Review Reports - Equilibrium, Kinetic, and Thermodynamic Studies of Anionic Dyes Adsorption on Corn Stalks Modified by Cetylpyridinium Bromide

Round 1

Reviewer 1 Report

The manuscript reports the adsorption of two dyes, Acid Red and Acid Orange, onto modified corn stalks. The manuscript is correct, the work is correctly planned and the results are correctly presented and discussed. So, I recommend acceptation of the manuscript, once the authors have considered the following comments.

First of all, a nomenclature point. In my opinion “cationic surfactant cetylpyridinium bromide (CPB)” and similar sentences are not correct. Cetylpyridinium bromide is a neutral compound, when dissolved it produces the cation cetylpyridinium and the anion bromide, and only the cation interacts with the solid. So, the authors may talk of cetylpyridinium cation, and the abbreviation should be “CP” instead of “CPB”. Only when citing the original compound used for the treatments, the complete name should be given.

End of page 2: Why the treatment with NaOH is necessary? What is the effect of this treatment? What would be the adsorption behaviour of the adsorbent without carrying out this treatment? All these aspects should be commented.

Line 80: The abbreviation CPC is not correct.

Why the adsorption was carried out at pH 3? Was the effect of pH investigated? What should be the behaviour at other pH value?

Equations 10-12: Three different constant values are used here, K, κ and K_L. Is this correct?

Equations 13-18: What is the interest of evaluating the errors by six different methods? I think that this is clearly excessive and not additional information is obtained.

Line 193: Band at 1631 cm^-1 should be due to bending vibration of water.

Table 5: Please revise the calculations. For the first ΔG value, if I understand correctly, it may be ΔG^o= -8.31 x 303 ln(1.60 10^-5). But I find a value of -27.8 kJ/mol, instead of the reported value of -30.2 kJ/mol. The same for the other values.

Author Response

Response to Reviewer 1 Comments

Point 1: First of all, a nomenclature point. In my opinion “cationic surfactant cetylpyridinium bromide (CPB)” and similar sentences are not correct. Cetylpyridinium bromide is a neutral compound, when dissolved it produces the cation cetylpyridinium and the anion bromide, and only the cation interacts with the solid. So, the authors may talk of cetylpyridinium cation, and the abbreviation should be “CP” instead of “CPB”. Only when citing the original compound used for the treatments, the complete name should be given.

Response 1: Authors have agreed with Reviewer 1 and corrected the name of the obtained adsorbent using abbreviation ‘’CS-CP” instead “CS-CPB”.

Point 2: End of page 2: Why the treatment with NaOH is necessary? What is the effect of this treatment? What would be the adsorption behaviour of the adsorbent without carrying out this treatment? All these aspects should be commented.

Response 2: Authors have taken into account the remarks of Reviewer 1 and supplemented information about NaOH treatment in the following edition (Lines 78-87):

Corn stalks are lignocellulose material and under alkaline treatments of corn stalks, ester linkages and glycosidic side chains of the lignin are cleaved. In addition to delignification, this treatment also results in partial hydrolysis of hemicelluloses, cellulose swelling, cellulose decrystallization, lower degree polymerization of cellulose, increased internal area and porosity - essentially required for adsorption application [12].

Our preliminary studies have shown that treatment of corn stalks by alkaline solution increased adsorption of CPB on the surface of corn stalks comparing with the adsorption of CPB on the surface of corn stalks without this treatment in 1.6 times. These studies are in good agreement with the data [13] according to which adsorption of CPB on barley straw treated by NaOH solution also was higher then adsorption of CPB on barley straw without this treatment.

Point 3: Line 80: The abbreviation CPC is not correct.

Response 3: Authors have agreed with Reviewer 1 and the abbreviation CPC has been corrected as CPB (Line 89)

Point 4: Why the adsorption was carried out at pH 3? Was the effect of pH investigated? What should be the behaviour at other pH value?

Response 4: Authors have taken into account the remarks of Reviewer 1 and gave clarification in the next edition (Line 111-116)

Our preliminary results showed that adsorption of anionic dyes on CS-CP had maximum values at pH=2-3, while at higher pH values the adsorption of anionic dyes on CS-CP had decreased (The Figure is not shown). Therefore all of the next adsorption experiments for dye removal using CS-CP adsorbent were carried out at the pH value of 3. This pH value also is in line with the studies reported on the removal of Methyl Orange on wheat straw modified by cetyltrimethylammonium bromide [8] and pineapple leaf modified by hexadecyltrimethylammonium bromide [10].

Point 5: Equations 10-12: Three different constant values are used here, K, κ and K_L. Is this correct?

Response 5: There are two constant values K and K_Lin the Equations 10-12_.It was a technical error. Equation 11 has been corrected (Line 175).

Point 6: Equations 13-18: What is the interest of evaluating the errors by six different methods? I think that this is clearly excessive and not additional information is obtained.

Response 6: Authors have taken into account the recommendation of Reviewer 1 and left standard error (SE) and Chi-square test (c2) which are employed to find out the best-fit model for the experimental kinetic and equilibrium data.

Point 7: Line 193: Band at 1631 cm^-1 should be due to bending vibration of water.

Response 7:

Authors have agreed with Reviewer 1 and corrected the text in manuscript in the following edition (Line199-200):

The peak at 1631 cm⁻¹ was related to the deformation vibration for H-O-H bonds of the physically adsorbed water in CS.

Point 8. Table 5: Please revise the calculations. For the first ΔG value, if I understand correctly, it may be ΔG^o= -8.31 x 303 ln(1.60 10-5). But I find a value of -27.8 kJ/mol, instead of the reported value of -30.2 kJ/mol. The same for the other values.

Response 8:

We have revised the calculations.

For the first ΔG value: ΔG^o= -8.31 x 303 ln(1.60×10⁵)= 30,2 kJ/mol.

It was a technical error in first line of Table 5 for K value: K×10⁵ was corrected as K× 10^-5.

Authors would like to thank Reviewer 1 for the comments which are aimed at improving the article.

Authors:

Soldatkina Liudmyla,

Zavrichko Marianna

Reviewer 2 Report

The paper is studying the adsorption of anionic dyes on modified natural adsorbents. The applied methodology (batch experiments, equilibrium and kinetic analysis) are very conventional. Desorption behavior is also shortly discussed. Overall, my evaluation is positive even though the work has limited originality and novelty contents. The few comments below should be accounted for before publication.

- line 19: the adsorption process is exothermic, not endothermic.

- lines 77-78: it is mentioned that CS is treated with alkaline solution ti increase the adsorption of the cationic surfactant. Why this should be the case? Did the authors compared the adsorption of the same surfactant with and without this treatment?

- the values of total pore volume in Table 2 are larger after the adsorption of the cationic surfactant: why the pore volume should significantly increase after adsorbing some species into the pores? This is fully counter-intuitive and should be carefully motivated.

- lines 112-114: desorption experiments are carried out after washing with water and drying: why is this the case? Actually, in an adsorption process the solid regeneration is carried out without any drying! Did the authors run any desorption experiments right after adsorption equilibration? These experiments would be much more meaningful.

- Equation (2): the variable qe is mentioned but it is defined later (line 130). Please, anticipate.

- line 131: KF is mentioned as "measure of adsorption capacity" but, in analogy with the definition provided for the Langmuir constant, I would propose to call it "Freundlich constant".

- Equation (11): the thermodynamic analysis is performed with reference to the Langmuir isotherm (KL is involved in the equation). Why? Is this because this is the only isotherm with thermodynamic foundation or because it is the one better fitting the data? This is quite confusing and should be well explained.

- subsection 2.2.8: many different error functions are considered but the most meaningful one or two could be used. I do not see the value of being so comprehensive here, these error functions are quite comparable in terms of model discrimination (cf. Table 3 and 4).

- section 3.3: did the authors analyze different particle size? These experiments would provide very valuable information about the kinetic mechanism.

- lines 268: the third interval (slowest equilibrium adsorption stage) is not a kinetic interval: equilibrium is established at 150 min and nothing changes in time later on. Remove this thrid section from the kinetic discussion.

- lines 292-295: it is mentioned that "chemical adsorption is the retarder stage": what this means? Is it the limiting or rate determining stage?

- Figure 4: the regeneration extent is less than 50% in the best case: is this an acceptable value for practical applications? Moreover, while the comment about the regeneration by NaOH solution is quite expected, why the effective regeneration by an organic solvent should prove the chemisorption mechanism? Please, explain in detail.

Author Response

Response to Reviewer 2 Comments

Point 1: Line 19: the adsorption process is exothermic, not endothermic.

Response 1: Authors made a correction on Line 19: exothermic instead endothermic. It was a technical error.

Point 2: Lines 77-78: it is mentioned that CS is treated with alkaline solution ti increase the adsorption of the cationic surfactant. Why this should be the case? Did the authors compare the adsorption of the same surfactant with and without this treatment?

Response 2: Authors have taken into account the remarks of Reviewer 1 and supplemented information about NaOH treatment in the following edition (Lines 78-87):

Point 3: The values of total pore volume in Table 2 are larger after the adsorption of the cationic surfactant: why the pore volume should significantly increase after adsorbing some species into the pores? This is fully counter-intuitive and should be carefully motivated.

Response 3:

Authors believe that in order to explain these results, it should be conducted additional research, which we will be carried out later. In this regard, the information about total pore volume was removed from Table 2.

Point 4: Lines 112-114: desorption experiments are carried out after washing with water and drying: why is this the case? Actually, in an adsorption process the solid regeneration is carried out without any drying! Did the authors run any desorption experiments right after adsorption equilibration? These experiments would be much more meaningful.

Response 4: Authors carried out desorption experiments using the adsorbent after washing with water and drying. The dye-loaded adsorbent was washed with distilled water to remove any unadsorbed dye. Then it was dried to evaporate the solvent because high humidity of the adsorbent sample affects on desorption results. Such experiments are generally accepted and used by many researchers. For example:

Oei, B.C.; Ibrahim, S.; Wang, S.; Ang, H.M. Surfactant modified barley straw for removal of acid and reactive dyes from aqueous solution. Bioresour. Technol. 2009. 100, 4292–4295. DOI: https://doi.org/10.1016/j.biortech.2009.03.063

Su, Y.; Jiao, Y.; Dou, Ch.; Han R. Biosorption of methyl orange from aqueous solutions using cationic surfactant-modiﬁed wheat straw in batch mode. Desalination and Water Treatment. 2014. 52. 6145–6156. DOI: 10.1080/19443994.2013.811121

Kamaru, A.A.; Sani, N. S.; Malek, N.A.N.N. Raw and surfactant-modified pineapple leaf as adsorbent for removal of methylene blue and methyl orange from aqueous solution. Desalination and Water Treatment. 2015. 1-15. DOI: 10.1080/19443994.2015.1095122

Authors have supplemented information about desorption in the following edition (Lines 127-128):

Dye-loaded adsorbent was washed with distilled water to remove any unadsorbed dye and dried at 323 K according to [6, 8, 10].

Point 5: Equation (2): the variable qe is mentioned but it is defined later (line 130). Please, anticipate.

Response 5: Authors have supplemented information about qe after Equation (2) (Line 134).

Point 6: Line 131: K_F is mentioned as "measure of adsorption capacity" but, in analogy with the definition provided for the Langmuir constant, I would propose to call it "Freundlich constant".

Response 6: Authors have agreed with Reviewer 2 and called it as (Line 146-147):

K_F is the Freundlich constant that related to the adsorption capacity.

Point 7: Equation (11): the thermodynamic analysis is performed with reference to the Langmuir isotherm (KL is involved in the equation). Why? Is this because this is the only isotherm with thermodynamic foundation or because it is the one better fitting the data? This is quite confusing and should be well explained.

Response 7:

The Langmuir constant (K_L ) is the adsorption equilibrium constant and the Langmuir model is the one better fitting for our experimental data. Authors have supplemented information about the Langmuir constant in the following edition (Lines 173-174):

If the experimental adsorption isotherms are adequately described by the Langmuir equation, thermodynamic parameters can be calculated by Eqs. (10-12)

Point 8: Subsection 2.2.8: many different error functions are considered but the most meaningful one or two could be used. I do not see the value of being so comprehensive here; these error functions are quite comparable in terms of model discrimination (cf. Table 3 and 4).

Response 8: We have taken into account the recommendation of Reviewer 2 and left standard error (SE) and Chi-square test (c2) which are employed to find out the best-fit model for the experimental kinetic and equilibrium data.

Point 9: Section 3.3: did the authors analyze different particle size? These experiments would provide very valuable information about the kinetic mechanism.

Response 9: Authors plan to analyze different particle size of the adsorbent on adsorption capacity and kinetics and obtain more information about adsorption mechanism in another paper.

Point 10: Lines 268: the third interval (slowest equilibrium adsorption stage) is not a kinetic interval: equilibrium is established at 150 min and nothing changes in time later on. Remove this third section from the kinetic discussion.

Response 10: Authors have agreed with Reviewer 2 and removed third section from the kinetic discussion.

Point 11: Lines 292-295: it is mentioned that "chemical adsorption is the retarder stage": what this means? Is it the limiting or rate determining stage?

Response 11: Authors have supplemented information in the following edition (Lines 301-304):

The pseudo-second order kinetic model also indicates that chemical adsorption is the retarder stage [21]. This means that the rate limiting step in the process is the chemical adsorption between the adsorbate and the active sites of the adsorbent.

Point 12: Figure 4: the regeneration extent is less than 50% in the best case: is this an acceptable value for practical applications? Moreover, while the comment about the regeneration by NaOH solution is quite expected, why the effective regeneration by an organic solvent should prove the chemisorption mechanism? Please, explain in detail.

Response 12: Authors believe that for the practical application of desorption using an alkaline solution, it is necessary to carry out additional studies increasing the number of desorption cycles.

It is known that anionic dyes have more solubility in alcohol than in water. Thus, it can be assumed that if the forces of interaction between dyes and active centers of the adsorbent have chemical nature, the dye will not be desorbed by alcohol.

Authors would like to thank Reviewer 2 for the comments which are aimed at improving the article.

Authors:

Soldatkina Liudmyla,

Zavrichko Marianna

Round 2

Reviewer 1 Report

The authors have adequately considered the comments from the reviewers, and I feel that the manuscript can now be accepted. There are some grammar mistakes that should be considered when revising the authors’ proofs:

Line 31: “According to[3],” should be “According to [3],” or better “According to Zaharia and Suteu [3],”

Line 41: “due high effectively” should be “due to high effectiveness”

Line 87: “higher then” should be “higher than”

Line 210: “hexadceylpyridinum” should be “hexadecylpyridinum”

Line 241: “dataof” should be “data of”

Line 257: “thaythe” should be “that the”

Author Response

Response to Reviewer 1 Comments

Point 1:   Line 31: “According to[3],” should be “According to [3],” or better “According to Zaharia and Suteu [3].”

Response 1: Authors have agreed with Reviewer 1. Line 31: The words “According to[3]” have been corrected as “According to Zaharia and Suteu [3],”.

Point 2: Line 41: “due high effectively” should be “due to high effectiveness.”

Response 2: Authors have agreed with Reviewer 1. This fix has already been made by the editor.

Point 3: Line 87: “higher then” should be “higher than.”

Response 3: Authors have agreed with Reviewer 1. This fix has already been made by the editor (Line 90).

Point 4: Line 210: “hexadceylpyridinum” should be “hexadecylpyridinum”.

Response 4: Authors have agreed with Reviewer 1. This fix has already been made by the editor (Line 203).

Point 5: Line 241: “dataof” should be “data of”.

Response 5: Authors have agreed with Reviewer 1. This fix has already been made by the editor (Line 235).

Point 6: Line 257: “thaythe” should be “that the”

Response 6: Authors have agreed with Reviewer 1. This fix has already been made by the editor (Line 252).

Authors would like to thank Reviewer 1 for the comments which are aimed at improving the article.

Authors:

Soldatkina Liudmyla,

Zavrichko Marianna

Author Response File: Author Response.doc

Reviewer 2 Report

The Authors accounted for most of the comments in the revised manuscript. In some cases, they removed text and/or results and admitted that further work is still required, thus confirming the partial completeness of the manuscript. Nevertheless, I evaluate the revised version suitable for publication after accounting for the few minor comments below.

Response 2: please, improve the text as follows:
"Corn stalks are lignocellulose materials and their ester linkages and glycosidic side chains are cleaved under alkaline treatments. In addition to delignification, this treatment also results in partial hydrolysis of hemicelluloses, cellulose swelling, cellulose decrystallization, lower degree polymerization of cellulose, increased internal area and porosity - essentially required for adsorption application [12]. Our preliminary studies have shown that treatment of corn stalks by alkaline solution increased adsorption of CPB about 1.6 times with respect to the case without this treatment. These studies are in good agreement with the data in [13], where adsorption of CPB on barley straw treated by NaOH solution was also higher then adsorption of CPB on barley straw without this treatment."

Response 3: this is an example of data removal. Probably, a better approach is to leave the data and explicitly declare that further work is still required to explain them.

Response 11: please, use "rate determining stage" instead of "retarder stage".

Author Response

Response to Reviewer 2 Comments

Response 2: please, improve the text as follows:

"Corn stalks are lignocellulose materials and their ester linkages and glycosidic side chains are cleaved under alkaline treatments. In addition to delignification, this treatment also results in partial hydrolysis of hemicelluloses, cellulose swelling, cellulose decrystallization, lower degree polymerization of cellulose, increased internal area and porosity - essentially required for adsorption application [12]. Our preliminary studies have shown that treatment of corn stalks by alkaline solution increased adsorption of CPB about 1.6 times with respect to the case without this treatment. These studies are in good agreement with the data in [13], where adsorption of CPB on barley straw treated by NaOH solution was also higher then adsorption of CPB on barley straw without this treatment."

Response 2: Authors have agreed with Reviewer 1 and corrected the text in manuscript in the edition of Reviewer 2 (Line 79-90):

"Corn stalks are lignocellulose materials and their ester linkages and glycosidic side chains are cleaved under alkaline treatments. In addition to delignification, this treatment also results in partial hydrolysis of hemicelluloses, cellulose swelling, cellulose decrystallization, lower degree polymerization of cellulose, increased internal area and porosity - essentially required for adsorption application [12]. Our preliminary studies have shown that treatment of corn stalks by alkaline solution increased adsorption of CPB about 1.6 times with respect to the case without this treatment. These studies are in good agreement with the data in [13], where adsorption of CPB on barley straw treated by NaOH solution was also higher than adsorption of CPB on barley straw without this treatment."

Response 3: this is an example of data removal. Probably, a better approach is to leave the data and explicitly declare that further work is still required to explain them.
Response 3: Authors would like to carry out additional researches for confirmation the obtained data using another methods and only then to declare and explain them in another our article.

Response 11: please, use "rate determining stage" instead of "retarder stage".

Response 11: Authors have taken into account the recommendation of Reviewer 2 and used "rate determining stage" instead of "retarder stage" (Line 293).

Authors would like to thank Reviewer 2 for the comments which are aimed at improving the article.

Authors:

Soldatkina Liudmyla,

Zavrichko Marianna