Barium-Encapsulated Biodegradable Polycaprolactone for Sulfate Removal

Round 1

Reviewer 1 Report

In this paper, the authors present a study of the removal of sulfate from aqueous solution using a Polycaprolactone composite. The composite is characterized using XRD, SEM, EDS, TEM, HR-TEM, and TGA and column test are made in order to study the efficiency of sulfate removal using the PCL/BaCO3 composites.

 

The article presents some interesting results and proposes the use of a promising material for the elimination of sulfates in aqueous solution.

However, the manuscript reveals serious experimental errors and misconcepting interpretation of data by authors.

Composite characterization was carried out using a number of techniques, but the results of these are only qualitative no definitive quantitative data.

Plots sulfate concentration versus time exhibit erratic behavior (See detailed comments in the PDF document) possibly due to an erroneous determination of sulfate concentration. And the explanation for this behavior given by the authors is unacceptable (See detailed comments in the PDF document).

Comments for author File: Comments.pdf

Author Response

Detailed Responses to Reviewers.

Thank you very much for considering our manuscript for publication at the special issue of “Advances in Water and Wastewater Monitoring and Treatment Technology” in Water. We appreciate the raised comments of the reviewers and we tried to address them to our highest ability and to improve the submission accordingly. It is noted that applied changes in the revised version have been highlighted in yellow color. Please see attached file and you will find our point-by-point responses to the reviewers’ comments and questions.

Author Response File: Author Response.docx

Reviewer 2 Report

Manuscript ID: water-382391

Title: Barium-Encapsulated Biodegradable Polycaprolactone for Sulfate Removal

 

General comments for Authors

 

The manuscript deals with interesting environmental issue. It is very well constructed. The adsorbent material is very well characterized. Only the performed experiment has some shortcomings and must be expanded by examination of different important parameters, in order to achieve the best column design with the best column efficiency.

 

Detail comments:

 

Abstract

Line 13: The unused and used composites were… composites before and after sulphate adsorption sounds better. In this way in Abstract is also clear the applied method of removal.

 

Introduction

Introduction does not provide all necessary data - the information about sulphate sources in the environment should be added and supported by the references.

 

Line 27-28: Recently, the removal of sulfate from drinking water has been targeted due to deleterious effects of sulfate on the environment [7, 8]. Sulphates affect not only the environment but human health; it should be added here especially due to the next sentence.

 

Line 28-29: Abnormal changes resulting from sulfate consumption from drinking water may result in diarrhea and dehydration. These ill effects are more prominent during infant years and decline with age…These statements should be supported by the references.

 

Line 35-36. Monomethyl-, dimethyl-, monobutyl- and dibutyl levels have been found at values up to 291 ng Sn/L, 49.1 ng Sn/L, 28.5 ng Sn/L, and 52.3 ng Sn/L, respectively, in drinking water distributed through PVC pipes [12, 13]. It would be good to pointed that Sn means tin since it is the first time where Sn appears in the paper - tin (Sn).

 

In Table 1 I believe Polcaprolactone is Polycaprolactone.

 

Lines 85-96: From this part of the text it is not clear in which way exactly the column experiments were performed, by down-flow or up-flow mode? How the flow rate was kept constant? Authors must better explain the performed experiment. Moreover, based on what Authors chose the flow rate of 2 mL/min, the initial sulphates concentration of 1100 mg/L, and NOM with sulphates concentration of 328 mg/L? If by some previous studies, this should be supported by the references. Is this concentration too high for column experiments? It is very important to predict service time in which the effluent concentration would not exceed the maximum permissible levels.

What about the pH? Is it monitored? What was the initial pH of the sulphate solution? It is very important parameter in sorption processes. Authors investigated the influence of column diameter - D = 1.0 cm and D = 2.25 cm filled with the same adsorbent mass of 1 g. What are the heights of the adsorbent bed in each column? What are the heights of the column? What was the particle size of the adsorbent in the column?

 

Lines 86-87: First, sulfate removal using PCL/Ba samples was investigated using clean water. Sulfate solution of 1100 mg/L was prepared using deionized (DI) water. This sentence should be rearranged. This is aqueous solution prepared in deionised water by dissolving of some sulphate salt to concentration of 1100 mg/L.

 

Scheme 1 better belongs into Introduction section since this is not a result obtained by this research.

 

Figure 6: The titles of x and y axes are almost invisible, this should be enlarged.

 

 Line 196: The reference 20 is by Liu et al. not by Li et al. This should be corrected.

 

Line 200: However, different trend was observed in case of carbon nanotubes (CNT) dispersed polycaprolactone composites. In this sentence after However, different trend was observed, by Lee et al. should be added.

 

Line 230: Higher sulfate removal in the smaller column was a result of longer retention time and hence better diffusion of sulfate molecules through the polymer matrix. The retention time is equal since the flow rate of 2 mL/min is the same in both columns. The reason for earlier saturation in higher diameter column may be the lowest height of the adsorbent, since in both columns the equal mass (1 g) of the adsorbent was added. What was the height of the adsorbent in each column?

 

In higher bed height the mass transfer zone (MTZ) will not appear immediately at the end of the column. If height of the MTZ is equal to the height of the bed, then breakthrough will appear almost immediately when experiment starts. The increase of the bed height lowers the effect of axial dispersion on the mass transfer process. Thus evaluation of optimum operating conditions is necessary in column design. The ratios of the bed height H to the column diameter d (H/d) and to the adsorbent particle diameter dp (H/dp) are very important parameters in column design to avoid appearance of axial dispersion and to minimize the channelling in the adsorbent bed. On the basis of all this Authors must investigate larger number of parameters to get the best column design. Even the obtained removal of sulphates was good; still the higher number of parameters must be examined to find the best column efficiency. Please look the relevant literature, just as examples:

 

Himanshu Patel and R. T. Vashi (2015) Characterization and column adsorptive Treatment for COD and color removal using Activated neem leaf powder from textile Wastewater, Journal of Urban and Environmental Engineering, v.9, n.1, p.45-53, doi: 10.4090/juee.2015.v9n1.045053

 

Mahsa Jahangiri-rad, Arsalan Jamshidi, Mohammad Rafiee, and Ramin Nabizadeh (2014) Adsorption performance of packed bed column for nitrate removal using PAN-oxime-nano Fe₂O₃, J Environ Health Sci Eng. 2014; 12: 90. doi: [10.1186/2052-336X-12-90]

 

Al-Ghouti, M.A.; Khraisheh, M.A.M.; Ahmad, M.N.; Allen, S.J. (2007) Microcolumn studies of dye adsorption onto manganese oxide modified diatomite. J. Hazard. Mater., 146: 316. DOI: 10.1016/j.jhazmat.2006.12.024

 

Kumar, U.; Bandyopadhyay, M. (2006) Fixed bed column study for Cd(II) removal from wastewater using treated rice husk. J. Hazard. Mater., B129:253. DOI:10.1016/j.jhazmat.2005.08.038

 

Zhou, D.; Zhang, L.; Zhou, J.; Guo, S. (2004) Development of a fixed-bed column with cellulose/Chitin beads to remove heavy-metals ions. J Appl. Polym. Sci., 94: 684. https://doi.org/10.1002/app.20946

 

Figure 9: In title of Figure 9 the PCL/Ba (10/90) should be stated as well as the column dimeter.

 

Table 3: There is no need for columns with influent concentration as well as with flow rate since these two parameters are not changed. They can be stated in the table title.

Author Response

Detailed Responses to Reviewers.

Thank you very much for considering our manuscript for publication at the special issue of “Advances in Water and Wastewater Monitoring and Treatment Technology” in Water. We appreciate the raised comments of the reviewers and we tried to address them to our highest ability and to improve the submission accordingly. It is noted that applied changes in the revised version have been highlighted in yellow color. Please see attached file and you will find our point-by-point responses to the reviewers’ comments and questions.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors discussed the synthesis and characterization of barium-containing PCL polymer composites via reprecipitation. Fabrication of the composites with varied material content was well thought out and the systematic dimensional and physical characterization was very thorough. Additionally, analysis of the composite adsorption performance via sulfate and organic matter removal was well done. As a whole, the manuscript is well written and communicates the experimental motive and results exceptionally well and is recommended for publication.

Author Response

Detailed Responses to Reviewers.

Thank you very much for considering our manuscript for publication at the special issue of “Advances in Water and Wastewater Monitoring and Treatment Technology” in Water. We appreciate the raised comments of the reviewers and we tried to address them to our highest ability and to improve the submission accordingly. It is noted that applied changes in the revised version have been highlighted in yellow color. Please see attached file and you will find our point-by-point responses to the reviewers’ comments and questions.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript shows the results of the preparation of a biodegradable adsorbent for effective removal of sulfate from water. However, article has serious flaws and additional experiments needed.

 

Specific Comments.

 

Comments on authors reply to Referee 2 comments:

 

Authors have replied in an appropriate way to most of the comments of referee 2 and have followed his indications; however, some issues pointed out by referee 2 remain unchanged and/or without reply. Below are underlined the unanswered or uncorrected questions

 

    Lines 85-96:

What are the heights of the adsorbent bed in each column? What are the heights of the column? What was the particle size of the adsorbent in the column?

    Line 200: However, different trend was observed in case of carbon nanotubes (CNT) dispersed polycaprolactone composites. In this sentence after However, different trend was observed, by Lee et al. should be added.

A reference number should be given. 21, I guess.

    Line 230: Higher sulfate removal in the smaller column was a result of longer retention time and hence better diffusion of sulfate molecules through the polymer matrix. The retention time is equal since the flow rate of 2 mL/min is the same in both columns. The reason for earlier saturation in higher diameter column may be the lowest height of the adsorbent, since in both columns the equal mass (1 g) of the adsorbent was added. What was the height of the adsorbent in each column?

 

    In higher bed height the mass transfer zone (MTZ) will not appear immediately at the end of the column. If height of the MTZ is equal to the height of the bed, then breakthrough will appear almost immediately when experiment starts. The increase of the bed height lowers the effect of axial dispersion on the mass transfer process. Thus evaluation of optimum operating conditions is necessary in column design. The ratios of the bed height H to the column diameter d (H/d) and to the adsorbent particle diameter dp (H/dp) are very important parameters in column design to avoid appearance of axial dispersion and to minimize the channelling in the adsorbent bed. On the basis of all this Authors must investigate larger number of parameters to get the best column design. Even the obtained removal of sulphates was good; still the higher number of parameters must be examined to find the best column efficiency. Please look the relevant literature, just as examples:

 

Comments on authors reply to Referee 3 comments:

 

Authors have replied in an appropriate way to some of the comments of referee 3 and have followed his indications; however, some fundamental issues pointed out by referee 3 remain unchanged and/or without reply. Below are underlined the unanswered or uncorrected questions

Answer: The objective of this study was developing an innovative biodegradable adsorbent to remove sulfate from water for providing safe water to the public and proving if the developed biodegradable materials can effectively remove sulfate in DI water in the absence/presence of natural organic matter. Even though the concentration range of sulfate for the method is from 2 to 70 mg, the method still demonstrates linearity of the concentration range tested in this study, which clearly showing the developed biodegradable adsorbent effectively removed sulfate from water. In addition, 10^th edition of the method 8051, including Ba interference for sulfate quantification, was recently released in February, 2018. Therefore, we added relevant discussion in the revised manuscript.

“Recently updated SlulfaVer 4 method (10^th edition, released in February, 2018) reported that Ba interferes sulfate quantification. The maximum error may occur about 20% lower concentration than actual sulfate concentration when Ba concentration is very high.”

No evidence of the linearity of the concentration range tested in this study is given.

I would like to point out that the method for determining sulfate concentration is not adequate. Neither for the sulfate concentration range of the sample, nor for the presence of barium in the composite that can pass to the solution and distort the results of the analysis. Even though PCL and BaCO₃ composite showed significant sulfate removal capacity, the obtained results can be only be considered qualitatively.

 

Have the authors checked the precision and/or accuracy of obtained data by means of determination of sulfates concentration by another method?

Answer: We updated Figure 8 in the revised version. Even though the analytical method cause maximum 20% error in the presence of high Ba in samples, it showed clear trends of sulfate removal for each adsorbent with different polycaprolactone contents. We believe that new Figure 8 clearly shows that the developed adsorbents can effectively remove sulfate from water and the removal capacity of the adsorbents was dependent on the content of Ba. Additionally, as we discussed in the revised manuscript, such the pattern of sulfate removal using PCL/Ba media can be explained in terms of mass transfer principles. In the initial phase, barium available on the exterior of the packed column comes in contact with sulfate and quickly gets saturated. Thereafter, it takes a longer time period for the sulfate molecules to access the barium molecules buried in the inside the polymer matrix. Once all the barium molecules are saturated with adsorbed sulfate, the media reaches complete saturation and is not capable of any further sulfate removal.

In view of the results of column, it could be stablish, but only qualitatively, that initially a very rapid process of sulfate removal from solution. This process is responsible for the total and fast sulfate elimination in the case of 50% Polycaprolactone and 1.0 cm column diameter; precipitation reaction, which is very fast, can be responsible of sulfate removal in this initial period. A further process with a comparatively high incubation time, results in the complete elimination of sulfate

 

 

Have the authors considered the possibility that the PLC polymer removes sulfate?

 

 

Answer: Since the flow rate of this experiment was 1 mL/min, indicating the retention time of the sulfate solution in the reactor is very long, we believe that there is enough time for mass transfer. Also, even though most reaction will occur at the surface of the adsorbent, most parts of adsorbent could be utilized to remove sulfate because particle sizes of the developed adsorbents are less than 10 μm. When all surface of the adsorbent is saturated with sulfate, sulfate will be detected in discharged solution from the reactor.

What is the reason why no experimental points have been obtained for the 10% Polycaprolactone and 2.25 cm column diameter plot between 0.5 hours and 48 hours?

In figure 8, for the 10% Polycaprolactone and 1.0 cm column diameter plot, at 90 h, sulfate concentration data vary approx. between 680 and 820 mg/L. What is the actual sulfate concentration at 90 h?

 

 

Line 53 the use of PVC and BaCO3 composites is reported in reference 9.

 

Reviewer 2 Report

Manuscript ID: water-382391

Title: Barium-Encapsulated Biodegradable Polycaprolactone for Sulfate Removal

 

Comments for Authors

 

The manuscript was improved in accordance with most reviewers’ suggestions. I am aware of the main objective of this study. As Authors stated it was developing an innovative biodegradable adsorbent to remove sulfate from water for providing safe water to the public and proving if the developed biodegradable materials can effectively remove sulfate in DI water in the absence/presence of natural organic matter. And I am aware that thus limited experimental conditions were tested to verify the performance of the developed biodegradable adsorbent, especially knowing how the column performance is very long lasting.

 

I was most worried that the Authors did not explain the experiment well. Now the performed experiment is clearer. But there is a part of the text which I still cannot accept in this form and must be better explained:

 

Lines 235-237: Although saturation in the larger diameter column (d=2.25 cm) was achieved much quicker compared to the smaller diameter (d=1.0 cm) column, the percent sulfate removal was higher in the smaller.

 

This sentence is not well designed. Please rearrange the sentence. Because it is obvious that if the saturation in the larger column was achieved earlier means that the percent of sulphate removal is higher in the smaller one.

 

Lines 237-239: Higher sulfate removal in the smaller column was a result of longer retention time and hence better diffusion of sulfate molecules through the polymer matrix.

 

I am sorry but I cannot accept this statement in present form. Based on what the Authors concluded this? Where are the values of retention times? Did authors calculate them? Or they based the conclusion on some reference? If so, please add the references.

 

In literature can be find that empty bed contact time (EBCT) is a critical parameter that represents the residence time during which the solution being treated is in contact with the sorbent, and includes bed height in the column and the linear flow rate. I agree that column diameter also affected it (because it is contained in the expression for linear flow rate in the cross-sectional area of the column) but cannot be considered without the height of the sorbent in the column and flow rate. Since the flow rate in this study is constant (1 mL/min), then column diameter affects the bed height especially if the same adsorbent mass of 1 g was added in each column. This is the reason why I asked for the height of the adsorbent in each column. The Authors can give this data in the manuscript based on performed experiment.

 

The same is in regard to the part of the text in Conclusions.

Lines 276-277: The sulfate removal was higher in the small column due to longer retention time for mass transfer compared to the large column.