Methylparaben Adsorption onto Activated Carbon and Activated Olive Stones: Comparative Analysis of Efficiency, Equilibrium, Kinetics and Effect of Graphene-Based Nanomaterials Addition

Round 1

Reviewer 1 Report

The authors carried out a comparative study on the adsorption of methylparaben on commercial activated carbon and onto olive stones doped with graphene oxide and reduced graphene oxide, examined the influences of the initial concentration of methylparaben, adsorbent dose, stirring speed and pH on methylparaben adsorption capacity of both adsorbents, analyzed the equilibrium data and kinetic data using six isotherm models and four kinetic models, respectively.  This work contains some new results and could be considered for publication. However, the authors should revise their manuscript before acceptance for publication according to the following comments:

1.        How does the doping of graphene oxide ot reduced graphene oxide increase methylparaben adsorption capacity of the activated carbon or activated olive stones adsorbent?

2.        Which parameter is the dominant factor influencing the adsorption capacity of the adsorbent?

3.        What is the main reason that the adsorption capacity of methylparaben on activated carbon is about 4 times higher than that of activated olive stones?

4.        What are the active sites in the adsorbents for the adsorption of methylparaben?

5.        A comparison on adsorption capacity of the as-obtained typical sample should be made with those of the related samples reported in the literature.

6.        There are some inappropriate English words or expressions in the manuscript. The authors should carefully polish the English of the whole manuscript.

There are some inappropriate English words or expressions in the manuscript. The authors should carefully polish the English of the whole manuscript.

Author Response

Authors comments to Reviewer 1 comments

The authors would like to acknowledge the valuable comments of Reviewer 1, which have undoubtedly contributed to improving the quality of our manuscript.

  

Comments and Suggestions for Authors

The authors carried out a comparative study on the adsorption of methylparaben on commercial activated carbon and onto olive stones doped with graphene oxide and reduced graphene oxide, examined the influences of the initial concentration of methylparaben, adsorbent dose, stirring speed and pH on methylparaben adsorption capacity of both adsorbents, analyzed the equilibrium data and kinetic data using six isotherm models and four kinetic models, respectively.  This work contains some new results and could be considered for publication. However, the authors should revise their manuscript before acceptance for publication according to the following comments:

 

1. How does the doping of graphene oxide or reduced graphene oxide increase methylparaben adsorption capacity of the activated carbon or activated olive stones adsorbent?

As discussed on page 14, graphene-based nanomaterials are characterised by their high porosity and high surface area, so their presence on the surface of the adsorbents under study should increase their surface area and, consequently, their adsorption capacity.

In addition, both GO and rGO maintains the polyaromatic p-electron system of graphene and contains oxygenated polar functional groups (in greater quantity and variety in the case of GO than rGO).

All this favours the interactions of MPB with the modified adsorbents and increases its adsorption onto them. Consequently, the adsorption capacity is increased.

2. Which parameter is the dominant factor influencing the adsorption capacity of the adsorbent?

As can be seen in Figure 1 (page 7), the four parameters analysed (initial MPB concentration, adsorbent dose, stirring rate and pH) have an important influence on the adsorption capacity of the adsorbents, and no significant and clear importance of one of them with respect to the others can be observed, at least in the experimental ranges studied.

3. What is the main reason that the adsorption capacity of methylparaben on activated carbon is about 4 times higher than that of activated olive stones?.

The fundamental reason, as indicated throughout the article (for example in page 7, lines 302-305), is that activated carbon has both a higher surface area and a higher proportion of polar oxygenated groups (carboxyl, phenol, anhydride, lactone, quinone and ether groups) than activated olive stones. This leads to a higher number of MPB interactions with the AC than with the AOS, resulting in a higher adsorption capacity of the AC.

4. What are the active sites in the adsorbents for the adsorption of methylparaben?

The active sites of the adsorbents (page 8, lines 329-331) are the polar oxygenated groups that exist on the surface of the adsorbents and that we have just mentioned in the previous point.

When the adsorbents are modified with GO or rGO, the functional groups provided by the GO and rGO, that is, polyaromatic p-electron system (hydrophobic interactions) and oxygenated polar functional groups that include hydroxyl, epoxide, carbonyl and carboxyl groups (hydrophilic interactions), are added to these active sites (page 14, lines 473-483).

5. A comparison on adsorption capacity of the as-obtained typical sample should be made with those of the related samples reported in the literature.

The authors have searched the literature for articles on the removal of methylparaben by adsorption onto activated carbon and onto activated olive stones.

There are no articles on methylparaben removal by adsorption on activated olive stones.

With regard to activated carbon, we have found 3 articles on methylparaben removal by adsorption on this adsorbent:

1. M.A. Bernal-Romero, N. Boluda-Botella, D. Prats-Rico. Removal of emerging pollutants in water treatment plants: adsorption of methyl and propylparaben onto powdered activated carbon. Adsorption (2019) 25:983-999.
2. V. Bernal, L. Giraldo, J.C. Moreno-Piraján, A. Erto. Mechanisms of Methylparaben Adsorption onto Activated Carbons: Removal Tests Supported by a Calorimetric Study of the Adsorbent-Adsorbate Interactions. Molecules 2019, 24, 413.
3. V. Bernal, L. Giraldo, J.C. Moreno-Piraján. Physicochemical Parameters of the Methylparaben Adsorption from Aqueous Solution Onto Activated Carbon and Their Relationship with the Surface Chemistry. ACS Omega 2021, 6, 13, 8797-8807.

Article 1 uses different experimental conditions than ours ([MPB]0 = 1 mg/L, AC dose 0-20 mg/L, stirring rate 150 rpm and temperature 25ºC), so the results of this article are not fully comparable with those we have obtained.

In article 2, an adsorption capacity of 1.15 mmol/g (174.97 mg/L) is obtained, but the experimental conditions corresponding to this adsorption capacity are not clearly specified.

We do not have access to article 3. In the abstract it speaks of a q_max of 1.64 mmol/g (249.5 mg/g), but we do not know the experimental conditions of the article.

Given the difficulty to carry out a proper comparative analysis of our results with those of the other articles found, as either we do not have access to the experimental conditions of those articles or those experimental conditions are different from ours, we kindly ask the reviewer to release us from this request.

6. There are some inappropriate English words or expressions in the manuscript. The authors should carefully polish the English of the whole manuscript.

The English style of the article has been revised using two different translators.

Reviewer 2 Report

This paper reports on a study of Methylparaben (MPB) extraction from aqueous solutions using commercial active carbon and chemically activated olive stones. The manuscript provides important scientific information on adsorption and kinetic properties of the adsorbents in reference to MPB however it is strongly recommended to make important corrections before making the decision for publishing.

1.            It is necessary to give an explanation for used method and selected conditions for the treatment of olive stones, e.g. chemical activation agent, temperatures etc. There are plenty papers dedicated to the topic of OS activation, please give more explanation.

2.            It is very difficult to analyze experimental information on MPB without understanding basic properties of materials. It is recommended to give more information about adsorbents, such as origin (for DARCO, graphene), textural properties, porous structure parameters (BET surface area, pore volume etc., obtained from N2 adsorption, for example), as well as the chemical state of the surface, particle sizes, etc.

3.            The results on MPB adsorption on activated OS are quite poor in comparison with the commercial AC. Does this mean that this raw material is unpromising for this technology? Are there ways to improve?

4.            Please check and open all abbreviations (GO, rGO, HAC etc.).

5.            Figures 2- 4, 6. The overall representation of the results is rather poor. Perhaps colored symbols will improve them. Figure 7. Please provide conditions for obtained results in the caption.

Author Response

Authors comments to Reviewer 2 comments

 

Authors would like to acknowledge the valuable comments of Reviewer 2, which have undoubtedly contributed to improving the quality of our manuscript.

 

Comments and Suggestions for Authors

This paper reports on a study of Methylparaben (MPB) extraction from aqueous solutions using commercial active carbon and chemically activated olive stones. The manuscript provides important scientific information on adsorption and kinetic properties of the adsorbents in reference to MPB however it is strongly recommended to make important corrections before making the decision for publishing.

 

1. It is necessary to give an explanation for used method and selected conditions for the treatment of olive stones, e.g. chemical activation agent, temperatures etc. There are plenty papers dedicated to the topic of OS activation, please give more explanation.

In subsection 2.2.1. Adsorbents preparation (lines 91-98), the experimental conditions used (temperature, activating agent) in the preparation of the activated olive stones are specified.

2. It is very difficult to analyze experimental information on MPB without understanding basic properties of materials. It is recommended to give more information about adsorbents, such as origin (for DARCO, graphene), textural properties, porous structure parameters (BET surface area, pore volume etc., obtained from N2 adsorption, for example), as well as the chemical state of the surface, particle sizes, etc.

The determination of all the parameters requested by the reviewer would require the development and performance of a large number of analytical determinations (some of them not available at our university). We therefore ask the reviewer to release us from this request.

3. The results on MPB adsorption on activated OS are quite poor in comparison with the commercial AC. Does this mean that this raw material is unpromising for this technology? Are there ways to improve?

Activated olive stones have a methylparaben adsorption capacity about 4 times lower than that of activated carbon. We believe that this difference is sufficiently low to consider that, as indicated in section “Featured Application”, activated olive stones can be considered as an alternative adsorbent to activated carbon in the removal of organic compounds, mainly considering their low/no cost.   

4. Please check and open all abbreviations (GO, rGO, HAC etc.).

Thanks, all abbreviations have been checked.

5. Figures 2- 4, 6. The overall representation of the results is rather poor. Perhaps colored symbols will improve them. Figure 7. Please provide conditions for obtained results in the caption.

The authors once again ask the reviewer to release us from the requirement to color the symbols in the figures, since this would mean a significant temporary delay in the possible publication of the article without resulting in a radically significant improvement of it.

Conditions for obtained results were typical experimental conditions described in Materials and Methods section. The sentence “(typical experimental conditions)” has been included in caption of Figure 7.

 

Reviewer 3 Report

This work is well done, and the manuscript is potential to be published in Applied Sciences after a few corrections and suggestion, as follows:

1.   

The title would be: “Methylparaben adsorption onto activated carbon and activated olive stones: comparative analysis of efficiency, equilibrium, kinetics and effect of graphene-based nanomaterials addition

 

2.   

Line 18: It should “… six isotherm models (Langmuir, Freundlich, ….”

 

3.   

Line 114-115: Authors should explain why 1 M NaOH is necessary to be added to the samples before the absorbance measurement at 289 nm.

 

4.   

Table 1: “Jovanovic” instead of “Jovanovik”.

 

5.   

Line 219-225: The effect of stirring speed from 80 rpm to 240 rpm that enhances the adsorption capacity of methylparaben increases by a factor of 4.5-5.8 is an interesting question. Here, authors addressed this effect to an increase in diffusion of methylparaben, but I do not think it is plausible, as enhancement diffusion of the organic compounds due to stirring would affect their adsorption rate but not their adsorption capacity. Please refer a relevant paper; https://doi.org/10.1007/s10661-022-10332-0. However, the mechanical stirring could break down the activated carbon and activated olive stones, so that the smaller particles of the adsorbents have larger surface area, and thus higher adsorption capacity of the methylparaben.    

 

6.   

Line 248: It should be “… adsorption capacity values of 89.66 mg/g for activated carbon and 18.26 mg/g for activated olive …”

 

7.   

Line 255: It should be “…to values of 77.18 mg/g for AC (13.92% decrease) and 15.95 mg/g for AOS …”

 

8.   

Line 260: It should be “… and of 10.03 mg/g in AOS …”

 

9.   

Line 270: “Jovanovic” instead of “Jovanovik”.

 

10.

Table 3: It should be “18.7006” instead of “18,7006”

 

11.

Table 3: Authors should explain why the Temkin isotherm constants A and B at 313 K are abruptly increased as compared to those at lower temperatures.

 

12.

Table 3: Kinetics. It should be “0.9956” instead of “0,9956”

 

13.

Section 3.4; Line 357-371: I think the interpretation of the Weber-Morris intraparticle and Boyd diffusion models needs to be improved. See and refer the following papers for more detail:

https://doi.org/10.3390/molecules27051718

https://doi.org/10.1016/j.emcon.2022.100199

 

14.    

Figure 7: The values of the y-axis should be started from 0, to avoid misunderstanding. 

Comments for author File: Comments.pdf

Author Response

Authors comments to Reviewer 3 comments

 

Authors would like to acknowledge the valuable comments of Reviewer 3, which have undoubtedly contributed to improving the quality of our manuscript.

 

Comments and Suggestions for Authors

This work is well done, and the manuscript is potential to be published in Applied Sciences after a few corrections and suggestion, as follows:

 

1. The title would be: “Methylparaben adsorption onto activated carbon and activated olive stones: comparative analysis of efficiency, equilibrium, kinetics and effect of graphene-based nanomaterials addition

Title of the paper has been modified according to reviewer suggestion.

2. Line 18: It should “… six isotherm models (Langmuir, Freundlich, ….”

Thanks, the term “Freunlich” has been corrected by “Freundlich”.

3. Line 114-115: Authors should explain why 1 M NaOH is necessary to be added to the samples before the absorbance measurement at 289 nm.

We studied the possibility of carrying out the analysis of MPB by VIS-UV spectroscopy at acid pH (addition of HCl), at the pH of the MPB solution (addition of distilled water) and at basic pH (addition of NaOH). We observed that the absorption band at basic pH was more intense (higher absorption) and somewhat wider, so this was the one selected.

The following sentence has been included in the text: “(basic pH provides a higher intensity of the MPB absorption band)”.

4. Table 1: “Jovanovic” instead of “Jovanovik”.

Thanks, the term “Jovanovik” has been corrected by “Jovanovic”.

5. Line 219-225: The effect of stirring speed from 80 rpm to 240 rpm that enhances the adsorption capacity of methylparaben increases by a factor of 4.5-5.8 is an interesting question. Here, authors addressed this effect to an increase in diffusion of methylparaben, but I do not think it is plausible, as enhancement diffusion of the organic compounds due to stirring would affect their adsorption rate but not their adsorption capacity. Please refer a relevant paper; https://doi.org/10.1007/s10661-022-10332-0. However, the mechanical stirring could break down the activated carbon and activated olive stones, so that the smaller particles of the adsorbents have larger surface area, and thus higher adsorption capacity of the methylparaben.

The results we obtained showed that, keeping all other parameters constant, as the stirring rate increased, in the range studied, the MPB concentration in the equilibrium decreased, therefore, the adsorption capacity increased.

On the other hand, the agitation was carried out by means of an orbital shaker, and no breakage of the AC or AOS granules was observed at the studied stirring rates (80 to 240 rpm).

The relevant article you indicate has been included as a reference.

6. Line 248: It should be “… adsorption capacity values of 89.66 mg/g for activated carbon and 18.26 mg/g for activated olive …”

The sentence has been modified according to the referee suggestion by adding mg/g to the adsorption capacity value.

7. Line 255: It should be “…to values of 77.18 mg/g for AC (13.92% decrease) and 15.95 mg/g for AOS …”

The sentence has been modified according to the referee suggestion by adding mg/g to the adsorption capacity value.

8. Line 260: It should be “… and of 10.03 mg/g in AOS …”

According to the referee suggestion, the unit mg/g has been added to the adsorption capacity value.

9. Line 270: “Jovanovic” instead of “Jovanovik”.

Thanks, the term “Jovanovik” has been corrected by “Jovanovic”.

10. Table 3: It should be “18.7006” instead of “18,7006”

Thanks, “18,7006” has been corrected by “18.7006”.

11. Table 3: Authors should explain why the Temkin isotherm constants A and B at 313 K are abruptly increased as compared to those at lower temperatures.

Thank you for your appreciation. This is an error, because as you can see in the table, the values of the constants A and B of the Temkin model coincide at all temperatures. We have recalculated the constants and their correct values are included in the table.

12. Table 3: Kinetics. It should be “0.9956” instead of “0,9956”

Thanks, “0,9956” has been corrected by “0.9956”.

13. Section 3.4; Line 357-371: I think the interpretation of the Weber-Morris intraparticle and Boyd diffusion models needs to be improved. See and refer the following papers for more detail:

https://doi.org/10.3390/molecules27051718

https://doi.org/10.1016/j.emcon.2022.100199

Although the article includes some paragraphs on the interpretation of these models in the subsections "Adsorption Mechanism" included in the "Materials and Methods" and "Results and Discussion" sections, in order to improve the interpretation of the Weber-Morris and Boyd models, the articles you indicate have been included as references.

14. Figure 7: The values of the y-axis should be started from 0, to avoid misunderstanding

According to the reviewer suggestion, figure 7 has been modified with the y-axis starting from 0.

Round 2

Reviewer 2 Report

-

Author Response

As the second review by Reviewer 2 does not contain any comments to the authors, authors do not include any comments to that second review.