Removal of Cr(VI) from an Aqueous Solution via a Metal Organic Framework (Ce-MOF-808)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. What structural and electronic mechanisms specific to Ce-MOF-808 explain its superior adsorption capacity compared to the other MOFs studied (Ce-UiO-66 and Ce-UiO-66-NO₂)?
2. What structural or chemical aspects of Ce-MOF-808 explain its superior Cr(VI) removal performance?
3. How does the presence of an NO₂ group on Ce-UiO-66-NO₂ influence its interaction with Cr(VI) ions and its adsorption efficiency?
4. How does the selection of Ce-MOF-808 for further studies illustrate the importance of MOF structure and functionalization for targeted pollutant adsorption?
5. How does the positive charge state of Ce-MOF-808 in the pH range 2.8–8.6 influence electrostatic interactions with the different chromate species present in solution?
6. Why is Cr(VI) adsorption strongly favored under acidic conditions, and how does the evolution of chromate species as a function of pH influence the process?
7. What role do the Ce³⁺/Ce⁴⁺ and Cr(VI)/Cr(III) valence transitions observed by XPS play in the Cr(VI) reduction-adsorption mechanism?
8. What combined mechanisms (chemical adsorption, electrostatic interactions, redox) explain the overall effectiveness of Ce-MOF-808 for Cr(VI) remediation?
9. How do the redox properties of Ce-MOF-808 influence its durability and potential for recycling under real-world water treatment conditions?
10. How can the redox properties and adsorption capacity of Ce-MOF-808 be optimized for large-scale practical applications?
11. How does Cr(VI) adsorption alter the surface morphology of Ce-MOF-808, and what implications does this have for its subsequent adsorption capacity?
12. Why do the particles appear more agglomerated and with less defined contours after Cr(VI) adsorption?
13. How could the increased surface roughness after adsorption influence interactions with Cr(VI) ions?
14. What role does the elemental distribution of chromium and cerium (observed by EDS mapping) play in the overall adsorption efficiency?
15. How does the presence of a mass weight of 3.63% Cr confirmed by EDS compare to the maximum adsorption capacity determined by the isotherms?
16. What microscopic mechanisms could explain the binding of Cr(VI) to the surface of Ce-MOF-808?
17. To what extent can post-adsorption agglomeration affect the accessible specific surface area and adsorption kinetics?
18. How does the combination of SEM and EDS data confirm the success of Cr(VI) adsorption?
19. What potential effects could Cr(VI) surface saturation have on the reuse or recycling of Ce-MOF-808?
20. How can these morphological observations guide the design of more effective MOFs for the removal of metal pollutants in aqueous solution?
21. Why are the XRD diffraction peaks of Ce-MOF-808 strongly attenuated or almost absent after Cr(VI) adsorption?
22. What mechanisms could explain the structural distortion or partial amorphization of the MOF framework following interaction with Cr(VI)?
23. How can the decrease in crystallinity affect the adsorption capacity and stability of Ce-MOF-808 during repeated use?
24. What links can be established between the changes observed by XRD and those observed by SEM/EDS concerning agglomeration and surface roughness?
25. Could the modification of the crystal structure influence the redox properties of Ce-MOF-808 and its ability to reduce Cr(VI) to Cr(III)?
26. How could partial amorphization affect the diffusion of Cr(VI) ions to the active sites of the MOF?
27. What experimental factors (pH, Cr(VI) concentration, adsorption time) could accentuate or limit the observed loss of crystallinity?
28. What synthesis adjustments or post-synthesis modifications could improve the resistance of Ce-MOF-808 to amorphization while maintaining its adsorption efficiency?
29. What kinetic parameters justify adjusting the process to the pseudo-second-order model, and what does this reveal about the adsorption mechanism?
30. Why do the equilibrium data follow the Langmuir isotherm model, and what does the maximum adsorption capacity of 42.74 mg/g reveal?
31. Why is the adsorption process spontaneous and exothermic according to thermodynamic analysis, and what implications does this have for large-scale applications?
32. How does varying the adsorbent dose change the kinetics and overall efficiency of Cr(VI) adsorption?
33. What is the impact of contact time on the saturation of active sites and on reaching adsorption equilibrium in Ce-MOF-808?
34. What effects could the presence of competitive ions or organic matter in real water have on the stability and efficiency of Ce-MOF-808?
35. To what extent can these experimental results be extrapolated to real water conditions with competing ions or organic matter?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript by Hongfei Zhang et al. reports an experimental study on the removal of Cr(VI) from aqueous solution using a Ce-based metal-organic framework (Ce-MOF). The work contains several strengths, but there are important deficiencies that must be addressed before the manuscript is suitable for publication.

1. The Title, Introduction, and Experimental sections indicate that three distinct MOFs were prepared and investigated: Ce-UiO-66, Ce-UiO-66-NO₂, and Ce-MOF-808. However, the section devoted to characterization (SEM-EDS, PXRD, XPS, FTIR, TGA, and zeta potential) reports only the characterization results for MOF-808; the corresponding data for Ce-UiO-66 and Ce-UiO-66-NO₂ are absent. This omission is problematic because all three materials are used in one of the adsorption studies. Regardless of which material ultimately exhibits the best Cr(VI) removal uptake, each material should be characterized to the same extent and documented in the manuscript.
2. Figure 3 (PXRD) appears to show that the Ce-MOF-808 sample became nearly amorphous after the adsorption experiment. If the post-adsorption PXRD pattern was collected after only one adsorption cycle, the observed amorphization raises serious concerns about the material’s structural stability and practical applicability: a sorbent that loses crystallinity after the first use is unlikely to be suitable for repeated environmental remediation. The authors should answer if the material can be regenerated and recovered after adsorption, and if so, what are the recovery conditions and the efficiency after successive cycles? If such studies were not performed, the authors should conduct them or clearly acknowledge this limitation.
3. The manuscript lacks any description of post-synthetic activation procedures applied to the MOFs prior to adsorption experiments. Activation (solvent exchange, thermal evacuation, vacuum treatment) is essential for removing uncoordinated linkers, trapped solvents, and synthesis by-products that occupy pore volume and reduce available adsorption capacity. The authors must state whether the materials were activated prior to adsorption; if so, they must provide detailed activation protocols. They should also report whether activation altered PXRD or IR spectra (i.e., whether structural or chemical changes were introduced by activation). If the three MOFs were not activated similarly, differences in measured adsorption capacities may simply reflect inconsistent sample preparation rather than intrinsic material performance.
4. A critical omission is the lack of nitrogen physisorption (77 K) data for all samples. The high specific surface area and pore volume of MOFs are fundamental to their adsorption properties. Without BET surface areas, pore volumes, and (ideally) pore size distributions, the adsorption results cannot be fully interpreted (as we don’t know if the synthesis was fully successful). The authors should report N₂ adsorption-desorption isotherms, BET surface area, total pore volume, and micropore/mesopore distributions for all three MOFs, both before adsorption and (if possible) after adsorption or regeneration. This information is necessary to understand whether the superior performance of Ce-MOF-808 is intrinsic (pore structure, functional groups) or arises from differences in sample activation or residual synthesis solvents/linkers.
5. For a complete assessment, it would be informative to compare the experimentally measured Cr(VI) uptake with the theoretical maximum uptake estimated from crystal structure models (CIF files). Such a comparison would indicate whether the pores were accessible to Cr(VI) and whether adsorption is limited by pore blocking or residual synthesis species. The authors should calculate the theoretical pore-occupancy capacity for each MOF and compare these values to the experimentally observed uptake.
6. The manuscript notes that Ce-MOF-808 exhibits the highest adsorption efficiency among the three materials and that it was selected for further study, but it does not attempt to explain why Ce-MOF-808 outperformed Ce-UiO-66 and Ce-UiO-66-NO₂. Given that UiO-66 is reported in the literature to have a higher internal surface area than MOF-808, the lack of discussion is a major omission. The authors should provide a mechanistic rationale for MOF-808’s superior performance.
7. The manuscript presents a comprehensive set of adsorption studies only for Ce-MOF-808 (eg. effect of dose, effect of pH). However, these important parameters were not investigated for Ce-UiO-66 and Ce-UiO-66-NO₂. As a reviewer, I would strongly recommend extending these adsorption tests to the other two materials as well. According to Figure 8, their adsorption efficiencies (assuming proper sample activation) are not markedly inferior to that of MOF-808. Therefore, exploring how those additional parameters influence their adsorption performance could provide valuable insight into the underlying adsorption mechanisms and enable a more meaningful comparison between the three Ce-MOF systems. Such comparative studies are crucial for understanding whether the superior performance of MOF-808 is intrinsic or merely a result of experimental conditions.

In summary, the manuscript addresses an important problem and provides potentially useful experimental data, but it currently suffers from significant gaps in experimental characterization and interpretation. The principal deficiencies are: incomplete characterization of two of the three claimed Ce-MOFs; absence of N₂ physisorption and surface-area data; missing information about sample activation before all the adsorption experiments; lack of regeneration studies given apparent amorphization after adsorption; and inadequate discussion of why MOF-808 outperforms the UiO-66 derivatives. I therefore recommend a major revision. The authors should address the points above by performing the missing characterizations (or providing the omitted data), reporting activation protocols and N₂ sorption measurements, conducting regeneration and additional adsorption parameter studies, and substantially expanding the discussion to explain the observed material performance.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors reported Cr(VI) removal using Ce-MOF. The authors investigated isotherm, kinetic and thermodynamic studies. I decided that this work cannot be considered for publication in its current form and needs major revision. Therefore, I suggest the following modifications to the manuscript:

1. MOF-related studies on Cr removal should be included in the introduction.
2. Fig. 2a and b should be on the same scale with the same magnification.
3. Cr weight percentage has to be included in the EDS table.
4. Discuss the results XRD study in detail; investigate the impact on structural (lattice) parameters and crystallite size and micro-strain.
5. The authors need to describe the reason for Ce4+ formation before the Cr adsorption.
6. The reason should be provided for the highest adsorption using Ce-MOF-808 compared to other adsorbents.
7. How does protonated chromium have attraction in an acidic medium?
8. A mechanistic study should be provided.
9. Could you briefly discuss the physical meaning of the PSO model being the best fit?

 

 

 

 

Comments on the Quality of English Language

 

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It's a good work

Reviewer 2 Report

Comments and Suggestions for Authors

The authors responded to most of the comments, after modifying the title and content, omitting the mention of UiO-66 analogues, the current version is suitable for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

 

      Comments on the Quality of English Language