Continuous Electrocoagulation for a Sustainable Water Treatment: Effects of Electrode Configuration, Electrical Connection Mode, and Polarity Reversal on Fluoride Removal

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Point-by-point response to Comments and Suggestions for Authors

Comments 1: By configuring electrodes perpendicular to the direction of water flow, the authors enhanced the dissolution effect of aluminum through electrocoagulation. Does an excessive concentration of aluminum ions in water also pose a threat to human health? How did the authors approach this concern?

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, the residual aluminum concentration in the treated water was monitored to prevent the occurrence of new health issues. However, the residual metal concentration did not exceed the detection limit of the HACH colorimeter (DR/890) used, which was less than 0.013 mg/L, well below the concentration limit set by the WHO. This is indicated on page 18, line 19.

Comments 2: The overall manuscript appears more akin to an experimental report, lacking a discussion of underlying mechanisms. The authors are advised to incorporate a more in-depth analysis to enrich the manuscript.

Response 2: Thank you for your feedback. We understand the importance of including a discussion on the underlying mechanisms to provide a comprehensive understanding of our findings. We hope that the incorporation of a more in-depth analysis into the manuscript will enrich the content and offer a clearer explanation of the processes involved. For example, on page 3, line 22, we developed the defluoridation mechanism by EC, and on page 19, line 21, we discussed the mechanism of the deposit’s dissolution.

Comments 3: Has the author calculated the treatment cost per ton of water using this technological approach?

Response 3: In the manuscript, we presented the energy consumption of the process under different operating conditions. The total cost value of the process is added on page 18 line 12 under the optimal operating conditions (0.154 €/m³), calculated by Equation E6 added on page 8.

Comments 4: On Introduction: It is recommended that the authors provide a more in-depth comparison of this technology with other relevant technologies, as well as a more detailed discussion of the advantages and limitations of this technology. Merely summarizing with a single sentence, such as "These diverse methods offer various advantages and may be selected based on factors such as cost, efficiency, and the specific requirements of the water treatment scenario," is insufficient.

Response 4:  We appreciate the suggestion for a more comprehensive comparison of our technology with other relevant methods. In response, we have expanded the introduction to provide a detailed discussion of the advantages and limitations of various technologies, including our proposed method to fluoride removal. Please see page 2 line 32-42 and page 3 line 1 to 8.

Comments 5: On Figure 5a, For BP and S-MP, the authors are encouraged to include additional data points for the first 10 min.

Response 5: Additional data points for the first 10 minutes have been included for BP and S-MP in Figure 5a. page 16

Comments 6: The authors are recommended to standardize the units, for instance, using “min” consistently for minutes.

Response 6: The units have been standardized, with "min" consistently used for minutes throughout the manuscript.

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions highlighted in red color in the re-submitted files.

Point-by-point response to Comments and Suggestions for Authors

Comments 1:  What is the F concentration in the treated water. It is a very importing index.

Response 1: We appreciate your suggestion regarding the concentration of fluoride (F) in the treated water. It is indeed an important index for assessing the effectiveness of the electrocoagulation process. The figures presented in the manuscript depict the fluoride removal efficiency (Y), which is directly related to the residual fluoride concentration over time (calculated by (E2) expression). The fluoride concentration terms have been incorporated into the interpretations of our revised manuscript. Page 18 line 12.

Comments 2: How about the passivation of the Al electrodes? How to determine the actual rate of dissolved aluminum?

Response 2: The measurement of the experimental quantity of dissolved aluminum was conducted by measuring the mass of the electrodes before and after the EC treatment. After each experiment, the electrodes were cleaned to eliminate deposits and dried to quantify the difference between the mass before and after electrocoagulation, thus determining the amount of dissolved aluminum. Please refer to the protocol on page 8, line 22-26 for more details.

Comments 3: What is the volume of the pulsed injection KCl solution?

Response 3: 10 mL the information is added in page 8, line 7.

Thank you very much for taking the time to review this manuscript. We appreciate the constructive feedback provided by you and the reviewers. We have endeavored to address all your comments in our manuscript.

The choice of the three comparison scenarios was made to compare the three electrical connection modes under different operating conditions to determine the optimal connection mode for water treatment. In the literature, comparisons are generally made using batch electrocoagulation reactors with the same current density and the same number of electrodes for all three connection modes. In our study, we compared the three connection modes with: the same current density and the same number of electrodes, as previously studied, then same current and the same number of electrodes and finally the same current density and the same number of electrochemical cells, this study shows that the most important factors are the number of electrochemical cells and the current density, regardless of the connection mode. The BP mode was the least energy-intensive, hence its selection as the ideal connection mode. We chose to investigate “the same current density” in different configurations to maintain consistent operating conditions across both configurations. By using the same current density, flow rate, number of electrochemical cells, and connection mode, we ensure a fair and controlled comparison, allowing us to determine the optimal configuration.

All experiments were conducted in duplicate or triplicate to ensure accuracy and reproducibility of the results. The synthesized tap water used in this work was precisely prepared by dissolving specific amounts of salts (NaF, NaHCO₃, KH₂PO₄, Na₂SO₄, MgSO₄·H₂O, CaCl₂·2H₂O) in deionized water to match the ion concentrations found in real Metlaoui tap water. The standard error was very small because the analyses were conducted on the same water collected on the same day.

We thank you for your valuable feedback, which has helped us improve the quality and clarity of our paper. Please find below our detailed responses to the comments and suggestions, along with the corresponding revisions and corrections highlighted red color in the resubmitted files

Point-by-point response to Comments and Suggestions

Comments 1: Is the tap water used in the study the same water coming out from the residential taps

Response 1: The water used in this study is deionized water in which we dissolved the appropriate amounts of salts to achieve the same chemical composition as presented in Table 1. We added this information in part 2.2 Experimental procedures. We treated simulated Metlaoui tap water, prepared by dissolving various salts (CaCl₂, 2H₂O, MgSO₄,7H₂O, NaHCO₃, KH₂PO₄, CaSO₄,2H₂O, NaF), in deionized water.

Comments 2: 2.1 – Aluminum electrodes were used in this study. – replace the sentence.

Response 2: This change can be found in the revised manuscript in page 4, line 15

Comments 3: Did the authors measure conductivity? Was mentioned in the methods, but no data was shown.

Response 3: The conductivity measurements conducted in this study were used in the hydrodynamic analysis to determine the KCl concentration at the reactor outlet (Figure 4). Additionally, the conductivities of natural and synthesized tap water from Metlaoui were measured and are reported on page 12, figure 4 and page 6 line 7.

Comments 4: Was pH measured?

Yes, the pH of the water was measured over time, but it was not presented in the article because we did not find a significant difference in the pH evolution over time essentially in the steady state. It remains at a neutral pH of (7.0 ± 0.3), slightly lower than the initial pH (pH=(7.5±0.2)). The presence of hydrogenocarbonate ions in the water acted as a buffer, maintaining constant pH values under different operating conditions. This clarification is added in the revised manuscript in page 17 line 24

Comments 5: The term “defluoridation yield” is awkward, consider “F- removal”

Response 5: The term “defluoridation yield” has been replaced with “Fluoride removal” throughout the manuscript as suggested.

Comments 6: What is the purpose of “faradaic yield”?

Response 6: The purpose of "faradaic yield" is to measure the efficiency of the electrocoagulation process under different conditions in terms of the desired reaction occurring at the electrode surface. It quantifies the conversion of electrical energy into chemical energy during an electrochemical reaction. For instance, in the case of parallel positioning, electrocoagulation was not effective, and the dissolution reactions were not fully completed (µ = 0.63, 0.64, and 0.71). However, in perpendicular positioning, the faradaic yield increased to (1.06, 1.13, and 1.22), indicating that electrochemical reactions occur more effectively during electrocoagulation using this configuration. (page 9 line 6).

4. Response to Comments on the Quality of English Language