Synthesis of Pectin Hydrogels from Grapefruit Peel for the Adsorption of Heavy Metals from Water
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis work reports the synthesis of pectin hydrogels derived from grapefruit peel and their application in the adsorption of Cu(II) and Ni(II) ions from aqueous solutions. In my opinion, the manuscript is worthy of publication in Technologies following the below minor revisions:
1) Fig. 4 should be redesigned more carefully because the current Fig. 4 looks ugly. Each step should also be explained exactly.
2) The name (% Transmittance) and data of the Y-axis in Fig. 5 should be moved to the left side.
3) The "cm-1" in Fig.5 should be corrected to "cm-1".
4) The "Cu2+" in Figs. 9 and 12 should be corrected to "Cu2+".
5) "(mg/g)" must be added after the “Adsorption capacity" in Figs. 9 and 10, 11, 12.
6) The "Ni2+" in Figs. 11 should be corrected to "Ni2+".
7) Some closely relevant references suggested below could be useful to support this research:
[1] Li, X. G., Huang, M. R. (Eds.). Milestones in Powerful Adsorbents of Heavy-Metal Ions. Cambridge Scholars Publishing, Newcastle upon Tyne, UK: 2024, 853 pages.
[2] Tariq, W., Arslan, C., et al. Application of agro-based adsorbent for removal of heavy metals. In Emerging Techniques for Treatment of Toxic Metals from Wastewater 2023, 157-182.
Author Response
Comment 1: Fig. 4 should be redesigned more carefully because the current Fig. 4 looks ugly. Each step should also be explained exactly.
Response 1: Thank you for pointing this out, I partially agree with this comment. Therefore I have redesigned Fig. 4 to make it look less “ugly” and more succinct. The steps are also clearer and easier to follow. The updated Fig. 4 can be found on page 8, line 356.
Comment 2: The name (% Transmittance) and data of the Y-axis in Fig. 5 should be moved to the left side.
Response 2: Thank you for pointing this out. I agree with this comment. Therefore I have modified Fig. 5 and moved the y-axis label “% Transmittance” to the left side, on page 11, line 477.
Comment 3: The "cm-1" in Fig.5 should be corrected to "cm-1".
Response 3: Thank you for pointing this out. I agree with this comment. Therefore I have modified Fig. 5 and corrected the x-axis label to “cm-1”, on page 11, line 477.
Comment 4: The "Cu2+" in Figs. 9 and 12 should be corrected to "Cu2+".
Response 4: Thank you for pointing this out. I agree with this comment. Therefore I have modified Figs. 9 and 12 and corrected the y-axis label to “Cu2+”. For Fig. 9 this is found on page 17, line 676, and for Fig. 12 on page 21, line 826.
Comment 5: "(mg/g)" must be added after the “Adsorption capacity" in Figs. 9 and 10, 11, 12.
Response 5: Thank you for pointing this out. I agree with this comment. Therefore I have modified Figs. 9, 10, 11 and 12 and corrected the y-axis label to “(mg/g)” after “Adsorption capacity”. For Fig. 9 this is found on page 17, line 676, Fig 10. on page 18, line 719, Fig. 11 on page 19, line 764, and Fig. 12 on page 21, line 826.
Comment 6: The "Ni2+" in Figs. 11 should be corrected to "Ni2+".
Response 6: Thank you for pointing this out. I agree with this comment. Therefore I have modified Fig. 11 and corrected the y-axis label to “Ni2+”, in Fig. 11 on page 19, line 764.
Comment 7: Some closely relevant references suggested below could be useful to support this research:
[1] Li, X. G., Huang, M. R. (Eds.). Milestones in Powerful Adsorbents of Heavy-Metal Ions. Cambridge Scholars Publishing, Newcastle upon Tyne, UK: 2024, 853 pages.
[2] Tariq, W., Arslan, C., et al. Application of agro-based adsorbent for removal of heavy metals. In Emerging Techniques for Treatment of Toxic Metals from Wastewater 2023, 157-182.
Response 7: thank you for your comment. I agree, however, since I have graduated I am no longer able to access the literature reference listed as [1], and was therefore unable to incorporate it into the manuscript. However, I was able to incorporate reference [2] into the discussion section. This can be found on page 23, lines 945-949.
Reviewer 2 Report
Comments and Suggestions for AuthorsI have read the manuscript in detail and I believe it addresses a relevant topic by proposing pectin-based hydrogels derived from grapefruit peel for heavy metal removal. However, I have several important observations that the authors should address before the work can be considered for publication.
1. I am particularly concerned about the quality of the data presented in Figures 9 and 10. In both figures, the error bars are extremely large, with relative standard deviations exceeding 40 % at certain time points. This indicates very low experimental reproducibility, which may stem from issues such as inadequate homogenization of samples, dilution errors, poor calibration of the FAAS instrument, or inaccuracies when weighing such small amounts of hydrogel. I strongly recommend that the authors thoroughly recalculate all adsorption capacities, ensuring unit conversions and dilution factors are correct. Additionally, providing the raw data as supplementary material would increase transparency and allow for verification of these calculations.
2. I noticed that in some cases in this same figures, adsorption capacities are reported as negative values, which is physically implausible (the error bars include negative values as possible values). A negative adsorption capacity suggests a fundamental experimental or analytical error, implying that the final metal concentration in solution was higher than the initial concentration. This could result from contamination, leaching of interfering components from the hydrogel, or inadequate blank correction during FAAS measurements. The authors should carefully investigate these possibilities, repeat measurements if necessary, and clarify why such values were obtained.
3. The study only reports time-dependent adsorption data without performing any kinetic modelling or equilibrium isotherm analysis. Fitting the data to pseudo-first-order or pseudo-second-order kinetic models, as well as Langmuir or Freundlich isotherms, would strengthen the manuscript significantly by providing mechanistic insights and allowing comparisons with existing literature.
4. There is insufficient discussion about the effect of incorporating MOFs into the hydrogels. For instance, the PHM composites are reported to show low or potentially negative adsorption capacities in some cases. The authors should clearly explain the amount of MOF incorporated, its contribution to adsorption, and whether any metal leaching from the MOF could interfere with measurements, artificially increasing metal concentrations in solution.
5. The manuscript would benefit from deeper mechanistic discussion. For example, why does Ni(II) show such high adsorption capacities compared to Cu(II), despite similar functional groups being involved in binding? Is this related to differences in ionic radius, hydration enthalpy, or simply an artefact of measurement errors? Expanding the discussion with literature-supported explanations would strengthen the overall impact of the study.
6. The reported adsorption capacities, especially for Ni(II), seem unusually high (e.g., over 250 mg/g within one minute). These values should be critically evaluated, as they surpass most biosorbents reported in the literature. The authors should confirm whether such capacities are realistic, given the experimental conditions and the calculated mass balance. Otherwise, they should check if concentration units or dilution calculations are incorrect.
7. The adsorption tests were performed using lyophilized hydrogels. However, it is unclear whether the beads were rehydrated before the adsorption assays, or whether the adsorption was performed directly with the dried beads. Using dried beads could significantly limit swelling and diffusion of metal ions into the hydrogel matrix, and may not reflect realistic conditions in water treatment. The authors should clarify this point and, if possible, compare adsorption performance between fresh hydrated beads and lyophilized beads.
8. In the Materials and Methods section, the pH control is described briefly. However, it is unclear whether the pH remained constant during the experiments, especially in the presence of metal nitrates and hydrogels that may exchange protons. Adsorption is strongly pH-dependent, and slight changes could affect results. The use of proper buffering systems is recommended.
9. In the results and discussion sections, the authors primarily describe observations without critical interpretation. For example, why is the adsorption capacity higher for Ni(II) than Cu(II)? What is the role of ionic radius, hydration energy, or coordination preference with carboxyl and hydroxyl groups of pectin? Moreover, how do their maximum adsorption capacities compare to other pectin-based hydrogels or biosorbents reported in recent literature? Adding such comparisons would contextualize the significance of the findings.
10. The manuscript contains minor grammatical errors and repetitive sentences, especially in the introduction, which could be improved through careful proofreading.
Author Response
Comment 1: I am particularly concerned about the quality of the data presented in Figures 9 and 10. In both figures, the error bars are extremely large, with relative standard deviations exceeding 40 % at certain time points. This indicates very low experimental reproducibility, which may stem from issues such as inadequate homogenization of samples, dilution errors, poor calibration of the FAAS instrument, or inaccuracies when weighing such small amounts of hydrogel. I strongly recommend that the authors thoroughly recalculate all adsorption capacities, ensuring unit conversions and dilution factors are correct. Additionally, providing the raw data as supplementary material would increase transparency and allow for verification of these calculations.
Response 1: Thank you for pointing this out, and I agree with the comment. The adsorption capacities were revised and there was a calculation error involving the dilution factor, where there was an upscale of 1000x instead of 100x. The adsorption capacities and standard deviations were recalculated using the correct dilution factor. The corrections are seen in Fig. 9 found on page 17, line 676, and Fig 10. on page 18, line 719.
Comment 2: I noticed that in some cases in this same figures, adsorption capacities are reported as negative values, which is physically implausible (the error bars include negative values as possible values). A negative adsorption capacity suggests a fundamental experimental or analytical error, implying that the final metal concentration in solution was higher than the initial concentration. This could result from contamination, leaching of interfering components from the hydrogel, or inadequate blank correction during FAAS measurements. The authors should carefully investigate these possibilities, repeat measurements if necessary, and clarify why such values were obtained.
Response 2: Thank you for pointing this out. I agree with this comment. As mentioned in response 1, I revised the data and noticed a calculation error in the dilution factor, hence, the adsorption capacities and standard deviations for Figs 9 and 10 were recalculated, and the revised data revealed standard deviations that were above 0. However, since this was an undergraduate thesis project, I am unable to repeat measurements or experiments because I have graduated. The adjustments can be found in Fig. 9 on page 17, line 676, and Fig 10. on page 18, line 719.
Comment 3: The study only reports time-dependent adsorption data without performing any kinetic modelling or equilibrium isotherm analysis. Fitting the data to pseudo-first-order or pseudo-second-order kinetic models, as well as Langmuir or Freundlich isotherms, would strengthen the manuscript significantly by providing mechanistic insights and allowing comparisons with existing literature.
Response 3: Thank you for pointing this out, and we agree that kinetic and isotherm modeling would provide a deeper insight. However, in this study kinetic modeling was not pursued due to the limited number of time points collected during the adsorption experiments. We think that attempting to fit the models with insufficient data points may lead to non-robust interpretations. Similarly, equilibrium isotherm analysis was not feasible because the experiment was not designed to explore a range of initial metal concentrations, as we chose one starting concentration. The primary focus was to evaluate the time-dependent uptake behavior of the hydrogel under a single concentration. To address this limitation, we have included in the conclusion that fitting the data to these models can help deepen our understanding of the adsorption behavior. This is on page 23, line 958-961.
Comment 4: There is insufficient discussion about the effect of incorporating MOFs into the hydrogels. For instance, the PHM composites are reported to show low or potentially negative adsorption capacities in some cases. The authors should clearly explain the amount of MOF incorporated, its contribution to adsorption, and whether any metal leaching from the MOF could interfere with measurements, artificially increasing metal concentrations in solution.
Response 4: Thank you for pointing this out. I agree that understanding the amount of MOF incorporated into the hydrogel matrix and its effect on adsorption is important. In this study, a fixed mass of MOF was added per gram of pectin hydrogel precursor solution to ensure consistent synthesis conditions across the batch. While individual bead variation may slightly affect MOF loading, the batch-wise synthesis of beads for all assays helped minimize this variability. The applicability of the PHM composites was discussed and is found on pages 21-22, lines 846-861.
Comment 5: The manuscript would benefit from deeper mechanistic discussion. For example, why does Ni(II) show such high adsorption capacities compared to Cu(II), despite similar functional groups being involved in binding? Is this related to differences in ionic radius, hydration enthalpy, or simply an artefact of measurement errors? Expanding the discussion with literature-supported explanations would strengthen the overall impact of the study.
Response 5: Thank you for pointing this out. We agree. Along with the calculation edits previously made, we have done literature research and included a deeper explanation of the adsorption results in the discussion. We delved deeper into the physicochemical properties of Cu(II) and Ni(II), and how differences in these properties showed a difference in the adsorption behaviour. This can be found on page 22, lines 870-902.
Comment 6: The reported adsorption capacities, especially for Ni(II), seem unusually high (e.g., over 250 mg/g within one minute). These values should be critically evaluated, as they surpass most biosorbents reported in the literature. The authors should confirm whether such capacities are realistic, given the experimental conditions and the calculated mass balance. Otherwise, they should check if concentration units or dilution calculations are incorrect.
Response 6: Thank you for pointing this out and I agree with your comment. I revisited the Ni(II) adsorption data and noticed that the mass of adsorbent used was not correctly converted from grams to milligrams. After the correction, the relative adsorption capacity of Ni(II) decreased from 250-300mg/g to between 25-30mg/g. These values are more consistent with literature, and these changes can be found in Fig. 11 on page 19 lines 765, 748 and 757, page 18 lines 733 and 739, page 1 line 23, and page 21 line 859.
Comment 7: The adsorption tests were performed using lyophilized hydrogels. However, it is unclear whether the beads were rehydrated before the adsorption assays, or whether the adsorption was performed directly with the dried beads. Using dried beads could significantly limit swelling and diffusion of metal ions into the hydrogel matrix, and may not reflect realistic conditions in water treatment. The authors should clarify this point and, if possible, compare adsorption performance between fresh hydrated beads and lyophilized beads.
Response 7: Thank you for pointing this out. I agree. I included a clearer description of the protocol by including that the adsorption assays were conducted only using lyophilized beads on page 9 line 390, in the methods section. Furthermore, the impact of lyophilization on the hydrogels size, shape and swelling capacity was previously discussed on page 13 lines 517-527, and lines 544-553. I agree that a comparison study between the adsorption of hydrated and dehydrated beads would be beneficial, however, as I have already graduated I am unable to conduct these additional experiments. As such, I have modified the future directions to include that using hydrated beads would simulate a more accurate real-life protocol for the adsorption of heavy metals, and that a comparison study should be conducted. This can be found on page 23, lines 955-958.
Comment 8: In the Materials and Methods section, the pH control is described briefly. However, it is unclear whether the pH remained constant during the experiments, especially in the presence of metal nitrates and hydrogels that may exchange protons. Adsorption is strongly pH-dependent, and slight changes could affect results. The use of proper buffering systems is recommended.
Response 8: Thank you for pointing this out, and I agree with this comment. We agree that pH can influence the adsorption behaviour of the hydrogels. In our study, we carefully adjusted the pH of the metal solution prior to adsorption, and the samples were handled consistently under the same conditions. We acknowledge that direct measurement of pH during the adsorption process would be ideal, however, due to the small volume of solution used in each vial (20mL), it was not feasible to continuously monitor pH with a pH meter without disturbing the system. Importantly, similar studies in the literature have conducted adsorption experiments under comparable conditions without monitoring pH during the actual experiment (Mahmoud and Mohamed, 2020). This study reported the initial pH values, under the assumption that any pH variation during the adsorption period is minimal or consistent across replicates. Nonetheless, we acknowledge this limitation and have included a statement in the conclusion to clarify the constraints regarding pH monitoring during the experiments, and to acknowledge the potential for slight pH variation. This is on page 24, line 963-965.
Comment 9: In the results and discussion sections, the authors primarily describe observations without critical interpretation. For example, why is the adsorption capacity higher for Ni(II) than Cu(II)? What is the role of ionic radius, hydration energy, or coordination preference with carboxyl and hydroxyl groups of pectin? Moreover, how do their maximum adsorption capacities compare to other pectin-based hydrogels or biosorbents reported in recent literature? Adding such comparisons would contextualize the significance of the findings.
Response 9: Thank you for pointing this out. We agree with this comment. As mentioned in response 5, we have taken into consideration the physiochemical properties of Cu(II) and Ni(II) and how those factors may have influenced adsorption. This can be found on This can be found on page 22, lines 870-902. Furthermore, I discussed the adsorption capacities, and similarities in other biosorbents reported in recent literature and can be found on page 23 lines 911-933
Comment 10: The manuscript contains minor grammatical errors and repetitive sentences, especially in the introduction, which could be improved through careful proofreading.
Response 10: Thank you for pointing this out and I agree with this comment. I have proofread the manuscript and fixed the grammatical errors, and made the overall writing more succinct and clear.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsAccept.
