Electrocoagulation of Spent Coolant by Dissimilar Fe-Al Combination

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present an electrocoagulation process utilizing aluminum (Al) and iron (Fe) electrodes for wastewater treatment, emphasizing its efficiency and environmental friendliness.

1. Please reformat the equation for clarity and consistency with standard formatting conventions.
2. In the Introduction, the authors highlight the advantage of their method being low-cost and sustainable, without the need for additional chemical inputs. However, in the actual experiments, NaCl and HCl are added to the solution. Could the authors justify this discrepancy? Specifically, what is the performance of the Fe and Al electrodes in the absence of any added chemicals?
3. In Figure 3, the red curve shows a COD reduction exceeding 100%. Could the authors explain how this is possible? Does this suggest oxygen generation or another reaction mechanism?
4. The authors are encouraged to provide a comparative figure illustrating the differences between the agitation conditions shown in Figures 5 and 7. If aeration-induced electrocoagulation leads to faster separation, could increasing the stirring speed in the mechanically agitated electrocoagulation achieve similar performance?
5. For universal comparison, the potential should be reported as V vs. RHE. Additionally, the authors should include a voltage range below 0 V to provide a more comprehensive electrochemical profile.

Author Response

Response to Reviewer 1
1. Summary
Thank you very much for taking the time to review this manuscript. Please find detailed responses below and the corresponding revisions highlighted in turquoise in the re-submitted files.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Can be improved	Thank you for your comments. We have updated the introduction to enhance the clarity.
Is the research design appropriate?	Can be improved	Thank you for the suggestions. We have addressed the relevant questions by providing additional explanations and information.
Are the methods adequately described?	Can be improved	Thank you for the suggestions. We have addressed the relevant questions by providing additional explanations and information.
Are the results clearly presented?	Yes	Thank you for acknowledging the clarity of the results presented in our work.
Are the conclusions supported by the results?	Yes	Thank you for recognising the logical connection between our results and conclusions.
Are all figures and tables clear and well-presented?	Yes	Thank you for confirming the clarity and effective presentation of our figures and tables.
3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: Please reformat the equation for clarity and consistency with standard formatting conventions.

Response 1: Thank you for pointing out our overlooking. We had updated the format of all the equations. The updated equations can be found in Line 58-77, Page 2.

Revised version (Line 58-71, Page 2):

Faradic efficiency (FE) of the process was calculated as equation (5), based on mass loss of anode during electrocoagulation:

 

Where  is the mass of metal ions released (g),  is the valency of the metallic anode,  is the Faraday’s constant (96,485 C/mol),  is the current (A),  is the process time (s) and  is the molar mass of the metallic electrode (g/mol).

 

Comments 2: In the Introduction, the authors highlight the advantage of their method being low-cost and sustainable, without the need for additional chemical inputs. However, in the actual experiments, NaCl and HCl are added to the solution. Could the authors justify this discrepancy? Specifically, what is the performance of the Fe and Al electrodes in the absence of any added chemicals?

Response 2: Thank you for bringing up this point which we made an unprecise statement. For electrocoagulation, it is necessary to maintain sufficient conductivity. For many wastewater streams, it is not necessary to add salt for enhanced conductivity. In this study, we add NaCl or other types of salt to maintain the conductivity, due to the special properties of spent coolant. Without the use of salt, the performance electrocoagulation in this case will be significantly suppressed.  Figure 3a showed the performance of electrocoagulation without HCl added and indicated that the addition of HCl is not a must. In this research, HCl was used to investigate the effect of pH and electrocoagulation efficiency. Without pH adjustment, the electrocoagulation process is slower but not critically inefficient.

To correct the point on additional chemical inputs, we had updated the sentence in Line 50, Page 2.

Revised version (Line 50, Page 2):

This process facilitates the removal of contaminants with less reliance on additional chemicals.

 

Comments 3: In Figure 3, the red curve shows a COD reduction exceeding 100%. Could the authors explain how this is possible? Does this suggest oxygen generation or another reaction mechanism?

Response 3: Thank you for highlighting this point, which we acknowledge was not explained in the original manuscript. The observation of COD values exceeding 100% can be attributed to two potential factors. Firstly, dissolved Fe²⁺ from the anode can undergo further oxidisation by the COD testing solution, thereby elevating the COD reading. Secondly, the inherent margin of error in COD testing vials (typically 5-10%) can also lead to unexpectedly high COD values. To enhance clarity for readers, a brief explanation of these factors has now been added to the manuscript in Line 157, Page 5.

Revised version (Line 157, Page 5):

The observation of COD value surpassing 100% may result from the dissolved Fe²⁺ ions from the anode, together with the inherent margin of error in COD testing vials.

 

Comments 4: The authors are encouraged to provide a comparative figure illustrating the differences between the agitation conditions shown in Figures 5 and 7. If aeration-induced electrocoagulation leads to faster separation, could increasing the stirring speed in the mechanically agitated electrocoagulation achieve similar performance?

Response 4: Thank you for raising this insightful point. While we were unable to investigate this factor in the current project due to resource limitations, existing literature provides valuable insights.

• https://www.sciencedirect.com/science/article/abs/pii/S0011916411005893
• https://www.mdpi.com/2071-1050/16/15/6383

Generally, higher stirring speeds can facilitate the separation of flocs from the solution. However, vigorous agitation may also lead to the breakage of larger flocs into smaller, less settleable particles, thereby complicating subsequent separation.

 

Comments 5: For universal comparison, the potential should be reported as V vs. RHE. Additionally, the authors should include a voltage range below 0 V to provide a more comprehensive electrochemical profile.

Response 5: Thank you for your comment, and we have further improved the LSV investigation accordingly by including the voltage range of – 0.5 V to 0 V (vs Ag/AgCl) and converting the applied potential to V vs SHE. After the modification, the full scan range for LSV was – 0.5 V to 3 V (vs Ag/AgCl) or – 0.29 to 3.21 V (vs SHE) (Figure 9). In the updated LSV results, we observed the linear response of working current to applied potential below 0 V, and the onset potential of Al anodes was found to be – 0.35 V. Another finding was that the LSV plots of Al(A)-Al(C) and Al(A)-SS(C) exhibited similar pattern, which further confirmed that the contribution of electrochemical dissolution of Al anodes was not affected by the cathode material (Line  459 – 461, Page 13) and the finding was consistent with the similar Al anode dissolution weight (Table 9). We have revised the manuscript (Line 455 to 463, Page 13) to include the improvements.

Revised version (Line 455, Page 13):

However, when using Al-6061 as the anode, the two LSV plots of Al(A)-Al(C) and Al(A)-SS(C) configurations were quite similar and exhibited linear response to the applied potential. The current was around 0.3 A at 0 V (vs SHE), and the corresponding onset potential for both Al(A)-Al(C) and Al(A)-SS(C) was – 0.35 V (vs. SHE) by extending the LSV plots. As a result, less applied voltage was required when using Al anode compared to SS anode, demonstrating advantage in energy consumption. In addition, the similar electrochemical behaviour of Al(A)-Al(C) and Al(A)-SS(C) in LSV experiment also aligned with the similar dissolution weight of Al anodes (Table 9), indicating the contribution of electrochemical dissolution of Al anodes was not affected by the cathode material.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper "Electrocoagulation of spent coolant by dissimilar Fe-Al combiation" is an interesting paper, clearly written, carefully described so that it is easy to understand what was attempted to be researched. The bibliography is up-to-date, the conclusion is in accordance with the results obtained, therefore I propose publishing the article after making small changes:

• -in line 63 check equation 4
• -Check the number of tables
• -I recommend mentioning the pH as a value in graphs fig 5-8

Author Response

Response to Reviewer 2
1. Summary
Thank you very much for taking the time to review this manuscript. Please find detailed responses below and the corresponding revisions highlighted in turquoise in the re-submitted files.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Yes	Thank you for acknowledging the clarify of background provided in our work.
Is the research design appropriate?	Yes	Thank you for recognising the appropriateness of our research design.
Are the methods adequately described?	Yes	Thank you for confirming the adequateness of the methodology.
Are the results clearly presented?	Yes	Thank you for acknowledging the clarity of the results presented in our work.
Are the conclusions supported by the results?	Yes	Thank you for recognising the logical connection between our results and conclusions.
Are all figures and tables clear and well-presented?	Can be improved	Thank you for your suggestions. We have updated figure 3-8 to enhance the clarify.
3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: In line 63 check equation 4

Response 1: Thank you for pointing out the typo in equation 4. We had updated this Equation 4 in Line 63, Page 2. The updated equation can be found below.

Revised version (Line 63, Page 2):

 

 

Comments 2: Check the number of tables

Response 2: Thank you for the comment and we checked the number of tables, as well as the cross-reference used. We hope the number of tables are fine in the latest version of manuscript.

 

Comments 3: I recommend mentioning the pH as a value in graphs fig 5-8

Response 3: Thank you for the suggestion. We agree that stating the pH values directly will enhance the clarity of the figures. We have updated Figures 3-8 accordingly.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this study, the performance of stainless steel (SS-304) and aluminium (Al-  6061) electrodes in both similar and dissimilar configurations for 120-minute electrocoagulation treatment of spent machinery coolant was analyzed. Supplementary, effects of initial pH and agitation methods on the process performance were studied. While many studies have focused on similar electrode materials, such as Al-Al or Fe-Fe configurations, the authors focused on dissimilar configuration of electrodes  which represents the novelty of this study. The manuscript is clear, quite relevant in the field and well structured. Abstract is explanatory. In materials and methods chapter is necessary to specify other parameters for coolant  like  SS, TDS and oil and grease content. The study is detailed and well structured and the experimental design is  appropriate to test the hypotheses.  The conclusions are consistent with the evidence and arguments presented. The findings are stated but is not clear if this process is a final  or is a pre-treatment one as is mentioned in other paper ( see https://doi.org/10.2166/wrd.2017.057) Please explain!!!

Author Response

Response to Reviewer 3
1. Summary
Thank you very much for taking the time to review this manuscript. Please find detailed responses below.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Yes	Thank you for acknowledging the clarify of background provided in our work.
Is the research design appropriate?	Yes	Thank you for recognising the appropriateness of our research design.
Are the methods adequately described?	Yes	Thank you for confirming the adequateness of the methodology.
Are the results clearly presented?	Yes	Thank you for acknowledging the clarity of the results presented in our work.
Are the conclusions supported by the results?	Yes	Thank you for recognising the logical connection between our results and conclusions.
Are all figures and tables clear and well-presented?	Yes	Thank you for confirming the clarity and effective presentation of our figures and tables.
3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: The findings are stated but is not clear if this process is a final  or is a pre-treatment one as is mentioned in other paper ( see doi.org/10.2166/wrd.2017.057) Please explain!!!

Response 1: Thank you for this comment. We acknowledge that the cited paper utilised electrocoagulation as a pre-treatment. Indeed, for certain wastewater characteristics, subsequent treatment, such as separation, is often necessary.

However, electrocoagulation is also highly capable of serving as a standalone treatment to achieve direct discharge. In our study, for instance, the water obtained after electrocoagulation treatment alone met the discharge requirements stipulated by the relevant local regulatory body.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The present study reports the treatment of coolant wastewater from a precision engineering company using electrocoagulation cell/method by employing stainless steel and aluminium electrode materials. The authors evaluated different anode-cathode configurations, pH levels and agitation methods to optimize wastewater treatment. The work is interesting for the readers; however, the choice of method, energy consumption during the process, generation of sludge containing high concentration of Al and Fe are concerning points. Additional concerns are outlined below.

-Have the authors analysed the wastewater content prior to treatments? The data showing the content of water before and after treatment should be included.

-When the solution was stirred, we observe higher Fe dissolution, but low COD removal. Can the authors explain this point.

- The applied current (app. 2 A) and resulting potential (app. 5-7 V) are high for the system. How is the energy efficient of the system, can the authors comment on this point? If the cell heated to 30 to 50 °C can the yield/efficiency be improved?

-What is the Faradaic efficiency of the cell? This should be included in the revised version.

-With this process more waste/sludge containing higher amount of Fe, Al is produced, which is harmful for the environment. Did the authors check the amount of Fe, Al, content in the sludge and if there is a method to regain the metals in the sludge?

-During the process, H₂ gas formation is observed. Can the wastewater cleaning technology be combined with H₂ production by improving the cell design?

Minor mistakes

Typos in the text

-2H2O (l) + 2e- à H2 (g) + 2OH-(aq) (line 63 in page 2)

Author Response

Response to Reviewer 4
1. Summary
Thank you very much for taking the time to review this manuscript. Please find detailed responses below and the corresponding revisions highlighted in turquoise in the re-submitted files.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Can be improved	Thank you for your comments. We have updated the introduction to enhance the clarity.
Is the research design appropriate?	Must be improved	Thank you for the suggestions. We have addressed the relevant questions by providing additional explanations and information.
Are the methods adequately described?	Yes	Thank you for confirming the adequateness of the methodology.
Are the results clearly presented?	Yes	Thank you for acknowledging the clarity of the results presented in our work.
Are the conclusions supported by the results?	Yes	Thank you for recognising the logical connection between our results and conclusions.
Are all figures and tables clear and well-presented?	Yes	Thank you for confirming the clarity and effective presentation of our figures and tables.
3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: Have the authors analysed the wastewater content prior to treatments? The data showing the content of water before and after treatment should be included.

Response 1: Thank you for your interest in the properties of the spent coolant. Based on industrial knowledge and our prior analysis, this spent coolant was categorised as an organic wastewater. Therefore, in this paper, we focused on key parameters (COD, pH, EC, metallic element concentrations) measured both before and after treatment to evaluate the electrocoagulation performance. The relevant information can be found in Table 1 (Line 114, Page 3).

 

Comments 2: When the solution was stirred, we observe higher Fe dissolution, but low COD removal. Can the authors explain this point.

Response 2: Thank you for the comment and we had checked our results in Figure 5a, Figure 7a, Table 3 and Table 4. We found that when SS used as anode, regardless of similar or dissimilar electrode configuration, higher Fe dissolution based on anode weight loss accompanies with faster COD removal. A quick comparison can be found in below table. Meanwhile, we agree that we should make it clearer in the manuscript and we had updated the manuscript (Line 421, Page 12) to make it clearer.

Electrode configuration	Initial pH (pH pre-adjusted)	Agitation	Weight loss of Anode (mg)	COD removal speed (minute)	Energy consumption (kWh/m3)
SS(A)-SS(C)	6	Stirring	2,196.35	80	18.81
		Aeration	2769.52	60	14.19
SS(A)-Al(C)		Stirring	2,856.38	60	14.90
		Aeration	3256.63	20	5.17

Revised version (Line 421, Page 12):

Meanwhile, it was found that when SS was used as anode, regardless of similar or dissimilar electrode configuration, higher Fe dissolution accompanies with faster COD removal.

 

Comments 3: The applied current (app. 2 A) and resulting potential (app. 5-7 V) are high for the system. How is the energy efficient of the system, can the authors comment on this point? If the cell heated to 30 to 50 °C can the yield/efficiency be improved?

Response 3: Thank you for the comment. Our calculations indicate a process energy consumption of approximately 30 kWh/m³ for a 2-hour experimental run. However, the process duration can be optimised by monitoring the COD removal rate, rather than adhering to a fixed 2-hour operation. For instance, the SS(A)-Al(C) configuration demonstrated the fastest COD removal, achieving optimal COD removal within just 20 minutes. This would reduce the estimated energy consumption to 5.17 kWh/m³, representing an 83% reduction compared to a 2-hour operational period.

The COD removal rate can theoretically be enhanced by heating the electrolytic cell. However, heating necessitates additional energy input and could negatively impact subsequent downstream processes if electrocoagulation is employed as a pre-treatment. For certain wastewater streams, heating the cell may also exacerbate odour release. Therefore, a comprehensive, systematic approach is required when determining the optimal cell temperature.

 

Comments 4: What is the Faradaic efficiency of the cell? This should be included in the revised version.

Response 4: Thank you for pointing out our mistake that the equation for Faradaic efficiency (FE) calculation was missing in our previous version of manuscript. We had updated it as Equation 5 in Line 71, Page 2. The values of FE under different process conditions can be found in Table 2, 3 and 4.

Revised version (Line 68-75, Page 2):

Faradic efficiency (FE) of the process was calculated as equation (5), based on mass loss of anode during electrocoagulation:

Where  is the mass of metal ions released (g),  is the valency of the metallic anode,  is the Faraday’s constant (96,485 C/mol),  is the current (A),  is the process time (s) and  is the molar mass of the metallic electrode (g/mol).

 

Comments 5: With this process more waste/sludge containing higher amount of Fe, Al is produced, which is harmful for the environment. Did the authors check the amount of Fe, Al, content in the sludge and if there is a method to regain the metals in the sludge?

Response 5: Prior analysis of sludge obtained from a different wastewater stream demonstrated significant metal content. Following a procedure involving rinsing, drying, and acid digestion, subsequent ICP-OES analysis revealed an approximate Fe concentration of 370 mg/g in the dried sludge.

While metal recovery is not the primary focus of this paper, we possess prior knowledge regarding this area. Various established methods, including separation and leaching, are widely employed for the recovery of metals from sludge. Although these processes are relatively mature, many generate additional waste and pose negative environmental impacts from a sustainability perspective. Developing a truly sustainable process for recovering valuable components from sludge remains a significant challenge.

 

Comments 6: During the process, H2 gas formation is observed. Can the wastewater cleaning technology be combined with H2 production by improving the cell design?

Response 6: Thank you for the comment. We agree that the H₂ generated by the cathode could be potential fuel source. According to some published works, the collection of H₂ is possible by employing a closed electrochemical reactor with gas separation device (doi.org/10.1016/j.ijhydene.2024.03.310, doi.org/10.1016/j.ijhydene.2010.06.100). However, the purity of H₂ collected was reported to range from 47.6% to 89.9% (doi.org/10.1016/j.ijhydene.2024.03.310) and may not warrant high value adding applications such as hydrogen fuel cells, which require highly purified H₂ gas. In addition, H₂ could have additional benefit of providing flotation to some pollutants such as oil and grease, which works better in an open electrochemical reactor and may restrict the H₂ collection. In summary, the collection of H₂ during electrocoagulation is possible by modifying the reactor design but also needs considering the type of wastewater treated and the subsequent application of the collected H₂.

 

Comments 7: Typos in the text: 2H2O (l) + 2e- à H2 (g) + 2OH-(aq) (line 63 in page 2)

Response 7: Thank you for pointing out this typo. We had updated this Equation 4 in Line 63, Page 2. The updated equation can be found below.

Revised version (Line 63, Page 2):

 

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have considered the reviewer`s comments and amended the article accordingly.