O-Modified Activated Carbon Fiber Electrode Efficiently Adsorption of Cu (II) in Wastewater

Round 1

Reviewer 1 Report

In this study, activated carbon fiber was subjected to processing to become O-modified activated carbon fiber (O-ACF) through modification using a nitric acid solution. The aim was to enhance the material's capacity for adsorbing Cu+2 from wastewater. The efficiency of O-ACF was evaluated through electrode adsorption tests.

 

The discussion section should be strengthened and supported by relevant literature, providing arguments, and supporting evidence. It would be beneficial to identify the limitations of the study and areas for improvement. Specifically, it is important to consider the conditions present in actual wastewater, such as the presence of coexisting ions and other pollutants, as they could potentially influence the efficiency of O-ACF and ACF. Addressing these factors would lead to a more comprehensive conclusion and highlight potential directions for future research. Here are some comments:

 

Comments to the Author:

 

Introduction:

• Line 25-32: This paragraph is not necessary and can be omitted.

• The introduction needs to explain the research gap or gaps and highlight the study's novelty.

 

Materials and Methods:

• This section needs to be reorganized and provide more details.

• • Give separately:  the modification procedure of ACF. 
•      Characterization of modified ACF
• . Adsorption/electrosorption capacity measurements (Adsorption kinetics, Intraparticle diffusion model, and isotherms?)

• Table 1 is not clear. Please explain the purpose of using these instruments in this research.

• Specify whether the wastewater used is a synthetic solution or real wastewater and provide its specifications.

• It would be helpful to include diagrams of the reactors and experimental setup, possibly using Figure 1 (the diagram of the electro-adsorption device) as a graphical abstract.

 

Results and Discussion:

• Provide a subsection titled "3.1. Characterization and Adsorption Properties of ACF and O-ACF" (Here you can only discuss between ACF and Modified ACF. It is not necessary to give AC.)

• In Figure 3, consider adding Figure 3.a). after Figure 3 or give it as a separate figure. 

• Line 185-188: Discuss the addition of oxygen during the modification process more thoroughly, referring to the relevant figures.

• Instead of discussing AC, It should be focused on comparing ACF and O-ACF.

 

Conclusion:

 

It should give Including the real-world application or future perspectives for the process, and any constraints or development prospects would strengthen the conclusion.

Comments for author File: Comments.pdf

Manuscript Englis is satisfied. 

Author Response

Dear Editor,

Thank you for your letter comments concerning our manuscript entitled “O-modified activated carbon fiber electrode efficiently adsorption of Cu (Ⅱ) in wastewater” (ID: Sustainability-2441551). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

Review 1

In this study, activated carbon fiber was subjected to processing to become O-modified activated carbon fiber (O-ACF) through modification using a nitric acid solution. The aim was to enhance the material's capacity for adsorbing Cu+2 from wastewater. The efficiency of O-ACF was evaluated through electrode adsorption tests.

The discussion section should be strengthened and supported by relevant literature, providing arguments, and supporting evidence. It would be beneficial to identify the limitations of the study and areas for improvement. Specifically, it is important to consider the conditions present in actual wastewater, such as the presence of coexisting ions and other pollutants, as they could potentially influence the efficiency of O-ACF and ACF. Addressing these factors would lead to a more comprehensive conclusion and highlight potential directions for future research. Here are some comments:

Comments to the Author:

Introduction:

• Line 25-32: This paragraph is not necessary and can be omitted.

Response: Thank you very much for your comments. At your suggestion, we had deleted lines 25-32 from the manuscript.

• The introduction needs to explain the research gap or gaps and highlight the study's novelty.

Response: Thank you very much for your comments. According to your suggestions, we had supplemented the introduction and highlighted it in red font. Modified as follows: At present, many novel methods have been studied, but complex chemical technology modification or cumbersome synthesis steps limit the application of technology or material in actual production. The acid oxidation modification method used in this study has simple steps, low cost, and is easy to be popularized and used on a large scale, and its performance has been significantly improved after modification, which is also one of the highlights of this study.

Materials and Methods:

• This section needs to be reorganized and provide more details.

• • Give separately:  the modification procedure of ACF. 
• Characterization of modified ACF
• Adsorption/electrosorption capacity measurements (Adsorption kinetics, Intraparticle diffusion model, and isotherms?)

Response: Thank you very much for your comments. We had adjusted this part of the content and made relevant additions. Modified as follows:

2.1. Preparation and adsorption test of O-ACF

2.1.1. Preparation of O-ACF

The activated carbon fiber (ACF) is purchased from the Chinese market, the ACF was cut into a 20 cm × 20 cm square, washed with deionized water, and soaked in ethanol for 2 h to eliminate bubbles inside the material. Next rinse with deionized water for 3 times, and dry the rinsed carbon fiber at 60℃ for 4 h. The dried activated carbon fibers were put into a beaker filled with 20% (W/V) HNO₃, soaked for 4 hours. Then, the activated carbon fibers were washed with deionized water for about 10 times until the pH of the material was 7 and dried in a thermostat at 80 °C to obtain O-ACF.

2.1.2. Characterization of O-ACF

The Fourier infrared spectra of ACF and O-ACF were measured by FTIR-650 spectrometer in the range of 4000 cm^-1-500 cm^-1. The surface morphology of ACF and O-ACF samples were determined by scanning electron microscopy (SEM). The acceleration voltage and operating distance of the instrument are set to 10.00 kV, 6.8 mm.

2.1.3. The electric adsorption device

The experimental device is shown in Fig. 1 and Fig. S1. The specific composition and operation of the device are described in Text S1.

2.1.4. Experimental operation and methods

Electro-adsorption test adopts the method of single factor experiment. The effects of different pH, initial concentration of Cu (Ⅱ), voltage and plate distance on the removal of Cu (Ⅱ) were tested. The concentration of Cu (Ⅱ) was determined by flame atomic absorption spectrometry. In addition, the number of electrodes in the experimental electro-adsorption device is a pair.

The effect of solution pH: Under the conditions of voltage of 1.0 V, plate distance of 15 mm and original concentration of 300 mg/L, the adsorption capacity of O-ACF on Cu (Ⅱ) at pH of 2, 4, 6, 8 and 10 was tested. The initial concentration of target pollutants: Under the conditions of voltage of 1.0 V, plate distance of 15 mm and pH of 6, the effect of initial concentration of Cu (Ⅱ) (20-300 mg/L) on the electro-adsorption capacity of O-ACF was tested. Study on the effect of voltage: Under the conditions of plate distance of 15 mm, pH of 6 and original concentration of Cu (Ⅱ) of 20 mg/L, the effects of voltage of 0.6, 0.8, 1.0 and 1.2 V on the electro-adsorption capacity of O-ACF were tested. Study on the effect of plate distance: Under the conditions of voltage of 1.0 V, pH of 6 and original concentration of Cu (Ⅱ) of 20 mg/L, the effects of plate distance of 5, 10, 15 and 20 mm on the electro-adsorption capacity of O-ACF were studied. All experiments were repeated 3 times.

2.1.5. Model fitting of Cu (Ⅱ) adsorption by O-ACF

In order to investigate the adsorption mechanism of O-ACF, kinetic and in-particle diffusion models were fitted to the adsorption data.

Pseudo-first-order kinetic model equation:

 

Pseudo-second-order kinetic model equation:

 

where q_t (mg/g) is the adsorption capacity at time, t; t (min) is the adsorption time; q_e (mg/g) is the equilibrium adsorption capacity; k₁ (min⁻¹) is the rate constant of the pseudo-first-order kinetic model; and k₂ (g/(mg·min)) is the rate constant of the pseudo-second-order kinetic model.

The intraparticle diffusion model equation:

 

where q_t (mg/g) is the adsorption capacity at time, t; K_id (mg/g·min^0.5) is the intraparticle diffusion rate constant; and C is the model constant.

• Table 1 is not clear. Please explain the purpose of using these instruments in this research.

Response: Thank you very much for your comments. We had changed Table 1 to text and added the purpose of administering the instrument. Modified as follows: The instruments used in the test mainly include: The surface morphology of the material was observed by scanning electron microscope (TM4000, Hitachi of Japan), test sample quality was weighed by analytical balance (EX series, American OHAUS), using fourier infrared spectrometer (FTIR-650, Tianjin Port East Technology) to measure the surface functional groups of materials, the content of Cu (Ⅱ) in the solution was determined by ultraviolet visible spectrophotometer (UV-1800, Shanghai Mepinda Instrument Co. LTD), electric blast drying oven (8401A-2, Shanghai Xuji Electric Co., LTD) was used to dry the test samples.

• Specify whether the wastewater used is a synthetic solution or real wastewater and provide its specifications.

Response: Thank you very much for your comments. The wastewater used in our test was a solution prepared with reagents, the specification of which was supplemented in the paper. Modified as follows: Cu (Ⅱ) solution was prepared from copper and nitric acid solution.

• It would be helpful to include diagrams of the reactors and experimental setup, possibly using Figure 1 (the diagram of the electro-adsorption device) as a graphical abstract.

Response: Thank you very much for your comments. A graphic summary is not required according to journal requirements, so Figure 1 is placed in the text.

Results and Discussion:

• Provide a subsection titled "3.1. Characterization and Adsorption Properties of ACF and O-ACF" (Here you can only discuss between ACF and Modified ACF. It is not necessary to give AC.)

Response: Thank you very much for your comments. At your suggestion we had removed the discussion of AC. Modified as follows: The maximum adsorption quantity of Cu (Ⅱ) on ACF and O-ACF is shown in Fig. 4. The maximum adsorption quantity of commercial ACF and O-ACF as adsorption electrodes were 33.56 and 48.60 mg/g, respectively. Compared with the AC commonly used in the market, the maximum adsorption capacity of ACF and O-ACF were significantly improved. The maximum adsorption quantity of the O-ACF was also significantly higher than that of the ACF, because micropore volume and adsorption site of O-ACF were significantly improved [38,39]. This was also consistent with the conclusion obtained from the characterization above. In addition, O-ACF had excellent adsorption performance for Cu (II) compared with other materials (Table 2).

References

38. Zhang, H.; Xing, L.; Liang, H.; Ren, J.; Ding, W.; Wang, Q.; Geng, Z.; Xu, C. Efficient removal of Remazol Brilliant Blue R from water by a cellulose-based activated carbon. Int. J. Biol. Macromol. 2022, 207, 254-262.
39. Duan, J.; Ji, H.; Xu, T.; Pan, F.; Liu, X.; Liu, W.; Zhao, D. Simultaneous adsorption of uranium (VI) and 2-chlorophenol by activated carbon fiber supported/modified titanate nanotubes (TNTs/ACF): Effectiveness and synergistic effects. Chem. Eng. J. 2021, 406, 126752.

• In Figure 3, consider adding Figure 3.a). after Figure 3 or give it as a separate figure. 

Response: Thank you very much for your comments. Fig. 3 and Fig. 4 were re-adjusted. Modified as follows:

 

Fig. 3. (a) FTIR spectra of ACF and O-ACF; (b-c) Scanned electronic images of ACF and O-ACF; (d-e) spectral images of ACF and O-ACF.

 

Fig. 4. Maximum adsorption of Cu (Ⅱ) at optimal conditions for AC, ACF and O-ACF.

• Line 185-188: Discuss the addition of oxygen during the modification process more thoroughly, referring to the relevant figures.

Response: Thank you very much for your comments. We discussed the role of oxygen groups in Section 3.2. As shown in Fig.3a, ACF had C-H, O-H, C-H, C = O, C = C, C-O, C-N and other functional groups. After modification, these functional groups are significantly enhanced, among which oxygen-containing functional groups can fix Cu²⁺ through complexation, and most oxygen-containing groups are in a negative form, thus enhancing the electroadsorption effect of O-ACF on Cu²⁺

 

• Instead of discussing AC, it should be focused on comparing ACF and O-ACF.

Response: Thank you very much for your comments. At your suggestion, I reduced the discussion of AC. Modified as follows:

Fig. 8 shows the regeneration rates of three kinds of adsorption electrodes. After 5 times of backwashing regeneration, the regeneration rates of ACF and O-ACF were maintained at about 87% and 95%, respectively (Fig. 8). The regeneration rate of O-ACF was higher than that of ACF, and reached a very significant difference. After backwashing and regeneration, the electrode remained intact, no scaling, passivation and other phenomena were found, and the regeneration rate did not change much with the increase of times, indicating that the O-ACF electrode had good regeneration and adsorption performance and could meet the requirements of practical applications [50,51]. In addition, flotation is one of the effective methods to treat solid particles, so the adsorbed carbon materials can be recovered by this method in practical applications [52-53].

References

50. Zhao, C.; Zhang, L.; Ge, R.; Zhang, A.; Zhang, C.; Chen, X. Treatment of low-level Cu (II) wastewater and regeneration through a novel capacitive deionization-electrodeionization (CDI-EDI) technology. Chemosphere. 2019, 217, 763-772.
51. Srivastava, A.; Gupta, B.; Majumder, A.; Gupta, A.K.; Nimbhorkar, S.K. A comprehensive review on the synthesis, performance, modifications, and regeneration of activated carbon for the adsorptive removal of various water pollutants. J. Environ. Chem. Eng. 2021, 9, 106177.
52. Feng, Q.C.; Wang, M.L.; Zhang, G.; Zhao, W.J.; Han, G. Enhanced adsorption of sulfide and xanthate on smithsonite surfaces by lead activation and implications for flotation intensification. Sep. Purif. Technol. 2023, 307, 122772.
53. Feng, Q.; Yang, W.; Wen, S.; Wang, H.; Zhao, W.; Han, G. Flotation of copper oxide minerals: a review. Int. J. Min. Sci. Technol. 2022, 32, 1351-1364.

Conclusion:

• It should give Including the real-world application or future perspectives for the process, and any constraints or development prospects would strengthen the conclusion.

Response: Thank you very much for your comments. We had supplemented the conclusion according to your suggestion. Modified as follows: In summary, O-ACF had good regeneration performance, which greatly reduced its application cost, it has great application prospect in the removal of Cu (Ⅱ) in industrial wastewater.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript introduced a conventional method to prepare carbon material for heavy metals removal. The related context was conventional and not interesting to the journal readership. So, I recommended a reject suggestion. The related comments were as follows.

1. Abstract. A sentence should be added to introduce the background. Some spill mistakes were found here, for example, Cu2+. In addition, the Cu-bearing species should be added in the abstract, especially at pH 6. The special functional groups should be cleared, to show the related adsorption mechanism.

2. Introduction. Why O-ACF should be prepared for the Cu-bearing wastewater treatment? The related reason should be added, to highlight the research target.

3. It is my first time to see the table of equipment in the submitted manuscript. Revised the manuscript according to the journal requirement. Somewhere, the formatted text should be noticed and carefully considered.

4. In the manuscript, the modification method of carbon materials has been reported. The related synthesis steps, characterization methods, and the interpretation were not interesting.

5. Free Cu was targeted to determine the adsorption performance of prepared material, but the investigation method was also conventional.

 

need to be improved.

Author Response

Dear Editor,

Thank you for your letter comments concerning our manuscript entitled “O-modified activated carbon fiber electrode efficiently adsorption of Cu (Ⅱ) in wastewater” (ID: Sustainability-2441551). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

The manuscript introduced a conventional method to prepare carbon material for heavy metals removal. The related context was conventional and not interesting to the journal readership. So, I recommended a reject suggestion. The related comments were as follows.

1. Abstract. A sentence should be added to introduce the background. Some spill mistakes were found here, for example, Cu2+. In addition, the Cu-bearing species should be added in the abstract, especially at pH 6. The special functional groups should be cleared, to show the related adsorption mechanism.

Response: Thank you very much for your comments. We had added a sentence to the abstract to introduce the background. Modify as follows: “At present, wastewater discharged from many industries contains a large amount of Cu (Ⅱ).” In addition, we changed “Cu²⁺” to “Cu (II)”. In addition, in the abstract we have made changes according to your suggestions. The functional group analysis is supplemented in the FTIR analysis in Section 3.2.

 

2. Introduction. Why O-ACF should be prepared for the Cu-bearing wastewater treatment? The related reason should be added, to highlight the research target.

Response: Thank you very much for your comments. We supplemented the relevant reasons in the introduction part. Modify as follows: The key to electro-adsorption is the choice of electrode materials. Carbon materials are usually used as electrodes and are mainly porous structures. However, there are few adsorption sites of heavy metals such as oxygen-containing groups on carbon materials, and the adsorption activity of heavy metals is not enough. It is necessary to increase its adsorption sites by artificial treatment. Activated carbon fiber is a cheap and easily available material with a large number of pore structures on the surface and excellent electrical conductivity. It is one of the excellent choices for adsorption electrodes [27,28]. Similarly, activated carbon fibers have fewer oxygen-containing functional groups, which is not conducive to adsorption. Therefore, it needs to be oxidized to increase the adsorption capacity [29-31]. Nitric acid oxidation modification is one of the most widely used methods, which is due to the strong oxidation of nitric acid. Different from sulfuric acid, nitric acid has low corrosion resistance to materials and will not destroy the main structure of materials. Therefore, it is more appropriate to use nitric acid as an oxidation modification solution [32,33]. In this study, nitric acid was used to oxidize the activated carbon fiber (ACF) to prepare a nitric acid modified activated carbon fiber (O-ACF) as an electrode material for electro-adsorption.

References

27. Huang, Z. H.; Yang, Z.; Kang, F.; Inagaki, M. Carbon electrodes for capacitive deionization. J. Mater. Chem. A. 2017, 5, 470–496.
28. Jiao, Y.; Ma, L.; Tian, Y.; Zhou, M. A flow-through electro-Fenton process using modified activated carbon fiber cathode for orange II removal. Chemosphere. 2020, 252, 126483.
29. Rahmi, R.; Lelifajri, L.; Iqbal, M.; Fathurrahmi, F.; Jalaluddin, J.; Sembiring, R.; Farida, M.; Iqhrammullah, M. Preparation, characterization and adsorption study of PEDGE-cross-linked magnetic chitosan (PEDGE-MCh) microspheres for Cd²⁺ Removal. Arab. J. Sci. Eng. 2022, 2–10.
30. Saiful; Riana, U.; Ramli, M.; Iqrammullah, M.; Raharjo, Y.; Wibisono, Y. Development of Chitosan/Rice Husk-Based Silica Composite Membranes for Biodiesel Purification. Membranes. 2022, 12, 435.
31. Iqhrammullah, M.; Suyanto, H.; Pardede Rahmi, M.; Karnadi, I.; Kurniawan, K.H.; Chiari, W.; Abdulmadjid, S.N. Cellulose acetate-polyurethane film adsorbent with analyte enrichment for in-situ detection and analysis of aqueous Pb using LaserInduced Breakdown Spectroscopy (LIBS), Environ. Nanotechnol. Monit. Manag. 2021, 16, 100516.
32. Öter, Ç.; Selçuk Zorer, Ö. Adsorption behaviours of Th (IV) and U (VI) using nitric acid (HNO₃) modified activated carbon: equilibrium, thermodynamic and kinetic studies. INT. J. ENVIRON. AN. CH. 2021, 101, 1950–1965.
33. Huang, G.; Shi, J.X.; Langrish, T.A. Removal of Cr (VI) from aqueous solution using activated carbon modified with nitric acid. Chem. Eng. J. 2009, 152, 434-439.
34. It is my first time to see the table of equipment in the submitted manuscript. Revised the manuscript according to the journal requirement. Somewhere, the formatted text should be noticed and carefully considered.

Response: Thank you very much for your comments. In the main equipment section, we change the table to text. Modified as follows: The instruments used in the test mainly include: scanning electron microscope (TM4000, Hitachi of Japan), analytical balance (EX series, American OHAUS), specific surface analyzer (JW-BK122W, Beijing Jingwei Gao Bo), fourier infrared spectrometer (FTIR-650, Tianjin Port East Technology), X-ray powder diffractometer (TDM-10, Dandong Tongda Technology Co., LTD), ultraviolet visible spectrophotometer (UV-1800, Shanghai Mepinda Instrument Co. LTD), dc power supply (PWS2000 series, Xi'an Antai Test Equipment Co., LTD), peristaltic pump (TT-30C Miniature, Wuxi Tianli Fluid Technology Co., LTD), ultrasonic cleaning instrument (JH-5108VT, Suzhou Juhong Ultrasonic Equipment Co., LTD), electric blast drying oven (8401A-2, Shanghai Xuji Electric Co., LTD).

4. In the manuscript, the modification method of carbon materials has been reported. The related synthesis steps, characterization methods, and the interpretation were not interesting.

Response: Thank you very much for your comments. Sorry for the lack of clarity in our description of the highlights of the study. At present, many novel methods have been studied, but complex chemical technology modification or cumbersome synthesis steps limit the application of technology or material in actual production. The acid oxidation modification method used in this study has simple steps, low cost, and is easy to be popularized and used on a large scale, and its performance has been significantly improved after modification, which is also one of the highlights of this study.

5. Free Cu was targeted to determine the adsorption performance of prepared material, but the investigation method was also conventional.

Response: Thank you very much for your comments. The method we used is a reference to previous studies and belongs to the classical adsorption test method. This method has been proved to be accurate and simple by predecessors. Our focus is on providing a technical solution that is suitable for large-scale application, and therefore requires the adoption of today's widely used testing methods.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Journal Name: sustainability

Recommendation: Minor

Manuscript Id: sustainability-2441551

Comments to Authors

In the manuscript entitled, “O-modified Activated Carbon Fiber Electrode Efficiently Adsorption of Cu2+ in Wastewater” researchers prepared a highly active O-modified activated carbon fiber (O-ACF) by oxidizing activated carbon fiber with nitric acid. The O-ACF demonstrated excellent electro-adsorption performance for Cu2+ ions, with a maximum adsorption capacity of 48.60 mg/g, 1.63 times higher than commercial activated carbon. Optimal working conditions for the O-ACF electrode were determined as 1.0 V voltage, pH 6 solution, and a plate distance of 10 mm. The study suggests that O-ACF could be a promising material for capacitive deionization systems used in wastewater treatment, seawater desalination, and nutrient recovery due to its practicality and regeneration capabilities. This work is good but needs some specific improvement. Therefore, the publication of this work in its present form is not recommended for publication in sustainability journal and can be justified after the authors consider the following points.

The following are some specific comments regarding the work covered in this manuscript.

1. Keywords are missing.

2. Write some optimizing parameters also in the abstract so that a clear picture of optimization is present to the reader.

3. In introduction, point out some more specific sources of copper contamination in water.

4. Discuss some more examples of study performed to eliminate the copper contamination in introduction section. Also write the chemical formulae for the materials used.

5. Rearrange your section 2, discuss first the materials, and then instruments used.

6. The details of carbon fibre are missing. specify its properties in the materials section.

7. Provide the actual image of your electric adsorption device setup in supplementary.

8. Why have you not performed the BET characterization. Authors are strongly encouraged to include the BET characterization results.

9. The authors should also provide a comparison table with other similar studies for better understanding.

Given the above-mentioned points, the current version of the manuscript is recommended minor revision before the publication in the sustainability Journal.

 

Comments for author File: Comments.pdf

Author Response

Dear Editor,

Thank you for your letter comments concerning our manuscript entitled “O-modified activated carbon fiber electrode efficiently adsorption of Cu (Ⅱ) in wastewater” (ID: Sustainability-2441551). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

 

In the manuscript entitled, “O-modified Activated Carbon Fiber Electrode Efficiently Adsorption of Cu2+ in Wastewater” researchers prepared a highly active O-modified activated carbon fiber (O-ACF) by oxidizing activated carbon fiber with nitric acid. The O-ACF demonstrated excellent electro-adsorption performance for Cu2+ ions, with a maximum adsorption capacity of 48.60 mg/g, 1.63 times higher than commercial activated carbon. Optimal working conditions for the O-ACF electrode were determined as 1.0 V voltage, pH 6 solution, and a plate distance of 10 mm. The study suggests that O-ACF could be a promising material for capacitive deionization systems used in wastewater treatment, seawater desalination, and nutrient recovery due to its practicality and regeneration capabilities. This work is good but needs some specific improvement. Therefore, the publication of this work in its present form is not recommended for publication in sustainability journal and can be justified after the authors consider the following points.

The following are some specific comments regarding the work covered in this manuscript.

1. Keywords are missing.

Response: Thank you very much for your comments. We added key words and highlighted them in red in the manuscript. Keywords are as follows: hydrothermal carbon; slow-release nitrogen fertilizer; nitrogen utilization.

2. Write some optimizing parameters also in the abstract so that a clear picture of optimization is present to the reader.

Response: Thank you very much for your comments. We added optimization parameters in the summary and marked them in red font. Modify as follows: After optimizing and adjusting the voltage (0.6-1.2 V), pH (2-10) and electrode plate spacing (5-20 mm), it was found that the most favorable working conditions for electro-adsorption of O-ACF electrode were voltage of 1.0 V, solution pH of about 6, and electrode plate spacing of 10 mm.

3. In introduction, point out some more specific sources of copper contamination in water.

Response: Thank you very much for your comments. In the introduction part, we supplemented the specific sources of copper pollution in water. Modify as follows: Copper ion pollution is mainly caused by wastewater discharged from metal processing and metallurgical industry, which poses a serious risk to organisms [9].

References

9. Li, M.; Liu, Q.; Guo, L.; Zhang, Y.; Lou, Z.; Wang, Y.; Qian, G. Cu (II) removal from aqueous solution by Spartina alterniflora derived biochar. Bioresour. Technol. 2013, 141, 83–88.
10. Discuss some more examples of study performed to eliminate the copper contamination in introduction section. Also write the chemical formulae for the materials used.

Response: Thank you very much for your comments. We added more research examples to remove copper pollution in the introduction. Modify as follows: Katiyar et al. examined the adsorption performance of seaweed-based biochar for Cu (II). The results showed that the highest removal rate of Cu (II) in water was more than 99% [21]. Xu et al. prepared porous nanofiber membranes and studied their performance in removing copper ions from water. The results showed that the maximum adsorption capacity for copper ions was 354 mg/g [22].

References

21. Katiyar, R., Patel, A.K., Nguyen, T.B., Singhania, R.R., Chen, C.W., Dong, C.D. Adsorption of copper (II) in aqueous solution using biochars derived from Ascophyllum nodosum seaweed. Bioresour. Technol. 2021, 328, 124829.
22. Xu, X.; Zhang, M.; Lv, H.; Zhou, Y.; Yang, Y.; Yu, D.G. Electrospun polyacrylonitrile-based lace nanostructures and their Cu (II) adsorption, Sep. Purif. Technol. 2022, 288, 120643.
23. Rearrange your section 2, discuss first the materials, and then instruments used.

Response: Thank you very much for your comments. We re-adjusted the order of the second part of the content, modified as follows:

2.1. Test equipment and electrode materials

2.2. Main experimental instruments

2.3. The main reagents of the experiment

6. The details of carbon fibre are missing. specify its properties in the materials section.

Response: Thank you very much for your comments. We added the relevant content in Section 2.1.1.

7. Provide the actual image of your electric adsorption device setup in supplementary.

Response: Thank you very much for your comments. We provided the image of the electroadsorption device in Fig. S1.

8. Why have you not performed the BET characterization. Authors are strongly encouraged to include the BET characterization results.

Response: Thank you very much for your comments. We supplemented the BET representation data of ACF and O-ACF in Table S1, and analyzed them in Section 3.2 of the manuscript.

9. The authors should also provide a comparison table with other similar studies for better understanding.

Given the above-mentioned points, the current version of the manuscript is recommended minor revision before the publication in the sustainability Journal.

Response: Thank you very much for your comments. We added a comparison table with other similar studies. Modified as follows:

Table 2. Comparison of adsorption properties of different adsorbents for Cu (Ⅱ).

Adsorbents	Initial pH	qmax (mg/g)	References
Diethylenetriamine-bacterial cellulose	4.5	63.09	[40]
amino-functionalized magnetic nanoparticles	6	28.70	[41]
Chemically modified orange peel	5	15.27	[42]
Succinylated mercerized cellulose modified with triethylenetetramine	5.8	56.80	[43]
O-modified activated carbon fiber	6	48.60	This study

References

40. Shen, W.; Chen, S.; Shi, S.; Li, X.; Zhang, X.; Hu, W.; Wang, H. Adsorption of Cu (II) and Pb (II) onto diethylenetriamine-bacterial cellulose. Carbohydr. Polym. 2009, 75, 110–114.
41. Li, H.; Xiao, D.L.; He, H.; Lin, R.; Zuo, P.L. Adsorption behavior and adsorption mechanism of Cu (II) ions on amino-functionalized magnetic nanoparticles. Trans. Nonferrous Metals Soc. 2013, 23, 2657-2665.
42. Lasheen, M.R.; Ammar, N.S.; Ibrahim, H.S. Adsorption/desorption of Cd (II), Cu (II) and Pb (II) using chemically modified orange peel: equilibrium and kinetic studies. Solid State Sci. 2012, 14, 202-210.
43. Gurgel, L.V.A.; Gil, L.F. Adsorption of Cu (II), Cd (II), and Pb (II) from aqueous single metal solutions by succinylated mercerized cellulose modified with triethylenetetramine. Carbohydr. Polym. 2009, 77, 142-149.

Author Response File: Author Response.pdf

Reviewer 4 Report

[Introduction]

1. The introduction provides a sufficient theoretical justification for the study by discussing the limitations of current methods and the potential of electro-adsorption using O-ACF as an electrode material. It also presents the novelty of the study by highlighting the use of nitric acid oxidation modification and the investigation of various parameters and models. However, it is a good idea to incorporate the following articles that have emphasized the importance of O-containing functional group in adsorption:

Rahmi et al. Arabian Journal for Science and Engineering volume 48, pages159–167 (2023) doi: 10.1007/s13369-022-06786-6

Saiful et al., Membranes 2022, 12(4), 435; doi: 10.3390/membranes12040435

Iqhrammullah et al., Environmental Nanotechnology, Monitoring & Management Vol.16, 2021, 100516. Doi: 10.1016/j.enmm.2021.100516.

 

[Methods]

2. Authors rinsed the sample several time with dionized water to neutralize the pH. But the measurement on the pH level should be stated. Moreover, please provide how many time the rinsing was performed - not just 'several time'.

3. In the experimental operation. Initial pH, initial concentration of Cu2+, voltage, and plate distance for each stage should be provided. Number of replicates in each investigation should be provided.

4. How did author measured the amount of re-adsorption? Also statistical analysis for the comparison between the backwash and regeneration effect should be carried out.

[Results]

5. Figure 3 e-f is not EDS mapping? The presentation of mapping should include the morphological images. They are just spectral images. Also, add the y-axis and x-axis.

[Conclusion]

6. Please make the conclusion in a paragraph not point-by-point.

 

 

 

 

 

Overall, the English writing quality appears to be quite good. The sentences are generally well-structured, and the terminology used in the scientific context is appropriate. The information is presented in a clear and concise manner, allowing readers to understand the key findings and observations.

Author Response

Dear Editor,

Thank you for your letter comments concerning our manuscript entitled “O-modified activated carbon fiber electrode efficiently adsorption of Cu (Ⅱ) in wastewater” (ID: Sustainability-2441551). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

[Introduction]

1. The introduction provides a sufficient theoretical justification for the study by discussing the limitations of current methods and the potential of electro-adsorption using O-ACF as an electrode material. It also presents the novelty of the study by highlighting the use of nitric acid oxidation modification and the investigation of various parameters and models. However, it is a good idea to incorporate the following articles that have emphasized the importance of O-containing functional group in adsorption:

Rahmi et al. Arabian Journal for Science and Engineering volume 48, pages159–167 (2023) doi: 10.1007/s13369-022-06786-6

Saiful et al., Membranes 2022, 12(4), 435; doi: 10.3390/membranes12040435

Iqhrammullah et al., Environmental Nanotechnology, Monitoring & Management Vol.16, 2021, 100516. Doi: 10.1016/j.enmm.2021.100516.

Response: Thank you very much for your comments. We quoted the above articles in the introduction part of the manuscript and marked them in red. Modify as follows: Similarly, activated carbon fibers have fewer oxygen-containing functional groups, which is not conducive to adsorption. Therefore, it needs to be oxidized to increase the adsorption capacity [29-31].

References

29. Rahmi, R.; Lelifajri, L.; Iqbal, M.; Fathurrahmi, F.; Jalaluddin, J.; Sembiring, R.; Farida, M.; Iqhrammullah, M. Preparation, characterization and adsorption study of PEDGE-cross-linked magnetic chitosan (PEDGE-MCh) microspheres for Cd²⁺ Removal. Arab. J. Sci. Eng. 2022, 2–10.
30. Saiful; Riana, U.; Ramli, M.; Iqrammullah, M.; Raharjo, Y.; Wibisono, Y. Development of Chitosan/Rice Husk-Based Silica Composite Membranes for Biodiesel Purification. Membranes. 2022, 12, 435.
31. Iqhrammullah, M.; Suyanto, H.; Pardede Rahmi, M.; Karnadi, I.; Kurniawan, K.H.; Chiari, W.; Abdulmadjid, S.N. Cellulose acetate-polyurethane film adsorbent with analyte enrichment for in-situ detection and analysis of aqueous Pb using LaserInduced Breakdown Spectroscopy (LIBS), Environ. Nanotechnol. Monit. Manag. 2021, 16, 100516.

[Methods]

2. Authors rinsed the sample several time with dionized water to neutralize the pH. But the measurement on the pH level should be stated. Moreover, please provide how many time the rinsing was performed - not just 'several time'.

Response: Thank you very much for your comments. The activated carbon fibers were washed with deionized water for about 10 times until the pH of the material was 7 and dried in a thermostat at 80 °C to obtain O-ACF. In addition, we had made changes in the manuscript and marked them in red fonts.

3. In the experimental operation. Initial pH, initial concentration of Cu2+, voltage, and plate distance for each stage should be provided. Number of replicates in each investigation should be provided.

Response: Thank you very much for your comments. Influence of operating parameters on electro-adsorption of O-ACF: (a) pH (voltage of 1.0 V, plate distance of 15 mm and original concentration of 300 mg/L); (b) voltage (plate distance of 15 mm, pH of 6 and original concentration of Cu²⁺ of 20 mg/L); (c) initial concentration (voltage of 1.0 V, plate distance of 15 mm and pH of 6,); (d) plate spacing (voltage of 1.0 V, pH of 6 and original concentration of Cu²⁺ of 20 mg/L). We supplemented it in Fig. 2. In addition, all experiments were repeated 3 times. We also supplemented it in the manuscript and marked it in red.

 

4. How did author measured the amount of re-adsorption? Also statistical analysis for the comparison between the backwash and regeneration effect should be carried out.

Response: Thank you very much for your comments. We tested the adsorption amount of O-ACF in the next adsorption test directly after the regeneration treatment, so we did not test the concentration of desorption. In future studies, we will consider quantifying the desorption effect as well.

[Results]

5. Figure 3 e-f is not EDS mapping? The presentation of mapping should include the morphological images. They are just spectral images. Also, add the y-axis and x-axis.

Response: Thank you very much for your comments. Due to our negligence, there was a writing error, and we had corrected it and added the y-axis and the x-axis. Modify as follows:

 

Fig. 3. (a) Maximum adsorption of Cu (Ⅱ) at optimal conditions for AC, ACF and O-ACF; (b) FTIR spectra of ACF and O-ACF; (c-d) Scanned electronic images of ACF and O-ACF; (e-f) spectral images of ACF and O-ACF.

[Conclusion]

6. Please make the conclusion in a paragraph not point-by-point.

Response: Thank you very much for your comments. We described the conclusion in the form of paragraphs and modify as follows: In this work, the influence factors of O-ACF on the electro-adsorption of Cu²⁺ in water were investigated. The maximum adsorption capacity of AC, ACF and O-ACF under optimal electro-adsorption conditions and the regeneration of O-ACF in different regeneration fluids were studied. The adsorption test showed that the O-ACF electro-adsorption of Cu²⁺ in water basically reached equilibrium about 4 h. Moreover, the optimal working conditions of O-ACF electrode electro-adsorption were voltage of 1.0V, solution pH of about 6, electrode plate spacing of 10 mm. Furthermore, the O-ACF electrode electro-adsorption process fit the pseudo-second-order kinetic equation and the particle diffusion adsorption model. The adsorption was mainly chemisorption, which was jointly controlled by a variety of factors. In addition, it was found in this study that the regeneration effect of O-ACF was the best when 20% HNO₃ was used as the regeneration solution. After several adsorption-regeneration cycles, the regeneration rate of O-ACF electrode can still reach about 95% by using the inverted electrode charging method. In summary, O-ACF had good regeneration performance, which greatly reduced its application cost.

 

Author Response File: Author Response.pdf

Reviewer 5 Report

In this paper, the effects of pH value, electrode plate spacing and Cu²⁺ concentration on the electro-adsorption capacity of O-ACF were explored. The adsorption kinetics model and intraparticle diffusion model were established for the process of electro-adsorption of Cu²⁺ by O-ACF to explore the mechanism and properties of electro-adsorption of copper ions by O-ACF. It is an interesting content, but arranged structure needs to be further improved. Therefore, it needs minor revision before it is published in this journal. Some issues should be carefully addressed. Please check the attachment.

Comments for author File: Comments.pdf

Minor editing of English language required.

Author Response

Dear Editor,

Thank you for your letter comments concerning our manuscript entitled “O-modified activated carbon fiber electrode efficiently adsorption of Cu (Ⅱ) in wastewater” (ID: Sustainability-2441551). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

 

In this paper, the effects of pH value, electrode plate spacing and Cu2+ concentration on the electro-adsorption capacity of O-ACF were explored. The adsorption kinetics model and intraparticle diffusion model were established for the process of electro-adsorption of Cu2+ by O-ACF to explore the mechanism and properties of electro-adsorption of copper ions by O-ACF. It is an interesting content, but arranged structure needs to be further improved. Therefore, it needs minor revision before it is published in this journal.

1. The upper and lower scripts should be written correctly, including abstract.

Response: Thank you very much for your comments. We examined the entire manuscript and corrected the incorrect writing.

2. The English of this manuscript is largely okay. But occasionally the texts get mixed up and require the reader to guess what the authors were trying to say. I suggest a careful and thorough editing.

Response: Thank you very much for your comments. We invited native English scholar to improve the English quality of the manuscript to ensure the accuracy of the English expression.

3. What is the particle size of O-modified activated carbon fiber?

Response: Thank you very much for your comments. According to the results of SEM images (Fig. 3d), the fiber thickness of O-MCF was concentrated within 100 μm.

4. I wonder how the authors treat modified activated carbon fiber retained in the solution in the subsequent process. Flotation is a useful method to treat solid particles, and it is widely used in the efficient recovery of minerals, solid waste, heavy metal ions in water, etc. Thus, it may be introduced to recover activated carbon retained in the solution in the subsequent process, and several relevant references may be added to support this point, such as Int. J. Min. Sci. Technol. 32 (2022) 1351–1364; Sep. Purif. Technol. 307 (2023) 122772.

Response: Thank you very much for your comments. We supplemented the discussion on flotation recovery of carbon fibers in the manuscript and cited the above references. Modify as follows: In addition, flotation is one of the effective methods to treat solid particles, so the adsorbed carbon materials can be recovered by this method in practical applications [52-53].

References

52. Feng, Q.C.; Wang, M.L.; Zhang, G.; Zhao, W.J.; Han, G. Enhanced adsorption of sulfide and xanthate on smithsonite surfaces by lead activation and implications for flotation intensification. Sep. Purif. Technol. 2023, 307, 122772.
53. Feng, Q.; Yang, W.; Wen, S.; Wang, H.; Zhao, W.; Han, G. Flotation of copper oxide minerals: a review. Int. J. Min. Sci. Technol. 2022, 32, 1351-1364.
54. Focus of aims should be further elevated in Kinetics model fitting.

Response: Thank you very much for your comments. We redrew the kinetic model image to make the results clearer.

6. It is suggested to compare the results of the present research with some similar studies which is done before

Response: Thank you very much for your comments. We added a comparison table with other similar studies. Modified as follows:

Table 2. Comparison of adsorption properties of different adsorbents for Cu (Ⅱ).

Adsorbents	Initial pH	qmax (mg/g)	References
Diethylenetriamine-bacterial cellulose	4.5	63.09	[40]
amino-functionalized magnetic nanoparticles	6	28.70	[41]
Chemically modified orange peel	5	15.27	[42]
Succinylated mercerized cellulose modified with triethylenetetramine	5.8	56.80	[43]
O-modified activated carbon fiber	6	48.60	This study

References

40. Shen, W.; Chen, S.; Shi, S.; Li, X.; Zhang, X.; Hu, W.; Wang, H. Adsorption of Cu (II) and Pb (II) onto diethylenetriamine-bacterial cellulose. Carbohydr. Polym. 2009, 75, 110–114.
41. Li, H.; Xiao, D.L.; He, H.; Lin, R.; Zuo, P.L. Adsorption behavior and adsorption mechanism of Cu (II) ions on amino-functionalized magnetic nanoparticles. Trans. Nonferrous Metals Soc. 2013, 23, 2657-2665.
42. Lasheen, M.R.; Ammar, N.S.; Ibrahim, H.S. Adsorption/desorption of Cd (II), Cu (II) and Pb (II) using chemically modified orange peel: equilibrium and kinetic studies. Solid State Sci. 2012, 14, 202-210.
43. Gurgel, L.V.A.; Gil, L.F. Adsorption of Cu (II), Cd (II), and Pb (II) from aqueous single metal solutions by succinylated mercerized cellulose modified with triethylenetetramine. Carbohydr. Polym. 2009, 77, 142-149.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

As a reviewer, after  reading the revised version of the text and considering the corrections made, I am pleased to provide positive feedback:

 

It is satisfied with the revisions made to the text. The overall content and presentation have been significantly improved, resulting in a more cohesive and engaging discussion. Overall, the revisions have significantly enhanced the clarity, cohesiveness, and readability of the text.

Reviewer 2 Report

The revised paper showed some interesting finding, so that I recommanded an acceptance.