Semi-Continuous Adsorption Processes with Multi-Walled Carbon Nanotubes for the Treatment of Water Contaminated by an Organic Textile Dye
, Antonio Chiodo 2, Anastasia Macario 2, Carlo Siciliano 3 and Jànos B.Nagy 1Round 1
Reviewer 1 Report
Please see attached PDF.
Comments for author File: 
Comments.pdf
Author Response
ANSWER REVIEWER 1
Dear Reviewer,
Thank you for your time spent reviewing our manuscript and for giving us important tips for improving it. The manuscript has been revised in many parts. The changes have been highlighted in red. Here are the answers to your suggestions point by point. We hope that in this form the manuscript will find your approval to be considered for its publication.
We thank you and send you our best regards.
I will preface my review by stating that I am an analytical chemist whose expertise is the characterization of carbon nanotubes for biological applications; I do not have chromatography expertise with or without multi-walled carbon nanotubes (MWNTs). I would consider myself to be part of the “general audience” of Applied Sciences.
The Introduction
The Introduction provided sufficient background and was well-cited. I also appreciated problem statements and quantitative values of important metrics such as: Every year 20% of the synthetic dyes used in textile production are lost in the waste [22-24], with residual concentrations between 10 and 50 mg/L [25]. I also appreciated thoroughly-defined references to the current state of the art, such as: The purification processes for the treatment of textile wastewater provide for specific refining processes for the reduction of the quantity of color, degradation generally carried out by biological [34-40], oxidation[41-43], coagulation [44- 47], flocculation [48], membrane [49, 50], photocalalytic [51-57] processes are mainly used for the reduction of dyes in textile wastewater. And Among these are microporous materials [58-68]. Activated carbon are commonly used for these applications [69-72]. Other adsorbent materials are like silica [73], chitosan [74-76] and natural materials [77-81] are used.
The latter is important for later comparison of the analytical performance features of the authors’ columns to the current state of the art.
I finally appreciated the statement that MWNTs have been used to adsorb azo dyes, including two 2020 pubs by the authors: Particular application the carbon nanotubes have shown for the removal of dyes in waste water [96-99].
- Machado, F.M.; Bergmann, C.P.; Fernandes, T.H.M.; Lima, E.C.; Royer, B.; Calvete, T.; Fagan, S.B. Adsorption of Reactive Red M-2BE dye from water solutions by multi-walled carbon nanotubes and activated carbon. J. Hazard. Mater. 2011, 192, 570 1122–1131.
- Duman, O.; Tunç, S.; Polat, T.G.; Bozo˘ glan, B.K. Synthesis of magnetic oxidized multiwalled carbon nanotube-k-carrageenan-Fe3O4 nanocomposite adsorbent and its application in cationic Methylene Blue dye adsorption. Carbohydr. Polym. 2016, 147, 79–88.
- Duman, O.; Tunc, S.; Bozoglan, B.K.; Polat, T.G. Removal of triphenylmethane and reactive azo dyes from aqueous solution by magnetic carbon nanotube-k-carrageenan-Fe3O4 nanocomposite. J. Alloys Compd. 2016, 687, 370–383.
- De Benedetto, C.; Macario, A.; Siciliano, C.; B.Nagy, J.; De Luca, P. Adsorption of Reactive Blue 116 Dye and Reactive Yellow 81 Dye from Aqueous Solutions by Multi-Walled Carbon Nanotubes. Materials 2020, 13, 2757.
- De Luca, P.; B.Nagy, J. Treatment of Water Contaminated with Reactive Black-5 Dye by Carbon Nanotubes. Materials 2020, 13, 5508; doi:10.3390/ma13235508.
This is also important for later comparison of the analytical performance features of the authors’ carbon nanotube system to the carbon nanotube systems of other authors such as references 96-98. When I looked up the Abstract for Reference 100 (shown below), it is somewhat similar to the Abstract of the current manuscript. Most of the dyes used today by the textile industry are of synthetic origin. These substances, many of which are highly toxic, are in many cases not adequately filtered during the processing stages, ending up in groundwater and water courses. The aim of this work was to optimize the adsorption process of carbon nanotubes to remove an azo-dye, called Reactive Black-5, from aqueous systems. Particular systems containing carbon nanotubes and dye solutions were analyzed. Furthermore, the reversibility of the process and the presence of possible degradation phenomena by the dye molecules were investigated. For this purpose, the influence of different parameters on the adsorption process, such as the nature of the carbon nanotubes (purified and nonpurified), initial concentration of the dye, stirring speed, and contact times, were studied. The solid and liquid phases after the tests were characterized by chemical-physical techniques such as thermogravimetric analysis (TG, DTA), UV spectrophotometry, BET (Brunauer, Emmett, Teller), and TOC (total organic carbon) analysis. The data obtained showed a high adsorbingcapacity of carbon nanotubes in the removal of the Reactive Black-5 dye from aqueous systems. Furthermore, the efficiency of the adsorption process was observed to be influenced by the stirring speed of the samples and the contact time, while purified and nonpurified nanotubes provided substantially the same results I look forward to read-on to determine what the significant differences between reference-100 and the current manuscript are. I also say this because: while I enjoyed reading the first paragraph of the two paragraph Introduction, I felt the second paragraph of the Introduction could be improved. First, maybe this is a good place for the authors to provide more details about reference-100, which could lead into the differences between it and the current manuscript. Second, I also enjoyed reading the Abstract, and maybe the summarized findings from the Abstract could be included in this second paragraph, such as: The column containing 2.5 g of carbon nanotubes revealed to be the best one for the total amount of Reactive Black 5 adsorbed, i.e. 55 mg/g(MWCNTs) The research has shown the high adsorption efficiency of carbon nanotubes towards RB-5 dye, highlighting the degradation of the dye molecule and the stratification, inside the columns, of the adsorbed compound.
ANSWER:
We thank you for the appreciation and also for the suggestions you give us. To underline the difference between our previous work (now ref.79 )we have inserted the following sentence in the introduction paragraph in the manuscript:
“In our previous work, the removal of Reactive Black-5 dye from aqueous solutions with carbon nanotubes in static systems was investigated, evaluating the influence of the nature of the carbon nanotubes, the initial concentrations of dye, the stirring speed and the times on the adsorption process [79]
In this work, in particular, we wanted to continue the previous research [79] by studying the absorption of the Reactive Black-5 dye from aqueous solutions with carbon nano tubes in semi-continuous processes, in order to search for the optimal conditions for the purification of water contaminated.
Reactive black-5 dye was chosen as the representative dye present in textile wastewater.
For this purpose, different adsorption columns were studied and subjected to different adsorption cycles which were suspended only when the permeate was colorless. The characteristics of each column were studied and compared”. (Lines 63-75)
The Materials and Methods
I am somewhat surprised that reference-100 is not cited in the Materials section since part of reference- 100 describes optimized conditions involving the absorption of this dye by MWNTs.
ANSWER: Reference [79] has been inserted
Section 2.1: I wouldn’t mind more information about the MWNTs used. I looked up the Materials section of reference 89 (that was provided by the authors) and all I learned was that the BET SSA was 108.78 m2/g. For example, what are the average internal and outer diameters, the average lengths, the metal catalyst used, and degree of surface functionalization (e.g., even pristine MWNTs have some degree of oxygen surface groups, which can vary depending on the CVD method employed and post synthesis processing). This information will also make it easier for other researchers to reproduce the work.
ANSWER: We have inserted this other reference [80]: “Policicchio, A .; Vuono, D .; Rugiero, T.; De Luca, P.; B.Nagy, J. Study of MWCNTs adsorption perfomances in gas processes. J. CO2 Util. 2015, 10, 30–39 ”, where it is possible to have further information on the characteristics of the nanotubes used. The following sentence has been inserted in the manuscript: “Further information on the characteristics of the nanotubes used can be found in references [80]” (Lines 89-90).
Section 2.2: How was it determined that MWNTs did not transverse the borosilicate filter? How often was this monitored, etc.?
ANSWER: The following sentence was inserted in the manuscript: "Previous tests carried out with a dispersion of carbon nanotubes in water, allowed to exclude the passage of the carbon nanotubes through the borosilicalite filter. The use of water and not of the dye solution was adopted to avoid that the color of the solution could cover and not make visible a possible passage of the nanotubes through the filter. However, permeates were constantly observed and no loss of nanotubes through the filter was observed." (Lines 106-111)
Section 2.4: What wavelength was used to measure the absorbance of the dye? Is this dye absorbance measurement affected by the absorbance of MWNTs at that wavelength? What is the concentration level of MWNTs that does effect this dye measurement? In fact, I imaging the general audience wouldn’t mind seeing some raw data, maybe in an Appendix, or maybe by reference to a previous work. Obviously, this is also important with respect to a reader being able to critically intepre the results shown in Section 3.2.
ANSWER: The following sentence was added to the manuscript: “UV analyses were performed considering the three characteristic peaks of Reactive Black-5 at 307, 481, and 600 nm” (Lines 151-152).
Carbon nanotubes are not present in the permeate as they remain in the column. Only the permeate is subjected to UV analysis. Furthermore, thanking you for the opportunity you give us to explain ourselves better, we say that it is not possible to talk about the concentration of nanotubes because they form a dispersion in water and not a solution.
For clarity the following sentence has been added: “Previous tests carried out with a dispersion of carbon nanotubes in water, allowed to exclude the passage of the carbon nanotubes through the borosilicalite filter. The use of water and not of the dye solution was adopted to avoid that the color of the solution could cover and not make visible a possible passage of the nanotubes through the filter. However, permeates were constantly observed and no loss of nanotubes through the filter was observed.” (Lines 106-111)
Section 2.4: What were the sample sizes analyzed by the dryness assay, DTA, TOC, and TIC? In other words, more details would not hurt.
ANSWER: The following sentence was inserted into the manuscript: “The analysis was carried out through oxidative catalytic combustion and the NDIR (non‐dispersive infrared gas measurement device )method. Acidification was carried out using 0.1M HCl and the combustion temperature was 680 °C, allowing to calculate the TOC value after evaluating TC and TIC.” (Lines 161-165)
The weight of the samples subjected to thermal analysis was around 15 mg, but we decided not to indicate it in the manuscript because the instrument returns the data in percentages with respect to the weight of the sample.
The R&D
There was very little Discussion in the R&D, and in fact, there was not a single citation in the R&D. I believe the R&D is a perfect place for the authors to address my earlier two comments: The latter is important for later comparison of the analytical performance features of the authors’ columns to the current state of the art; and, this is also important for later comparison of the analytical performance features of the authors’ carbon nanotube system to the carbon nanotube systems of other authors such as references 96-98. This information not only useful to readers, but it helps reviewers give advice to the Editor about the novelty and impact of the authors’ results.
ANSWER: The following sentences are added: “In a recent review work it is reported that carbon nanotubes appear to be the best and most appropriate adsorbent materials for the removal of Reactive Black 5 dye having as its main feature a rapid adsorption rate [81]. By comparing our data with those reported in the literature [75-82], where adsorption processes in static conditions are studied in the latter, it is possible to deduce first of all that they are consistent and confirm a great propensity of carbon nanotubes for the treatment of waters contaminated by Reactive Black -5.
The semi-continuous adsorption process, studied in particular in this work, has shown that the rapid removal rate and efficiency of carbon nanotubes is maintained even after several consecutive adsorption cycles and that for example a column containing only 2.5 g of nanotubes was able to treat 3.8 L of Ractive Black-5 solution with a concentration equal to 37mg/L. The data obtained in this research are encouraging for a potential implementation of a continuous plant characterized by several columns that alternate during the regeneration phases of the nanotubes”. (Lines 407-420)
The Conclusions
The authors write the following: Using UV-visible spectroscopy for the analysis of Reactive Black 5, and analyzing alsothe TOC, it was demonstrated that the colorant was partially destroyed during the filtration. I have looked several times and I did not find any spectra in this manuscript, and I would like to see the spectra so that the authors could explain what spectral features support the finding that the azo dye was degraded? I also believe this discussion should include the degradation data for Reactive Black 5 from reference-100 as well, for completeness.
ANSWER: Data of total carbon (TOC), inorganic carbon (IC) and total organic carbon (TOC) analysis are reported now in Table 5 with following discussion: “Two important results can be noted. The first one is that all of the adsorbent columns report an excess of inorganic carbon (IC) compared to that obtained in the mother liquor of RB-5. It also stabilizes at low values after an initial high peak in the first filtration step. This result is probably related to a leaching effect of the MWCNTs. Initially, TOC is partially decreased, compared to mother liquor of RB-5. Then it increases as the permeate is near to the colour change state. TOC variation, for permeate and mother liquor, is to assume a degradation of dye molecule inside MWCNTs layer, where a chromophore group is adsorbed at least, as it can be demonstrated by UV-VIS, while part of organic carbon (OC) could remain in the permeate passing through the MWCNTs layer. It is possible to hypothesize that inorganic carbon is generated by the decomposition of the dye into carbonates, bicarbonates, and dissolved CO2., as discussed in previous works [79].” (Lines 340-352)
In the conclusions we have added the following sentence: “In agreement with previous research [79], the data obtained confirm that the dye undergoes partial destruction even during the adsorption process in semi-continuous conditions.” (Lines 434-436)
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript reports a study for investigating the performance of carbon nanotubes to remove a dye from water. Concrete experimental results have been shown. It can be seen that the experiments were conducted carefully. However, some obvious problems are found. The manuscript should undergo a major revision. Please see my comments below.
General comments:
The most outstanding problem is that the authors did not explain how and why the experiments were designed and conducted clearly. When reading the first half of the article, from the beginning to Fig 4, I was confused with the experiments and the results. The authors should explain the experimental procedures in detail in Section 2.
The authors showed the results, but did not make in-depth discussion to justify the results and explain any associated mechanisms. When delivering discussion, the authors should compare the results to other previous studies.
The article only shows very site-specific results. The authors should extrapolate the results to other situations. This study is only meaningful when the results can be applied broadly.
Specific comments:
Line 17: This sentence is confusing. Do you mean repeating adsorption until the permeate showed a distinguishable colour?
The first paragraph of Introduction: This paragraph is overly long. Please break it down.
Lines 59 to 62: Re-organize this sentence. It is confusing.
Lines 65 to 72: Introduction needs to be improved. Please condense the very basic information for pollution and dyes, but detail the information about dye treatment, especially for using carbon nanotubes. Based on a review of literature, discuss why you chose this material. Have previous studies investigated this material? What did they show? Please give more quantitative discussion about previous studies on this material. In addition, you need to indicate the research gap and highlight what was new in your study.
Line 76: Why did you choose Reactive Black-5 textile dye? Is it representative?
Lines 84 to 85: Please show some critical parameters of this material. It is better to conduct experiments to test different kinds of carbon nanotubes.
Line 88, “the silica gel spheres”: Indicate where you used them.
Table 1: Explain somewhere in the article how you determined how much nanotubes were going to be used. For example, you tested the heights of the layer from 0.4 to 4.5. You need to explain why they were selected.
Line 113: Did the solution only passed through the filter once? If yes, using 'circulation' is confusing. I think circulation means you pump the solution that has been passed through the filter back to the upstream of the filter, i.e., treating the same solution several times.
Line 115: Did you use the vacuum pump to control the flow rate? I think the contact time of the nanotubes with the solution is an important factor. You need to show the data (adsorption performance with different adsorption time) and make necessary discussion.
Line 122: The expression of 'no longer colorless' is confusing. Do you mean you stopped the run once the color after passing through the filter could be identified? How did you determine when it was not 'colorless'?
Sections 3.1 and 3.2: For Sections 3.1 and 3.2, you mainly showed the results, but with no enough in-depth discussion.
Line 144, “a completely colorless permeate”: Specify the determination criteria for colorlessness.
Fig 3: Please specify how to calculate the permeation time. Did you not fix the flowrate of the peristaltic pump? Did you conduct replicates? You should do so. Then when applicable, show the error bars in figures.
Lines 197 to 199: After reading this part back and forth and taking some reasoning, I think I understand the rationale now. If you fixed the vacuum pressure, a deeper carbon layer led to a higher resistance force and a slower filtration rate for every 100 mL solution. That is why the permeation time for C1 was much higher than other columns. Am I right? You did not explain the whole process very well. It makes it hard to understand the experiments and the discussion. Please improve Section 2 to provide all necessary information, so that audiences can understand Section 3. This deduces another important question. With the application of the same vacuum pressure, when a deeper carbon layer is used, the filtration rate is slower, which means the contact time (adsorption time) is longer. So you definitely get a better adsorption result. Is this experimental design reasonable? I think a better design could keep the filtration rate constant, i.e., the same 'permeation time'. In this case, it can better show how a higher amount of carbon impacts the adsorption.
Table 4: Tables 3 and 4 can be combined together. They are verifying the same observation that C1 was better. After showing the results, please discuss the mechanistic reason.
Fig 4: After you clearly indicated the threshold to determine a colorless solution, data in Fig 4 are not important. They only serve as an indicator to show when the run should be stopped. They can be incorporated into Fig 3.
Line 244: I think this is the criterion for determining the colorless solutions. This criterion is important information. You need to explicitly indicate it beforehand, for example, in the experimental section.
Table 5: I think Tables 3, 4, and 5 are showing the same thing. Please try to incorporate them into a single table and reduce the redundant data.
Fig 5: Figs 3, 4, and 5 can be combined together.
Table 6: Merge Table 6 into other tables.
Fig 6: Fig 6 and Fig 4 are actually showing the same thing.
Line 295: Expand the discussion. I think it is better to verify this phenomenon through tests.
Line 301: How was inorganic carbon generated from dye decomposition? Please discuss the mechanisms.
Section 3.3: You only showed the results, but did not explain why you did these tests. How do these results relate to previous results? Additionally, you need to explain the results according to relevant mechanisms.
Line 306: What are TG and DTA? What are they used for? Please explain them probably in Section 2.
Author Response
ANSWERS TO REVIEWER 2
Dear Reviewer,
Thank you for your time spent reviewing our manuscript and for giving us important tips for improving it. The manuscript has been revised in many parts. The changes have been highlighted in red. Here are the answers to your suggestions point by point. We hope that in this form the manuscript will find your approval to be considered for its publication.
We thank you and send you our best regards.
This manuscript reports a study for investigating the performance of carbon nanotubes to remove a dye from water. Concrete experimental results have been shown. It can be seen that the experiments were conducted carefully. However, some obvious problems are found. The manuscript should undergo a major revision. Please see my comments below.
General comments:
The most outstanding problem is that the authors did not explain how and why the experiments were designed and conducted clearly. When reading the first half of the article, from the beginning to Fig 4, I was confused with the experiments and the results. The authors should explain the experimental procedures in detail in Section 2.
Answer:
The following sentences have been added in the manuscript to explain the research more clearly
Introduction: “In our previous work, the removal of Reactive Black-5 dye from aqueous solutions with carbon nanotubes in static systems was investigated, evaluating the influence of the nature of the carbon nanotubes, the initial concentrations of dye, the stirring speed and the times on the adsorption process [79].
In this work, in particular, we wanted to continue the previous research [79] by studying the adsorption of the Reactive Black-5 dye from aqueous solutions with carbon nanotubes in semi-continuous processes, in order to search for the optimal conditions for the purification of water contaminated.
For this purpose, different adsorption columns were studied and subjected to different adsorption cycles which were suspended only when the permeate was colorless. The characteristics of each column were studied and compared.” (Lines 63-75)
In Section 2:
“The research was carried out through several consecutive steps. In the first phase, the raw materials were chosen and prepared. Subsequently, adsorption columns with different quantities and types of nanotubes were prepared. Adsorption tests in a semi-continuous process follow. Finally, the retentates and permeates were characterized. Below are the details of the different steps of the research. (Lines78-82)”
The authors showed the results, but did not make in-depth discussion to justify the results and explain any associated mechanisms. When delivering discussion, the authors should compare the results to other previous studies. The article only shows very site-specific results. The authors should extrapolate the results to other situations. This study is only meaningful when the results can be applied broadly.
Answer: The following sentences are added in the manuscript: “The data obtained allow us to say that during the adsorption process, two factors intervene simultaneously: the quantity of nanotubes used and the contact times. By increasing the thickness of the nanotubes inside the column, the solution finds greater resistance to its descent which also leads to an increase in contact times. Both the greater quantity of nanotubes and the longer contact time create the conditions for a greater adsorption of the column. For example, for the C1 column, which has a greater quantity of nanotubes and longer contact times, the result is greater adsorption. The adsorption is due to the great van der Waals forces between the dye and the carbon nanotubes.” (Section 3.2 – Lines 325-332)
Specific comments:
Line 17: This sentence is confusing. Do you mean repeating adsorption until the permeate showed a distinguishable colour?
Answer:
The following sentence was inserted in the manuscript: “A predetermined volume of dye solution, equal to 100 mL, was subjected to repeated cycles of adsorption until the eluted solution became colorless. This adsorption operation was carried out for different types of columns”. (Lines 17-20)
The first paragraph of Introduction: This paragraph is overly long. Please break it down.
Answer:
The introduction has been cut and edited
Lines 59 to 62: Re-organize this sentence. It is confusing.
Answer: The following sentence was modifed: “Textile wastewater treatments are carried out through different processes such as biological [13-19], oxidative [20-22], coagulation [23-26], flocculation [27], membranes [28-29], photocalaysis [30-36]”.(Lines 50-52)
Lines 65 to 72: Introduction needs to be improved. Please condense the very basic information for pollution and dyes, but detail the information about dye treatment, especially for using carbon nanotubes. Based on a review of literature, discuss why you chose this material. Have previous studies investigated this material? What did they show? Please give more quantitative discussion about previous studies on this material. In addition, you need to indicate the research gap and highlight what was new in your study.
Answer: The introduction has been cut and edited.
Line 76: Why did you choose Reactive Black-5 textile dye? Is it representative?
Answer:
The following sentences were added in the manuscript: “In our previous work, the removal of Reactive Black-5 dye from aqueous solutions with carbon nanotubes in static systems was investigated, evaluating the influence of the nature of the carbon nanotubes, the initial concentrations of dye, the stirring speed and the times on the adsorption process [79]. In this work, in particular, we wanted to continue the previous research [79] by studying the absorption of the Reactive Black-5 dye from aqueous solutions with carbon nanotubes in semi-continuous processes, in order to search for the optimal conditions for the purification of water contaminated. Reactive black-5 dye was chosen as the representative dye present in textile wastewater. For this purpose, different adsorption columns were studied and subjected to different adsorption cycles which were suspended only when the permeate was colorless. The characteristics of each column were studied and compared. (Lines 63-75)
Lines 84 to 85: Please show some critical parameters of this material. It is better to conduct experiments to test different kinds of carbon nanotubes.
Answer: In section 2.1 this sentence has been inserted: “Further information on the characteristics of the nanotubes used can be found in references [80].” (Lines 89-90)
Line 88, “the silica gel spheres”: Indicate where you used them.
ANSWER: In section 2.1 this sentences has been inserted: “Previous tests carried out with a dispersion of carbon nanotubes in water, allowed to exclude the passage of the carbon nanotubes through the borosilicalite filter. The use of water and not of the dye solution was adopted to avoid that the color of the solution could cover and not make visible a possible passage of the nanotubes through the filter. However, permeates were constantly observed and no loss of nanotubes through the filter was observed.” (Lines 106-111)
Table 1: Explain somewhere in the article how you determined how much nanotubes were going to be used. For example, you tested the heights of the layer from 0.4 to 4.5. You need to explain why they were selected.
ANSWER: The following sentence was added in the manuscript: “The quantities of nanotubes were chosen after a series of preliminary tests which had the criterion of preparing columns as efficient as possible and with the minimum quantities” (Lines 100-102).
Line 113: Did the solution only passed through the filter once? If yes, using 'circulation' is confusing. I think circulation means you pump the solution that has been passed through the filter back to the upstream of the filter, i.e., treating the same solution several times.
ANSWER: The following sentence was modified in the manuscript: “The passage of the solution inside the columns was guaranteed thanks to the use of a peristaltic pump”.(Lines 127-128)
Line 115: Did you use the vacuum pump to control the flow rate? I think the contact time of the nanotubes with the solution is an important factor. You need to show the data (adsorption performance with different adsorption time) and make necessary discussion.
ANSWER: The vacuum pump was used only to facilitate the descent of the dye solution into the column and was always kept constant. The descent times of the solution inside the column were variable and increased by increasing the amount of nanotubes inside the columns. Furthermore, within each single column, the solution descent time tends to increase in conjunction with its progressive congestion. However, the time data are shown in figure 3.
The following sentence was added: “The permeation time was calculated as the sum of the times used by each single adsorption cycle up to the last cycle, that is, until the solution was no longer colorless”. (Lines 185-186)
Line 122: The expression of 'no longer colorless' is confusing. Do you mean you stopped the run once the color after passing through the filter could be identified? How did you determine when it was not 'colorless'?
ANSWER: The following sentence was added in the manuscript:” The adsorption cycles were blocked when the permeate was visually colorless, confirming the saturation of the nanotubes” (Lines 137-138)
Sections 3.1 and 3.2: For Sections 3.1 and 3.2, you mainly showed the results, but with no enough in-depth discussion.
ANSWER: The following sentences were added in manuscript:
The values of the hydraulic parameters show a dependence on the quantity of nanotubes present in the columns. In short, it was found that the contact times, the total treatable volume and the Empity Bed Contact Time increase by increasing the quantity of nanotubes, while the Carbon Usage Rate value is inversely proportional. However, it must be emphasized that these dependencies are not linear. The treatable volume, for example, did not show a linear dependence with the quantity of carbon nanotubes. All this leads us to say that by increasing the quantity of nanotubes in the columns, not only does the quantity of adsorbent material increase but another parameter such as the resistance that opposes the descent of the solution and which determines an increase in contact times. A slight increase in the quantities of carbon nanotubes leads to a larger variation of the parameters. In fact, for example, by comparing the data of columns C1 and C2, it is highlighted that a doubling of the quantities of carbon nanotubes inside the column does not lead to an increase of twice the treatable volume but rather of about 4 times greater. (Lines 264-276)
Comments have also been added in section 3.2 (see above- Lines 325-332)
Line 144, “a completely colorless permeate”: Specify the determination criteria for colorlessness.
ANSWER: The following sentence was modified: “Table 2 reports the number of adsorption cycles, carried out in the different columns, which allow to obtain a visually colorless permeate, together with the respective total volume of treated solution.” (Lines 169-171)
Fig 3: Please specify how to calculate the permeation time. Did you not fix the flow rate of the peristaltic pump? Did you conduct replicates? You should do so. Then when applicable, show the error bars in figures.
ANSWER: The following sentence was added: “The permeation time was calculated as the sum of the times used by each single adsorption cycle up to the last cycle, that is, until the solution was no longer colorless”. (Lines 185-186)
Lines 197 to 199: After reading this part back and forth and taking some reasoning, I think I understand the rationale now. If you fixed the vacuum pressure, a deeper carbon layer led to a higher resistance force and a slower filtration rate for every 100 mL solution. That is why the permeation time for C1 was much higher than other columns. Am I right? You did not explain the whole process very well. It makes it hard to understand the experiments and the discussion. Please improve Section 2 to provide all necessary information, so that audiences can understand Section 3. This deduces another important question. With the application of the same vacuum pressure, when a deeper carbon layer is used, the filtration rate is slower, which means the contact time (adsorption time) is longer. So you definitely get a better adsorption result. Is this experimental design reasonable? I think a better design could keep the filtration rate constant, i.e., the same 'permeation time'. In this case, it can better show how a higher amount of carbon impacts the adsorption.
ANSWER: We agree perfectly with your speech, so we have included the following sentence in the manuscript:
“The data obtained allow us to say that during the adsorption process, two factors intervene simultaneously: the quantity of nanotubes used and the residence times. By increasing the thickness of the nanotubes inside the column, the solution finds greater resistance to its descent which also leads to an increase in contact times. Both the greater quantity of nanotubes and the longer contact time create the conditions for a greater adsorption of the column. For example, for the C1 column, which has a greater quantity of nanotubes and longer contact times, the result is greater adsorption.” (Lines 325-332)
Table 4: Tables 3 and 4 can be combined together. They are verifying the same observation that C1 was better. After showing the results, please discuss the mechanistic reason.
ANSWER : Tables 3 and 4 have been merged (now Table 3) and comments have been added (see above-Lines 264-276)
Fig 4: After you clearly indicated the threshold to determine a colorless solution, data in Fig 4 are not important. They only serve as an indicator to show when the run should be stopped. They can be incorporated into Fig 3.
ANSWER: We tried to unify some figures but this made it difficult to maintain the exhibition organization of the manuscript. If this does not create problems, we would like to leave the figures separate for a simpler reading of the reported values such as permeation times, residual concentration, adsorption capacity and removal efficiency.
Line 244: I think this is the criterion for determining the colorless solutions. This criterion is important information. You need to explicitly indicate it beforehand, for example, in the experimental section.
ANSWER: The following sentences were added: “The adsorption cycles were blocked when the permeate was visually colorless, confirming the saturation of the nanotubes. Furthermore, the maximum quantities of retainable dye in the various columns were calculated according to the volumes of solution treated until the color reappear.” (Lines 137-140)
Table 5: I think Tables 3, 4, and 5 are showing the same thing. Please try to incorporate them into a single table and reduce the redundant data.
ANSWER: Tables 3 and 4 have been merged into a single table (now Table 3); Table 5 and 6 have been merged into a single Table (now Table 4)
Fig 5: Figs 3, 4, and 5 can be combined together.
ANSWER: See previous answer.
Table 6: Merge Table 6 into other tables.
ANSWER: Table 6 has been inserted into Table 4
Fig 6: Fig 6 and Fig 4 are actually showing the same thing.
ANSWER: We thank you for the chance you give us to clarify. Figures 4 and 6 report the residual concentrations and removal efficiencies as a function of the number of adsorption cycles ,respectively. Practically, Figure 4 represents the data of the UV analyses while Figure 6 represents the processing of these data.
Line 295: Expand the discussion. I think it is better to verify this phenomenon through tests. How was inorganic carbon generated from dye decomposition? Please discuss the mechanisms.
ANSWER: The pandemic situation does not currently allow us to carry out further tests, but the following sentence has been added in the text: "It is possible to hypothesize that inorganic carbon is generated by the decomposition of the dye into carbonates, bicarbonates, and dissolved CO2”. (Lines 350-351).
Section 3.3: You only showed the results, but did not explain why you did these tests. How do these results relate to previous results? Additionally, you need to explain the results according to relevant mechanisms.
ANSWER: The following sentences were added: “This non-homogeneity in the nanotube filters, in which the higher layers have a greater weight loss and therefore a greater adsorption can be attributed to the fact that the solution as it arrives in the deeper layers of the filter will flow outwards in faster way by having an increasingly smaller layer in front that opposes its passage. On the other hand, in the higher layers of the filter, the descent of the solution will be hindered by the lower layers. All this determines within the filter, different contact times of the solution which will be greater in the higher layers and less in the lower ones. This gradient of contact times within the filter generates a stratification of the dye which will be more evident the greater the thickness of the layer” (Lines 398-406).
Line 306: What are TG and DTA? What are they used for? Please explain them probably in Section 2.
ANSWER: The following sentences were added : “Subsequently thermogravimetric analysis (TG) and differential thermal analysis (DTA) (Shimadzu-60, Kyoto, Japan), in order to detect the weight losses and the thermal characteristics of the nanotubes after the adsorption tests, at different depths of the nanotube layer present in the column (High, Medium, Low) were carried out. An air flow of 50 mL/min and heating rate of 10 °C/min were utilized.” (Lines 155-159)
Author Response File: Author Response.docx
Reviewer 3 Report
Although the topic addressed in this work is interesting and reveals an innovative technology for water purification, I cannot suggest its publication in the journal of Applied Sciences.
Along with numerous grammatical errors and editing errors, the manuscript lacks rigor in data treatment and there is no adequate statistical methodology.
Furthermore, a more detailed and in-depth explanation of the physical and chemical phenomena that take place would be desirable.
On the other hand, the presence of carbon nanotubes in the filtration and purification system is not adequately justified or discussed. In fact, can the authors explain the mechanism underlying the adsorption in the layer of MWCNTs? In my opinion, that would be the essence of the work and would allow a possible applicability.
Author Response
ANSWERS TO REVIEWER 3
Although the topic addressed in this work is interesting and reveals an innovative technology for water purification, I cannot suggest its publication in the journal of Applied Sciences. Along with numerous grammatical errors and editing errors, the manuscript lacks rigor in data treatment and there is no adequate statistical methodology. Furthermore, a more detailed and in-depth explanation of the physical and chemical phenomena that take place would be desirable. On the other hand, the presence of carbon nanotubes in the filtration and purification system is not adequately justified or discussed. In fact, can the authors explain the mechanism underlying the adsorption in the layer of MWCNTs? In my opinion, that would be the essence of the work and would allow a possible applicability.
Dear Reviewer,
First of all, thank you for taking the time to read our manuscript. We are sorry that the manuscript is not to your liking. Let us point out that this research is of an applicative nature and its main purpose is not to explain the reaction mechanisms, where other types analyses as NMR or IR would be needed. It wants to be a contribution to optimize a potential treatment of Reactive-Black-5 contaminated water, through a semi-continuous process using carbon nanotubes. The work aims to provide informative and useful parameters of the process, such as removal efficiency, permeation times, carbon usage rate, etc.
We justify the use of carbon nanotubes as they are very versatile materials and widely studied and used in water treatment. The reactive Black-5 dye molecules are adsorbed in MWNTs due to the great van der Waals forces between them. However, part of the dye molecules is decomposed as it is shown in the paper.
The manuscript, however, has been revisited, in several parts. We hope that in the current version it will find your approval.
Round 2
Reviewer 2 Report
I appreciate the authors’ efforts to make some revisions in the article based on my comments. I am now having some follow-up comments. Please see below.
Response to the first general comment: You wrote “For this purpose, different adsorption columns were studied and subjected to different adsorption cycles which were suspended only when the permeate was colorless” in Lines 73 to 75. To make it easier to be understood, please specify how you determined the colorless permeate.
Response to the second general comment: You wrote “The data obtained allow us to say that…” Did any other studies conduct similar tests? What did they report? Please provide such discussion somewhere in the article to make a comparison with your results. Please also discuss the application opportunity of this method somewhere in the article. As I said, only broader application can make this specific study useful.
Response to Line 17: You wrote ““A predetermined volume of dye solution… until the eluted solution became colorless.” Specify the quantitative criteria for “colorless”.
Response to Line 76: I now understand you chose Reactive black-5 because you used it before. However, this is not a scientific answer. Why did you choose it in your previous study? You need to convince audiences that this dye is the best or at least suitable for this kind of study.
Response to Lines 84 to 85: To make this paper standalone, it is suggested to show some critical parameters here.
Response to Line 88: You added a paragraph “Previous tests carried out with a dispersion of carbon nanotubes in water…” This paragraph is hard to be understood. Please make it better. It is not clear why you wrote it. Initially you mentioned “silica gel spheres”. My suggestion on that was that to briefly explain why you used it, maybe in one sentence.
Response to Line 113: You did not answer my question explicitly. Please indicate whether the solution was pumped through the filter once or several times.
Response to Line 122: Do you judge the colorless permeate by eyes? This is not rigorous. You need to detect the color using instruments and set a threshold to determine the colorlessness.
Response to Sections 3.1 and 3.2: You wrote “However, it must be emphasized that these dependencies are not linear…” For this nonlinear relationship, can you explain why?
Response to Fig 3: You did not fully address my comment. Did you conduct replicates? You should do so. Then when applicable, show the error bars in figures
Author Response
Answers to Reviewer 2
Dear Reviewer,
Thank you for your second time spent reviewing our manuscript. The changes have been highlighted in red. Here are the answers to your suggestions point by point. We hope that in this form the manuscript will find your approval to be considered for its publication.
We thank you and send you our best regards.
I appreciate the authors’ efforts to make some revisions in the article based on my comments. I am now having some follow-up comments. Please see below.
1) Response to the first general comment: You wrote “For this purpose, different adsorption columns were studied and subjected to different adsorption cycles which were suspended only when the permeate was colorless” in Lines 73 to 75. To make it easier to be understood, please specify how you determined the colorless permeate.
1) ANSWER. The lack of color in the permeate was verified visually by comparing the permeate with a sample of distilled water taken as a reference. Both, the sample and the reference, were subjected to the same light on a white background and compared. However, it is necessary to underline that the instrumental determination of color was not considered fundamental, but a clear visual determination of lack of color was sufficient, since each permeate was subsequently subjected to spectrophotometric UV analysis which determined the precise values of concentration of dye. For greater clarity the following sentence was added in the manuscript: “The lack of color in the permeate was visually verified by comparing the permeate with a distilled water sample taken as a reference. Both, the sample and the reference, were subjected to the same light on a white background and compared "(See Section 2.3)
2)Response to the second general comment: You wrote “The data obtained allow us to say that…” Did any other studies conduct similar tests? What did they report? Please provide such discussion somewhere in the article to make a comparison with your results. Please also discuss the application opportunity of this method somewhere in the article. As I said, only broader application can make this specific study useful.
2) ANSWER.
This part has been inserted in the manuscript with the relative references: “We are not aware of any research that has used columns of carbon nanotubes in semi-continuous processes, for the treatment of water specifically contaminated by Reactive Black 5. However, studies reported in the literature have used similar experimental conditions, although for the removal of other substances such as NOMs (Natural organic matter) [84], glimepiride [85], heavy metals [86], achieving results consistent with those reported by us, in which the quantity of carbon nanotubes, and therefore the height of the read in the column, it plays the fundamental role capable of amplifying the adsorption capacity”. (Section 3.3)
3)Response to Line 17: You wrote ““A predetermined volume of dye solution… until the eluted solution became colorless.” Specify the quantitative criteria for “colorless”.
3) ANSWER. See answer 1
it is necessary to underline that the instrumental determination of color was not considered fundamental, but a clear visual determination of lack of color was sufficient, since each permeate was subsequently subjected to spectrophotometric UV analysis which determined the precise values of concentration of dye. For greater clarity the following sentence was added in the manuscript: “The lack of color in the permeate was visually verified by comparing the permeate with a distilled water sample taken as a reference. Both, the sample and the reference, were subjected to the same light on a white background and compared "(See Section 2.3)
4)Response to Line 76: I now understand you chose Reactive black-5 because you used it before. However, this is not a scientific answer. Why did you choose it in your previous study? You need to convince audiences that this dye is the best or at least suitable for this kind of study.
4) ANSWER. For greater clarity, the following sentence has been added in the manuscript with the relative reference: "This dye was chosen because it is one of the most common dyes used in the textile industry thanks to the presence of reactive groups in the molecule that promote the formation of covalent bonds with fibers. Furthermore it was also chosen for its high solubility in water which allowed to prepare its aqueous solutions to be used in the experimental procedures [80]”. (See Introduciton)
5)Response to Lines 84 to 85: To make this paper standalone, it is suggested to show some critical parameters here.
5) ANSWER. The following sentence was added: “They have a specific BET area of 108.70 m2/g and an average pore width of 103.70 Å. (See section 2.1)
6)Response to Line 88: You added a paragraph “Previous tests carried out with a dispersion of carbon nanotubes in water…” This paragraph is hard to be understood. Please make it better. It is not clear why you wrote it. Initially you mentioned “silica gel spheres”. My suggestion on that was that to briefly explain why you used it, maybe in one sentence.
6) ANSWER. We added these sentences to answer the other reviewer who asked us: "How was it determined that MWNTs did not cross the borosilicate filter? How often was it monitored, etc.?"
We then replied that we had carried out exploratory tests with a dispersion of nanotubes in water in order to exclude the passage of nanotubes. However, the following sentence has been changed to "
“To exclude the passage of carbon nanotubes through the borosilicalite filter, preliminary tests, before the adsorption tests, were carried out with a dispersion of carbon nanotubes in water. The use of water only and not of the dye solution was adopted to prevent the color of the dye solution from covering and not making any passage of the nanotubes visible through the filter. However, permeate and no loss of nanotubes through the filter were consistently observed”. (See Section 2.1)
We actually confused the answer. To explain why silica gel spheres were used, this sentence was added.
"Over the carbon nanotube layer, inside each column, a layer of silica gel spheres was added to ensure the maintenance of the carbon nanotube compaction and to achieve a uniform impact of the dye solution on the section of the nanotube layer".(See Section 2.2)
7)Response to Line 113: You did not answer my question explicitly. Please indicate whether the solution was pumped through the filter once or several times.
New 100 ml of dye solution were used for each cycle.
The following sentence is present in the manuscript: “The process was powered semi-continuously: after every 100 ml of treated solution, the process was blocked to allow the collection of the permeate and subsequently loaded with another new100 ml of Reactive Black-5 solution.” (See Section 2.3)
8)Response to Line 122: Do you judge the colorless permeate by eyes? This is not rigorous. You need to detect the color using instruments and set a threshold to determine the colorlessness.
8)ANSWER. See answer 1
it is necessary to underline that the instrumental determination of color was not considered fundamental, but a clear visual determination of lack of color was sufficient, since each permeate was subsequently subjected to spectrophotometric UV analysis which determined the precise values of concentration of dye. For greater clarity the following sentence was added in the manuscript: “The lack of color in the permeate was visually verified by comparing the permeate with a distilled water sample taken as a reference. Both, the sample and the reference, were subjected to the same light on a white background and compared "(See Section 2.3)
9)Response to Sections 3.1 and 3.2: You wrote “However, it must be emphasized that these dependencies are not linear…” For this nonlinear relationship, can you explain why?
9) ANSWER.
The following part has been rewritten: “However, it should be emphasized that these dependencies are not linearly proportional to the quantities of nanotubes but these parameters undergo a sort of amplification even for small increases in carbon nanotubes. This is because when the nanotubes are not dispersed, unlike what happens in the case of a Batch system, but are compacted in a column, a small increase in their quantity leads as a consequence, in addition to the increase in the adsorbent material which determines a greater adsorption, also an amplification of the resistances that oppose the descent of the solution. This results in a longer contact time and EBCT. The synergistic action of the increase in the adsorbent material and in the contact times leads to a marked improvement in the adsorption capacity. For example, by comparing the data of the treatable volume in columns C1 and C2, it can be seen that a doubling of the quantities of carbon nanotubes inside the column does not lead to a double increase in the treatable volume but rather about 4 times greater”. (See Section 3.1)
10)Response to Fig 3: You did not fully address my comment. Did you conduct replicates? You should do so. Then when applicable, show the error bars in figures.
10) ANSWER. The following sentence was inserted in the manuscript:“Each single test was repeated twice and taking into account the average value of the data obtained”( See section 2.3)
In figure 3 the error bars have been inserted.
Author Response File: Author Response.docx
Reviewer 3 Report
The presentation and content
of the work have improved and it is now
Suitable for publication.
Author Response
Dear Reviewer 3,
We thank you for taking the time to review our work and are happy for your approval.
Best regards
The authors
