Metal–Organic Frameworks-Based Membranes with Special Wettability for Oil–Water Separation: A Review

Round 1

Reviewer 1 Report

Review

 

Manuscript ID:  coatings-2480737

Journal: Coatings

Title:

 

 Metal-organic frameworks-based membranes with special wet-2 tability for oil-water separation: A review

 

 

The author try to study  efficient methods to purify the oily  wastewater. In recent years, metal-organic frameworks (MOFs)-based membranes have emerged as  a promising solution for oil-water separation due to the low density, high porosity, and high surface  area of MOFs. This paper provides a comprehensive overview of the oil-water separation mechanism and the utilization of MOFs-based membranes, including ZIF series, UiO series, MIL series, etc. Furthermore, this review summarizes representative studies on each series of MOFs and gener-18 alizes the common preparation methods for MOFs-based oil-water separation membranes. Addi-19 tionally, we analyze the challenges faced by MOFs-based membranes in oil-water separation and 20 offers insights into their future developments and applications. But there are some comments as the following:

 

1-    The author must add the novelty of review in abstract

2-    The authors must add more references in introduction part

3-    The author must permissions for reuse figures 2-8

4-    The author must modify figure 6 because no units appear at figure

 

5-    In Conclusions and , authors must add future prospective before references as separate section

 

 

 

Recommendation: Minor revision

 

 

some grammer errors must be changed

Author Response

The author try to study efficient methods to purify the oily  wastewater. In recent years, metal-organic frameworks (MOFs)-based membranes have emerged as a promising solution for oil-water separation due to the low density, high porosity, and high surface area of MOFs. This paper provides a comprehensive overview of the oil-water separation mechanism and the utilization of MOFs-based membranes, including ZIF series, UiO series, MIL series, etc. Furthermore, this review summarizes representative studies on each series of MOFs and generalizes the common preparation methods for MOFs-based oil-water separation membranes. Additionally, we analyze the challenges faced by MOFs-based membranes in oil-water separation and offers insights into their future developments and applications. But there are some comments as the following:

1. The author must add the novelty of review in abstract.

Thank you very much for your valuable advice. There may be some shortcomings in abstract. The following is the revised abstract.

The presence of oily wastewater poses a significant threat to both the ecological environment and public health. In order to solve this problem, the design and preparation of efficient oil-water separation membrane is very important. Metal organic frameworks (MOFs) are currently a promising material for oil-water separation due to their tunable wettability, adjustable pore size and also low density, high porosity and high surface area. Therefore, MOFs-based membranes show great potential in the field of oil-water separation. In this paper, we firstly introduce the oil-water separation mechanism, and then comprehensively summarize the common preparation methods of MOFs-based oil-water separation membranes, and the research progress of different MOFs-based membranes, including ZIF series, UiO series, MIL series, etc.. Finally, we also analyze the challenges faced by MOFs-based membranes in oil-water separation and provide an outlook on their future development and application.

 

2. The authors must add more references in introduction part.

We sincerely appreciate the valuable comments. We have carefully checked the literature and added more new references in the introduction part of the revised manuscript.

Metal-organic frameworks (MOFs), a novel class of porous materials made of metal ions and organic ligands [16-18], hold huge potential in the field of oil-water separation. MOFs show significant advantages over zeolites, activated carbon and other porous materials in term of adjusted pore structure and properties [19-21]. Firstly, MOFs have the advantage of easy microstructure construction and pore size adjustment [22]. Secondly, their surface wettability can be easily tuned [23]. Additionally, due to their low density, high porosity, and high surface area, MOFs possess good adsorption properties, enabling them to effectively address complex pollutants such as heavy metal ions, dyes, drug, antibiotics and other pollutants in wastewater [24-29].

[19] Venkatesan, N.; Yuvaraj, P.; Fathima, N. N., Fabrication of non-fluorinated superhydrophobic and flame retardant porous material for efficient oil/water separation[J]. Mater. Chem. Phys. 2022, 286, 126190. https://doi.org/10.1016/j.matchemphys.2022.126190.

[20] Borazjani, A. R.; Akhlaghi, B.; Abbasi, M.; Osfouri, S., Investigation of petroleum products dehydration using natural zeolite and activated carbon[J]. Pet. Sci. Technol. 2023, 20. https://doi.org/10.1080/10916466.2023.2209124.

[21] Chen, X. P.; Li, Y. M.; Yang, Y. S.; Zhang, D.; Guan, Y. H.; Bao, M. T.; Wang, Z. N., A super-hydrophobic and antibiofouling membrane constructed from carbon sphere-welded MnO2 nanowires for ultra-fast separation of emulsion[J]. J. Membr. Sci. 2022, 653, 120514. https://doi.org/10.1016/j.memsci.2022.120514.

[24] Bhuyan, A.; Ahmaruzzaman, M., Metal-organic frameworks: A new generation potential material for aqueous environmental remediation[J]. Inorg. Chem. Commun. 2022, 140, 109436. https://doi.org/10.1016/j.inoche.2022.109436.

[25] Beydaghdari, M.; Saboor, F. H.; Babapoor, A.; Karve, V. V.; Asgari, M., Recent Advances in MOF-Based Adsorbents for Dye Removal from the Aquatic Environment[J]. Energies 2022, 15 (6), 34. https://doi.org/10.3390/en15062023.

[26] Dong, X. Y.; Li, Y. Y.; Li, D. Q. C.; Liao, D. H.; Qin, T. R.; Prakash, O.; Kumar, A.; Liu, J. Q., A new 3D 8-connected Cd(II) MOF as a potent photocatalyst for oxytetracycline antibiotic degradation[J]. Crystengcomm 2022, 24 (39), 6933-6943. https://doi.org/10.1039/d2ce01121b.

[27] Zheng, M. Y.; Chen, J. Y.; Zhang, L.; Cheng, Y.; Lu, C. Y.; Liu, Y. F.; Singh, A.; Trivedi, M.; Kumar, A.; Liu, J. Q., Metal organic frameworks as efficient adsorbents for drugs from wastewater[J]. Mater. Today Commun. 2022, 31, 103514. https://doi.org/10.1016/j.mtcomm.2022.103514.

[28] Li, L. T.; Zou, J. F.; Han, Y. T.; Liao, Z. H.; Lu, P. F.; Nezamzadeh-Ejhieh, A.; Liu, J. Q.; Peng, Y. Q., Recent advances in Al(iii)/In(iii)-based MOFs for the detection of pollutants[J]. New J. Chem. 2022, 46 (41), 19577-19592. https://doi.org/10.1039/d2nj03419k.

[29] Ke, F.; Pan, A.; Liu, J. Q.; Liu, X. X.; Yuan, T.; Zhang, C. Y.; Fu, G. N.; Peng, C. Y.; Zhu, J. F.; Wan, X. C., Hierarchical camellia-like metal-organic frameworks via a bimetal competitive coordination combined with alkaline-assisted strategy for boosting selective fluoride removal from brick tea[J]. J. Colloid Interface Sci. 2023, 642, 61-68. https://doi.org/10.1016/j.jcis.2023.03.137.

 

3. The author must permissions for reuse figures 2-8.

Thank you very much for your comment. We have applied for the copyright of the images in Figures 3-8, which were submitted together with the previous paper submission. However, one of them, Figure 2, depicts the underlying theory of solid surface roughness and wettability in air for the Young’s equtation, Wenzel’s equation and Cassie’s equation. Figure 4 is a schematic diagram of the situation when the membrane separates free oil-water mixtures and emulsions. Figure 2 and Figure 4 are our own diagrams made by ourselves and Figure 7C is an open source article. Therefore no copyright has been applied to these three images.

 

4. The author must modify figure 6 because no units appear at figure.

Thank you very much for your comment. We took a closer look at Figure 6 and found that the SEM image in Figure 6A does not have units. The image was taken at 10 μm, but it was not fit to add due to the scale of the image.

 

5. In conclusion and, authors must add future prospective before references as separate section.

Thank you very much for your valuable advice. The conclusions and outlook are explored in more depth in response to this question.

In this review, the mechanisms of oil-water separation are described and the preparation and recent progress of MOFs-based oil-water separation membranes are summarized. There are diverse methods for the preparation of MOFs, many of which have the potential for large-scale industrialization, opening up new avenues for future practical engineering applications. Furthermore, this paper focuses on various MOF-based membranes for oil-water separation, such as the ZIF series, the UiO series and the MIL series, and presents the functional properties and their related applications. The versatility of MOFs gives these oil-water separation membranes superior performances, such as antifouling properties, self-healing capabilities, high adsorption of heavy metal ions and dyes, and photocatalytic degradation of organic pollutants. To sum up, it can be concluded that MOFs-based membranes have a broader application in the treatment of oily wastewater.

However, in the existing literatures, most studies using MOF-based oil-water separation membranes are somewhat flawed and the following efforts are still needed to solve the environmental problems of wastewater. (1) The long-term performance of MOFs-based membranes has been neglected, especially in continuous oil-water separation processes. Researchers can therefore focus on ways to improve the long-term durability of the membranes. (2) Most MOFs materials are unstable in wet environments due to weak coordination between their own metals and organic ligands. How to improve the stability of MOFs without affecting their structural properties remains to be investigated. (3) The specific wastewater environment is very complex and may contain organic molecules, heavy metals, microorganisms, etc.. MOFs-based membranes need to be more stable and fouling resistant in the complex components. Based on this, there is a need for researchers to create materials with better stability and self-cleaning properties. (4) MOFs-based membranes should be used in a wider range of applications, and they must also be able to treat actual wastewater from municipal wastewater to industrial wastewater, broadening their practical applications. (5) In addition, there is still a lack of longitudinal comparisons of data on the separation efficiency, mechanical and chemical stability and pollution resistance of MOF-based membranes based on the separation of complex oil-water mixtures, which will need to be reviewed by subsequent researchers as this research progresses.

Author Response File: Author Response.pdf

Reviewer 2 Report

The study is effectively communicates the results. Such finding is important and could be expected to be of great interest to readers. I recommend publication after minor revision based on the following comments.
1. In the introduction, it should be discussed detailed about the difference of this material with other materials. 

2. The section of Conclusion and outlooks is too simple, the authors should give deeper insights into the advantages, loopholes and future development direction of MOFs.

3. Some of the work on the sensor for heavy metal ions and dyes. The versatility of MOFs also confers multifunctionality to oil-water separation materials, including photocatalysis, should be cited, such as J. Colloid. Interf. Sci, 2023, 642, 61–68; New J. Chem., 2022, 46, 19577–19592; CrystEngComm, 2022, 24, 6933–6943; Mater. Today. Commum., 2022, 31,103514

4.  The author needs to highlight the novel of work in abstract

5, There should be a new paragraph in this review article based on compares the findings of the most recent study.

6. There are many things wrong with English writing. The author needs to revise the manuscript thoroughly

  

 

see the below comment

Author Response

The study is effectively communicates the results. Such finding is important and could be expected to be of great interest to readers. I recommend publication after minor revision based on the following comments.

1. In the introduction, it should be discussed detailed about the difference of this material with other materials. 

Thank you very much for your valuable advice. The following is the revised content in manuscript.

Metal-organic frameworks (MOFs), a novel class of porous materials made of metal ions and organic ligands [16-18], hold huge potential in the field of oil-water separation. MOFs show significant advantages over zeolites, activated carbon and other porous materials in term of adjusted pore structure and properties[19-21].

[16] Kitao, T.; Zhang, Y. Y.; Kitagawa, S.; Wang, B.; Uemura, T., Hybridization of MOFs and polymers[J]. Chem. Soc. Rev. 2017, 46 (11), 3108-3133. https://doi.org/10.1039/c7cs00041c.

[17] Wang, C. H.; Liu, X. L.; Demir, N. K.; Chen, J. P.; Li, K., Applications of water stable metal-organic frameworks[J]. Chem. Soc. Rev. 2016, 45 (18), 5107-5134. https://doi.org/10.1039/c6cs00362a.

[18] Ryu, U.; Jee, S.; Rao, P. C.; Shin, J.; Ko, C.; Yoon, M.; Park, K. S.; Choi, K. M., Recent advances in process engineering and upcoming applications of metal-organic frameworks[J]. Coord. Chem. Rev. 2021, 426, 213544. https://doi.org/10.1016/j.ccr.2020.213544.

[19] Venkatesan, N.; Yuvaraj, P.; Fathima, N. N., Fabrication of non-fluorinated superhydrophobic and flame retardant porous material for efficient oil/water separation[J]. Mater. Chem. Phys. 2022, 286, 126190. https://doi.org/10.1016/j.matchemphys.2022.126190.

[20] Borazjani, A. R.; Akhlaghi, B.; Abbasi, M.; Osfouri, S., Investigation of petroleum products dehydration using natural zeolite and activated carbon[J]. Pet. Sci. Technol. 2023, 20. https://doi.org/10.1080/10916466.2023.2209124.

[21] Chen, X. P.; Li, Y. M.; Yang, Y. S.; Zhang, D.; Guan, Y. H.; Bao, M. T.; Wang, Z. N., A super-hydrophobic and antibiofouling membrane constructed from carbon sphere-welded MnO₂ nanowires for ultra-fast separation of emulsion[J]. J. Membr. Sci. 2022, 653, 120514. https://doi.org/10.1016/j.memsci.2022.120514.

 

2. The section of Conclusion and outlooks is too simple, the authors should give deeper insights into the advantages, loopholes and future development direction of MOFs.

Thank you very much for your advice. We have made changes to the conclusions and outlook. The revised manuscript is shown below.

In this review, the mechanisms of oil-water separation are described and the preparation and recent progress of MOFs-based oil-water separation membranes are summarized. There are diverse methods for the preparation of MOFs, many of which have the potential for large-scale industrialization, opening up new avenues for future practical engineering applications. Furthermore, this paper focuses on various MOF-based membranes for oil-water separation, such as the ZIF series, the UiO series and the MIL series, and presents the functional properties and their related applications. The versatility of MOFs gives these oil-water separation membranes superior performances, such as antifouling properties, self-healing capabilities, high adsorption of heavy metal ions and dyes, and photocatalytic degradation of organic pollutants. To sum up, it can be concluded that MOFs-based membranes have a broader application in the treatment of oily wastewater.

However, in the existing literatures, most studies using MOF-based oil-water separation membranes are somewhat flawed and the following efforts are still needed to solve the environmental problems of wastewater. (1) The long-term performance of MOFs-based membranes has been neglected, especially in continuous oil-water separation processes. Researchers can therefore focus on ways to improve the long-term durability of the membranes. (2) Most MOFs materials are unstable in wet environments due to weak coordination between their own metals and organic ligands. How to improve the stability of MOFs without affecting their structural properties remains to be investigated. (3) The specific wastewater environment is very complex and may contain organic molecules, heavy metals, microorganisms, etc.. MOFs-based membranes need to be more stable and fouling resistant in the complex components. Based on this, there is a need for researchers to create materials with better stability and self-cleaning properties. (4) MOFs-based membranes should be used in a wider range of applications, and they must also be able to treat actual wastewater from municipal wastewater to industrial wastewater, broadening their practical applications. (5) In addition, there is still a lack of longitudinal comparisons of data on the separation efficiency, mechanical and chemical stability and pollution resistance of MOF-based membranes based on the separation of complex oil-water mixtures, which will need to be reviewed by subsequent researchers as this research progresses.

 

3. Some of the work on the sensor for heavy metal ions and dyes. The versatility of MOFs also confers multifunctionality to oil-water separation materials, including photocatalysis, should be cited, such as J. Colloid. Interf. Sci, 2023, 642, 61–68; New J. Chem., 2022, 46, 19577–19592; CrystEngComm, 2022, 24, 6933–6943; Mater. Today. Commum., 2022, 31,103514.

Thank you very much for your valuable advice. The following is the revised content in manuscript.

due to their low density, high porosity, and high surface area, MOFs possess good adsorption properties, enabling them to effectively address complex pollutants such as heavy metal ions, dyes, drug, antibiotics and other pollutants in wastewater [24-29].

[24] Bhuyan, A.; Ahmaruzzaman, M., Metal-organic frameworks: A new generation potential material for aqueous environmental remediation[J]. Inorg. Chem. Commun. 2022, 140, 109436. https://doi.org/10.1016/j.inoche.2022.109436.

[25] Beydaghdari, M.; Saboor, F. H.; Babapoor, A.; Karve, V. V.; Asgari, M., Recent Advances in MOF-Based Adsorbents for Dye Removal from the Aquatic Environment[J]. Energies 2022, 15 (6), 34. https://doi.org/10.3390/en15062023.

[26] Dong, X. Y.; Li, Y. Y.; Li, D. Q. C.; Liao, D. H.; Qin, T. R.; Prakash, O.; Kumar, A.; Liu, J. Q., A new 3D 8-connected Cd(II) MOF as a potent photocatalyst for oxytetracycline antibiotic degradation[J]. Crystengcomm 2022, 24 (39), 6933-6943. https://doi.org/10.1039/d2ce01121b.

[27] Zheng, M. Y.; Chen, J. Y.; Zhang, L.; Cheng, Y.; Lu, C. Y.; Liu, Y. F.; Singh, A.; Trivedi, M.; Kumar, A.; Liu, J. Q., Metal organic frameworks as efficient adsorbents for drugs from wastewater[J]. Mater. Today Commun. 2022, 31, 103514. https://doi.org/10.1016/j.mtcomm.2022.103514.

[28] Li, L. T.; Zou, J. F.; Han, Y. T.; Liao, Z. H.; Lu, P. F.; Nezamzadeh-Ejhieh, A.; Liu, J. Q.; Peng, Y. Q., Recent advances in Al(iii)/In(iii)-based MOFs for the detection of pollutants[J]. New J. Chem. 2022, 46 (41), 19577-19592. https://doi.org/10.1039/d2nj03419k.

[29] Ke, F.; Pan, A.; Liu, J. Q.; Liu, X. X.; Yuan, T.; Zhang, C. Y.; Fu, G. N.; Peng, C. Y.; Zhu, J. F.; Wan, X. C., Hierarchical camellia-like metal-organic frameworks via a bimetal competitive coordination combined with alkaline-assisted strategy for boosting selective fluoride removal from brick tea[J]. J. Colloid Interface Sci. 2023, 642, 61-68. https://doi.org/10.1016/j.jcis.2023.03.137.

 

4. The author needs to highlight the novel of work in abstract.

Thank you very much for your valuable advice. We have made changes to the abstract. The following is the modification.

The presence of oily wastewater poses a significant threat to both the ecological environment and public health. In order to solve this problem, the design and preparation of efficient oil-water separation membrane is very important. Metal organic frameworks (MOFs) are currently a promising material for oil-water separation due to their tunable wettability, adjustable pore size and also low density, high porosity and high surface area. Therefore, MOFs-based membranes show great potential in the field of oil-water separation. In this paper, we firstly introduce the oil-water separation mechanism, and then comprehensively summarize the common preparation methods of MOFs-based oil-water separation membranes, and the research progress of different MOFs-based membranes, including ZIF series, UiO series, MIL series, etc.. Finally, we also analyze the challenges faced by MOFs-based membranes in oil-water separation and provide an outlook on their future development and application.

 

5. There should be a new paragraph in this review article based on compares the findings of the most recent study.

Thank you very much for your valuable advice. We have taken your suggestions seriously. Because of the framing of the article, it was not appropriate to compare recent studies in a separate paragraph, so we have added some recent studies to different paragraphs.

MOFs show significant advantages over zeolites, activated carbon and other porous materials in term of adjusted pore structure and properties [20].

Additionally, due to their low density, high porosity, and high surface area, MOFs possess good adsorption properties, enabling them to effectively address complex pollutants such as heavy metal ions, dyes, drug, antibiotics and other pollutants in wastewater [29].

In addition, MOF-808 has adsorption property for copper ion and there are some recent research such as Cr-soc-MOF and HKUST-1 which can be applied to oil-water separation, especially SSM@HKUST-1 can separate oil-in-water and water-in-oil emulsions on demand [82,84-85].

[20] Borazjani, A. R.; Akhlaghi, B.; Abbasi, M.; Osfouri, S., Investigation of petroleum products dehydration using natural zeolite and activated carbon[J]. Pet. Sci. Technol. 2023, 20. https://doi.org/10.1080/10916466.2023.2209124.

[29] Ke, F.; Pan, A.; Liu, J. Q.; Liu, X. X.; Yuan, T.; Zhang, C. Y.; Fu, G. N.; Peng, C. Y.; Zhu, J. F.; Wan, X. C., Hierarchical camellia-like metal-organic frameworks via a bimetal competitive coordination combined with alkaline-assisted strategy for boosting selective fluoride removal from brick tea[J]. J. Colloid Interface Sci. 2023, 642, 61-68. https://doi.org/10.1016/j.jcis.2023.03.137.

[82] Obaid, M.; Alsadun, N.; Shekhah, O.; Almahfoodh, S.; Zhou, S.; Ghaffour, N.; Eddaoudi, M., Deployment of superhydrophilic and super-antifouling Cr-soc-MOF-1-based membrane for ultrafast separation of stabilized oil-in-water emulsions[J]. ACS Appl. Mater. Interfaces 2023, 15 (25), 31067-31076. https://doi.org/10.1021/acsami.3c05285.

[84] Deng, Y. Y.; Bian, H. Z.; Dai, M.; Liu, X.; Peng, C. S., Underwater superoleophobic HKUST-1/PDA@SM membrane with excellent stability and anti-fouling performance for oil-in-water emulsion separation[J]. J. Membr. Sci. 2023, 678, 121655. https://doi.org/10.1016/j.memsci.2023.121655.

[85] Li, J. H.; Ding, S. L.; Wu, J.; Guo, Z. G., Underwater superoleophobic and underoil superhydrophilic copper benzene-1,3,5-tricarboxylate (HKUST-1) mesh for self-cleaning and on-demand emulsion separation[J]. Langmuir 2023, 39 (17), 6201-6210. https://doi.org/10.1021/acs.langmuir.3c00331.

 

6. There are many things wrong with English writing. The author needs to revise the manuscript thoroughly.

Thanks for your suggestion. We tried our best to improve the manuscript and made some changes to the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in yellow in the revised paper. We appreciate for reviewer’s warm work earnestly and hope that the correction will meet with approval.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors presented a review article of the MOF based membranes technology for the oil contaminants separation from the wastewater. The manuscript is well written and organized. I have some comments to improve the manuscript.

My comments as below:

1.       Please add space between the words and references in for the whole citations for example: emulsion[63] to emulsion [63].

2.       Please add the concentration of pollutants and the catalyst in all the tables.

3.       Please provide more details for each deposition method. How the material formed by each method was not explained clearly in detail.

4.       Please update all the old references with recent references.

5.       The conclusion and outlook are too short. The authors must expand the discussion here. The potential and the future of this technology should be stated clearly. In addition, the challenges along with limitations must be included.

Extensive editing of English language required

Author Response

The authors presented a review article of the MOF based membranes technology for the oil contaminants separation from the wastewater. The manuscript is well written and organized. I have some comments to improve the manuscript.

My comments as below:

1. Please add space between the words and references in for the whole citations for example: emulsion[63] to emulsion [63].

Thank you very much for your careful review. We apologize for our carelessness. We have carefully revised the manuscript according to your comment. Thanks for your corrections.

 

2. Please add the concentration of pollutants and the catalyst in all the tables.

Thank you very much for your valuable advice. We checked all the tables carefully and then we added oil to water volume ratios as well as pollutant concentrations to all the tables, but many of the literature did not mention catalysts for the time being, so they were not added.

Table 1. A summary and comparison of ZIF series membranes in oil/water separation.

Material	Preparation method	Liquids of separation	O/W  Volume ratio	Separation efficiency (%)	Permeation flux (L·m-2·h-1)	Ref.
ZIF-8/GO	LBL	Toluene in water	—	—	110±6	[58]
ZIF-67@Cu(OH)2	In-suit growth	Oil-water mixture	1:1	99	23854	[59]
ZIF-71/PVDF-HFP	Electrospinning and in-suit growth	Water-in-oil emulsions	99:1	>99	6577.68	[60]
ZIF-9-III@PVDF	Phase inversion	Water-in-oil emulsions	99:1	99.8	14.3	[61]
ZIF-90	Solvothermal method	Water-in-oil emulsions	—	99.98	1260	[62]
CPE-ZIF-7-coated SSM	Hydrothermal growth and dip coating	Water-in-oil emulsion	9:1	99.9	1056	[63]
ZIF-8/PAN	Coprecipitation and electrospinning	Oil-in-water emulsions	1:50	99.92	2514	[52]
ZIF-L(Co)@SSM	One-pot method	Polar/nonpolar liquids	1:1	>96	—	[64]
ZIF-L(Zn)@SSM	Pre-seeding and secondary growth	Oil-water mixture	—	99.98	1.24×105	[65]

 

Table 2. A summary and comparisons of UiO series membranes in oil/water separation (Pollutant concentrations are shown in brackets for other functions).

Material	Preparation method	Liquids of separation	O/W  Volume ratio	Separation efficiency (%)	Permeation flux  (L·m-2·h-1)	Other functions	Ref.
OPASS/UiO-66	Electrospinning	Water-in-oil emulsions	40:1	98.44	636	Adsorption of dyes (10 mg/L)and heavy metal ions (4 mg/L)	[66]
SSM/UiO-66-NH2/CMn	Hydrothermal	Oil-water mixtures	1:1	>99	—	Degradation of dyes (10mg/L)	[67]
UiO-66-F4@rGO	Dip-coating	Water-in-oil enulsions	—	99.73	990.45	—	[68]
MXene@UiO-66-(COOH)2	Vacuum assisted self-assembly	Toluene-in-water emulsion	—	99.54	498.91	—	[69]

 

Table 3. A summary and comparisons of MIL series membranes in oil/water separation (Pollutant concentrations are shown in brackets for other functions).

Material	Preparation method	Liquids of separation	O/W  Volume ratio	Separation efficiency  (%)	Permeation flux (L·m-2·h-1)	Other functions	Ref.
SPAN@GO/M88A	Hydrothermal	Oil-in-water emulsions	1:100	>99	920-7083 (20-100kpa)	Photo-Fenton self-cleaning properties	[70]
MIL-53(Fe)	Solvothermal and electrospinning	Oil-in-water emulsions	1:100; 1:20; 1:10	>90	380	Dye degradation (100 ppm)	[71]
NH2-MIL-125@PAA	Hydrothermal and vacuum-assisted self-assembly process	Oil-in-water emulsions	1:100	99.5	500	Dyes separation (20 ppm)	[72]
PAN/PEI/MIL	Electrospinning and hydrothermal	Oil-in-water emulsions	1:1	>99	4000	Dyes adsorption (10 ppm)	[73]
GO/NH2-MIL-101(Fe)	One-pot method	gasoline-water mixtures	1:1408	—	120	Tetracycline degradation (100 mg/L)	[74]
Ag NPs@MIL-100(Fe)/GG	Blending and self-crosslinking	Oil-water mixtures	1:1	>97.82	—	Photocatalytic performance (40 mg/L)	[75]

 

Table 4. A summary and comparisons of other MOFs membranes in oil/water separation (Pollutant concentrations are shown in brackets for other functions).

Material	Preparation method	Liquids of separation	O/W  Volume ratio	Separation efficiency (%)	Permeation flux (L·m-2·h-1)	Other functions	Ref.
PDMS/(Cu-MOFs)5@paper	LBL and in-suit growth	Oil-water mixtures	1:1	>98	55.8	Antibacterial activity	[77]
prGO@cHKUST-1	Hydrothermal	Oil-water emulsion	—	99.6	28.6	Dye adsorption (20mg/L-100mg/L)	[78]
PCN-224/TA/PVDF	In-suit deposition	Oil-in-water emulsions	1:99	>99	1542	Dye adsorption (10-50 ppm)	[79]
Ce/Cu-MOF	Dip-coating/in-suit growth	Oil-water mixtures	1:4	>98	—	—	[80]
Co-CAT-1	Vapor-assisted conversion	Oil-water mixtures	1:1	99.98	8.4×105	—	[81]
Cr-soc-MOF	Vacuum-assisted self-assemly method	Oil-in-water emulsions	1:100	98.7	7040.1	—	[82]
MOF-808-EDTA	Solvothermal and electrospinning	Oil-in-water emulsions	1:10	99.97	379.3	Heavy metal ions adsorption (Cu2+: 50 ppm)	[83]
HKUST-1/PDA@SM	Step-by-step deposition and hydrothermal synthesis	Oil-in-water emulsions	1:9	95	200	—	[84]
SSM@HKUST-1	Electrochemical and in suit growth method	Oil-in-water and water-in-oil emulsions	1:100; 100:1	97.46; 99.63	569.7; 88.1	—	[85]

 

3. Please provide more details for each deposition method. How the material formed by each method was not explained clearly in detail.

Thank you very much for your valuable advice. We have made some changes to the deposition method to provide some details. The modifications are shown below.

The direct deposition method involves the direct synthesis of MOFs composite membranes by placing the substrate into a mixture of metal ions and organic ligands. For example, a PP/ZIF-8 composite membrane can be prepared by simply adding polypropylene (PP) non-woven fabric into a mixed solution of zinc nitrate and 2-methylimidazole at room temperature, and it has the ability to separate free oil-water mixtures [46]. Yang et al. prepared a PDMS/Ti-MOFs composite cotton fabric by placing cotton fabric in a blend of titanium isopropanol and 2-amino-terephthalic acid for 24 h and modifying it with PDMS after drying [47]. In both cases, PP non-woven fabric and cotton fabric are used as substrates, zinc nitrate and titanium isopropanol provide zinc and titanium ions, and 2-methylimidazole and 2-amino-terephthalic acid are organic ligands that are mixed together to form composite membranes of MOFs on the substrate. The direct deposition method is simple in its approach, but due to a deficiency of sufficient nucleation sites on the pristine substrate to interact with MOFs, ensuring good dispersity of the MOFs and strong bonding with the substrate can be difficult.

The electrochemical deposition method, similar to the direct deposition method, involves the rapid growth of MOFs on the substrate through the use of metal cations and organic ligands in the presence of an applied voltage. For example, after only 200 s of applied voltage, Co²⁺ and 2-methylimidazole react and accelerate to produce electrochemical deposition that aggregate onto the cppper mesh, resulting in the formation of a uniform and dense ZIF-67 film [48]. This method offers advantages such as a short process time, uniform growth, and a high deposition rate.

The filtration deposition method is a technique for creating functional membranes by depositing solid suspensions on a substrate using vacuum filtration. For example, Zhu et al. first synthesized UiO-66-NH₂ powder using Zr⁴⁺ and 2-NH₂-benzenedicarboxylate by the solvent thermal method, added it to chitosan solution and attached it to a cellulose membrane through vacuum filtration obtain a superhydrophilic and underwater superoleophobic membrane, which can effectively separate oil-in-water emulsions and has excellent corrosion resistance [50]. Similarly, You et al. synthesised micro and nano-sized ZIF-8 particles, which was then deposited onto a cellulose membrane by vacuum filtration. Both water-in-oil and oil-in-water emulsions can be separated by the resultant membrane [51].

 

4. Please update all the old references with recent references.

Thank you very much for your valuable advice. We have updated some of our old references. Some of the references are replaced as follows.

In addition, MOF-808 has adsorption property for copper ion and there are some recent research such as Cr-soc-MOF and HKUST-1 which can be applied to oil-water separation, especially SSM@HKUST-1 can separate oil-in-water and water-in-oil emulsions on demand [82,84-85].

[82] Obaid, M.; Alsadun, N.; Shekhah, O.; Almahfoodh, S.; Zhou, S.; Ghaffour, N.; Eddaoudi, M., Deployment of superhydrophilic and super-antifouling Cr-soc-MOF-1-based membrane for ultrafast separation of stabilized oil-in-water emulsions[J]. ACS Appl. Mater. Interfaces 2023, 15 (25), 31067-31076. https://doi.org/10.1021/acsami.3c05285.

[84] Deng, Y. Y.; Bian, H. Z.; Dai, M.; Liu, X.; Peng, C. S., Underwater superoleophobic HKUST-1/PDA@SM membrane with excellent stability and anti-fouling performance for oil-in-water emulsion separation[J]. J. Membr. Sci. 2023, 678, 121655. https://doi.org/10.1016/j.memsci.2023.121655.

[85] Li, J. H.; Ding, S. L.; Wu, J.; Guo, Z. G., Underwater superoleophobic and underoil superhydrophilic copper benzene-1,3,5-tricarboxylate (HKUST-1) mesh for self-cleaning and on-demand emulsion separation[J]. Langmuir 2023, 39 (17), 6201-6210. https://doi.org/10.1021/acs.langmuir.3c00331.

 

5. The conclusion and outlook are too short. The authors must expand the discussion here. The potential and the future of this technology should be stated clearly. In addition, the challenges along with limitations must be included.

Thank you very much for your valuable advice. We have discussed in the conclusion and outlook, giving an in-depth description of the potential and challenges of MOFs materials. The revised manuscript is presented below.

In this review, the mechanisms of oil-water separation are described and the preparation and recent progress of MOFs-based oil-water separation membranes are summarized. There are diverse methods for the preparation of MOFs, many of which have the potential for large-scale industrialization, opening up new avenues for future practical engineering applications. Furthermore, this paper focuses on various MOF-based membranes for oil-water separation, such as the ZIF series, the UiO series and the MIL series, and presents the functional properties and their related applications. The versatility of MOFs gives these oil-water separation membranes superior performances, such as antifouling properties, self-healing capabilities, high adsorption of heavy metal ions and dyes, and photocatalytic degradation of organic pollutants. To sum up, it can be concluded that MOFs-based membranes have a broader application in the treatment of oily wastewater.

However, in the existing literatures, most studies using MOF-based oil-water separation membranes are somewhat flawed and the following efforts are still needed to solve the environmental problems of wastewater. (1) The long-term performance of MOFs-based membranes has been neglected, especially in continuous oil-water separation processes. Researchers can therefore focus on ways to improve the long-term durability of the membranes. (2) Most MOFs materials are unstable in wet environments due to weak coordination between their own metals and organic ligands. How to improve the stability of MOFs without affecting their structural properties remains to be investigated. (3) The specific wastewater environment is very complex and may contain organic molecules, heavy metals, microorganisms, etc.. MOFs-based membranes need to be more stable and fouling resistant in the complex components. Based on this, there is a need for researchers to create materials with better stability and self-cleaning properties. (4) MOFs-based membranes should be used in a wider range of applications, and they must also be able to treat actual wastewater from municipal wastewater to industrial wastewater, broadening their practical applications. (5) In addition, there is still a lack of longitudinal comparisons of data on the separation efficiency, mechanical and chemical stability and pollution resistance of MOF-based membranes based on the separation of complex oil-water mixtures, which will need to be reviewed by subsequent researchers as this research progresses.

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