this work.