Cross-Linking Combined with Surfactant Bilayer Assembly Enhances the Hydrophilic and Antifouling Properties of PTFE Microfiltration Membranes

Round 1

Reviewer 1 Report

This manuscript demonstrates the hydrophilic modification on PTFE membranes for improved anti-fouling property. While the results are reasonable, the reviewer has following concerns:

1. The authors listed many literature reports in the Introduction, but it is more like a compulsory work. Please clearly clarify the background, the motivation, and strategy proposed in this work.
2. Please provide the pore size distribution results of the membranes shown in Fig. 3.
3. Could it be possible that with low amount of CA, the cross-linked coating layer is not stable under water?
4. Could the authors replot Fig.10 with normalized flux? Flux of pure water before filtration of BSA solution also should be included in this figure.
5. Is it reasonable to conduct BSA filtration test as BSA cannot be rejected by these membranes?

Author Response

Reviewer 1:

1. The authors listed many literature reports in the Introduction, but it is more like a compulsory work. Please clearly clarify the background, the motivation, and strategy proposed in this work.

Response: Thank you for your suggestion. We have modified the introduction to clearly clarify the background, motivation and methods of this work.

Changes in: What we changed is the part marked in blue in the introduction.

 

2. Please provide the pore size distribution results of the membranes shown in Fig. 3.

Response: Thank you for raising this very valuable question and we have discussed and thought about it. We are very sorry that we did not consider the pore size distribution test when we started designing the experiment. We mainly observe the deposition of the hydrophilic coating on the membrane surface and the effect on the membrane pore size through the test results of the scanning electron microscope.

 

3. Could it be possible that with low amount of CA, the cross-linked coating layer is not stable under water?

Response:Thank you for raising this very valuable question and we have discussed and thought about it. We think this should be one of the important reasons .When the content of citric acid increases from 1 g·L-1 to 2 g·L-1, the degree of crosslinking of the PVA gel increases, making the crosslinking structure more stable. Therefore, when performing pure water filtration, the water resistance of the PVA cross-linked coating in water is improved.

Changes in: We made changes in lines 362 to 364 in blue.

 

4.Could the authors replot Fig.10 with normalized flux? Flux of pure water before filtration of BSA solution also should be included in this figure.

Response: Thank you for your suggestion. The normalized flux can indeed compare the antifouling performance of different PTFE microfiltration membranes more clearly. The water flux at 0 min for each membrane is the pure water flux after the initial stabilization.

Changes in: We have made changes in Figure 10 and lines 406 to 408 in blue.

 

5.Is it reasonable to conduct BSA filtration test as BSA cannot be rejected by these membranes?

Response: Although BSA will not be retained by the membrane, during the filtration process, due to the strong hydrophobic interaction between the BSA and the membrane, the protein will adsorb to the membrane surface and contaminate the membrane. After the hydrophilic modification, the hydrophilic layer will reduce the adsorption of BSA on the membrane surface. Therefore, we used the BSA filtration experiment to compare the degree of contamination of the membrane surface with BSA before and after the hydrophilization modification, and to evaluate the anti-fouling performance of the PTFE microfiltration membrane. In recent studies, some researchers have used similar methods to evaluate the antifouling performance of PTFE microfiltration membranes.(10.1016/j.memsci.2021.119301;10.1016/j.jtice.2019.01.001)

Author Response File: Author Response.pdf

Reviewer 2 Report

1. 2-5 g/L crosslinking were found to be the best. How much of the used crosslinker is incorporated into the membrane, and how much remains in solution or degrades? The mass balance, through quantification of the crosslinker, should be closed.
2. It is seems contraintuitive to perform a hydrophilic modification on a hydrophobic membrane. Either hydrophilic of hydrophobic membranes are needed, and what is the benefit of combining the two? PTFE is very robust hydrophobic, and therefore why is it necessary to perform the crosslinking in a way to make it more hydrophilic and lose the great hydrophobic character?
3. The results should be put into context by comparing the performance of the used PTFE membrane directly with other, conventional or already reported membranes for the same purpose. What has been achieved? What are the advantages of the new system?
4. Some errors are reported as error bars but it is unclear how they were derived. Are they standard deviations? How many independently prepared membranes were fabricated and tested to yield the error? It should be reported in the experimental section as well as in the corresponding results figures’ caption.
5. The purity and grade of all the chemicals, solvents and materials should be listed under the section 2.1 Materials in order to facilitate reproduction of the work and better understanding.
6. The effect of crosslinking on various membranes is an increasingly important area for fine-tuning separation properties, and recent broad examples should be briefly mentioned (10.1016/j.memsci.2021.119724; 10.1016/j.memsci.2021.119963; 10.1016/j.cej.2021.129443; 10.1016/j.memsci.2020.118912).
7. The molecular weight and its distribution of the used polymers is an important parameter that determines the membrane fabrication as well as the final separation performance. Therefore it should be reported in the manuscript.
8. Figure 1 should be more informative by including parameter ranges and other practical information.
9. The scale bars are not legible in Figure 3 SEM and Figure 4 images. The images cannot be interpreted in their current form.
10. Figure 9b caption should explicitly mention the applied pressure as flux on its on is insufficient.
11. Both the quotient (“x/y”) and negative exponent (“x y-1”) formats are used in the manuscript for units. Either of them should be used consistently, preferably the negative exponent format, which is recommended by the IUPAC.
12. The conclusion section is too short and vague. A good amount of results are presented, and compared to that only some of them made it into the conclusion section. The main research findings should be briefly summarized in quantitative statements.

Author Response

Reviewer 2:

1.2-5 g/L crosslinking were found to be the best. How much of the used crosslinker is incorporated into the membrane, and how much remains in solution or degrades? The mass balance, through quantification of the crosslinker, should be closed.

Response: Thank you for your suggestions.We did not consider the remaining cross-linking agent content in the test solution when we were designing the experiment. We first set a gradient for the content of the cross-linking agent. Then by studying the relationship between the change in the content of the crosslinking agent and the hydrophilicity of the membrane, the optimal addition amount of the crosslinking agent was obtained.

 

2.It is seems contraintuitive to perform a hydrophilic modification on a hydrophobic membrane. Either hydrophilic of hydrophobic membranes are needed, and what is the benefit of combining the two? PTFE is very robust hydrophobic, and therefore why is it necessary to perform the crosslinking in a way to make it more hydrophilic and lose the great hydrophobic character?

Response:The PTFE porous membrane is manufactured by a stretching process and has achieved great success as an air filter. However, when the PTFE membrane is processed in water for filtration and separation, the strong hydrophobicity of PTFE significantly reduces its performance. First, the strong hydrophobicity prevents water from penetrating into the PTFE membrane, which not only requires higher pressure to consume more energy, but also reduces flux. At the same time, due to the strong hydrophobic interaction between PTFE and hydrophobic solutes in water, the PTFE membrane is easily absorbed and contaminated, resulting in reduced flux. Therefore, when the PTFE porous membrane is used in the water treatment field, it needs to be hydrophilized to improve the flux and antifouling performance.

 

3.The results should be put into context by comparing the performance of the used PTFE membrane directly with other, conventional or already reported membranes for the same purpose. What has been achieved? What are the advantages of the new system?

Response: Thank you for your suggestions. We have compared the work of this research with related work in recent years.

Changes in: We made changes in Table 4 and lines 395 to 400 in red.

 

4.Some errors are reported as error bars but it is unclear how they were derived. Are they standard deviations? How many independently prepared membranes were fabricated and tested to yield the error? It should be reported in the experimental section as well as in the corresponding results figures’ caption.

Response: The error bars in the article are all standard deviations.

Changes in: We made changes in the experimental section in red.

 

5.The purity and grade of all the chemicals, solvents and materials should be listed under the section 2.1 Materials in order to facilitate reproduction of the work and better understanding.

Response:. Thank you for your suggestion.

Changes in: We have made changes in the section 2.1 Materials in red.

 

6.The effect of crosslinking on various membranes is an increasingly important area for fine-tuning separation properties, and recent broad examples should be briefly mentioned (10.1016/j.memsci.2021.119724; 10.1016/j.memsci.2021.119963; 10.1016/j.cej.2021.129443; 10.1016/j.memsci.2020.118912).

Response:. Thank you for your suggestion.

Changes in: We made changes in lines 64 to 69 in blue.

 

7.The molecular weight and its distribution of the used polymers is an important parameter that determines the membrane fabrication as well as the final separation performance. Therefore it should be reported in the manuscript.

Response:. Thank you for your suggestion.

Changes in: We have made changes in the section 2.1 Materials in red.

 

8.Figure 1 should be more informative by including parameter ranges and other practical information.

Response:. Thank you for your suggestion. We have marked the corresponding operating temperature and the addition ratio of PVA and CA in Figure 1.

Changes in: We have made changes in figure 1.

 

9.The scale bars are not legible in Figure 3 SEM and Figure 4 images. The images cannot be interpreted in their current form.

Response:. We have adjusted the scale in Figure 3 and the size of Figure 4 to make it clearer.

Changes in: We have made changes in figure 3 and figure 4.

 

10.Figure 9b caption should explicitly mention the applied pressure as flux on its on is insufficient.

Response:. Thank you for your suggestion.

Changes in: We made a modification on line 375.

 

11.Both the quotient (“x/y”) and negative exponent (“x y-1”) formats are used in the manuscript for units. Either of them should be used consistently, preferably the negative exponent format, which is recommended by the IUPAC.

Response:. Thank you for your suggestion.

Changes in: We have modified the relevant units in the article in red.

 

12.The conclusion section is too short and vague. A good amount of results are presented, and compared to that only some of them made it into the conclusion section. The main research findings should be briefly summarized in quantitative statements.

Response:. Thank you for your suggestion. We have modified the conclusion section to clarify the main findings of this work

Changes in: We have modified the relevant units in the article in red.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have addressued most of my cocerns on their previous manuscript. The manuscript now is acceptable for publication.

Reviewer 2 Report

The comments have been address, the manuscript has improved considerably.