Precise Manipulation of Amino Groups in Zr-MOFs for Efficient Adsorption Performance

Round 1

Reviewer 1 Report

The manuscript "Precise Manipulation of Pores and Defects Structure in Zr-MOFs for Efficient Adsorption Performance" addresses a crucial topic in MOFs and pollutant adsorption. Despite the paper's noteworthy findings, some grammatical and English errors require attention. The manuscript's evaluation and analysis of Chromium adsorption performance are interesting. Authors should address the below points before accepting the manuscript.

 

 

 

1. The BET surface areas are not provided in the manuscript. However, the text mentions that UiO-66-COOH-3 has the highest negative surface charge, the smallest BET surface area, and the lowest adsorption capacity. The authors are advised to include the BET surface area in the text and provide separate images.
2. The authors are advised to explain the absence of a doublet signal at ~3500 cm-1 in the IR spectra of UiO-66-NH2, which interestingly reappears in the Chromium-loaded sample.
3. The pore size distributions in Figure 3 need to be corrected, and the NLDFT-PSD should be presented.
4. In Figure 2, the authors should explain the other signals of the IR spectra of UiO-66-NH2.
5. The experimental section should ensure that the correct chemical formula for anhydrous methanol, which is provided as C2H5OH, is used, and the empirical formula should be included whenever applicable.
6. Consistency in chemical names and proper elaboration of chemical formulas and their nomenclature should also be ensured throughout the manuscript. 
7. Figure 2b should include the yield % in the y-axis in addition to the amount of benzoic acid added. 
8. BET and Langmuir graphs should be provided in the manuscript, and the surface areas for all samples (U-N-0.5, (b) U-N-1, (c) U-N-2, and (d) U-N-4) should be compared, as the authors stated that standard multi-point techniques of N2 adsorption determined BET specific surface areas. 
9. CHNS and ICP-MS analysis should be performed on different amounts of benzoic acid synthesized samples and post-Cr adsorbed samples to explain how benzoic acid moderates the UiO-66 and UiO-66-NH2 synthesis and provide purity. 

I suggest that the manuscript be thoroughly reviewed for clarity as certain sections (experimental, and Synthesis of compounds) appear difficult to comprehend. It would be beneficial to consider conducting a comprehensive English language review to improve the quality of the writing

Author Response

Reviewer #1:

The manuscript "Precise Manipulation of Pores and Defects Structure in Zr-MOFs for Efficient Adsorption Performance" addresses a crucial topic in MOFs and pollutant adsorption. Despite the paper's noteworthy findings, some grammatical and English errors require attention. The manuscript's evaluation and analysis of Chromium adsorption performance are interesting. Authors should address the below points before accepting the manuscript.

The BET surface areas are not provided in the manuscript. However, the text mentions that UiO-66-COOH-3 has the highest negative surface charge, the smallest BET surface area, and the lowest adsorption capacity. The authors are advised to include the BET surface area in the text and provide separate images.

Responses and changes: We sincerely thank you for your valuable feedback. According to the suggestions, we wrote in detail the BET surface area of UiO-66-COOH-1, UiO-66-COOH-2 and UiO-66-COOH-3 prepared by Zhang et al in the introduction section, and we separately listed the specific surface area, pore size and pore volume of various materials in the revised manuscript. (see line 7 to line 11 from the top of page 2)

 

The authors are advised to explain the absence of a doublet signal at ~3500 cm^-1 in the IR spectra of UiO-66-NH₂, which interestingly reappears in the Chromium-loaded sample.

Responses and changes: We appreciate this valuable advice. The signals at 3552~3300 cm^-1 (Figure 11) are characteristic peaks of aromatic amines, in which the primary amine generally has a bimodal signal. The double signal peak of the materials bearing Cr (Ⅵ) may be de to the involvement of -N-H in the adsorption process of Cr (Ⅵ), which distorts the vibration of-N-H and promotes the appearence of the double signal peak.

 

The pore size distributions in Figure 3 need to be corrected, and the NLDFT-PSD should be presented.

Responses and changes: We appreciate the valuable comments. The pore size is mainly an adsorption channel, which affects the adsorption performance of materials with specific surface area and pore size. Accordingly, we provide information on the average distribution of specific surface area, pore size, and pore volume, as shown in Table 1 of the revised manuscript. (see line 1 to line 7 from the bottom of page 6)

   The Table 1 has been provided in the revised manuscript.

 

 

In Figure 2, the authors should explain the other signals of the IR spectra of UiO-66-NH₂.

Responses and changes: Thanks for your comments sincerely. As shown in Figure 11, the signals at 3552~3300 cm^-1 are the characteristic peaks of aromatic amine. The signals at 1612~1523 cm^-1 are the expansion vibration of the benzene ring skeleton. The signals at 1440~1363 cm^-1 are mainly the stretching vibration of O-C-O. The signals at 1284-1234 cm^-1 are the stretching vibration of C-N. The signals at ~768 cm^-1, ~660 cm^-1, ~472 cm^-1, and ~418 cm^-1 are the mixing of Zr-O stretching vibration and the bending vibration of O-H and C-H. This information has been provided in the revised manuscript. (see line 3 to line 15 from the top of page 10)

 

 

The experimental section should ensure that the correct chemical formula for anhydrous methanol, which is provided as C₂H₅OH, is used, and the empirical formula should be included whenever applicable.

Responses and changes: As suggested, the correct chemical formula for anhydrous methanol has been provided in the revised manuscript.

 

 

Consistency in chemical names and proper elaboration of chemical formulas and their nomenclature should also be ensured throughout the manuscript.

Responses and changes: We appreciate the valuable comments. Accordingly, we have carefully polished the entire manuscript to minimize the mistakes.

 

 

Figure 2b should include the yield % in the y-axis in addition to the amount of benzoic acid added.

Responses and changes: Actually, the molecular weight of the product cannot be quantified, and when the amount of benzoic acid is 50 equivalent, the ratio of the actual yield and the theoretical yield calculated based on ZrCl₄ is about 1.3, which is greater than the theoretical yield, so the yield cannot be expressed.

 

 

BET and Langmuir graphs should be provided in the manuscript, and the surface areas for all samples (U-N-0.5, (b) U-N-1, (c) U-N-2, and (d) U-N-4) should be compared, as the authors stated that standard multi-point techniques of N₂ adsorption determined BET specific surface areas.

Responses and changes: Accordingly, we provide information on the average distribution of specific BET surface area, pore size, and pore volume, as shown in Table 1 of the revised manuscript. (see line 1 to line 7 from the bottom of page 6)

   The Table 1 has been provided in the revised manuscript.

 

CHNS and ICP-MS analysis should be performed on different amounts of benzoic acid synthesized samples and post-Cr adsorbed samples to explain how benzoic acid moderates the UiO-66 and UiO-66-NH₂ synthesis and provide purity.

Responses and changes: We agree with the reviewer that characterization techniques should be provided to detect UiO-66 containing different amounts of benzoic acid to further determine the regulation of benzoic acid on UiO-66. According to the suggestions, we conducted DTA-TG testing on UiO-66 containing different amounts of benzoic acid (U-N-x (x=0, 0.5, 1, 2, 4)). From the DTA-TG curves, we found that the DTA differential thermal peak appeared only at ~550 °C, indicating that oxidative cleavage of the ligands was used as the main reaction during combustion. The purity of the synthesized material can be illustrated.

   The sentence “DTA-TG curves of the samples were recorded using a thermal analyzer (NETZSCH STA 449C, Germany).” has been added in the revised manuscript. (see line 18 to line 19 from the top of page 3).

   The sentences “Fig. 5 shows the DTA-TG curves of U-N-x, the introduction of the amino group will reduce the thermal stability of the material. The pyrolysis process of materials is mainly divided into the loss of guest molecules and the collapse of inorganic-organic frame. Compared with the UiO-66 material which begins to collapse at 500 °C, UiO-66-NH₂ series materials collapse about 400 °C because of the oxidability and high activity of amino group. Since the loss of guest molecules cannot be accurately defined stoichiometrically, the ligands lost by MOFs after dehydroxylation are defect-quantified. The structural formula of UiO-66-NH₂ is Zr₆O₆(NH₂BDC)₆, that is, the percentage of ligand loss should be 60% theoretically, assuming the final residual amount at 800 °C is ZrO₂, calculate the temperature defect of U-N-x at 373 °C ~800 °C, x=1, the percentage of ligand loss is the least, 39%, that is, the most linker defects.” have been added in the revised manuscript. (see line 4 from the bottom of page 5 to line 7 from the top of page 6).

   The corresponding Figure 5 and its figure caption have been added in the revised manuscript.

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript reported a study focusing on the impact of amino groups-modified UiO-66 (UiO-66-NH2) for its adsorption properties on Cr(VI) ions in aqueous solution. The results do provide additional insight into understanding the performance and the mechanism of such adsorbents for the treatment of pollutants in water system. However, there are major issues associated with the current manuscript, which need additional work to be done in order for them to be resolved or clarified. Based on such evaluation, this manuscript is not recommended for being considered for publication.

 

The major issues are:

1)      The title of the manuscript seems to indicate that its work focuses on the impact of pore and defects on the adsorption performance of UiO-66. However, most of the results included in the manuscript are originated from the addition of functional group to UiO-66. There is no clear discussion which connects the existence of function group in UiO-66, its impact on pores and defects, and then on adsorption performance.

2)      In the manuscript, there are several claims about the impact of the addition of amino groups. For example, the authors state that based on Figure 3, “Clearly, the introduction of amino groups effectively adjusts the microstructure of UiO-66.” Similarly, it is also stated that Figure 4 indicates “The results confirm that the introduction of amino groups effectively regulates the specific surface area and pore structure of UiO-66.” However, all the samples tested and compared in Figure 3 and Figure 4 are just those with different initial contents of 2-aminoterephthalic acid. The samples without the addition of amino group are missing. Without such information, it is not conclusive to make the aforementioned results. Figure 6 also has similar issues.

 

Below are a few minor issues:

1)      It is recommended that the authors explain the novelty of their research, which is not clearly stated in “introduction” section.

2)      In 2.2, is “200 C for 400 min” degassing condition? N₂ adsorption-desorption cannot be done at this temperature.

3)      “2.3. Synthesis of compounds” should be placed before “2.2. Characterization”.

4)      In “3.1. Synthesis of UiO-66”, it should be clearly stated the purpose of adding benzoic acid in synthesis, which is not explained anywhere in the text.

5)      In Figure 2, the benzoic acid contents are not consistent with what is stated in the text. – “The effects of benzoic acid contents (0, 0.07 mol, 0.21 mol, 0.35 mol, 0.42 mol, and 0.49 mol) on the crystal structure and yield of UiO-66 were studied.”

6)      In 3.2, it is stated “The above UiO-66 with benzoic acid content of 50 equiv was used to adsorb Cr (VI) solution, with a unit adsorption capacity of 45 mg g-1 and a removal rate of 23%.” Please specify the initial concentration of Cr(VI) solution.

7)      In “3.3. Stability of UiO-66-x(NH2)”, please explain why the same concentration of H+ were chosen for testing, instead of different concentrations.

8)      To confirm the successful addition of amino group to UiO-66, it is recommended that the authors should compare the spectra of IR with and without the addition of amino group. This would be the best use of IR. Using IR to confirm the adsorption of Cr is not convincing, let alone the fact that the authors did XPS analysis for the same purpose.

9)      The authors also made both claim that adsorption is mainly as the results of electrostatic interaction as well as that “the adsorbent is mainly monolayer adsorption in the adsorption process” based on Langmuir model. From the reviewer’s understanding, Langmuir model assumes that there are no preferred sites on adsorbents. But electrostatic interaction infers to the assumption that there are preferred sites for adsorption to occur. Some clarification may help.

 

Proofreading is strongly recommended for future submission.

Author Response

Reviewer #2:

This manuscript reported a study focusing on the impact of amino groups-modified UiO-66 (UiO-66-NH₂) for its adsorption properties on Cr(VI) ions in aqueous solution. The results do provide additional insight into understanding the performance and the mechanism of such adsorbents for the treatment of pollutants in water system. However, there are major issues associated with the current manuscript, which need additional work to be done in order for them to be resolved or clarified. Based on such evaluation, this manuscript is not recommended for being considered for publication.

The major issues are:

1) The title of the manuscript seems to indicate that its work focuses on the impact of pore and defects on the adsorption performance of UiO-66. However, most of the results included in the manuscript are originated from the addition of functional group to UiO-66. There is no clear discussion which connects the existence of function group in UiO-66, its impact on pores and defects, and then on adsorption performance.

Responses and changes: We appreciate the valuable comments. Accordingly, the title has been changed to “Precise manipulation of amino groups in Zr-MOFs for efficient adsorption performance” in the revised manuscript. Furthermore, the thermal weight curve of UiO-66-NH₂ material with different amino-functionalized ligand content is provided in the revised manuscript, as shown in Figure 5. In addition, more details about the UiO-66 without amino group, such as yield (Figure 2d), XRD patterns (Figure 3a), SEM images (Figure 3b,3c), nitrogen adsorption-desorption isotherms (Figure 6a), and pore diameter distributions (Figure 6b), have been provided in the revised manuscript to further discuss the influence of function group in UiO-66 on its pores and defects, and adsorption performance.

   The revised Figure 2d and Figure 6 have been updated in the revised manuscript. The corresponding description has been modified in the revised manuscript.

   The Figure 3 and Figure 5 have been added in the revised manuscript. The corresponding description has been added in the revised manuscript.

 

 

2) In the manuscript, there are several claims about the impact of the addition of amino groups. For example, the authors state that based on Figure 3, “Clearly, the introduction of amino groups effectively adjusts the microstructure of UiO-66.” Similarly, it is also stated that Figure 4 indicates “The results confirm that the introduction of amino groups effectively regulates the specific surface area and pore structure of UiO-66.” However, all the samples tested and compared in Figure 3 and Figure 4 are just those with different initial contents of 2-aminoterephthalic acid. The samples without the addition of amino group are missing. Without such information, it is not conclusive to make the aforementioned results. Figure 6 also has similar issues.

Responses and changes: Thanks for your comments sincerely. According to the comments, more details about the UiO-66 without amino group, such as yield (Figure 2d), XRD patterns (Figure 3a), SEM images (Figure 3b,3c), nitrogen adsorption-desorption isotherms (Figure 6a), pore diameter distributions (Figure 6b), and the DTA-TG curves (Figure 5) have been provided in the revised manuscript to further discuss the influence of function group in UiO-66 on its microstructure, specific surface area, pore structure, and adsorption performance.

   The revised Figure 2d and Figure 6 have been updated in the revised manuscript. The corresponding description has been modified in the revised manuscript.

   The Figure 3 and Figure 5 have been added in the revised manuscript. The corresponding description has been added in the revised manuscript.

 

 

Below are a few minor issues:

1) It is recommended that the authors explain the novelty of their research, which is not clearly stated in “introduction” section.

Responses and changes: We appreciate this valuable advice. As suggested, the “introduction” section has been modified to further explain the novelty of this work.

   The sentence “On this basis, 2-amino terephthalic acid was used to endow UiO-66 with amino functional groups to fine-tune the pore and defect of UiO-66.” has been changed to “By introducing amino groups on the surface of UiO-66, the size, microstructure, pores, and defects of UiO-66 were finely regulated.” in the revised manuscript. (see line 13 to line 15 from the bottom of page 2).

 

 

2) In 2.2, is “200 C for 400 min” degassing condition? N₂ adsorption-desorption cannot be done at this temperature.

Responses and changes: Thank you for pointing out this point. This is an expression error, and “200 C for 400 min” is the degassing condition. The temperature for N₂ adsorption-desorption is 77 k. The text in the revised manuscript has been updated.

 

 

3) “2.3. Synthesis of compounds” should be placed before “2.2. Characterization”.

Responses and changes: As suggested, “2.3. Synthesis of compounds” has been placed before “2.2. Characterization”.

 

 

4) In “3.1. Synthesis of UiO-66”, it should be clearly stated the purpose of adding benzoic acid in synthesis, which is not explained anywhere in the text.

Responses and changes: We appreciate this valuable suggestion. Accordingly, the purpose of adding benzoic acid in synthesis has been stated in the revised manuscript.

   The sentence “The concentration of the regulator can affect the crystal structure and morphology^[19].” Has been added in the revised manuscript. (see line 11 from the top of page 4).

[19] A. Schaate, P. Roy, A. Godt, J. Lippke, F. Waltz, M. Wiebcke, P. Behrens, Modulated Synthesis of Zr-Based Metal-Organic Frameworks: From Nano to Single Crystals, Chem. Eur. J. 17 (2011) 6643–6651.

 

 

5) In Figure 2, the benzoic acid contents are not consistent with what is stated in the text. – “The effects of benzoic acid contents (0, 0.07 mol, 0.21 mol, 0.35 mol, 0.42 mol, and 0.49 mol) on the crystal structure and yield of UiO-66 were studied.”

Responses and changes: Actually, the specific mole concentration of the different equivalent of benzoic acid is stated in parentheses to avoid causing misunderstanding again. As suggested, the description has been modified in the revised manuscript to ensure accuracy.

   The sentence “The effects of benzoic acid contents (0, 0.07 mol, 0.21 mol, 0.35 mol, 0.42 mol, and 0.49 mol) on the crystal structure and yield of UiO-66 were studied.” has been changed to “The effect of benzoic acid contents on the crystal structure and yields of UiO-66 were studied.” in the revised manuscript. (see line 12 to line 13 from the top of page 4).

 

 

6) In 3.2, it is stated “The above UiO-66 with benzoic acid content of 50 equiv was used to adsorb Cr (VI) solution, with a unit adsorption capacity of 45 mg g-1 and a removal rate of 23%.” Please specify the initial concentration of Cr(VI) solution.

Responses and changes: The adsorption conditions used when comparing UiO-66 with UiO-66-NH₂ are consistent. Accordingly, the initial concentration of Cr(VI) solution (100 mg·L^-1) has been provided in the revised manuscript.

 

 

7) In “3.3. Stability of UiO-66-x(NH₂)”, please explain why the same concentration of H⁺ were chosen for testing, instead of different concentrations.

Responses and changes: We appreciate the valuable comments. Actually, the “3.3. Stability of UiO-66-x(NH2)” section in this work is mainly used to characterize the chemical stability of the material in the acid-base environment, to ensure the stable presence of the materials, and to explore the influence of pH on the adsorption capacity. The main purposed of discussing different types of acids rather than their concentrations is to ensure that the structure of the adsorbent does not collapse in different types of acid solvents.

 

 

8) To confirm the successful addition of amino group to UiO-66, it is recommended that the authors should compare the spectra of IR with and without the addition of amino group. This would be the best use of IR. Using IR to confirm the adsorption of Cr is not convincing, let alone the fact that the authors did XPS analysis for the same purpose.

Responses and changes: We appreciate the valuable advices. Actually, the introduction of amino functional groups can be illustrated by a significant increase in adsorption amount and a forward shift of the ligand decomposition temperature in the thermal weight (DTA-TG) curve. In this work, the IR spectra of UiO-66 before and after loading Cr are mainly used to further prove whether the adsorption of Cr is in the form of bonding or pure electrostatic attraction.

According to the suggestion, the DTA-TG curves of the samples have been provided in the revised manuscript.

   The sentence “DTA-TG curves of the samples were recorded using a thermal analyzer (NETZSCH STA 449C, Germany).” has been added in the revised manuscript. (see line 18 to line 19 from the top of page 3).

   The sentences “Fig. 5 shows the DTA-TG curves of U-N-x, the introduction of the amino group will reduce the thermal stability of the material. The pyrolysis process of materials is mainly divided into the loss of guest molecules and the collapse of inorganic-organic frame. Compared with the UiO-66 material which begins to collapse at 500 °C, UiO-66-NH₂ series materials collapse about 400 °C because of the oxidability and high activity of amino group. Since the loss of guest molecules cannot be accurately defined stoichiometrically, the ligands lost by MOFs after dehydroxylation are defect-quantified. The structural formula of UiO-66-NH₂ is Zr₆O₆(NH₂BDC)₆, that is, the percentage of ligand loss should be 60% theoretically, assuming the final residual amount at 800 °C is ZrO₂, calculate the temperature defect of U-N-x at 373 °C ~800 °C, x=1, the percentage of ligand loss is the least, 39%, that is, the most linker defects.” have been added in the revised manuscript. (see line 4 from the bottom of page 5 to line 7 from the top of page 6).

   The corresponding Figure 5 and its figure caption have been added in the revised manuscript.

 

 

9) The authors also made both claim that adsorption is mainly as the results of electrostatic interaction as well as that “the adsorbent is mainly monolayer adsorption in the adsorption process” based on Langmuir model. From the reviewer’s understanding, Langmuir model assumes that there are no preferred sites on adsorbents. But electrostatic interaction infers to the assumption that there are preferred sites for adsorption to occur. Some clarification may help.

Responses and changes: We appreciate the valuable comments. The adsorption process is a complex process that includes physical adsorption and chemical adsorption, as well as hydrogen bonding. However, through XPS, it can be concluded that the adsorption mechanism is mainly electrostatic attraction and reduction. By investigating the effect of the initial concentration and temperature on the adsorption, a highly fitted Langmuir model can be obtained, indicating the monolayer adsorption and the electrostatic attraction generated by changes in amino functional groups before and after adsorption.

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

In this paper, the authors reported the preparation of pores Zr₆O₄(OH)₄(2-amino terephthalate)₆ (UiO-66-NH₂) in the condition of various ratio of NH₂-BDC and ZrCl₄ to find useful material for removing the harmful Cr(VI) from industrial wastewater.

The prepared complexes (UiO-66-NH₂) characterized suitably by various measurements such as XRD, SEM and BET surface area.

The adsorption performance for Cr(VI) analyzed by measuring the adsorption capacity and removal rate.

They investigated the effect of pH value, initial concentration, adsorption temperature and adsorption time for Cr(VI) adsorption using the best sample of UiO-66-NH₂, and they found the best condition for the Cr(VI) adsorption.

They also investigated the adsorption mechanism of pores UiO-66-NH₂ for Cr(VI) carefully by analyzing adsorption isotherms and XPS.

Although there are some questions which I don’t understand, the results reported in this paper are useful to develop the field of Cr(VI) elimination.

Minor comments

1.     P. 3, line 13

  The meaning of x is not clear. Does the x mean the ratio of NH₂BDC to ZrCl₄ in the first stage of the synthesis for UiO-66-NH₂?

  Did you try the same experiment for UiO-66?

2.     P.3, line 16

K₂CrO₄      ?

3.     Fig. 4

The inset is not clear.

4.     Fig.6

Please check the figure caption and figure 6 (b).

5.     P. 8, line 2

d       ?

6.     P. 9, line 19

r² → R²

7.     Table 1

K₂ → k₂

8.     Table 2

Please check the first column.

9.     P. 11, line 2

The 398.18ev is higher than 398.08ev.

10.  P. 11, line 17-19

I don’t find the experimental results for the conclusion.

11.  P. 6, line 18

Please check the following sentence.

“The low content of….., …..the  adsorption site.

Author Response

Reviewer #3:

In this paper, the authors reported the preparation of pores Zr₆O₄(OH)₄(2-amino terephthalate)₆ (UiO-66-NH₂) in the condition of various ratio of NH₂-BDC and ZrCl₄ to find useful material for removing the harmful Cr(VI) from industrial wastewater.

The prepared complexes (UiO-66-NH₂) characterized suitably by various measurements such as XRD, SEM and BET surface area.

The adsorption performance for Cr(VI) analyzed by measuring the adsorption capacity and removal rate.

They investigated the effect of pH value, initial concentration, adsorption temperature and adsorption time for Cr(VI) adsorption using the best sample of UiO-66-NH₂, and they found the best condition for the Cr(VI) adsorption.

They also investigated the adsorption mechanism of pores UiO-66-NH₂ for Cr(VI) carefully by analyzing adsorption isotherms and XPS.

Although there are some questions which I don’t understand, the results reported in this paper are useful to develop the field of Cr(VI) elimination.

Minor comments

1. P. 3, line 13

The meaning of x is not clear. Does the x mean the ratio of NH₂BDC to ZrCl₄ in the first stage of the synthesis for UiO-66-NH₂?

Did you try the same experiment for UiO-66?

Responses and changes: We appreciate the valuable advices. The meaning of x is detailed in the "2.2 Synthesis of compound" section. As stated in the original manuscript, “U-N-x (x=0.5, 1, 2, 4. when x=1, n_(NH2BDC)=0.70 mmol) according to the amino functionalized ligands content, the remaining steps were consistent with the preparation process of UiO-66.” (see line 6 to line 8 from the top of page 3). In addition, we tested UiO-66 under the same adsorption conditions. (see line 8 to line 10 from the bottom of page 4)

 

 

2. P.3, line 16

K₂CrO₄      ?

Responses and changes: We appreciate the valuable comments. Accordingly, the “K₂CrO₄” has been changed to “K₂Cr₂O₇”” in the revised manuscript.

 

 

3. Fig. 4

The inset is not clear.

Responses and changes: We agree with review that the inset of Figure 4 is not clear. As suggested, the revised Figure 4 (Figure 6 in the revised manuscript) have been updated in the revised manuscript.

 

 

4. Fig.6

Please check the figure caption and figure 6 (b).

Responses and changes: According to the suggestion, the figure caption and figure 6 (b) (Figure 8a in the revised manuscript) have been corrected.

   The sentence “Figure 6. (a) XRD patterns of UiO-66-NH₂-x (x=0.5,1,2 and 4). Effect of amino (b) and adsorbent (c) content on adsorption.” has been changed to “Figure 8. (a) Effect of amino ligand concentration on the adsorption amount of Cr (VI). (b) Effect of ad-sorbent content on adsorption capacity.” in the revised manuscript. (see line 1 to line 3 from the bottom of page 8).

 

 

5. P. 8, line 2

d ?

Responses and changes: Accordingly, “d” has been deleted in the revised manuscript.

 

 

6. P. 9, line 19

r2 → R2

Responses and changes: Updated.

 

7. Table 1

K2 → k2

Responses and changes: Updated.

 

8. Table 2

Please check the first column.

Responses and changes: According to the comments, the first column of Table 2 (Table 3 in the revised manuscript) has been checked.

   The revised Table 3 has been updated in the revised manuscript.

 

 

9. P. 11, line 2

The 398.18ev is higher than 398.08ev.

Responses and changes: We appreciate the valuable comments. Accordingly, the corresponding description has been modified in the revised manuscript. (see line 5 from the bottom of page 12 to line 2 from the top of page 13).

 

 

10. P. 11, line 17-19

I don’t find the experimental results for the conclusion.

Responses and changes: As suggested, the corresponding conclusion has been modified in the revised manuscript.

 

 

11. P. 6, line 18

Please check the following sentence.

“The low content of….., …..the  adsorption site.

Responses and changes: Thanks for your comments sincerely. Accordingly, the sentence The low content of adsorbent, the less the adsorption sites.” has been changed to “This is because the increase of adsorbent in the system can bring more active site, thus capturing more Cr (VI) ions.” in the revised manuscript. (see line 9 to line 10 from the bottom of page 8).

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

 

It is appreciated for the authors’ effort in providing additional results and revising the manuscript. The study presented in the manuscript is aimed at explaining the expectation that the manipulation of Zr-MOFs by the ligand functionalization could improve MOFs’ adsorption performance. As such, the first question to be addressed by such a study should be whether or not the functionalization could achieve such a goal. As stated in the reviewer’s comment on the previous version of the manuscript, the authors need present the adsorption results between the samples with amino groups and those without. However, the revised manuscript is still lack of such a comparison.

 

This comparison can be also achieved by using the data from the reference. In the section of 3.2, the authors state that “The above UiO-66 with benzoic acid content of 50 equiv was used to adsorb 100 mg·L-1 Cr (VI) solution, with a unit adsorption capacity of 45 mg·g-1 and a removal rate of 23%. It can be preliminarily inferred that only 23% of Cr (VI) adsorption is the adsorption site. However, Wu[23] used UiO-66-NH2 to remove 100 mg·L-1 Cr (VI) solution, when pH=6.5, the unit adsorption capacity reached 32.36 mg·g-1. Based on this, the ligand of UiO-66 was replaced with 2-amino terephthalic acid containing amino functional group.”. Based on such a statement, it seems that the pristine UiO-66 shows a larger capacity that the functionalized one (45 mg·g-1 vs 32.36 mg·g-1). If it is the case, it will be less appealing to study the functionalization. Before such a fundamental question is answered, therefore, the manuscript is not recommended for publication.

Extensive editing of English language is recommended.

Author Response

Reviewer #2:

It is appreciated for the authors’ effort in providing additional results and revising the manuscript. The study presented in the manuscript is aimed at explaining the expectation that the manipulation of Zr-MOFs by the ligand functionalization could improve MOFs’ adsorption performance. As such, the first question to be addressed by such a study should be whether or not the functionalization could achieve such a goal. As stated in the reviewer’s comment on the previous version of the manuscript, the authors need present the adsorption results between the samples with amino groups and those without. However, the revised manuscript is still lack of such a comparison.

   This comparison can be also achieved by using the data from the reference. In the section of 3.2, the authors state that “The above UiO-66 with benzoic acid content of 50 equiv was used to adsorb 100 mg·L^-1 Cr (VI) solution, with a unit adsorption capacity of 45 mg·g^-1 and a removal rate of 23%. It can be preliminarily inferred that only 23% of Cr (VI) adsorption is the adsorption site. However, Wu [23] used UiO-66-NH₂ to remove 100 mg·L^-1 Cr (VI) solution, when pH=6.5, the unit adsorption capacity reached 32.36 mg·g^-1. Based on this, the ligand of UiO-66 was replaced with 2-amino terephthalic acid containing amino functional group.”. Based on such a statement, it seems that the pristine UiO-66 shows a larger capacity that the functionalized one (45 mg·g^-1 vs 32.36 mg·g^-1). If it is the case, it will be less appealing to study the functionalization. Before such a fundamental question is answered, therefore, the manuscript is not recommended for publication.

Responses and changes: We appreciate the valuable comments.

Actually, we report a simple hydrothermal method to fabricate Zr-MOFs (UiO-66), which are innovatively modified by amino groups to enhance the adsorption capacity of Cr (VI). The effects of amino group content, adsorbent content, pH value of the solution, adsorption time, and adsorption temperature on the adsorption process are systematically investigated. Importantly, the pore structure and defect structure of UiO-66 can be finely regulated by regulating the amino modification process. The adsorption process is analyzed by the kinetic simulation model and isotherm model. Furthermore, the adsorption capacity and stability of amino groups-modified UiO-66 (UiO-66-NH₂) are greatly improved.

  This work enriches the adsorbent-family and presents an alternative and efficient route to accelerate the adsorption effect of MOF-based adsorbent. Moreover, it indicates that the regulation of pore and defect structure for MOF through functional group modification is an effective strategy to enhance the intrinsic activity of MOF-based materials toward heavy metal removal.

For this comment, Wu [23] reported the UiO-66-NH₂ with unit adsorption capacity of 32.36 mg·g^-1 (4 mg·L^-1 Cr (VI) solution). Obviously, the concentration of Cr (VI) solution at 4 mg·L^-1 is too low, which does not represent the removal of Cr (VI) from industrial wastewater. At the same time, this work was actually carried out five years ago. According to this comment, we have provided a more reliable reference work in the revised manusciript. Specifically, Zhang used UiO-66-NH₂ to remove 100 mg·L^-1 Cr (VI) solution, and the q_m from Langmuir model was 252 mg·g^-1. (Korean J. Chem. Eng. 39 (2022) 1839–1849.)

   The sentence “Zhang^[23] used UiO-66-NH₂ to remove 100 mg·L^-1 Cr (VI) solution, and the q_m from Langmuir model was 252 mg·g^-1.” Has been added in the revised manuscript. (see line 6 to line 7 from the top of page 4).

[23] X. Zhang, S. Zhang, G. Ouyang, R. Han, Removal of Cr(VI) from solution using UiO-66-NH₂ prepared in a green way, Korean J. Chem. Eng. 39 (2022) 1839–1849.

   We really hope the reviewers can see the significant effort we have put into completing and revising this manuscript. At the same time, this work has its own characteristics and innovation, as mentioned above. Thus, we hope the reviewers can give us the opportunity to publish.

 

Author Response File: Author Response.docx

Reviewer 3 Report

The revised manuscript is better than the original one.

However, there are some comments as follows.

1.     P. 3, line 15

77 k → 77 K

2.     P. 7, line fig. 6

The old figures (a)-(d) should be removed.

3.     P. 8, line 10, fig. 8 (b)

The following sentence is not consistent with the fig. 8.

“the adsorption capacity increases with the increase of the content of adsorption”

Author Response

Reviewer #3:

The revised manuscript is better than the original one.

However, there are some comments as follows.

1. P. 3, line 15

77 k → 77 K

Responses and changes: The text has been updated.

 

2. P. 7, line fig. 6

The old figures (a)-(d) should be removed.

Responses and changes: According to the suggestion, we have removed the old figures 6(a)-6(d).

 

3. P. 8, line 10, fig. 8 (b)

The following sentence is not consistent with the fig. 8.

“the adsorption capacity increases with the increase of the content of adsorption”

Responses and changes: Thanks for your comments sincerely. Accordingly, the sentence “the adsorption capacity increases with the increase of the content of adsorption” has been changed to “the adsorption capacity decreases with the increase of the content of adsorption” in the revised manuscript. (see line 5 to line 6 from the top of page 8).

 

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

N/A

Further editing is recommended.

Author Response

Reviewer #2:

Comments on the Quality of English Language

Further editing is recommended.

Responses and changes: We really appreciate your valuable suggestions, which have made our paper greatly improved and suitable for publication.

  According to the suggestion, we have carefully polished the entire manuscript to minimize the mistakes.

Author Response File: Author Response.docx