Mechanistic Evaluation of Pb(II) Adsorption on Magnetic Activated Carbon/Fe₃O₄ Composites: Influence of Hydrothermal and Ultrasonic Synthesis Routes

Round 1

Reviewer 1 Report

It seems that the study aimed to check the potential of each material prepared as an adsorbent for lead under fixed experimental conditions. However, many parameters such as initial concentration, pH, adsorbent dosage, contact time and others can affect significantly metal adsorption efficiency, it would be important to highlight the scope and limitations of this study in relation to the goals indicated in the introduction. Additionally, the relevance and novelty of their work must be demonstrated along the manuscript, and especially, in conclusions.

I include some comments, suggestions and doubts as follow:

Introduction:

In Table 1, the reference [30] with information about a dye adsorption could be removed since it is the only not referred to lead.

Methods:

• After reading the manuscript, since authors have shown results for magnetite in Figure 4 and 5, this material should be included in the section corresponding to Materials.
• How were the conditions for activation carbon synthesis selected? It would be appropriate that authors could introduce a reference.
• In addition, I appreciate that the authors have specified the exact amounts of reagents and volumes used to prepare the material. In any case, it may be more appropriate to express quantities in terms of concentrations or ratios as in the ultrasonication-assisted process rather than absolute amounts for clarity of the results and comparison with other authors. In any case, this raises a question about the yield at each stage, How much material is lost before obtaining the product? For example, they used a total of 5.8175 g of FeSO₄·7H₂O + FeCl₃·6H₂O + activated carbon, but then they took only 0.232 g of the composite for hydrothermal step. Which is the final weight of h-AC-Fe₃O₄? The same question about the yield is for ultrasonicated carbon.
• Regarding the adsorption experiments, authors differentiate those to evaluate the adsorption performance and the experiments to investigate the kinetic models. Then, would it not be more logical to perform the kinetic experiments until more than 4 h instead of 2 h to verify the equilibrium time and after, to compare the adsorption efficiency at equilibrium time for both materials? Why did they select an initial concentration of 50 mg/L of Pb?
• In my opinion, the paragraph about the determination of residual Pb concentration should be in the section 2.3 of adsorption test and not in the section 2.4 as it is not strictly a method for material characterization.

Results and discussion

• Authors describe in page 7 (Lines 260-261) the nitrogen adsorption–desorption isotherms of the samples (Figure 3a not 4a). However, only the points for the adsorption data can be observed, since the desorption data are either missed or not distinguised. Please, I ask to the authors to verify whether the desorption data are included. This would support the discussion in the text, demonstrating if the hystheresis loop characterizes the form of isotherms as H4.
• Upon examining the BET plots, it appears that the slope of the line is very similar for AC and us-AC-Fe₃O₄, however it is higher for h-AC-Fe₃O₄. This raises the question of whether the reported difference in surface area for the first carbons can be so high among them, and only a half between both composites with magnetite. Could the authors clarify how the surface area values were derived? Which are the values for volume of nitrogen adsorbed for the monolayer? The fittings seem to be very good.
• Authors indicate that "Following modification with iron oxide (Fe₃O₄) nanoparticles, a significant reduction in nitrogen uptake is observed, indicating a decrease in accessible surface area and/or pore volume (Figure 3a)". However, the surface area values (accesible or not) are already lower to S BETfor the pristine carbon. Additionally, if the authors attributed this behavior to size exclusion effects, it would be helpful to specify the approximate size of Pb²⁺ ions and discuss whether the pore dimensions of the modified material are indeed smaller than this threshold. Figure 3a is not for surface area or pore volume. Is it necessary to include Figure 3c? Would be it better to put the range of pore diameter in text?
• Related to the previous comment, authors must explain more thoroughly this observation "The pore size distribution plot (Figure 3c) further confirms that the incorporation of iron oxide nanoparticles leads to blockage of the porous structure of the activated carbon". It is not evident from figure.
• The manuscript includes some results about the characterization of magnetite nanoparticles in Figure 4 and 5, which are commented in text. However, it is not clear whether this material is for using as a reference or as part of the carbons prepared. In this case, they are prepared in different way, (can be the black precipitate separated as magnetite be considered to have the same purity in both cases?) Moreover, the results for Fe₃O₄ are shown without relation to the other materials. In my opinion, it is important, first to clarify this point, and then doing the discussion properly.
• Were also the magnetic separation checked after adsorption experiments?
• At which value does neutral pH correspond? As I commented before, the pH is a very important parameter influencing on adsorption efficiency, and also, for being done the comparison of adsorption capacities found with those for other adsorbents in the same conditions.
• Kinetic experiments demonstrated that it is not necessary to perform the adsorption experiments at 4 h, For all materials, the use of 1 h is sufficient. Why did the authors decide to perform the experiments at 4 h? If the experiments were replicated, then the authors must add, at least, the error bars. In other case, they should be replicated.
• Once the best fit to the pseudo-second-order kinetic model was confirmed, the authors were able to display in a figure the corresponding calculated vs. experimental curves for adsorption capacity with time or calculated vs. experimental adsorption capacities, as preferred.
• To extend the explanation for effects of diffusion stages, authors could fit the experimental data to diffusion intraparticular model or others.
• In addition, authors refer to Figure 10 for explanation about two distinct stages observing the adsorption curves, but in this figure, they show the linear fitting to pseudo-first and pseudo-second order kinetic models. This point needs a revision.
• In the introduction, the authors present a table, Table 1, summarizing adsorption capacities for Pb²⁺ ions using various adsorbents reported in literature. Considering the main objectives of their work, it would be valuable to compare the performance of the materials developed with those more recent published by other authors.

Figures and Tables

Please revise Figs. 1, 4, 6 and 7 with different images which must be clearly labeled as a), b), etc., both within the figure and in the corresponding captions.

Figures 2 and 8 must be placed after their first appearance in text.

In Fig. 3, authors must revise the decimal places, subtituting the commas by dots for correct numeric formatting.

All Y-axis in Figure 10 need units.  In Table 2, the units for kinetic coefficients must be also included, and a reduction of significant figures for these parameters and correlation coefficients must be done.

Author Response

We sincerely thank the reviewer for the thorough and comprehensive evaluation of our manuscript. The comments and recommendations provided have been extremely valuable, as they not only helped us to eliminate inaccuracies but also significantly improved the overall quality of the paper by adding important clarifications and a deeper analysis of the processes under consideration. 

Introduction:

Comments 1: In Table 1, the reference [30] with information about a dye adsorption could be removed since it is the only not referred to lead.

Response 1: We thank the reviewer for this valuable observation. We agree with the suggestion and have removed reference [30] from Table 1, since it refers to dye adsorption and is not directly related to lead removal. The revised Table 1 now only includes references relevant to Pb²⁺ adsorption.

Methods:

Comments 2: After reading the manuscript, since authors have shown results for magnetite in Figure 4 and 5, this material should be included in the section corresponding to Materials.

Response 2: In line with the suggestion, we have revised the Materials section to explicitly include magnetite (Fe₃O₄), as its results are presented in Figures 4 and 5. In addition, a new subsection 2.2 describing the synthesis of magnetite nanoparticles has been added to provide clarity and completeness.

Comments 3: How were the conditions for activation carbon synthesis selected? It would be appropriate that authors could introduce a reference.

Response 3: The synthesis of activated carbon from rice husk for various applications, including energy storage systems, is one of the research directions of our group. In the present work, the synthesis was carried out under optimal conditions that provide high specific surface area values of the material. Subsection 2.1 has been revised to include the corresponding reference [48].

Comments 4: In addition, I appreciate that the authors have specified the exact amounts of reagents and volumes used to prepare the material. In any case, it may be more appropriate to express quantities in terms of concentrations or ratios as in the ultrasonication-assisted process rather than absolute amounts for clarity of the results and comparison with other authors. In any case, this raises a question about the yield at each stage, How much material is lost before obtaining the product? For example, they used a total of 5.8175 g of FeSO₄·7H₂O + FeCl₃·6H₂O + activated carbon, but then they took only 0.232 g of the composite for hydrothermal step. Which is the final weight of h-AC-Fe3O4? The same question about the yield is for ultrasonicated carbon.

Response 4: The description has been corrected in Section 2.3.2. During the preparation of the composites, mass loss during washing and filtration did not exceed 5% of the total mass. The synthesis of the composites was carried out considering the required amount of material for triplicate experiments at each stage. After synthesis, 50 mg portions of each sorbent were taken from the total mass for the adsorption experiments.

Comments 5: Regarding the adsorption experiments, authors differentiate those to evaluate the adsorption performance and the experiments to investigate the kinetic models. Then, would it not be more logical to perform the kinetic experiments until more than 4 h instead of 2 h to verify the equilibrium time and after, to compare the adsorption efficiency at equilibrium time for both materials? Why did they select an initial concentration of 50 mg/L of Pb?

Response 5: The kinetic experiments (Figure 11 in the revised manuscript) demonstrated that all sorbents reached equilibrium before 120 minutes: the us-AC/Fe₃O₄ composite at ~2 h, and the h-AC/Fe₃O₄ and pristine AC even faster. Therefore, a contact time of 4 h was applied in the adsorption capacity experiments to ensure equilibrium was reached for all materials and to allow a correct comparison of maximum adsorption capacities. The initial Pb(II) concentration of 50 mg/L was selected because it reflects realistic levels reported in industrial effluents (e.g., 51.8–414.4 mg/L in wood-processing wastewater [DOI: 10.1007/s11696-022-02302-9]) and (ii) it is widely used in adsorption studies, ensuring comparability with literature results [DOI: 10.1038/s41598-017-09700-5; DOI: 10.1007/s13201-022-01703-6]. This justification has been added in Section 2.4 (Adsorption Performance Test).

Comments 6: In my opinion, the paragraph about the determination of residual Pb concentration should be in the section 2.3 of adsorption test and not in the section 2.4 as it is not strictly a method for material characterization.

Response 6: The paragraph describing the determination of residual Pb concentration has been moved from Section 2.4 to Section 2.3 in the revised manuscript, since Section 2.4 now corresponds to Adsorption Performance Test. 

Results and discussion 

Comments 7: Authors describe in page 7 (Lines 260-261) the nitrogen adsorption–desorption isotherms of the samples (Figure 3a not 4a). However, only the points for the adsorption data can be observed, since the desorption data are either missed or not distinguised. Please, I ask to the authors to verify whether the desorption data are included. This would support the discussion in the text, demonstrating if the hystheresis loop characterizes the form of isotherms as H4.

Response 7: We thank the reviewer for the important comment regarding the adsorption isotherms. To ensure the accuracy and reliability of the reported data, the specific surface area analysis and the adsorption/desorption isotherms were re-measured. The updated isotherms are presented in Figure 9 of the revised manuscript and clearly show distinguishable adsorption and desorption branches.

Comments 8: Upon examining the BET plots, it appears that the slope of the line is very similar for AC and us-AC-Fe3O4, however it is higher for h-AC-Fe3O4. This raises the question of whether the reported difference in surface area for the first carbons can be so high among them, and only a half between both composites with magnetite. Could the authors clarify how the surface area values were derived? Which are the values for volume of nitrogen adsorbed for the monolayer? The fittings seem to be very good.

Response 8: In accordance with the response to the previous comment, the adsorption and desorption isotherms were re-measured, and new BET plots were constructed. The presented data are more accurate and reflect the true values of the specific surface area. Figure 9 in the revised manuscript has been reconstructed to provide clearer and more interpretable profiles. The report on the specific surface area analysis is provided below:

	AC	us-AC/Fe₃O₄	h-AC/Fe₃O₄
BET SSA[m^2/g]:	1818.6165	1007.6166	833.1416
Slope:	0.002390	0.004314	0.005222
Y-Intercept:	0.000004	0.000007	0.000003
Correlation coefficient:	0.998754	0.998678	0.998467
Monolayer volume[cc/g]:	417.765533	231.465775	191.386063
BET C-Value:	575.951660	650.870850	1586.302490
Single BET SSA[m^2/g]:	1711.2714	949.6948	780.7532

Comments 9: Authors indicate that "Following modification with iron oxide (Fe₃O₄) nanoparticles, a significant reduction in nitrogen uptake is observed, indicating a decrease in accessible surface area and/or pore volume (Figure 3a)". However, the surface area values (accesible or not) are already lower to S BET for the pristine carbon. Additionally, if the authors attributed this behavior to size exclusion effects, it would be helpful to specify the approximate size of Pb²⁺ ions and discuss whether the pore dimensions of the modified material are indeed smaller than this threshold. Figure 3a is not for surface area or pore volume. Is it necessary to include Figure 3c? Would be it better to put the range of pore diameter in text?

Response 9: In accordance with the previous comments, the adsorption/desorption isotherms were re-measured using the BET method. It was determined that the specific surface area is 1818.2 m²/g for AC, 1007.2 m²/g for us-AC/Fe₃O₄, and 833.1 m²/g for h-AC/Fe₃O₄. The authors suggest that the key effect of introducing magnetite nanoparticles into the composite structure is not a reduction in pore size, but rather their partial blockage. As a result, the overall number of pores available for effective Pb²⁺ ion sorption decreases. A discussion of the correlation between the actual pore diameter and the ionic diameter of lead has been added to the revised manuscript (p. 13, lines 449-459). The updated isotherm and BET plots, and pore size distribution (Figure 9 in the revised manuscript) are accurate and reflect the actual values of the specific surface area and porosity of the material. 

Comments 10: Related to the previous comment, authors must explain more thoroughly this observation "The pore size distribution plot (Figure 3c) further confirms that the incorporation of iron oxide nanoparticles leads to blockage of the porous structure of the activated carbon". It is not evident from figure.

Response 10: We agree with this comment. Figure 3c (Figure 9c in the revised manuscript) has been updated with new data, making the presented profiles clearer and more readily interpretable. The incorporation of iron oxide nanoparticles reduces the available pore volume, which we attribute to their partial filling and/or blocking of the pores by the nanoparticles.

Comments 11: The manuscript includes some results about the characterization of magnetite nanoparticles in Figure 4 and 5, which are commented in text. However, it is not clear whether this material is for using as a reference or as part of the carbons prepared. In this case, they are prepared in different way, (can be the black precipitate separated as magnetite be considered to have the same purity in both cases?) Moreover, the results for Fe₃O₄ are shown without relation to the other materials. In my opinion, it is important, first to clarify this point, and then doing the discussion properly.

We agree with this comment. Section “2. Materials and Methods” has been revised and expanded to provide a clearer description of the steps involved in obtaining the composites via ultrasonic and hydrothermal treatment. Magnetite synthesized by co-precipitation was used to prepare the composites by mixing with activated carbon followed by ultrasonic or hydrothermal processing. To confirm the structure of the initial magnetite and verify its preservation after modification, additional XRD patterns of both composites were recorded (Figures 6, 8). The results confirmed that the magnetite retained its structure after ultrasonic and hydrothermal treatment.

Comments 12: Were also the magnetic separation checked after adsorption experiments?

Response 12: A photograph of the suspension after adsorption experiments and magnetic separation has been provided in the Supporting Information (Figure S2).

Comments 13: At which value does neutral pH correspond? As I commented before, the pH is a very important parameter influencing on adsorption efficiency, and also, for being done the comparison of adsorption capacities found with those for other adsorbents in the same conditions.

Response 13: We agree that pH is a crucial parameter in adsorption studies. In our experiments, the initial pH of the Pb(NO₃)₂ solution was not adjusted and was measured at 5.7 ± 0.2. In the revised manuscript, the term “neutral pH” has been replaced with this specific value to provide clarity and ensure comparability with other studies (p. 13, lines 466-468).

Comments 14: Kinetic experiments demonstrated that it is not necessary to perform the adsorption experiments at 4 h, For all materials, the use of 1 h is sufficient. Why did the authors decide to perform the experiments at 4 h? If the experiments were replicated, then the authors must add, at least, the error bars. In other case, they should be replicated

Response 14: Kinetic analysis shows that equilibrium is reached within 1–2 h depending on the sorbent, and therefore longer adsorption times are not strictly necessary. However, additional adsorption experiments were performed at 2, 4, and 6 h (Table S1) in order to (i) determine the maximum adsorption capacity and (ii) ensure comparability with literature data, where adsorption times of 3–4 h are commonly used. These tests confirmed that the maximum adsorption capacity is achieved at 4 h. Accordingly, Section 2.3 (Section 2.4 in the revised version) has been revised, and a clarification about the choice of 4 h as the reference contact time has been added.

All adsorption experiments were performed in at least triplicate. Standard deviations were calculated, and the corresponding values have been added in the revised manuscript (p. 14, lines 492-494) to reflect data variability.

Comments 15: Once the best fit to the pseudo-second-order kinetic model was confirmed, the authors were able to display in a figure the corresponding calculated vs. experimental curves for adsorption capacity with time or calculated vs. experimental adsorption capacities, as preferred.

Response 15: We agree with this comment. For a more comprehensive interpretation of the results, additional calculations were performed using the Weber–Morris, Bangham, Elovich, and Boyd models, and adsorption isotherms were constructed according to the Langmuir and Freundlich models. The calculations showed that the maximum adsorption capacity (qₘₐₓ) for activated carbon is 63.69 mg/g, while for the us-AC/Fe₃O₄ and h-AC/Fe₃O₄ composites it is 41.32 and 24.16 mg/g, respectively. These results have been incorporated into the “Results and Discussion” section.

Comments 16: To extend the explanation for effects of diffusion stages, authors could fit the experimental data to diffusion intraparticular model or others.

Response 16: In accordance with the previous comment, the additional calculations have been performed and incorporated into the manuscript.

Comments 17: In addition, authors refer to Figure 10 for explanation about two distinct stages observing the adsorption curves, but in this figure, they show the linear fitting to pseudo-first and pseudo-second order kinetic models. This point needs a revision.

Response 17: We agree with this comment. To confirm the multistage nature of the sorption process, calculations were carried out using the Weber–Morris model. The results indicate that adsorption occurs not only on the sorbent surface at the initial stage but is also limited by intraparticle diffusion at later stages of the process.

Comments 18: In the introduction, the authors present a table, Table 1, summarizing adsorption capacities for Pb²⁺ ions using various adsorbents reported in literature. Considering the main objectives of their work, it would be valuable to compare the performance of the materials developed with those more recent published by other authors.

Response 18: We have included a comparison of the obtained results with the data reported in the literature (p. 22, Table 5).

Figures and Tables

Comments 19: Please revise Figs. 1, 4, 6 and 7 with different images which must be clearly labeled as a), b), etc., both within the figure and in the corresponding captions.

Response 19: Figures 1, 7 (and 4, 6 (Figures 3, 5 in the revised manuscript)) have been revised with improved images. Each subfigure is now clearly labeled as (a), (b), etc., within the figure itself, and the corresponding captions have been updated accordingly.

Comments 20: Figures 2 and 8 must be placed after their first appearance in text.

Response 20: Figures have been relocated to appear immediately after their first mention in the text.

Comments 21: In Fig. 3, authors must revise the decimal places, subtituting the commas by dots for correct numeric formatting.

Response 21: In Figure 3 (Figures 9 in the revised manuscript), the decimal formatting has been revised by substituting commas with dots for consistency and correctness. 

Comments 22: All Y-axis in Figure 10 need units.  In Table 2, the units for kinetic coefficients must be also included, and a reduction of significant figures for these parameters and correlation coefficients must be done.

Response 22: The Y-axes in Figure 10 (Figure 11 in the revised manuscript) have been revised to include the appropriate units. In Table 2, the units for the kinetic coefficients have been added, and the number of significant figures for both the parameters and the correlation coefficients has been reduced for clarity and consistency.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled “Mechanistic Evaluation of Pb(II) Adsorption on Magnetic Activated Carbon/Fe₃O₄ Composites: Influence of Hydrothermal and Ultrasonic Synthesis Routes” is a relevant contribution for C. However, from my perspective, this manuscript needs major changes to be considered for publication.

The manuscript entitled “Mechanistic Evaluation of Pb(II) Adsorption on Magnetic Activated Carbon/Fe₃O₄ Composites: Influence of Hydrothermal and Ultrasonic Synthesis Routes” is a relevant contribution for C. However, from my perspective, this manuscript needs major changes to be considered for publication.

In Introduction:

It is important to emphasize and highlight the relevance of the research presented in this work.

In Results:

Regarding the discussion of textural properties:

i) In Figure 3a, it is necessary to present the complete nitrogen adsorption-desorption isotherms. Furthermore, in the discussion, the authors state, “The nitrogen adsorption-desorption isotherms of the samples corresponded to Type Ib with an H4 hysteresis loop (Figure 4a), suggesting the predominance of wider micropores and narrow mesopores with slit-shaped geometry and high specific surface area.” Since the complete isotherms are not presented, this discussion lacks foundation.

ii) The caption of Figure 3a incorrectly references the isotherms.

iii) Based on Figure 3b, the relative pressure range used to apply the BET method for each sample has not been correctly selected, as it should follow the Rouquerol criteria recommended by IUPAC 2015.

iv) The pore size distributions presented in Figure 3c lack a clear profile from which pore size information can be derived. What method or model was used to determine these distributions?

Regarding the Pb2+ adsorption tests:

i) At what pH were the Pb2+ removal tests conducted? What were the final pH values after the study? Was there significant variation between the initial and final pH that could alter the discussion? What is the point of zero charge (PZC) of the materials? The performance of the materials largely depends on this property. It is necessary to present the PZC of the materials or, at the very least, the effect of pH on the Pb2+ removal capacity.

ii) Why was an initial Pb2+ concentration of 50 mg/L selected? Why were Pb2+ adsorption isotherms not determined?

iii) The reported removal capacity of activated carbon (42 mg/g) is underestimated, as according to Figure 9, this material achieved a 100% removal percentage, indicating that this adsorbent is not saturated.

iv) The results presented in Table 2 contain too many numerical figures. What are the units of each parameter? The units of the parameters in Table 2 should be added.

v) Based on the results presented, what is the role of the textural characteristics of these materials (e.g., specific surface area) in their Pb2+ removal performance? It would be important to conduct a textural characterization of these materials or, alternatively, explain why the authors consider it unnecessary.

Author Response

We sincerely thank the reviewer for the thorough and comprehensive evaluation of our manuscript. The comments and recommendations provided have been extremely valuable, as they not only helped us to eliminate inaccuracies but also significantly improved the overall quality of the paper by adding important clarifications and a deeper analysis of the processes under consideration.

Comments 1: It is important to emphasize and highlight the relevance of the research presented in this work.

Response 1: We agree with this comment. The Introduction has been revised and expanded to better emphasize and highlight the relevance of the research presented in this work.

Comments 2: In Figure 3a, it is necessary to present the complete nitrogen adsorption-desorption isotherms. Furthermore, in the discussion, the authors state, “The nitrogen adsorption-desorption isotherms of the samples corresponded to Type Ib with an H4 hysteresis loop (Figure 4a), suggesting the predominance of wider micropores and narrow mesopores with slit-shaped geometry and high specific surface area.” Since the complete isotherms are not presented, this discussion lacks foundation. 

Response 2: We agree with this comment. The adsorption isotherms were re-measured using the BET method for all three investigated sorbents. The complete adsorption/desorption isotherms are now provided in Figure 9 of the revised manuscript.

Comments 3: The caption of Figure 3a incorrectly references the isotherms. 

Response 3: We agree with the comment. The captions for Figure 3a (Figures 9а in the revised manuscript) have been corrected.

Comments 4: Based on Figure 3b, the relative pressure range used to apply the BET method for each sample has not been correctly selected, as it should follow the Rouquerol criteria recommended by IUPAC 2015.

Response 4: We thank the reviewer for this important remark. The BET isotherms were remeasured for all investigated sorbents using the appropriately selected relative pressure range in accordance with the criteria proposed by Rouquerol for the BET equation. This valuable comment allowed us to remove incorrect data from the manuscript and thereby improve the reliability of the presented results.

Comments 5:  The pore size distributions presented in Figure 3c lack a clear profile from which pore size information can be derived. What method or model was used to determine these distributions? 

Response 5: We fully agree with this comment. It has allowed us to correct the errors made in the analysis of adsorption/desorption isotherms using the BET method. In the revised version of the manuscript, the updated pore size distribution plot is presented in Figure 3C.

Comments 6:  At what pH were the Pb2+ removal tests conducted? What were the final pH values after the study? Was there significant variation between the initial and final pH that could alter the discussion? What is the point of zero charge (PZC) of the materials? The performance of the materials largely depends on this property. It is necessary to present the PZC of the materials or, at the very least, the effect of pH on the Pb2+ removal capacity.

Response 6: The reviewer’s comments and questions were taken into account during the revision of the manuscript. The Pb²⁺ removal experiments were carried out without adjusting the initial solution pH. The actual pH value of the solution with a Pb²⁺ concentration of 50 mg/L was 5.7. These data are provided in Section 3 (pp. 13, lines 466-68). Information on the changes in solution pH after adsorption is also presented in Section 3 (pp. 13, lines 469-470). In addition, the points of zero charge (PZC) were determined for all three investigated composites. The pH values corresponding to the PZC are reported in Section 3 (pp. 13, lines 473-484), while the plots illustrating the determination of PZC are included in the Supporting Information. The pH undoubtedly has a significant effect on the sorption properties of the materials, particularly activated carbon. However, conducting adsorption studies at high pH values (alkaline medium) is not reasonable, as the formation of poorly soluble lead hydroxide precipitate may distort the true adsorption capacity values. On the other hand, additional acidification of the solution leads to competing effects between positively charged lead ions and protons, which also complicates the correct interpretation of the results. As shown in the studies [https://doi.org/10.3390/molecules29112489; DOI: 10.1016/j.arabjc.2016.04.017], conducting adsorption experiments at high pH values leads to distorted data on lead ion removal due to the formation of poorly soluble lead hydroxide.

Comments 7:  Why was an initial Pb2+ concentration of 50 mg/L selected? Why were Pb2+ adsorption isotherms not determined?

Response 7: We agree with this comment. The initial Pb2+ concentration of 50 mg/L was deliberately chosen based on two primary considerations to ensure both the practical relevance and the scientific comparability of our study. Firstly, we aimed to use a concentration that is representative of contaminated industrial wastewater. While Pb(II) concentrations can vary significantly depending on the industrial source, the chosen value of 50 mg/L falls within the range reported for highly polluted effluents. For instance, a recent review by Wang et al. indicates that wastewater from the wood processing industry can contain Pb(II) in a range of 51.8–414.4 mg/L [DOI: 10.1007/s11696-022-02302-9]. Thus, our selected concentration of 50 mg/L represents a challenging yet realistic scenario for water treatment, rather than an artificially high or low value. Secondly, the concentration of 50 mg/L is widely employed in the literature for evaluating the performance of various adsorbents. This allows for a direct and meaningful comparison of our materials' efficiency with previously published data. This is exemplified by the work [DOI: 10.1038/s41598-017-09700-5], who also used a 50 mg/L solution to simulate industrial conditions. Furthermore, a comprehensive review on lead removal confirms that 50 mg/L is a frequently used concentration in adsorption studies [DOI: 10.1007/s13201-022-01703-6]. Thus, our choice of 50 mg/L is a well-justified compromise that ensures our results are both relevant to practical environmental challenges and comparable to the existing body of scientific work in this field. We have added this detailed justification to the 2.4. Adsorption Performance Test section p. 5, lines 194-203 of our revised manuscript for clarity.

The adsorption isotherms of Pb²⁺ ions at various initial concentrations were obtained. The detailed experimental procedure is described in Section 2.4 (pp. 5-6, lines 218-226). The corresponding plots and calculated adsorption isotherm parameters according to the Langmuir and Freundlich models are presented in Section 3 (pp. 19-21, lines 682-751).

Comments 8: The reported removal capacity of activated carbon (42 mg/g) is underestimated, as according to Figure 9, this material achieved a 100% removal percentage, indicating that this adsorbent is not saturated. 

Response 8: We fully agree with this comment. From the adsorption isotherm analysis, it was determined that the maximum sorption capacity of activated carbon was 63.69 mg/g according to the calculations. This information is provided in Table 4 (p. 20).

Comments 9: The results presented in Table 2 contain too many numerical figures. What are the units of each parameter? The units of the parameters in Table 2 should be added.

Response 9: We agree with this comment. The numerical values have been rounded to four decimal places, and the corresponding measurement units have been specified in the text.

Comments 10: Based on the results presented, what is the role of the textural characteristics of these materials (e.g., specific surface area) in their Pb2+ removal performance? It would be important to conduct a textural characterization of these materials or, alternatively, explain why the authors consider it unnecessary.

Response 10: We agree that textural characteristics, such as specific surface area and pore distribution, have a significant effect on the sorption capacity of the materials. In the present work, these parameters were investigated using low-temperature nitrogen adsorption (BET analysis). The corresponding values of specific surface area and pore distribution are provided in Section 3 (pp. 12-13, lines 426-459) and in Figure 9. These results demonstrate that the decrease in specific surface area after modification of the carbon material is accompanied by a reduction in its sorption capacity toward Pb²⁺. Thus, the textural characteristics are directly related to the efficiency of lead ion removal and have been taken into account in the discussion of the obtained data.

Author Response File: Author Response.pdf

Reviewer 3 Report

The innovative points of this study are as follows:

（1）A comparative investigation of hydrothermal and ultrasonic synthesis routes for AC/Fe₃O₄ composites was conducted for the first time, revealing that ultrasonic treatment preserved porosity and achieved 92.84% Pb(II) removal, rivaling pristine AC with 99.0%.

（2）The us-AC/Fe₃O₄ magnetically separable composite was synthesized via ultrasonication, which exhibited near-pristine AC adsorption capacity 39.15 mg/g while enabling rapid magnetic recovery within 300 seconds.

（3）A dual adsorption mechanism was identified through kinetic modeling，physisorption dominated initial stages, while chemisorption controlled later phases, attributed to pore diffusion and surface complexation.

The specific modification suggestions are as follows:

 (1)The ultrasonication parameters such as frequency mode, power density were incompletely specified in Section 2.2.2. So the clarification is required to ensure reproducibility. 

(2) Figure 4 , Fe₃O₄ morphology was referenced prematurely in Page 8 before its introduction in Page 9. Sequential numbering and contextual descriptions need to be revised.

(3) Equations for pseudo-second-order kinetics were omitted in Section 3.2 (Page 12). These should be added as:

           t/Q_t=(1/k₂ Q_e²)+ t/Q_e​

(4) The correlation between Fe₃O₄ distribution and chemisorption in Page 13 was inadequately explained. The references to surface defects/O-functional groups such as Raman/BET data should be incorporated.

Author Response

We sincerely thank the reviewer for the thorough and comprehensive evaluation of our manuscript. The comments and recommendations provided have been extremely valuable, as they not only helped us to eliminate inaccuracies but also significantly improved the overall quality of the paper by adding important clarifications and a deeper analysis of the processes under consideration. 

Comments 1: The ultrasonication parameters such as frequency mode, power density were incompletely specified in Section 2.2.2. So the clarification is required to ensure reproducibility.

Response 1: The missing ultrasonication parameters, including frequency mode and power density, have been specified in Section 2.2.2 (Section 2.3.1 in the revised manuscript) of the revised manuscript. The corresponding lines (p. 4, lines 161-164) now contain this information to ensure reproducibility.

Comments 2: Figure 4 , Fe₃O₄ morphology was referenced prematurely in Page 8 before its introduction in Page 9. Sequential numbering and contextual descriptions need to be revised.

Response 2: The sequential numbering and contextual descriptions have been revised. The reference to Fe₃O₄ morphology now appears in the correct order.

Comments 3: Equations for pseudo-second-order kinetics were omitted in Section 3.2 (Page 12). These should be added as:

           t/Qt=(1/k2 Qe2)+ t/Qe.

Response 3: The equations for the pseudo-first-order and pseudo-second-order kinetics have been added in the corresponding lines 509-511 of the revised manuscript. 

Comments 4: The correlation between Fe₃O₄ distribution and chemisorption in Page 13 was inadequately explained. The references to surface defects/O-functional groups such as Raman/BET data should be incorporated.

Response 4: We agree with this comment. The explanations of the physical and chemical sorption stages were provided in the discussion of the Weber–Morris intraparticle diffusion model. In addition, the points of zero charge (pHₚ_zc) were determined for the investigated sorbents to characterize the surface functional groups responsible for the surface charge. The BET isotherms were remeasured for all investigated sorbents. The corresponding values of specific surface area and pore distribution are provided in Section 3 (pp. 12, lines 430-432, 447-452) and in Figure 9. These results demonstrate that the decrease in specific surface area after modification of the carbon material is accompanied by a reduction in its sorption capacity toward Pb²⁺. Thus, the textural characteristics are directly related to the efficiency of lead ion removal and have been taken into account in the discussion of the obtained data (p. 13, lines 452-455).

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript entitled “Mechanistic Evaluation of Pb(II) Adsorption on Magnetic Activated Carbon/Fe₃O₄ Composites: Influence of Hydrothermal and Ultrasonic Synthesis Routes” is a relevant contribution for C. However, from my perspective, this manuscript needs major changes to be considered for publication.

a) Regarding the discussion of textural properties:

1. The authors mentioned the BET as an analysis (BET surface area analysis demonstrated that the….), but BET is an equation, it is not a characterization technique, the technique is N2 adsorption-desorption at 77 K.
2. Based on Figure 9b, the relative pressure range used to apply the BET method for each sample has not been correctly selected, as it should follow the Rouquerol criteria recommended by IUPAC 2015. For example, in this type of material, the BET method must be applied to values of relative pressures greater than 0.05.
3. The authors employed the Barrett–Joyner–Halenda (BJH) method to determine the pore size distribution of micropores, which is inappropriate because this method is designed for mesopores. However, the materials studied are predominantly microporous, as evidenced by Figure 9c. To accurately analyze the pore size distribution, the authors should use a method suitable for micropores, such as the Horvath-Kawazoe method, which is available for various pore geometries (e.g., slit, cylindrical, and spherical). The authors must present the micropore size distribution, accompanied by a detailed discussion and comparison with relevant results reported in the scientific literature.

b) Figure 10 shows the adsorption kinetics of Pb²⁺, which lacks data points at short times that would allow the application of different kinetic models and their corresponding fittings. Without these data points, the analysis lacks reliability since most of the experimental points were taken at equilibrium. This issue needs to be clarified. Consequently, it is evident that in Figures 11–14, the models were fitted using data points where the material is already fully saturated with Pb²⁺, rather than in the region where the amount of adsorbed Pb²⁺ increases with time (i.e., at short times). This renders the presented fittings physically meaningless and not representative of a proper kinetic analysis.

c) Regarding the fittings of the Pb²⁺ adsorption isotherms shown in Figure 15, it is necessary and important to first present the adsorption isotherms (qₑ Cₑ).

The manuscript entitled “Mechanistic Evaluation of Pb(II) Adsorption on Magnetic Activated Carbon/Fe₃O₄ Composites: Influence of Hydrothermal and Ultrasonic Synthesis Routes” is a relevant contribution for C. However, from my perspective, this manuscript needs major changes to be considered for publication.

a) Regarding the discussion of textural properties:

1. The authors mentioned the BET as an analysis (BET surface area analysis demonstrated that the….), but BET is an equation, it is not a characterization technique, the technique is N2 adsorption-desorption at 77 K.
2. Based on Figure 9b, the relative pressure range used to apply the BET method for each sample has not been correctly selected, as it should follow the Rouquerol criteria recommended by IUPAC 2015. For example, in this type of material, the BET method must be applied to values of relative pressures greater than 0.05.
3. The authors employed the Barrett–Joyner–Halenda (BJH) method to determine the pore size distribution of micropores, which is inappropriate because this method is designed for mesopores. However, the materials studied are predominantly microporous, as evidenced by Figure 9c. To accurately analyze the pore size distribution, the authors should use a method suitable for micropores, such as the Horvath-Kawazoe method, which is available for various pore geometries (e.g., slit, cylindrical, and spherical). The authors must present the micropore size distribution, accompanied by a detailed discussion and comparison with relevant results reported in the scientific literature.

b) Figure 10 shows the adsorption kinetics of Pb²⁺, which lacks data points at short times that would allow the application of different kinetic models and their corresponding fittings. Without these data points, the analysis lacks reliability since most of the experimental points were taken at equilibrium. This issue needs to be clarified. Consequently, it is evident that in Figures 11–14, the models were fitted using data points where the material is already fully saturated with Pb²⁺, rather than in the region where the amount of adsorbed Pb²⁺ increases with time (i.e., at short times). This renders the presented fittings physically meaningless and not representative of a proper kinetic analysis.

c) Regarding the fittings of the Pb²⁺ adsorption isotherms shown in Figure 15, it is necessary and important to first present the adsorption isotherms (qₑ Cₑ).

Author Response

We sincerely thank the reviewer for the constructive and detailed comments. The remarks not only improved the manuscript but also drew our attention to important aspects that we have now addressed in a more comprehensive manner. This feedback provided valuable perspectives that enhanced both the scientific rigor and the overall readability of the paper. 

a) Regarding the discussion of textural properties:

Comment a1: The authors mentioned the BET as an analysis (BET surface area analysis demonstrated that the…), but BET is an equation, it is not a characterization technique, the technique is N2 adsorption-desorption at 77 K.

Response a1: We agree with the remark. The wording in Section 2.5 (p. 6, lines 269–272) and Section 3 (p. 12, lines 422–434) has been revised: it is now specified that N₂ adsorption–desorption measurements at 77 K were performed, and the surface area was calculated using the BET equation.

Comment a2: Based on Figure 9b, the relative pressure range used to apply the BET method for each sample has not been correctly selected, as it should follow the Rouquerol criteria recommended by IUPAC 2015. For example, in this type of material, the BET method must be applied to values of relative pressures greater than 0.05.

Response a2: We agree with the remark. The criteria described in DOI: 10.1515/pac-2014-1117 were reviewed. Figure 9b in the previous version was included only to illustrate the applicability of the linear section; it has been removed in the revised manuscript. The BET surface area was calculated by the instrument software within the relative pressure range P/P₀ ≥ 0.05, in accordance with the IUPAC (2015) recommendations. For all three samples, the C-value was positive, and the BET plot retained linearity over the selected interval.

Comment a3: The authors employed the Barrett–Joyner–Halenda (BJH) method to determine the pore size distribution of micropores, which is inappropriate because this method is designed for mesopores. However, the materials studied are predominantly microporous, as evidenced by Figure 9c. To accurately analyze the pore size distribution, the authors should use a method suitable for micropores, such as the Horvath-Kawazoe method, which is available for various pore geometries (e.g., slit, cylindrical, and spherical). The authors must present the micropore size distribution, accompanied by a detailed discussion and comparison with relevant results reported in the scientific literature.

Response a3: We fully agree with the remark. The pore size distribution for the microporous materials was recalculated using the Horvath–Kawazoe method. The corresponding plot is included in the revised manuscript (Fig. 9b), and the description has been updated and supplemented with appropriate literature references (Section 3, pp. 12–13, lines 436–457).

Comment b: Figure 10 shows the adsorption kinetics of Pb²⁺, which lacks data points at short times that would allow the application of different kinetic models and their corresponding fittings. Without these data points, the analysis lacks reliability since most of the experimental points were taken at equilibrium. This issue needs to be clarified. Consequently, it is evident that in Figures 11–14, the models were fitted using data points where the material is already fully saturated with Pb²⁺, rather than in the region where the amount of adsorbed Pb²⁺ increases with time (i.e., at short times). This renders the presented fittings physically meaningless and not representative of a proper kinetic analysis.

Response b: The authors thank the reviewer for the fair remark and agree that, for a more reliable kinetic analysis, it is desirable to have experimental points in the short-time region. However, in our study the minimum registration time was 5 minutes, which is determined by the specifics of both the material and the procedure. The sorbent used is magnetic, and at the initial stage a very rapid interaction with the solution is observed, followed by nearly instantaneous separation under the magnetic field. Obtaining reliable measurements within the first seconds is practically infeasible, while shorter sampling intervals lead to high error and poor reproducibility. It should be emphasized that a similar approach is widely used in the literature on magnetic adsorbents (e.g. https://doi.org/10.1016/j.isci.2025.113266), where kinetic studies also start from 5 minutes or longer; other publications (https://doi.org/10.1016/j.sajce.2023.06.007; https://doi.org/10.1016/j.dwt.2025.101099; https://doi.org/10.1016/j.nxmate.2025.101061; https://doi.org/10.1016/j.chphi.2023.100181; https://doi.org/10.1039/d3ra06244a; https://doi.org/10.1016/j.arabjc.2024.106067) likewise employ comparable time windows. Therefore, although the absence of very short sampling intervals is acknowledged as a limitation, the obtained data remain valid, and the kinetic models were fitted within the growth region up to the equilibrium plateau, which is consistent with established practice for such materials.

Comment c: Regarding the fittings of the Pb²⁺ adsorption isotherms shown in Figure 15, it is necessary and important to first present the adsorption isotherms (qₑ Cₑ).

Response c: The remark is accepted. The corresponding adsorption isotherms (qₑ vs Cₑ) have been added to the Supporting Information (Figure S6) and included in the main text on p. 20 (lines 732–734).

Author Response File: Author Response.pdf