One-Pot Synthesis of Chitosan/Layered Double Hydroxide Composite and Its Sorption Properties Toward Hexavalent Chromium

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present the preparation of a composite consisting of layered double hydroxides (LDH) and chitosan with the aim of removing chromium from aqueous media.

The characterization of the composites include i.a. 1H NMR, FTIR, X-ray diffraction and determination of sorption capacity and kinetics – both for pure chromate solutions and in simulated groundwater as well as the reusability. The characterisation of the composite is always compared to results of the same experiments performed on the pure LDH and chitosan. The methods are clearly described and the results thoroughly discussed, also evaluating competing mathematical models to describe the observations, and the conclusion.

The only part that seemingly is less well performed is testing of the reusability of the composite. The composite is subjected to a series of sorption/desorption, but since the desorption process don’t seem to work (only 3-5% removed), it is hard to understand how these experiments can indicate anything about the reusability of the composite. Paragraph 3.7 therefore has to be completely rewritten or omitted all together.

Although one could always appreciate if further evaluation of the composition (e.g. varying ratio of chitosan:LDH) and characterisation of the composite (e.g. determination of particle size or surface area/porosity) had been performed, the study serves as a good “proof of concept” and is in my opinion sufficient to warrant publication.

I therefore have only a few minor comments:

There are some appearances of Russian text that should be translated (line 58-59 and 203-204)

The alpha in the equation in line 195 should be an exponent.

In line 382 the legend for B and C are the same (B should probably be pseudo first-order)

In line 483 the legend to fig. 13 should specify the meaning of the red/grey colours.

Author Response

The authors are sincerely grateful to Editor and Reviewers for their unselfish and extremely important work, for thoroughly checking the manuscript, for valuable comments and advices, which have significantly improved the starting text of the submitted manuscript.

The characterization of the composites include i.a. 1H NMR, FTIR, X-ray diffraction and determination of sorption capacity and kinetics – both for pure chromate solutions and in simulated groundwater as well as the reusability. The characterisation of the composite is always compared to results of the same experiments performed on the pure LDH and chitosan. The methods are clearly described and the results thoroughly discussed, also evaluating competing mathematical models to describe the observations, and the conclusion.

- We are deeply grateful to the reviewer for the very positive, thoughtful, and encouraging evaluation of our manuscript. We sincerely appreciate that the reviewer recognized the value of the study as a proof of concept, the clarity of the methods, the comparison of the composite with the individual starting components, and the discussion of the experimental results, including the mathematical treatment of sorption equilibria and kinetics. Such a careful and balanced assessment is extremely valuable to us. We are also grateful for the reviewer’s constructive comments, which helped us improve several important points in the revised version.

The only part that seemingly is less well performed is testing of the reusability of the composite. The composite is subjected to a series of sorption/desorption, but since the desorption process don’t seem to work (only 3-5% removed), it is hard to understand how these experiments can indicate anything about the reusability of the composite. Paragraph 3.7 therefore has to be completely rewritten or omitted all together.

- We thank the reviewer for pointing out this important issue. We agree that the original presentation of the reusability experiment was not sufficiently clear and could lead to misunderstanding. In particular, the distinction between chromate uptake during the sorption step and chromate release during the NaOH desorption step was not explained clearly enough, and the figure caption did not explicitly define the meaning of the plotted values. In the old version we failed to correctly convey our idea that desorption is very high, 3-5% sometimes a little more remains, and everything else is desorbed (from 97.3% to 90.3%). We have therefore completely rewritten Section 3.7. In the revised version, we clearly define the experimental logic of the sorption/desorption cycles and distinguish between two parameters: chromate uptake during the sorption step and desorption efficiency during treatment with 1 M NaOH. The experiment shows that the composite retained complete chromate uptake during five cycles under the tested conditions, whereas desorption was not complete and gradually decreased from 97.3% to 90.3%. Thus, the experiment is now interpreted more cautiously as a primary operational reusability test rather than as proof of complete regeneration or full post-cycling structural stability. We have also avoided overclaiming the result. The revised paragraph now states that the composite retains sorption performance under the selected cycling conditions, while more detailed post-cycling structural analysis would be required to confirm complete structural stability after repeated use. The caption of Figure 13 has also been revised to define the meaning of the colors and plotted values more explicitly.

Although one could always appreciate if further evaluation of the composition (e.g. varying ratio of chitosan:LDH) and characterisation of the composite (e.g. determination of particle size or surface area/porosity) had been performed, the study serves as a good “proof of concept” and is in my opinion sufficient to warrant publication.

- We are sincerely grateful to the reviewer for this positive and encouraging assessment. We fully agree that additional optimization of the chitosan:LDH ratio and further characterization of the composite, including particle-size analysis, surface area, and porosity measurements, would be valuable. However, as the reviewer correctly noted, the present manuscript was designed primarily as a proof-of-concept study demonstrating the possibility of one-pot in situ formation of an LDH phase within a chitosan matrix and evaluating the resulting material as a chromate sorbent. To avoid overstating the scope of the present work, we have clarified in the revised manuscript that further optimization of the composite composition and more detailed structural/textural characterization should be considered as directions for future research.

I therefore have only a few minor comments:

There are some appearances of Russian text that should be translated (line 58-59 and 203-204)

- We thank the reviewer for noticing these remaining non-English fragments. The indicated Russian text fragments have been removed or translated into English in the revised manuscript. We have also checked the manuscript again to eliminate any remaining Russian comments or placeholders.

The alpha in the equation in line 195 should be an exponent.

- We thank the reviewer for this correction. The equation has been corrected so that α is presented as an exponent in the Mark-Houwink-Sakurada equation.

In line 382 the legend for B and C are the same (B should probably be pseudo first-order)

- We thank the reviewer for carefully checking the figure caption. The reviewer is correct: the legend contained an error, and panels B and C were incorrectly described in the same way. We have corrected the caption of Figure 12. Panel B is now identified as the pseudo-first-order fit, and panel C as the pseudo-second-order fit.

In line 483 the legend to fig. 13 should specify the meaning of the red/grey colours.

- We thank the reviewer for this helpful comment. We agree that the caption of Figure 13 was insufficiently informative. The caption has been revised to specify the meaning of the red and grey colors. The corrected legend now explicitly distinguishes the chromate uptake during the sorption step from the desorption efficiency during treatment with NaOH solution. This revision also helps prevent misinterpretation of the reusability experiment.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper the authors used chitosan and LDH to obtain a composite capable of absorbing chromium from water. The whole idea is interesting, moreover, they are proposing a Sigle step method for its production. Even if the results are attractive, the study must be improved. Unfortunately, I cannot recommend accepting it in the actual form. My comments are below.

1 Introduction, line 46-48, you claimed that amount of D-glucosamine is over 60 %. It is better to specify that it may very depend on it DD and to give a range ex. Between 60-90% for example.

2 I am not sure that chitosan is the most common biopolymer after cellulose, first, chitin is the biopolymer and chitosan is a derivate of chitin. Moreover, it is not in top 3 polysaccharides. You should reconsider this statement.

3 Line 58-59 – you forgot to translate the phrase from Russian to English

4 In the IR spectrum please mark the bands

5 for NMR spectrum of chitosan please indicate the integrals and mark the peaks. Insert the entire spectrum too

6, Figure 3, If [Ac] is indicated, what is the purpose of CH3?

7 The determination method of DD and molar mass should be moved in method section. It is not a new approach, so it is better to indicate it there. Chitosan characterization can be briefly described in results.

8 Pay attention to the formulas, there are letters from Cyrillic alphabet (line 215)

9 Line 219-222 it must be reformulated. It is not cursive.

10 LDH synthesis must be moved to methods section

11 For composites preparation the preparation method must be indicated in method section too and only the results to be discussed.

12 In Figure 6 please indicate the diffraction bands. Figure 7 – the wavelengths are not visible. Make them bigger or mark them. None of them have legend.

 13 The TGA and DSC are not explained. Please explain why the in TGA analysis the mass increase till 100 ^oC? Acording to DSC, there are no reactions. Also it is necessary to explain the degradation process of the composites and to compare with the chitosan sample without LDH. Put the legend too.

14 The LDH qualitative presence was demonstrated, but no quantitative detrmination of it in the composite. It is necessary to confirm the amount of the LDH.

15 Line 284-286 indicate the reference

16  Figure 9 10 11 12 and 13 must have legend.

17 The determination method for Qmax must be indicated. How it was done?

 It will be better to use a direct determination method to evaluate the absorbed quantity. Atom absorption for example or something else.

18 Regarding the heterogeny nature of the obtained composites, how can it be controled? Is it possible or will it vary at each production trial?

19 How were the kinetic tests done? What analytical method was use, what instrument?

20 Is this composite good only for chromium or it can be applied for other metals?mIf so, dose it present selectivity?

 21 Why didn’t you compared you results with other studies on the chromium absorbency and reusability?

22 The stability of the composite should be checked too

 

I hope it will help improve the manuscript.

Best regards.

Author Response

The authors are sincerely grateful to Editor and Reviewers for their unselfish and extremely important work, for thoroughly checking the manuscript, for valuable comments and advices, which have significantly improved the starting text of the submitted manuscript.

1 Introduction, line 46-48, you claimed that amount of D-glucosamine is over 60 %. It is better to specify that it may very depend on it DD and to give a range ex. Between 60-90% for example.

- Thank you for this helpful comment. We agree that the amount of D-glucosamine units in chitosan is not fixed and depends on the degree of deacetylation (DD). We have revised the corresponding sentence in the Introduction to clarify this point and to indicate a representative range.

2 I am not sure that chitosan is the most common biopolymer after cellulose, first, chitin is the biopolymer and chitosan is a derivate of chitin. Moreover, it is not in top 3 polysaccharides. You should reconsider this statement.

- We agree that the previous statement was not sufficiently precise. Chitosan is a partially deacetylated derivative of chitin, whereas chitin is commonly referred to as one of the most abundant natural polysaccharides. Therefore, we have revised the sentence to avoid an inaccurate ranking of chitosan among biopolymers.

3 Line 58-59 – you forgot to translate the phrase from Russian to English

- Thank you very much, we have corrected this omission.

4 In the IR spectrum please mark the bands

- Thank you very much, we marked the bands in the IR spectrum.

5 for NMR spectrum of chitosan please indicate the integrals and mark the peaks. Insert the entire spectrum too

- We thank the reviewer for the advice and have done everything in the revised version of the manuscript according to the reviewer's advice.

6, Figure 3, If [Ac] is indicated, what is the purpose of CH3?

- The reviewer is absolutely correct; CH3 is meaningless, since the methyl group is a component of Ac. We have amended Figure 2.

7 The determination method of DD and molar mass should be moved in method section. It is not a new approach, so it is better to indicate it there. Chitosan characterization can be briefly described in results.

- We fully agree with the reviewer and have corrected this oversight in the revised version of the manuscript.

8 Pay attention to the formulas, there are letters from Cyrillic alphabet (line 215)

- We are very grateful to the reviewer for his careful and thorough reading of the manuscript. We have corrected this error.

9 Line 219-222 it must be reformulated. It is not cursive.

- In our format, the text corresponding to these lines displays correctly. Perhaps there was an error opening the file?

10 LDH synthesis must be moved to methods section

- We thank the reviewer for this comment, which helped us improve the consistency and organization of the manuscript. The sentence directly related to the preparative procedure “Mg/Fe LDH was prepared using the standard unified [31] method, differing only in that we used 12.82 g Mg(NO₃)₂×6H₂O instead of 19.23 g in order to achieve a Mg/Fe ratio of 2:1” has been moved to the Experimental section. The brief discussion of the synthesis results, namely the characterization of the obtained LDH, has been retained in the Results and Discussion section.

11 For composites preparation the preparation method must be indicated in method section too and only the results to be discussed.

- We fully agree with the reviewer and we have made appropriate changes in the revised version of the manuscript.

12 In Figure 6 please indicate the diffraction bands. Figure 7 – the wavelengths are not visible. Make them bigger or mark them. None of them have legend.

- We thank the reviewer for this helpful comment. We have revised Figures 6 and 7 accordingly. In Figure 6, the characteristic diffraction reflections have been indicated. In Figure 7, the wavenumber labels have been enlarged to improve readability, and the LDH characteristic bands have been additionally marked. Legend has also been added to Figure 6.

 13 The TGA and DSC are not explained. Please explain why the in TGA analysis the mass increase till 100 oC? Acording to DSC, there are no reactions. Also it is necessary to explain the degradation process of the composites and to compare with the chitosan sample without LDH. Put the legend too.

- We agree that the TGA/DSC data required a more detailed explanation. We have expanded the discussion of Figure 8 in the revised manuscript. In particular, we clarified that the apparent mass increase observed below 100 °C is not associated with a chemical process in the sample, but is  caused by instrumental effects at the initial stage of the TG experiment, such as baseline stabilization and buoyancy effects. This is a completely normal effect and there is nothing extraordinary about this instrumental effect. Therefore, this region was not interpreted as a real mass-gain process.

We also agree that the DSC curve does not show sharp peaks that could be assigned to individual well-resolved reactions. The revised text now states that the DSC signal reflects broad and overlapping thermal processes accompanying the degradation of the organic chitosan matrix and the inorganic LDH phase rather than discrete reactions.

In addition, we added an explanation of the degradation stages of the chitosan/LDH composite and included a comparison with the well-known thermal behavior of pristine chitosan. Specifically, the main mass-loss stage was assigned mainly to degradation of the chitosan matrix, whereas the higher-temperature mass loss was associated with further decomposition/carbonization of the polymer residue and dehydroxylation/decomposition of the LDH component. The presence of LDH was also discussed as the reason for the higher residual mass of the composite compared with pristine chitosan.

14 The LDH qualitative presence was demonstrated, but no quantitative detrmination of it in the composite. It is necessary to confirm the amount of the LDH.

- We thank the reviewer and we agree that the previous version of the manuscript demonstrated the formation of the LDH phase qualitatively, mainly by XRD and FTIR, but did not provide a quantitative estimate of the LDH component. In the revised manuscript, we added ICP analysis data for the chitosan/Mg–Fe LDH composite. The ICP analysis showed that the composite contains 0.74 wt.% Mg and 0.86 wt.% Fe. These values correspond to a Mg/Fe molar ratio of 1.98, which is very close to the expected 2:1 ratio for Mg₂Fe-LDH. Since the formation of the LDH phase was confirmed by XRD, the Mg and Fe contents were used to estimate the amount of the Mg–Fe LDH component as LDH-equivalent content. The calculated content is approximately 4.1 wt.% for the anhydrous Mg–Fe LDH equivalent, or approximately 4.5–5.0 wt.% when interlayer/structural water is considered. This quantitative information has been added to the revised manuscript.

15 Line 284-286 indicate the reference

- We are grateful to the reviewer and have provided the reference.

16  Figure 9 10 11 12 and 13 must have legend.

- We are grateful to the reviewer and we have corrected the figures.

17 The determination method for Qmax must be indicated. How it was done?
It will be better to use a direct determination method to evaluate the absorbed quantity. Atom absorption for example or something else.

- We agree that the method used for determining Qmax should be stated more explicitly. In the revised manuscript, we clarified that Qmax ​ was determined experimentally from the adsorption isotherm as the maximum equilibrium adsorption value qe reached in the plateau region. q_e were calculated from the difference between the initial and equilibrium Cr(VI) concentrations according to the mass-balance equation: q_e = (С₀ – С_е)V/m, where C₀ is the initial Cr(VI) concentration, C_e is the equilibrium Cr(VI) concentration after sorption, V is the solution volume, and m is the mass of the sorbent. The experimental Q_max ​value was determined as the maximum q_e value reached in the plateau region of the adsorption isotherm. We also clarified the analytical method used to determine Cr(VI) concentration. The residual chromate concentration in the solution was determined spectrophotometrically at 540 nm according to Vogel’s classical method for Cr(VI) determination. This method is well established and included in standard textbooks of inorganic analysis: Vogel, A. Vogel’s Textbook of Macro and Semimicro Qualitative Inorganic Analysis; Svehla, G.; Longman Group Limited: New York, NY, 1979. We respectfully note that atomic absorption spectroscopy is not necessarily more appropriate for this particular task, because it determines total chromium rather than selectively chromate/Cr(VI), unless additional speciation procedures are used. Since the target species in this work is chromate, i.e., hexavalent chromium, the spectrophotometric method used here is selective for Cr(VI), sufficiently sensitive for the studied concentration range, and fully suitable for calculating the adsorbed amount by solution depletion. This clarification has been added to the revised manuscript.

18 Regarding the heterogeny nature of the obtained composites, how can it be controled? Is it possible or will it vary at each production trial?

- We thank the reviewer for this relevant question. The heterogeneous nature of the chitosan/LDH composite is expected, since the material consists of an organic polymer matrix and an inorganic LDH phase. Therefore, complete molecular-level homogeneity cannot be expected for this type of composite. However, the heterogeneity can be controlled and minimized at the preparation level by maintaining fixed synthesis parameters, including the chitosan/Mg/Fe ratio, Mg/Fe precursor ratio, rate of alkali addition, vigorous stirring, washing procedure, and drying conditions. Thus, although the composite is intrinsically heterogeneous, its heterogeneity is not random but controlled by the synthesis protocol. We have clarified this point in the revised manuscript. We also note that batch-to-batch reproducibility can be monitored by the Mg/Fe ratio and LDH-equivalent content from ICP analysis, characteristic LDH reflections in XRD, FTIR bands of the LDH component, and sorption performance. Under unchanged synthesis conditions, only minor batch-to-batch variations are expected.

19 How were the kinetic tests done? What analytical method was use, what instrument?

- We agree that the description of the kinetic experiments was not sufficiently detailed. In the revised manuscript, we clarified the experimental procedure and the analytical method used.

20 Is this composite good only for chromium or it can be applied for other metals? If so, dose it present selectivity?

- The present study was specifically focused on the sorption of hexavalent chromium in the form of chromate anions. Therefore, we do not claim that the obtained composite is universally applicable to all metal ions, nor do we claim general selectivity toward metals as a class. Other metal-containing species, especially anionic oxyanions, may in principle interact with LDH-containing materials, but this was not experimentally investigated in the present work and would require a separate systematic study. In this manuscript, selectivity was evaluated only in terms of chromate sorption in the presence of common competing inorganic anions in simulated groundwater, not toward a series of different metal ions.

 21 Why didn’t you compared you results with other studies on the chromium absorbency and reusability?

- In the original version, we initially focused mainly on the internal comparison between the materials studied in this work: chitosan, Mg–Fe LDH, and the chitosan/Mg–Fe LDH composite. In the revised manuscript, we added a concise comparison with literature data on chromium sorption and reusability. We also note that direct comparison of adsorption capacities should be made with caution, since reported values strongly depend on pH, initial Cr(VI) concentration, sorbent dosage, contact time, ionic strength, competing ions, and regeneration conditions. Nevertheless, the obtained chitosan/Mg–Fe LDH composite shows competitive chromate adsorption capacity and pronounced reusability, retaining complete chromate uptake over five cycles, with desorption efficiency decreasing only from 97.3% to 90.3%.

22 The stability of the composite should be checked too

- The stability of the composite was assessed in terms of its operational stability during repeated sorption/desorption cycles. In the reusability experiment, the chitosan/LDH composite was subjected to five consecutive chromate sorption cycles followed by regeneration with 1 M NaOH. The composite retained complete chromate uptake throughout all five cycles, while the desorption efficiency decreased only moderately, from 97.3% to 90.3%. Thus, the material demonstrates good stability under the regeneration conditions used in this work. In addition, the composite retained its sorption ability in simulated groundwater containing competing chloride, bicarbonate, and sulfate anions. This also supports the stability of its sorption performance under more complex aqueous conditions. We agree that a detailed long-term structural stability study, including post-cycling XRD/FTIR and metal leaching analysis, would be valuable. However, such a study is beyond the scope of the present work. In the current manuscript, we therefore limit our conclusion to operational stability/reusability under the tested sorption–desorption conditions, rather than claiming complete long-term structural stability.

Reviewer 3 Report

Comments and Suggestions for Authors

The article investigates Cr(VI) removal of Chitosan/LDH composite prepared in single step.

The approach is novel. However, the significance of the results is unclear. Below are my specific comments.

1. Abstract. The applied one-pot synthesis of chitosan/LDH is novel.
2. Abstract. There are no numerical values. Thus it is difficult to evaluate the results of the study with existing literature.
3. Introduction. The literature review of existing co-precipitation studies with chitosan and LDH studies are missing. The novelty of the study must be supported by improved literature review.
4. Introduction. The introduction lines 58-59 includes Russian text.
5. Materials and method. What pH condition was used during composite synthesis? Why did you not study BET surface area?
6. Materials and methods. Thermogravimetric analysis explanation is insufficient without detailed description of mass used, inert gas flow rate, heating program used, etc.
7. Results and discussion. The major issue here is insufficient discussion. This section is similar to a report.
8. Figure 5. What does that Russian text mean? Similar to food?
9. Some results and discussions are too superficial. For example TGA results are described with a 3 line text (line 264-266) without a proper discussion in the literature.
10. Table 3. Langmuir results of the composite are strange. Pseudo-second order kinetics were already reported for similar studies. What is the novelty of this study?
11. Figure 13. Unclear. Which column shows what?
12. Conclusions. They must be rewritten. The conclusions must include the significant results with numerical values. The first and fourth results seem to be significant while others are redundant.

Author Response

The authors are sincerely grateful to Editor and Reviewers for their unselfish and extremely important work, for thoroughly checking the manuscript, for valuable comments and advices, which have significantly improved the starting text of the submitted manuscript.

 

The approach is novel. However, the significance of the results is unclear. Below are my specific comments.

- We thank the reviewer for recognizing the novelty of the proposed approach. We agree that the significance of the results was not sufficiently clear in the previous version. Therefore, the manuscript has been substantially revised. In the revised version, the significance of the results is emphasized through quantitative comparison with the starting components. The chitosan/Mg–Fe LDH composite showed the highest experimental plateau adsorption capacity, 137.4 mg/g, compared with 92.2 mg/g for chitosan and 53.5 mg/g for Mg–Fe LDH. The composite also demonstrated the best performance in simulated groundwater, removing 82% of Cr(VI) at a sorbent content of 1.0 g/L, compared with 52% for chitosan and 31% for Mg–Fe LDH. In addition, the composite retained complete chromate uptake during five sorption/desorption cycles under the tested conditions. These quantitative results are now included in the Abstract, Results and Discussion, and Conclusions.

1. Abstract. The applied one-pot synthesis of chitosan/LDH is novel.

- We thank the reviewer for this positive assessment. We agree that the one-pot synthesis is the main novelty of the study. The Abstract has been revised to make this point clearer. It now explicitly states that the chitosan/Mg–Fe LDH composite was prepared by alkaline coprecipitation from an acidic chitosan solution containing Mg(II) and Fe(III) precursors, avoiding the separate synthesis of LDH and its subsequent introduction into an acidic chitosan solution.

2. Abstract. There are no numerical values. Thus it is difficult to evaluate the results of the study with existing literature.

- We agree with the reviewer. The Abstract has been rewritten and now includes the key numerical results.

3. Introduction. The literature review of existing co-precipitation studies with chitosan and LDH studies are missing. The novelty of the study must be supported by improved literature review.

- We agree with the reviewer. The Introduction has been substantially revised and expanded. We added recent literature on chitosan–LDH composite sorbents, including studies from 2022–2025, as well as recent LDH-containing systems for Cr(VI) adsorption. The revised Introduction now more clearly compares the commonly reported strategies for preparing chitosan–LDH composites with the approach proposed in this work. Most reported systems are prepared by multistep procedures involving separate LDH synthesis followed by mixing, coating, encapsulation, crosslinking, or incorporation into a chitosan matrix. In contrast, our method is based on direct one-pot in situ coprecipitation of Mg–Fe LDH within the chitosan matrix. Thus, the novelty of the present work is now supported by a more complete literature review and a clearer comparison with existing preparation strategies.

4. Introduction. The introduction lines 58-59 includes Russian text.

- We thank the reviewer for pointing this out. The Russian text in the Introduction has been translated into English. We also checked the manuscript for other remaining non-English internal comments or placeholders and removed or translated them where necessary.

5. Materials and method. What pH condition was used during composite synthesis? Why did you not study BET surface area?

- We thank the reviewer for this comment. The Materials and Methods section has been revised to describe the synthesis conditions more clearly. The composite was synthesized from an acidic chitosan solution prepared in 1% acetic acid. After addition of Mg(II) and Fe(III) nitrate precursors, sodium hydroxide solution was added dropwise under vigorous stirring and pulsed ultrasound treatment. This step caused neutralization/precipitation of chitosan and simultaneous formation of the Mg–Fe LDH phase. The resulting precipitate was then washed with distilled water to neutral pH before lyophilization. Regarding BET surface area, we agree that BET analysis would provide additional information about textural properties. However, the present work was designed as a proof-of-concept study focused on demonstrating the feasibility of one-pot in situ LDH formation within a chitosan matrix and evaluating the resulting composite as a chromate sorbent. Detailed textural characterization, including BET surface area, porosity, particle-size analysis, and optimization of the chitosan/LDH ratio, is beyond the scope of the present manuscript and is now indicated as a direction for future work.

6. Materials and methods. Thermogravimetric analysis explanation is insufficient without detailed description of mass used, inert gas flow rate, heating program used, etc.

- We agree with the reviewer that the experimental description of thermal analysis should be more complete. The Materials and Methods section has been revised to provide more details on the thermal analysis procedure, including the instrument, crucible type, heating program, temperature range, and heating rate. In addition, the Results and Discussion section has been substantially expanded. The revised manuscript now discusses the apparent low-temperature mass behavior, the main degradation stages of the composite, the contribution of the chitosan matrix, and the thermal transformations of the LDH component, including dehydroxylation and removal/decomposition of interlayer nitrate species. The discussion is now supported by relevant literature on chitosan thermal degradation and LDH thermal behavior.

7. Results and discussion. The major issue here is insufficient discussion. This section is similar to a report.

- We thank the reviewer for this important comment. We agree that several parts of the previous Results and Discussion section were too descriptive. This section has been substantially rewritten.

8. Figure 5. What does that Russian text mean? Similar to food?

- We thank the reviewer for noticing this inappropriate remaining text. It was an internal editing note that should not have appeared in the manuscript. This text has been removed. The manuscript has also been checked to eliminate other internal comments and non-English placeholders.

9. Some results and discussions are too superficial. For example TGA results are described with a 3 line text (line 264-266) without a proper discussion in the literature.

- We agree with the reviewer. The TGA/DSC section has been substantially expanded. In the previous version, the thermal analysis was described only briefly. In the revised manuscript, we provide a detailed discussion of the thermal behavior of the chitosan/Mg–Fe LDH composite.

10. Table 3. Langmuir results of the composite are strange. Pseudo-second order kinetics were already reported for similar studies. What is the novelty of this study?

- We agree with the reviewer that the Langmuir parameters in the previous version were problematic. In particular, the negative Langmuir adsorption capacity obtained from linearized fitting was physically meaningless and indicated that the fitting procedure had to be corrected. Therefore, the sorption equilibrium analysis has been completely revised.

In the revised manuscript, the Langmuir and Freundlich isotherms were fitted by nonlinear least-squares regression in the original qe​ versus Cecoordinates. The same objective function and the same statistical weights were used for both isotherms. Table 3 has been recalculated accordingly, and physically meaningless negative parameters have been eliminated. RMSE and R2 values calculated in the original adsorption scale are now reported as goodness-of-fit descriptors.

Regarding pseudo-second-order kinetics, we agree that pseudo-second-order behavior has been reported for many adsorption systems and is not the novelty of the present work. We have clarified this point in the revised manuscript. The novelty of the study is not the observation of pseudo-second-order kinetics, but the one-pot in situ preparation of a chitosan/Mg–Fe LDH composite, in which the LDH phase is generated directly within the chitosan matrix during alkaline coprecipitation. The kinetic analysis is included to characterize the sorption behavior of this new material, not as the primary novelty claim.

11. Figure 13. Unclear. Which column shows what?

- We thank the reviewer for this comment. The reusability section has been revised to clarify the meaning of the data shown in the figure. We now explicitly distinguish between chromate uptake during the sorption step and chromate desorption efficiency during treatment with 1 M NaOH. We also revised the interpretation of the reusability experiment. The test is now described as a primary operational reusability assessment rather than as proof of complete post-cycling structural stability. The composite retained 100% chromate uptake over five cycles under the tested conditions, while desorption efficiency gradually decreased from 97.3% to 90.3%. We also added a statement that post-cycle XRD, FTIR, and metal-leaching analyses would be required to confirm structural and compositional stability after repeated use.

12. Conclusions. They must be rewritten. The conclusions must include the significant results with numerical values. The first and fourth results seem to be significant while others are redundant.

- We agree with the reviewer. The Conclusions section has been rewritten to include the main significant quantitative results and to avoid unsupported general statements. The revised Conclusions now summarize the key outcomes of the work: successful one-pot formation of Mg–Fe LDH within the chitosan matrix; experimental plateau adsorption capacity of 137.4 mg/g for the composite, compared with 92.2 mg/g for chitosan and 53.5 mg/g for Mg–Fe LDH; improved performance in simulated groundwater, where the composite removed more than 80% of Cr(VI) at 1.0 g/L compared with 52% for chitosan and 31% for Mg–Fe LDH; and primary operational reusability over five cycles, with retained chromate uptake and desorption efficiency decreasing from 97.3% to 90.3%. The revised Conclusions also indicate future research directions, including testing with real contaminated water samples, dynamic flow conditions, long-term operational stability, possible metal leaching, extended sorption/desorption cycling, and application of the one-pot strategy to other chitosan/LDH systems.

Reviewer 4 Report

Comments and Suggestions for Authors

The file is attached.

Comments for author File: Comments.pdf

Author Response

The authors are sincerely grateful to Editor and Reviewers for their unselfish and extremely important work, for thoroughly checking the manuscript, for valuable comments and advices, which have significantly improved the starting text of the submitted manuscript.

1. 1. Introduction. The aim of the work is well defined and the novel elements are correctly indicated in relation to the literature data. However, after the sentence “Not surprisingly, in recent years, several composite sorbents have been obtained in which LDHs act as a filler distributed in a chitosan polymer matrix.” some papers presenting chitosan – LDH composites should be cited ([20-24]). Small correction – in paragraph 2, last sentence is in Russian.

- We thank the reviewer for this positive assessment and helpful suggestion. We have added several relevant references to the sentence discussing chitosan–LDH composites, as recommended. We also corrected the language issue in the Introduction: the Russian sentence in the second paragraph has been translated into English. The Introduction has been revised accordingly.

2. 2. Materials and Methods. Specify the type of spectrophotometer used to determine the Cr concentration.

- Thank you, we specified the type of the spectrophotometer.

3. 3.2. Synthesis of LDH and 3.3. Preparation and characterization of chitosan composite with magnesium-iron LDH. In the work, Figure 6 is cited first, and only then Figure 5.

- We agree with the reviewer and have corrected this defect.

4. 3.3. Preparation and characterization of chitosan composite with magnesium-iron LDH. The TGA and DSC analysis is limited to the determination of body weight loss in two stages, without any interpretation of the processes related to the observed effects.

- We fully agree with the reviewer's comments. In the revised version of the manuscript, we have significantly expanded the description of the TGA and DSC analysis.

5. Sorption equilibria study. “The experimentally determined maximum adsorption value (sorption capacity) Qmax was 137.4 mg/g for the synthesized composite, 92.2 mg/g for chitosan, and 53.5 mg/g for magnesium-iron LDH.” The adsorption capacities should be determined based on a specific model or equation. The sorption capacity values given in the publication reflect the approximate adsorption value for the experimentally measured isotherms in the region of reaching the plateau.

- We thank the reviewer for this clarification. The section “Sorption equilibria study” has been substantially revised. We now clearly distinguish the experimentally observed plateau adsorption capacity from model-derived parameters. The reported values of 137.4, 92.2, and 53.5 mg/g are now defined as experimental plateau adsorption capacities, Qplateau,exp​, calculated from the mass-balance equation and corresponding to the plateau region of the measured isotherms. Model-derived parameters are reported separately in the revised Table 3.

6. Sorption equilibria study. “These findings are consistent with recently reported data,…” These publications should be cited.

- We agree with the reviewer. The statement referring to previously reported data has now been supported by an appropriate citation. The corresponding reference has been added directly to the revised text.

7. Sorption equilibria study. “We analyzed the obtained data using two classical models, Freundlich and Langmuir…” The Langmuir equation was derived within a model based on strictly defined assumptions, while the Freundlich isotherm is an experimental equation. The text can be corrected as follows: “We analyzed the obtained data using the classical Freundlich and Langmuir isotherms…” Moreover, "Freundlich model" should be corrected to "Freundlich isotherm" throughout the text.

- We are grateful to. the reviewer, and we agree with the reviewer’s terminology correction. The text has been revised accordingly: “Freundlich model” has been replaced with “Freundlich isotherm” throughout the section. We also corrected the introductory sentence to “We analyzed the obtained data using the classical Langmuir and Freundlich isotherms” and clarified the difference between the model-based Langmuir equation and the empirical Freundlich isotherm.

8. Sorption equilibria study. Why are the linear theoretical relationships according to the Langmuir and Freundlich isotherms are not shown in Figures 10 and 11? I see that the deviations of the theoretical isotherms from the experimental data are large, and the determined values of the parameters (Qmax) are wrong or unphysical.

- We thank the reviewer, and we agree that the fitted theoretical relationships should be shown explicitly. Figures 10 and 11 have been replaced. The revised figures now show the experimental adsorption data together with the corresponding fitted Langmuir and Freundlich curves in the original qe versus Ce coordinates. This makes the agreement or deviation between the experimental data and fitted isotherms directly visible.

9. Sorption equilibria study. The another problem is connected with the fitting procedure. As I understand the authors use the linear fitting procedure using the different linear forms of Langmuir and Freundlich isotherms, and for comparison of these equations only R2 value is used. Taking into account different linear forms of isotherms the optimization target is also different. Thus, in the corrected optimization procedure the optimization target should be specified the same for all regarded equations. Only then the comparisons between equations are meaningful. Depending on the character of experimental data deviations different statistical weights should be used, however, they should be always the same independently on theoretical equation (e.g. Langmuir or Freundlich). Only then the comparisons between equations are meaningful.

- We completely agree with the reviewer’s methodological comment. The fitting procedure has been fully revised. Instead of using different linearized forms of the Langmuir and Freundlich isotherms, both isotherms were fitted by nonlinear least-squares regression in the original q e ​ versus C e ​ coordinates. The same objective function and the same statistical weights were used for both equations, which makes the comparison meaningful. Table 3 has been recalculated accordingly and now reports physically meaningful fitted parameters together with RMSE and R2 values calculated in the original adsorption scale.

10. 3.4. Sorption equilibria study and 3.5. Kinetics studies. The lines in Figures 9, 12A and B should be smooth.

 

- We are grateful to the reviewer and we have corrected the figures, however, due to the large number of dots in Figure 9, it is not possible to make the line even smoother.

11. 3.5. Kinetics studies. The caption under Figure 12B and C should be corrected.

- We are grateful to the reviewer and we have corrected the figure caption.

12. 3.5. Kinetics studies. As far as the procedure for optimizing kinetic data (pseudo-first and second-order equations) is concerned, the comments relating to the optimization of equilibrium adsorption data remain valid.

- We agree with the reviewer’s comment. The kinetic data treatment has been revised following the same principles as those applied to the equilibrium adsorption data. The pseudo-first-order and pseudo-second-order kinetic equations were fitted by nonlinear least-squares regression in the original qt versus t coordinates, without using linearized transformations. For both kinetic equations, the same objective function was minimized, namely the sum of squared residuals between experimental and calculated qtq_tqt​ values, and equal statistical weights were used for all experimental points. Figure 12 and Table 4 have been revised accordingly. The updated results confirm that the pseudo-second-order equation provides a better description of chromate adsorption kinetics for all studied sorbents.

13. Conclusions. The perspectives for further research should be indicated.

- We thank the reviewer and agree that the perspectives for further research should be indicated more explicitly. The Conclusion section has been revised accordingly. We added a short statement outlining future studies, including testing the composite with real contaminated water samples, evaluating long-term operational stability under flow conditions, and exploring the applicability of the one-pot synthesis strategy to other chitosan/LDH systems and environmentally relevant oxyanions.

Reviewer 5 Report

Comments and Suggestions for Authors

1. The abstract lacks essential quantitative results such as maximum adsorption capacity (e.g., 137.4 mg/g), and removal efficiency in simulated groundwater.
2. In introduction: (i) A non-English sentence (“Также СДГ интересны как катализаторы…”) is present in the manuscript. This is a critical issue and must be removed or translated. (ii) The literature review is insufficiently comprehensive and not up-to-date, as recent (2022–2025) studies on chitosan–LDH composites, and relevant works on Cr(VI) adsorption systems. (iii) The novelty claim is not convincingly supported as there is no direct comparison with existing synthesis strategies or the manuscript lacks a critical discussion of limitations in previous studies, which weakens the justification of novelty. (iv) The introduction contains repetitive descriptions of chitosan and LDH properties, which should be streamlined to improve readability.
3. The manuscript incorrectly refers to chromium as Cr(IV) in multiple sections. What is Cr(IV)? Incorrect oxidation state: Cr(IV) instead of Cr(VI).
4. Important experimental parameters are not reported such as pH conditions, number of replicates and error analysis, instrument calibration details, etc.
5. There are inconsistencies in concentration units: both g/L and mg/L are used without clear standardization.
6. Although ANOVA is mentioned in statistical analysis, no statistical results (p-values, significance levels) are presented.
7. The manuscript lacks surface and interaction analysis, such as: zeta potential measurements, XPS or other surface chemistry characterization for understanding of the adsorption mechanism.
8. The Langmuir model yields physically meaningless parameters, including negative adsorption capacity (Qmax = –303.3 mg/g). This indicates invalid model fitting. Despite acknowledging inconsistencies, the manuscript continues to over-discuss the Langmuir model, which is not appropriate. Additionally, the authors should include error bars, residual or goodness-of-fit analysis beyond R².
9. The adsorption experiments use very high concentration ranges, which may not reflect realistic environmental conditions.
10. The reported R² values (~1.0) are unrealistically high and may indicate overfitting or excessive rounding. The Weber–Morris intraparticle diffusion model should be included.
11. The use of simulated groundwater is a strength; however, the composition is not sufficiently justified and there is no comparison with real groundwater systems or literature data.
12. No structural or compositional analysis after cycles (e.g., XRD, FTIR). Also, the mechanism of performance retention is not discussed.
13. The manuscript requires major English language revision. E.g., “staring chitosan” instead of “starting chitosan”. Additionally, several sections contain unnecessary repetition, particularly, IR spectral assignments, general adsorption theory.

 

Overall, I am not very positive about the paper. Authors can still be given a chance to address as a fresh submission.

Comments on the Quality of English Language

The manuscript requires major English language revision.

Author Response

The authors are sincerely grateful to Editor and Reviewers for their unselfish and extremely important work, for thoroughly checking the manuscript, for valuable comments and advices, which have significantly improved the starting text of the submitted manuscript.

1. The abstract lacks essential quantitative results such as maximum adsorption capacity (e.g., 137.4 mg/g), and removal efficiency in simulated groundwater.

- We deeply thank the reviewer for their significant and important comment. We have rewritten the abstract in accordance with the reviewer's comments.

2. In introduction: (i) A non-English sentence (“Также СДГ интересны как катализаторы…”) is present in the manuscript. This is a critical issue and must be removed or translated. (ii) The literature review is insufficiently comprehensive and not up-to-date, as recent (2022–2025) studies on chitosan–LDH composites, and relevant works on Cr(VI) adsorption systems. (iii) The novelty claim is not convincingly supported as there is no direct comparison with existing synthesis strategies or the manuscript lacks a critical discussion of limitations in previous studies, which weakens the justification of novelty. (iv) The introduction contains repetitive descriptions of chitosan and LDH properties, which should be streamlined to improve readability.

- We agree with the reviewer’s comment. The Introduction has been substantially revised. We have updated the literature overview by adding recent studies from 2022–2025 on chitosan–LDH composites and Cr(VI) adsorption systems. We have also shortened the repetitive general descriptions of chitosan and LDHs to improve readability. In addition, we strengthened the novelty statement. The revised Introduction now directly compares the commonly reported strategies for preparing chitosan–LDH composites, which usually involve separate LDH synthesis followed by incorporation, coating, encapsulation, or crosslinking in a chitosan matrix, with the one-pot approach proposed in this work. We also clarified the limitations of previous approaches, including their multistep nature and the possible instability of preformed LDH particles under acidic chitosan-dissolution conditions. This provides a clearer rationale for the novelty of the present one-pot in situ coprecipitation method.

3. The manuscript incorrectly refers to chromium as Cr(IV) in multiple sections. What is Cr(IV)? Incorrect oxidation state: Cr(IV) instead of Cr(VI).

- We thank the reviewer for noticing this error. The incorrect notation Cr(IV) has been corrected throughout the manuscript to Cr(VI), since the studied species are chromate anions, i.e., hexavalent chromium. We have carefully checked the revised manuscript to ensure consistent use of Cr(VI) in all relevant sections.

 

4. Important experimental parameters are not reported such as pH conditions, number of replicates and error analysis, instrument calibration details, etc.

- We agree with the reviewer that these experimental details should be reported more explicitly. The Materials and Methods section has been revised to include the pH conditions of the adsorption experiments, the number of experimental replicates, and the calibration procedure used for spectrophotometric determination of Cr(VI).

 

5. There are inconsistencies in concentration units: both g/L and mg/L are used without clear standardization.

- We thank the reviewer for this comment. The use of both g/L and mg/L was intentional and reflects the different concentration ranges used in different experiments. In the sorption equilibrium experiments, relatively concentrated K₂CrO₄ solutions were used, and therefore the concentration range was reported in g/L. In the kinetic, simulated groundwater, and recyclability experiments, much lower Cr(VI) concentrations were used, and therefore mg/L was the more convenient unit.

To avoid ambiguity, we have carefully checked the manuscript and ensured that the units are explicitly indicated in each experimental section, table, and figure caption. All calculations were performed after conversion to consistent units where necessary. Thus, the use of g/L and mg/L does not affect the reported adsorption values or their interpretation.

6. Although ANOVA is mentioned in statistical analysis, no statistical results (p-values, significance levels) are presented.

- We deeply thank the reviewer for pointing out this inconsistency. The statement about ANOVA and Duncan’s test was mistakenly retained in the Materials and Methods section, although no formal inferential statistical analysis was used for the data presented in this manuscript. Therefore, this statement has been removed from the revised version.

In the revised manuscript, we report only the data-treatment procedures that were actually applied. In particular, adsorption isotherm and kinetic parameters were obtained by nonlinear least-squares fitting, and the quality of fitting was evaluated using RMSE and R2R^2R2 values calculated in the original experimental scales.

7. The manuscript lacks surface and interaction analysis, such as: zeta potential measurements, XPS or other surface chemistry characterization for understanding of the adsorption mechanism.

- We agree with the reviewer that zeta potential measurements, XPS, and other surface-sensitive techniques would provide additional information about the surface chemistry and adsorption mechanism. However, a detailed surface-chemistry/mechanistic study was not the main objective of the present manuscript. The aim of this work was to demonstrate a one-pot route to chitosan/Mg–Fe LDH composite preparation, confirm the formation of the LDH phase in the chitosan matrix, and evaluate the sorption performance of the obtained material toward chromate anions. Therefore, the mechanism is discussed in the manuscript only at the level supported by the available experimental data, namely XRD, FTIR, ICP analysis, adsorption isotherms, kinetic data, simulated groundwater experiments, and reusability tests. We have avoided making detailed claims about the molecular-level adsorption mechanism that would require XPS, zeta potential, or related surface-chemistry techniques. We agree that such measurements would be valuable, but they represent a separate, more detailed mechanistic study and are beyond the scope of the present work. This point has been clarified in the revised manuscript.

8. The Langmuir model yields physically meaningless parameters, including negative adsorption capacity (Qmax = –303.3 mg/g). This indicates invalid model fitting. Despite acknowledging inconsistencies, the manuscript continues to over-discuss the Langmuir model, which is not appropriate. Additionally, the authors should include error bars, residual or goodness-of-fit analysis beyond R².

- We agree with the reviewer that the negative Langmuir Qmax ​ value in the previous version indicated an invalid fitting procedure and should not have been discussed as a physically meaningful parameter. The whole “Sorption equilibria study” section has therefore been substantially revised. In the revised manuscript, the linearized fitting procedure was completely abandoned. The Langmuir and Freundlich isotherms are now fitted by nonlinear least-squares regression in the original qe versus Ce ​ coordinates, using the same objective function and the same statistical weights for both isotherms. Table 3 has been recalculated accordingly. As a result, all fitted parameters are positive and physically meaningful. We also revised the discussion of the Langmuir isotherm. The manuscript no longer over-interprets the Langmuir model; instead, it compares the applicability of the Langmuir and Freundlich isotherms based on the corrected nonlinear fitting. The revised analysis shows that the Freundlich isotherm better describes adsorption by Mg–Fe LDH, whereas the Langmuir isotherm better describes adsorption by chitosan and the chitosan/Mg–Fe LDH composite. In addition, Figures 10 and 11 were replaced with plots showing the experimental data together with the nonlinear fitted curves. The goodness of fit is now evaluated not only by R2, but also by RMSE values calculated in the original qeq_eqe​ scale. Thus, the revised version addresses the problem of physically meaningless parameters and provides a more appropriate comparison of the isotherms.

9. The adsorption experiments use very high concentration ranges, which may not reflect realistic environmental conditions. 10. The reported R² values (~1.0) are unrealistically high and may indicate overfitting or excessive rounding. The Weber–Morris intraparticle diffusion model should be included.

- We thank the reviewer for this comments. We agree that the concentration range used in the equilibrium adsorption experiments should be better explained. The relatively high concentration range was intentionally used to construct complete adsorption isotherms and to reach the plateau region required for estimating the experimental adsorption capacity and fitting the Langmuir/Freundlich isotherms. These experiments were not intended to reproduce directly the concentration level of natural waters. The environmental relevance of the material was evaluated separately in simulated groundwater experiments, where lower chromate concentration and competing anions were used. We also agree that the fitting quality should not be judged only from R2 values. In the revised manuscript, the kinetic and equilibrium fitting procedures were corrected: nonlinear fitting was performed in the original experimental coordinates, and RMSE values were added as additional goodness-of-fit criteria. We also avoided overinterpreting near-unity R2 values. The models used contain only two adjustable parameters, so the revised analysis is not based on excessive model complexity; nevertheless, RMSE and direct comparison between experimental and fitted curves are now provided to make the fitting assessment more transparent. Following the reviewer’s suggestion, we added the Weber-Morris intraparticle diffusion analysis as an additional diagnostic tool for the kinetic data and this has been added to the revised manuscript.

11. The use of simulated groundwater is a strength; however, the composition is not sufficiently justified and there is no comparison with real groundwater systems or literature data.

- We thank the reviewer for this comment. We agree that the composition and limitations of the simulated groundwater should be explained more clearly. In the revised manuscript, we clarified that the simulated groundwater composition was selected on the basis of natural groundwater from Aprelevka, Moscow Region, and was intended to reproduce the main inorganic competing anions relevant for chromate sorption, namely chloride, bicarbonate, and sulfate. At the same time, we realize that this system is a simplified model of natural groundwater. It does not include dissolved organic matter, humic substances, trace components, or possible seasonal variations in groundwater composition. Therefore, we do not claim that it fully reproduces all real groundwater systems. The purpose of this experiment was to provide a primary assessment of chromate uptake under more complex conditions than distilled water, especially in the presence of competing inorganic anions. A full comparison with real groundwater samples will require a separate study and is now indicated as a direction for further research.

12. No structural or compositional analysis after cycles (e.g., XRD, FTIR). Also, the mechanism of performance retention is not discussed.

- We agree with the reviewer that post-cycling XRD, FTIR, or compositional analysis would provide additional information about the structural stability of the composite after repeated use. However, the aim of the recyclability experiment in the present work was more limited: it was intended as a primary assessment of operational reusability under the selected sorption/desorption conditions, rather than as a complete post-cycling structural stability study. In the revised manuscript, we clarified this point and avoided making claims about unchanged structure after cycling. The retained sorption performance is discussed only as operational stability under the tested conditions. The likely reason for the performance retention is that alkaline treatment with NaOH efficiently desorbs chromate species and restores a significant fraction of the available sorption sites for the next cycle. At the same time, we agree that detailed confirmation of the structural and compositional stability after cycling, including post-cycle XRD, FTIR, and possible metal-leaching analysis, would require a separate study and is now indicated as a direction for further research.

13. The manuscript requires major English language revision. E.g., “staring chitosan” instead of “starting chitosan”. Additionally, several sections contain unnecessary repetition, particularly, IR spectral assignments, general adsorption theory.

- We thank the reviewer for carefully reading the manuscript; we have corrected the errors found.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I agree with manuscript publication

Author Response

We sincerely thank the reviewer!

Reviewer 3 Report

Comments and Suggestions for Authors

In general, the revised manuscript is acceptable and most of the previous concerns have been adequately addressed. However, a few minor issues still remain. Some figures still contain Russian text. In addition, the Abstract must include a brief statement acknowledging the limitations of the study. Finally, the Conclusions section is lengthy containing additional discussions and must be shortened to improve conciseness and readability.

Author Response

We sincerely thank the Reviewer for the positive overall assessment of the revised manuscript and for pointing out the remaining minor issues.
In response to the Reviewer’s comments, we have carefully checked all figures and replaced the remaining Russian text with the corresponding English labels and annotations throughout the manuscript.
We have also revised the Abstract by adding a brief statement acknowledging the limitations of the study. Specifically, we now note that the sorption performance was evaluated mainly in batch systems and simplified simulated groundwater, and that further validation using real contaminated waters and dynamic flow conditions is required.
In addition, the Conclusions section has been substantially shortened and rewritten to improve conciseness and readability. We removed excessive discussion-style statements and retained only the key findings, the main practical implication of the work, and a concise statement on the remaining limitations and future validation needs.
We believe that these revisions fully address the remaining comments and have improved the clarity and presentation quality of the manuscript.

Reviewer 4 Report

Comments and Suggestions for Authors

Authors corrected the manuscript according to remarks. The paper may be accepted for publication.

Author Response

We sincerely thank the reviewer!

Reviewer 5 Report

Comments and Suggestions for Authors

The authors have addressed well. Then accept for publication.

Author Response

We sincerely thank the reviewer!

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Tha article is accepted.