New Bioadsorbent Derived from Winemaking Waste Cluster Stalks: Application to the Removal of Toxic Cr(VI) from Liquid Effluents

Round 1

Reviewer 1 Report

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Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

The article is an interesting study and are very well described. They are described logically and coherently. The article should be published in a Applied Sciences journal. 

However, I have four comments.

1. 1. The authors in chapter 3.2 have only included the best fit results for both isotherms and kinetics. In such a situation it is difficult to figure out if other results diverge far from the best. It would be worth to include them, e.g. in the form of a table, because the reader lacks a list of other results. As you can see from other publications, it sometimes happens that R² for different series are so close to each other that it is difficult to clearly indicate which model was the best. How was it this time?
2. 2. Fig. 2 - some descriptions are mirror images. Therefore, the drawing is unreadable.
3. As you can see, the point of zero charge pH (pH_PZC) was not tested. Can the approximate value of this point be determined from the current research?
4. The methodology does not explain how the graph in Fig. 5 (section 3.1.4) was obtained.

Author Response

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Reviewer 3 Report

Dear Authors,

after long thought, I decided to reject your manuscript. In present form it has too much failures and needs the strong rewrite. Below I present the main comments.

1. The title is inadequate and it misleads the reader. It suggests the paper with wide characteristics of the new material. In the manuscript there is no material characteristics (surface, structure, chemistry) but adsorptive properties studies.
2. The way you cite the references is incorrect. I checked the references [2,4,5,6] and the papers are not exactly about you wrote. E.g. the sentence about the sources of chromium pollutants (lines 33-35) you documented by the paper [2] contains the comparison of isotherm models, whereas it should be an article about the topic presented in the sentence. So, the references cited in the paper must be checked and evaluated.
3. lines 59,60: The first sentence of the aim is “In present work an activated carbon (AC) …. has been described”. The Authors described only the preparation of AC, but the material wasn’t described.
4. The methodology section is incomplete and badly described. You described many tests differently designed (the influence of pH, ionic strength, T, stirring, equilibrium and kinetics) trying to unify these. However, the description omitted many information. I think, first the scope should be presented. Next the dependent and independent variables of different tests. Separately could be described the general conditions of tests. Next the analytical methods and data treatment. You presented the process parameters very fragmented – part in the methodology section and part in results section (under the Figures, but not all of them).
5. You presented results of pH measurement – how was the pH measured. Was the samples incubated in the thermal cabinet? What about the ionic strange? Was it theoretically calculated or conductivity was measured?
6. The data were fitted using presented models. What was the method of models fitting or software used?
7. The fitting was validated only by determination coefficient R2. It is strongly insufficient additionally in linear regression! Another error functions must be applied.
8. line 85: There is presented stirring range 250-1000 min-1, but in the Figure 1 is presented the wider range 250-1500.
9. The symbols applied in the manuscript are hard to distinguish, not obvious. This form is unacceptable. So:

9a.  First, the symbols are in brackets, what is reserved mainly for molar concentration. It could be ok in equations no. 6 and 7, but only here.

9b. You have differentiated the symbols by the expression in the subscript. It is hard to recognize and reading.

9c. The symbols used are inconsistent. Line 124: [Cr]ads,e is the concentration of metal at equilibrium, and previously in equations it was [Cr]aq,e. The same [Cr]ads,t and [Cr]aq,t.

9d. However, in lines 124,125 there is the inaccuracy. You wrote “the concentrations of metal adsorbed onto the carbon…” and put the unit (mg/L). I hope the ratio of adsorption (qt/qe) is in equation 13, not the concentration (Ct/Ce).

9e. In used kinetic equations there are reaction rate constants, but you used only one symbol (k) for all of them. How to conduct a discussion when symbols are not differentiated?

10. You didn't provide sources of the equations 2-9.
11. Consider whether you are providing original sources: e.g. the eq. 10 comes from the work of Boyd (1947).
12. In my opinion your consideration of thermodynamics needs the improvement. Please see the explanation in the paper (http://dx.doi.org/10.1016/j.watres.2017.04.014)

“In summary, to determine accurate thermodynamic parameters, several points should be thoroughly considered. First, the equilibrium constant (KC) must be dimensionless. Second, if the concentrations of adsorbate used to obtain the adsorption isotherm are very low (infinite dilution), the partition (Kp) or distribution coefficients (Kd) are appropriate for calculation of the thermodynamic parameters; otherwise, the Langmuir (KL) or Freundlich (KF) constant models may be more appropriate. Third, the plot of lnKC against 1/T corresponding to the van't Hoff equation must be linear with a high regression coefficient (R2). Additionally, it is necessary to consider whether the adsorption process can reach equilibrium and consideration of the adsorption isotherm shapes and the adsorption model fit are also recommended. Thus, presentation of the complete adsorption isotherm (plot of qe versus Ce) is strongly recommended. Finally, the determined thermodynamic parameters must have a logical relationship with the relevant experimental data.”

In your manuscript are presented only vestigial information. Please see also the approach to the distribution coefficient Kd. Were the solutions strongly diluted or not? (no information in the manuscript)

13. Figure 2. needs the correction.
14. Stirring test. Was the carbon previously degassed? The presence of air in pores strongly indicates the experimental results and high stirring favours gas removal.
15. The authors' approach to the chromium removal mechanism is unacceptable. Please see the explanation in the paper (http://dx.doi.org/10.1016/j.watres.2017.04.014) section 8.2.

“Generally, when Cr(VI) comes into contact with organic substances or reducing agents, especially in an acidic medium, it is easily or spontaneously reduced to Cr(III), as Cr(VI) has a high redox potential (above þ1.3 V under standard conditions). Therefore, there is a strong possibility that the mechanism of Cr(VI) removal by biomaterials or biomaterial-based activated carbon is not “anionic adsorption” but “adsorption-coupled reduction”. Various authors (Aoyama, 2003; Park et al., 2006a, 2006b, 2006c) also suggested that in Cr(VI) adsorption studies, it is necessary to analyze both the Cr(VI) and total Cr in aqueous solution, as well as determining the oxidation state of chromium bound to the biomaterial or activated carbon.”

The mechanism of chromium(VI) removal onto reducing surface is also investigated here: doi:10.3390/molecules25010047

16. What is the reason for presenting data in Table 1 when previously data from optimisation tests were presented in Figures 1,3,4,5?
17. The Authors forgot to present the research results!!!!!!!!!! The section 3.2. is so poor it discredits all manuscript. The graphical representation of data is needed to evaluate the equilibrium and kinetic tests results.
18. The discussion of results is very poor. What are the properties of this carbon compared to other similar materials? Should the reader do this analysis himself?
19. In section 3.3. the Authors present the results of chromium(III) concentration. The methodology omits the measurement procedure. There is only information about using the AAS technique, nothing about separation of chromium forms (VI) and (III).
20. Is the activated carbon the biosorbent when the biosorption is the process of accumulation the pollutants onto cellular surface? The paper is about the AC obtained from the waste biomass.

Best regards,

reviewer

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Activated carbon has been used for removal of heavy metals for a long while. Fabrication of activated carbon from renewable source and waste/byproducts has great relevance for the science and industry practice, as well.

Authors of manuscript applsci-999922 focused ont he preparation of activated carbon from  winemaking waste.

Manuscript is generally well written with a good structure. Introduction section is a good summary of the reserach motivations and solvende problems. materials and methods are described clearly int he manuscript.

Manuscript contains interesting and valuable results but need revision before publishing (see below).

Comments, suggestions

#1: It is not clearly given of manuscript why did the authors choose pH4 and temperature range of 20-60 °C for the experiments. I recommend amending the MS by this information (based on preliminary experiments and/or references).

#2: Figure 1, 3, 4, 5 and Table 1 and 2 do not contain the errors/deviations. I recommend to give these data.

#3: In Page 4 line 154-155 establishments of ’ The decrease in the adsorption of Cr(VI) at higher stirring speeds could be 154 due to the formation of local equilibria which hinders the adsorption process.’ is not supported by reference.

#4: In Figure 2 legends and the texts are not visible. I suggest the authors to correct the figure.

#5: results given in section 3.1.3-3.1.5 are not discussed in details with references.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

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Comments for author File: Comments.docx

Author Response

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Reviewer 3 Report

I am very sorry, but the new version of the manuscript does not meet expectations. Some comments are only slightly included without convincing arguments. Unfortunately, I cannot recommend this work.

Author Response

We regret that you do not approve our manuscript.  In any case, thank you for your comments.

Reviewer 4 Report

Manuscript applsci-999922 has an interesting topic. Manuscript is generally well written with a logic structure.

Authors have revised the manuscript thoroughly according to reviewers' comments and suggestion. After the revision the overall scientific quaility of MS improved significantly.

Amendments and rephrasings made the MS more clear and complete.

I accept the answers and explanations by the authors and recommend MS for publishing.

Author Response

Thank you for your positive comments about our manuscript

Round 3

Reviewer 1 Report

with reference 31 something went wrong, perhaps again with the submission system: "… of Mn( <scp>VII</scp> ) in liquid solutions. Can. J. Chem. Eng. 2020, cjce.23862."