Removal of Dyes and Cd2+ in Water by Kaolin/Calcium Alginate Filtration Membrane
Round 1
Reviewer 1 Report
The paper deals with the development of Kaolin/CaAlg membranes for Cd2+ and dyes removal. The paper is well structured and clear. However, some minor issues should be fixed prior to be published.
In general, the number of figure is excessive, maybe the authors should reduce them to clearly distinguish between main and secondary findings.
Which is the main novelty of the work? The authors should clearly state it. Please, clarify the contribution of this work respect to the literature.
Author Response
Comments and Suggestions for Authors
The paper deals with the development of Kaolin/CaAlg membranes for Cd2+ and dyes removal. The paper is well structured and clear. However, some minor issues should be fixed prior to be published. In general, the number of figure is excessive, maybe the authors should reduce them to clearly distinguish between main and secondary findings. Which is the main novelty of the work? The authors should clearly state it. Please, clarify the contribution of this work respect to the literature.
Response: Thank you for your advice. Some figures were deleted to clearly the distinguish findings. The main novelty of the work lies in the introduction of Kaolin in CaAlg filtration membrane in order to improve the mechanical behavior and the flux of the hydrogel membrane. The Kaolin/CaAlg filtration membrane was used to remove Cd2+ and dyes synchronously. The Kaolin/CaAlg membrane exhibited almost 100% rejection for Brilliant Blue G250 (Mw=854.02), and fairly high rejection (95.22%) for Congo red (Mw=696.68). And the removal rate of Cd2+ by Kaolin/CaAlg filtration membrane reached 99.69%. The novelty of the work was clearly stated and clarified respect to the literatures in the introduction.
Reviewer 2 Report
The manuscript entitled “Removal of dyes and Cd2+ in water by anti-fouling kaolin/calcium alginate filtration membrane” deals with the preparation and implementation of fouling-resistant membranes based on calcium alginate with kaolin as an additive and calcium as a pore-forming agent. The premise sounds very interesting and promising, but it has some critical points along the manuscript that make it susceptible to be rejected. There are some inconsistencies in the background exposed in the Introduction section as well as in the Results & discussion section, especially in terms and knowledge related to membrane technology. For this reason, and together with the following specific comments, I strongly recommend a major revision in order to improve the quality of the manuscript.
Specific comments:
1) General. The use of first-person pronouns is not appropriate for academic writing.
2) General. There are some grammatical mistakes along the whole manuscript. Please check the language and text composition.
3) Abstract. Authors claim that “the mechanical behaviors, anti-fouling properties of the Kaolin/CaAlg membrane were investigated”. Please rephrase this.
4) Introduction. “Many dyes are widely utilized in various industries and the dye pollution is also an important reason for water pollution”. Please indicate several industries where dyes are used and the reason why the dye pollution is so important.
5) Introduction. “Like Cadmium (II), dyes can also cause several damage”. Please add “damage to human health and the environment”.
6) Introduction. In line 50, authors indicate that membrane separation has its high separation efficiency as an advantage, but in line 54 indicate that a low separation efficiency is also related to membrane separation. There is a contradiction here, thus indicating that the paragraph has a poor composition. Please focus your attention on the writing and revise the overall structure of the manuscript.
7) Introduction. Authors claim that “the above modification methods are laborious and time-consuming, and have poor long-term stability”. It depends on the method. For example, blending is frequently used for modifying membranes due to its easy and fast operation and can be performed during membrane preparation but crosslinking or grafting are carried out after preparing the bare membrane. In this article, you propose a modification (with a duration of 12 hours) after preparing the bare membrane, so this statement undermines your own work. Authors point the little impact of their own work. I would like to think that the meaning of these sentences has been misunderstood by myself and I heavily suggest removing them. Can you explain your statements with other words?
8) Introduction. In addition, there is no study here about long-term stability of the membranes. There are some studies that analyze long-term stability and also leaching out effect in literature (García-Ivars et al., Separation and Purification Technology 135, 2014, 88-99; Qi et al., Journal of Membrane Science 508, 2016, 94-103; Mukherjee et al., Journal of Membrane Science 570-571, 2019, 204-214). I heavily suggest changing or removing this sentence. Be careful with your statements.
9) Introduction. “Some contaminants are adsorbed in the hydrophobic membrane during the separation process”. This sentence makes no sense here, because now you are talking about membrane modification. Remove it.
10) Introduction. The last paragraph in the Introduction section (“In this study…”) has a modified version of the abstract with some results and observations. THIS IS THE INTRODUCTION SECTION, SO PLEASE REMOVE YOUR RESULTS FROM HERE IMMEDIATELY.
11) Materials and methods. The parameter “Mw” should have units. Which units are?
12) Materials and methods. Please indicate the method you used for preparing membranes and its main conditions (solvent, temperature of the polymer solution, bath temperature, crosslinking conditions…). For instance, the temperature is a very influential and important factor during membrane preparation, being capable to define the pore size of the membrane and its working range. So, the membrane structure strongly depends on its value.
13) Materials and methods. How did authors ensure the degassing of the polymeric solution without carrying out any technique to this end?
14) Materials and methods. Can authors explain how crosslinking occurs?
15) Materials and methods. Authors selected CaAlg membranes prepared with 2.5 wt.% NaAlg and 2.5 wt.% urea…why?
16) Results and discussion. Almost all the absorption peaks of Kaolin, Kaolin/CaAlg and CaAlg in Figure 3 are the same, except for the absorption peak observed at 619.18 cm-1, which is attributed to the kaolin molecule. Can authors indicate how the presence of kaolin modifies the bare CaAlg membrane?
17) Results and discussion. The discussion of Figure 3 is not correctly related to the information shown in such a figure. Can authors link properly the explanation with the information shown in this figure?
18) Results and discussion. Please improve the discussion of the Section 3.4. This part is lack of discussion, poorly written and references are needed. More information can be extracted from Figure 4.
19) Results and discussion. “All the membranes flux increased with the increase in operating pressure”. This is very common and expected in pressure-driven membrane processes, so this cannot be seen as a result in any case. There is no relevance in this result.
20) Results and discussion. From the discussion in Section 3.5, it can be considered that kaolin improves the hydrophilic character of the modified membranes, right? Or make their pores bigger? Why did not authors discuss this? Where is the study of the membrane hydrophilicity?
21) Results and discussion. Please correct Figure 5.
22) Results and discussion. Figure 6 shows different filtration experiments with kaolin/CaAlg membranes. The statement “what’s more, the introduction of kaolin did not change the hydrophilicity of the CaAlg hydrogel filtration membrane and made it enable to withstand cleaning operation” makes no sense because authors are not comparing a control membrane with the modified ones. Previously, kaolin/CaAlg membranes showed higher water fluxes than CaAlg membranes (17.53 and 9.48 L/m2·h at 0.1 MPa, respectively). This fact could be also related to an improvement in hydrophilicity or an increase in the average pore size. For this reason, I suggest calculating the average pore radius using the Guerout-Elford-Ferry equation. This should be used in order to evaluate the structural changes and also the different hydrophilic characters.
23) Results and discussion. Can authors discuss in detail why these membranes can be considered as NF membranes? The transmembrane pressure is too low to reach a good performance for nanofiltration.
24) Results and discussion. Authors repeated twice the following statement in the same paragraph “the dyes removal of CaAlg membrane was due to the membrane rejection instead of adsorption”. Please remove it.
25) Results and discussion. The statements achieved in Section 3.7 should be proven in long-term stability experiments. What about the leaching out effect of additives? How can authors ensure the good properties achieved by membranes after modification?
26) Conclusions. Please reuse and rephrase the conclusions section.
Author Response
Specific comments:
1) General. The use of first-person pronouns is not appropriate for academic writing.
Response: Thank you for your advice. All the first-person pronouns were revised.
2) General. There are some grammatical mistakes along the whole manuscript. Please check the language and text composition.
Response: Thank you for your advice. We check the language and text composition to avoid the grammatical mistakes.
3) Abstract. Authors claim that “the mechanical behaviors, anti-fouling properties of the Kaolin/CaAlg membrane were investigated”. Please rephrase this.
Response: Thank you for your advice. The phrase “the mechanical behaviors, anti-fouling properties of the Kaolin/CaAlg membrane were investigated” was deleted in the revised manuscript.
4) Introduction. “Many dyes are widely utilized in various industries and the dye pollution is also an important reason for water pollution”. Please indicate several industries where dyes are used and the reason why the dye pollution is so important.
Response: Thank you for your advice. Many dyes are widely utilized in textile industry, paint industry and the dye pollution is also an important reason for water pollution. The dyes can contaminate bodies of water, modify the ecosystem and affect biological cycles. Like Cadmium (II), dyes can also cause severe damage to human health and the environment, such as the dysfunction of the liver, kidneys, brain and central nervous system.
5) Introduction. “Like Cadmium (II), dyes can also cause severe damage”. Please add “damage to human health and the environment”.
Response: Thank you for your advice. The phrase “damages to human health and the environment” was added.
6) Introduction. In line 50, authors indicate that membrane separation has its high separation efficiency as an advantage, but in line 54 indicate that a low separation efficiency is also related to membrane separation. There is a contradiction here, thus indicating that the paragraph has a poor composition. Please focus your attention on the writing and revise the overall structure of the manuscript.
Response: Thank you for your advice. The overall structure of the paragraph was revised. “Membrane separation has become an energy efficient new separation technology due to its easy operation, low energy consumption, and no secondary pollution [12,13]. Currently, membrane separation technology has been greatly developed in the treatment of waste water. However, due to the hydrophobic nature of most polymer separation membranes and the low surface energy, there are still significant challenges, such as the severe membrane fouling [14,15]. Much effort has been exerted to modify the membrane, including blending [16,17] surface coating [18] and surface grafting [19,20].”
7) Introduction. Authors claim that “the above modification methods are laborious and time-consuming, and have poor long-term stability”. It depends on the method. For example, blending is frequently used for modifying membranes due to its easy and fast operation and can be performed during membrane preparation but crosslinking or grafting are carried out after preparing the bare membrane. In this article, you propose a modification (with a duration of 12 hours) after preparing the bare membrane, so this statement undermines your own work. Authors point the little impact of their own work. I would like to think that the meaning of these sentences has been misunderstood by myself and I heavily suggest removing them. Can you explain your statements with other words?
Response: Thank you for your advice and the fowling statements were removed. “The above methods can partly alleviate membrane fouling. However, the above modification methods are laborious and time-consuming, and have poor long-term stability [20]. Some contaminants are adsorbed in the hydrophobic membrane during the separation process.”
8) Introduction. In addition, there is no study here about long-term stability of the membranes. There are some studies that analyze long-term stability and also leaching out effect in literature (García-Ivars et al., Separation and Purification Technology 135, 2014, 88-99; Qi et al., Journal of Membrane Science 508, 2016, 94-103; Mukherjee et al., Journal of Membrane Science 570-571, 2019, 204-214). I heavily suggest changing or removing this sentence. Be careful with your statements.
Response: Thank you for your advice and the statements were removed.
9) Introduction. “Some contaminants are adsorbed in the hydrophobic membrane during the separation process”. This sentence makes no sense here, because now you are talking about membrane modification. Remove it.
Response: Thank you for your advice and the statements were removed.
10) Introduction. The last paragraph in the Introduction section (“In this study…”) has a modified version of the abstract with some results and observations. THIS IS THE INTRODUCTION SECTION, SO PLEASE REMOVE YOUR RESULTS FROM HERE IMMEDIATELY.
Response: Thank you for your advice and the RESULTS were removed.
11) Materials and methods. The parameter “Mw” should have units. Which units are?
Response: The unit of the parameter “Mw” was g/mol or Dalton (D). The unit D was added.
12) Materials and methods. Please indicate the method you used for preparing membranes and its main conditions (solvent, temperature of the polymer solution, bath temperature, crosslinking conditions…). For instance, the temperature is a very influential and important factor during membrane preparation, being capable to define the pore size of the membrane and its working range. So, the membrane structure strongly depends on its value.
Response: Thank you for your advice. The main conditions of preparing membranes were as follows. Solvent: deionized water; temperature of the polymer solution: room temperature; bath temperature: room temperature; Crosslinking conditions: 2.5wt% CaCl2 water solution at room temperature. We haven't studied the effect of temperature on the properties of membranes, which will be taken into account in future research. The temperature is a very influential and important factor during membrane preparation for the conventional membrane such as PAN, PVDF and PSF. The calcium alginate hydrogel can be formed by sodium alginate in calcium chloride aqueous solution at 0-100 oC.
13) Materials and methods. How did authors ensure the degassing of the polymeric solution without carrying out any technique to this end?
Response: We ensure the degassing of the polymeric solution without carrying out any technique because the viscosity of the sodium alginate (2.5 wt%) aqueous solution is low and the bubbles can be eliminated by standing the solution in the air for 5-8 hours at room temperature.
14) Materials and methods. Can authors explain how crosslinking occurs?
Response: The crosslinking of Ca2+ and NaAlg was explain in detail in many literatures. Figure 1. Shows the scheme for the preparation of CaAlg filtration membrane. Figure 1. The scheme for the preparation of CaAlg filtration membrane. (J. Membr. Sci. 2015, 492, 536–546.)
15) Materials and methods. Authors selected CaAlg membranes prepared with 2.5 wt.% NaAlg and 2.5 wt.% urea…why?
Response: The formula is based on the results of the previous paper (J. Membr. Sci. 2015, 492, 536–546.). Therefore, this formula is directly used to prepare the membrane in this manuscript.
16) Results and discussion. Almost all the absorption peaks of Kaolin, Kaolin/CaAlg and CaAlg in Figure 3 are the same, except for the absorption peak observed at 619.18 cm-1, which is attributed to the kaolin molecule. Can authors indicate how the presence of kaolin modifies the bare CaAlg membrane?
Response: Figure 3 and the related descriptions were deleted.
17) Results and discussion. The discussion of Figure 3 is not correctly related to the information shown in such a figure. Can authors link properly the explanation with the information shown in this figure?
Response: Figure 3 and the related descriptions were deleted.
18) Results and discussion. Please improve the discussion of the section 3.4. This part is lack of discussion, poorly written and references are needed. More information can be extracted from figure 4.
Response: Thank you for your advice. The discussion of the section 3.4 was improved and the related references were added. The stress the Kaolin/CaAlg membranes initially increased and then decreased with the increase of Kaolin content. When the content of Kaolin in NaAlg was 70 %, the stress of Kaolin/CaAlg membrane reached 963.95 KPa, which was three times of the stress of CaAlg membrane (316.85 KPa). However, when the content of Kaolin in NaAlg reached 100 %, the stress of Kaolin/CaAlg membrane decreased because of the aggregation and non-uniform dispersion of Kaolin. The literature indicates that the interaction between kaolin particles and CaAlg hydrogel may be the result of a combination of hydrogen bonding and electrostatic forces, which increases the strength of kaolin mixed with CaAlg hydrogel [33, 34].
19) Results and discussion. “All the membranes flux increased with the increase in operating pressure”. This is very common and expected in pressure-driven membrane processes, so this cannot be seen as a result in any case. There is no relevance in this result.
Response: The related descriptions were deleted.
20) Results and discussion. From the discussion in section 3.5, it can be considered that kaolin improves the hydrophilic character of the modified membranes, right? Or make their pores bigger? Why did not authors discuss this? Where is the study of the membrane hydrophilicity?
Response: In section 3.5 we just investigated the anti-fouling of the kaolin/CaAlg membrane. We haven't paid much attention to the hydrophilic character and the pores of the kaolin/CaAlg membrane and we will discuss them in depth in future studies. In fact, the pore and distribution of hydrogels are very difficult to measure accurately. We will do further experiments to verify whether kaolin can improve the hydrophilicity of the CaAlg membranes. The water contact angle of the kaolin/CaAlg hydrogel had been tested, but the error was large.
21) Results and discussion. Please correct Figure 5.
Response: Thank you for your advice. Figure 5 (Figure 4 in the revised manuscript) was corrected.
22) Results and discussion. Figure 6 shows different filtration experiments with kaolin/CaAlg membranes. The statement “what’s more, the introduction of kaolin did not change the hydrophilicity of the CaAlg hydrogel filtration membrane and made it enable to withstand cleaning operation” makes no sense because authors are not comparing a control membrane with the modified ones. Previously, kaolin/CaAlg membranes showed higher water fluxes than CaAlg membranes (17.53 and 9.48 L/m2·h at 0.1 MPa, respectively). This fact could be also related to an improvement in hydrophilicity or an increase in the average pore size. For this reason, I suggest calculating the average pore radius using the Guerout-Elford-Ferry equation. This should be used in order to evaluate the structural changes and also the different hydrophilic characters.
Response: The related descriptions were deleted. The Figure 6 (A) (Figure 5 in the revised manuscript) was deleted. In fact, the pore and distribution of hydrogels are very difficult to measure accurately because the hydrogel is easy to swell. Guerout-Elford-Ferry equation can be used to evaluate the structural of microfiltration membrane and we will try it in our further research.
23) Results and discussion. Can authors discuss in detail why these membranes can be considered as NF membranes? The transmembrane pressure is too low to reach a good performance for nanofiltration.
Response: Thank you for your advice. These kaolin/CaAlg membranes can be considered as loose NF membranes and the transmembrane pressure is relatively low. As a hydrogel, it can reject dyes with high molecular weight (Mw>696.68 D) and has good permeability for various salts. This kind of membrane has important application prospects in the field of dye desalination.
24) Results and discussion. Authors repeated twice the following statement in the same paragraph “the dyes removal of CaAlg membrane was due to the membrane rejection instead of adsorption”. Please remove it.
Response: The related descriptions were deleted.
25) Results and discussion. The statements achieved in Section 3.7 should be proven in long-term stability experiments. What about the leaching out effect of additives? How can authors ensure the good properties achieved by membranes after modification?
Response: Thank you for your advice and question. The long-term stability experiment of the kaolin/CaAlg membrane has been done to investigate the removal of dyes by the membrane. The kaolin/CaAlg membrane maintained a steady flux and retention rate for a week and the kaolin did not leach out. However, after 10 days of immersion in water at room temperature, bacteria grow on the surface of the membrane, which reduces the mechanical properties of the membrane. We are trying to improve the antimicrobial properties of the kaolin/CaAlg membranes so that they can be used for a long time.
26) Conclusions. Please reuse and rephrase the conclusions section.
Response: Thank you for your advice. The conclusion section was revised.
Author Response File: 
Author Response.docx
Reviewer 3 Report
The authors provide no comparison with commercially available membranes. Without that, the motivation for making new types of membranes is not clear at all.
Author Response
Comments and Suggestions for Authors
The authors provide no comparison with commercially available membranes. Without that, the motivation for making new types of membranes is not clear at all.
Response: Thank you for your advice. The main novelty of the work lies in the introduction of Kaolin in CaAlg filtration membrane in order to improve the mechanical behavior and the flux of the Kaolin/CaAlg membrane. The Kaolin/CaAlg filtration membrane can be used to remove Cd2+ and dyes synchronously. The Kaolin/CaAlg membrane exhibited almost 100% rejection for Brilliant Blue G250 (Mw=854.02 Da), and fairly high rejection (95.22%) for Congo red (Mw=696.68 Da). And the removal rate of Cd2+ by Kaolin/CaAlg filtration membrane reached 99.69%.
Reviewer 4 Report
Comments on manuscript 444347
Title: Application of ultrafiltration in a paper mill: Process water reuse and membrane fouling analysis
Authors: Yujie Zhao, Xiaowei Liu, Meng Qi, Tian Bai, Kongyin Zhao and Xinxin Zhang.
In this study Kaolin/CaAlg composite membranes were prepared by adding different amounts of Kaolin into sodium alginate and by using urea as porogen agent. Subsequently, authors conducted a systematic study in order to characterize the mechanical behavior as well as the efficiency of the aforementioned membranes. BSA, oil-water emulsion and typical dyes such as Brilliant Blue G250, Congo red and methyelene blue were used to evaluate membrane rejection. In the referee’s opinion, the scope and content of the paper are of interest for the readers of the journal, therefore, I recommend that the manuscript could be published in Coatings, although a minor revision is necessary prior to publication. In specific, authors should consider the following comments and/or suggestions to further improve the quality of their manuscript:
Page 1, Line 37: Please add the word “However”, at the beginning of the sentence.
Page 2, Line 62: Authors are suggested to explain the meaning of the abbreviation “PEG”.
Based on the text in page 5, membrane thickness were 220 μm and 229 μm, however, values in Figure 2 are a little bit different. Please explain why is this happening.
Authors are suggested to replace the title of x-axis in Figure 4 as follows : “Strain (%)”
Page 7, Line 241: Replace the words “ many literatures” with “ many studies”.
5. Page 7, Line 241: Please explain the meaning of the abbreviation “CWF”.
In the caption of Figures 7 and 8 use the term “permeate solutions” instead of “penetrated solutions”.
Page 8, Lines 282-283. “The rejection of dyes by Kaolin/CaAlg membrane was a little lower than that of the CaAlg membrane at the same conditions “. Provide some numbers to support this argument.
Page 8, Lines 295. Please add the units of measurement in the end of the sentence. Thus you can read “690 Da”.
Comments for author File: Comments.docx
Author Response
In specific, authors should consider the following comments and/or suggestions to further improve the quality of their manuscript:
1. Page 1, Line 37: Please add the word “However”, at the beginning of the sentence.
Response: Thank you for your advice. The word “However” was added.
2. Page 2, Line 62: Authors are suggested to explain the meaning of the abbreviation “PEG”.
Response: Thank you for your advice. The meaning of the abbreviation “PEG”abbreviation was added and it was polyethylene glycol.
3. Based on the text in page 5, membrane thickness were 220 μm and 229 μm, however, values in Figure 2 are a little bit different. Please explain why is this happening.
Response: Thank you for your reminding. The description of membrane thickness were changed to 0.234 mm and 0.235 mm.
4. Authors are suggested to replace the title of x-axis in Figure 4 as follows : “Strain (%)”
Response: Thank you for your advice and the title of x-axis in Figure 4 was changed to “Strain (%). 5. Page 7, Line 241: Replace the words “ many literatures” with “ many studies”.
Response: Thank you for your advice and the words “ many literatures” was changed to “ many studies”.
6. 5. Page 7, Line 241: Please explain the meaning of the abbreviation “CWF”.
Response: Thank you for your advice and the abbreviation “CWF” should be “PWF”. And the meaning of the abbreviation “CWF” was explained.
7. In the caption of Figures 7 and 8 use the term “permeate solutions” instead of “penetrated solutions”.
Response: Thank you for your advice and the term was revised.
8. Page 8, Lines 282-283. “The rejection of dyes by Kaolin/CaAlg membrane was a little lower than that of the CaAlg membrane at the same conditions “. Provide some numbers to support this argument.
Response: Thank you for your advice. The sentence “The rejection of dyes by Kaolin/CaAlg membrane was a little lower than that of the CaAlg membrane at the same conditions.” was deleted.
9. Page 8, Lines 295. Please add the units of measurement in the end of the sentence, Thus you can read “690 Da”.
Response: Thank you for your advice and the unit was added.
Round 2
Reviewer 2 Report
After being corrected, it should be accepted by the journal in its current format.
Reviewer 3 Report
The motivation behind this work is still not clear and without any direct comparison with commercial UF/NF membranes, the paper cannot be accepted. Also, the writing needs to be improved.
