N-Cetyltrimethylammonium Bromide-Modified Zeolite Na-A from Waste Fly Ash for Hexavalent Chromium Removal from Industrial Effluent

Round 1

Reviewer 1 Report

In their manuscript, "N-cetyltrimethylaammonium bromide-modified zeolite Na-AA from waste fly ash for hexavalent chromium removal from industrial efflutent" the authors describe how cationic surfactant modification of zeolites can increase their sorptive capacity for Cr(VI). As the authors note in the outset, the zeolite, Na-A, has a negative surface charge which vastly limits its ability to remove Cr(VI) from wastewater. The cationic surfactant, CTAB, was used to coat the surface of the zeolite. As this is crucial to the capacity of the adsorbant material, I feel as though the authors have some missing analysis. 

1) What is the state of the CTAB on the zeolite? Is it as a monolayer? A bi-layer? If it is a monolayer, what is the driving force for Cr(VI) adsorption? The SEM (Figure 3) would seem to indicate that there isn't simple coverage of the individual zeolite crystallites. Rather, a more complex composite is produced. The authors show XRD and SEM, what happens to the internal surface cavities of Na-A? Does the added CTAB take up all/most of the cavity space? Is Cr(VI) adsorbed to the external faces of the composite? Is any adsorbed internally? If no Cr(VI) is adsorbed internally, what is the purpose of using a zeolite rather than some other low internal-surface area particle?

2) Again, the CTAB is the important factor in driving adsorption. There are several articles in the literature that discuss CTAB-modified particles and enhanced Cr(VI) adsorption. Yet the authors do not mention any of these in their paper. 

Because of these important deficiencies, I must suggest rejecting this manuscript. I do appreciate the analysis and use of their composites on true wastewater. But the characterization of their material leaves too much yet to be understood about their system. 

Author Response

Reviewer 1

In their manuscript, "N-cetyltrimethylammonium bromide-modified zeolite Na-A from waste fly ash for hexavalent chromium removal from industrial effluent" the authors describe how cationic surfactant modification of zeolites can increase their sorptive capacity for Cr(VI). As the authors note at the outset, the zeolite, Na-A, has a negative surface charge which vastly limits its ability to remove Cr(VI) from wastewater. The cationic surfactant, CTAB, was used to coat the surface of the zeolite. As this is crucial to the capacity of the adsorbent material, I feel as though the authors have some missing analysis. 

Reply: The authors thanking to the reviewer for spending their valuable time and  giving feedback/comments towards improve our article quality. We consider the reviewers suggesation and improved the revised version accordingly.

• What is the state of the CTAB on the zeolite? Is it a monolayer? A bi-layer? If it is a monolayer, what is the driving force for Cr(VI) adsorption? The SEM (Figure 3) would seem to indicate that there isn't simple coverage of the individual zeolite crystallites. Rather, a more complex composite is produced. Is Cr(VI) adsorbed to the external faces of the composite? Is any adsorbed internally? If no Cr(VI) is adsorbed internally, what is the purpose of using a zeolite rather than some other low internal-surface area particle?

Reply: Thank you for your valuable and constructive comment.
Cationic modification on the FZA surface has been performed in water using cationic surfactant N-cetyltrimethylammonium bromide (CTAB). CTAB ions adsorbed on the surface of the adsorbent are employed to change the surface charge from negative to positive. Cationic exchange and hydrophobic interactions can influence the sorption of a CTAB on the exterior surface of FZA. At a low surfactant loading, the surfactant cations are exchanged with the exchangeable cations of the FZA (i.e. Na⁺), connected to the external surface, with the outer layer of surfactant molecules bound forming a monolayer of surfactant cations on the external surface. At concentrations above the critical micelle concentration, a bilayer of surfactant molecules (admicelle) is by hydrophobic interactions. The external surface charge of the FZA is switched from negative to positive oriented in the direction of the solution and now displays Cr(VI) removal efficiency (figure given below).
This information has been added to the revised manuscript. (Section 1, Line 70-79)

The authors show XRD and SEM, what happens to the internal surface cavities of Na-A? Does the added CTAB take up all/most of the cavity space?

Reply: We provided XRD and SEM to the manuscript and discussed their results in the manuscript. Further understanding texture properties, we studied BET analysis and discussed in the revised manuscript. The same briefly given here for your perusal.

Table: Surface textural properties of CTAB@FZA and FZA

Based on the BET results table, the CTAB@FZA surface area and total pore volume are 0.78 m²/g and 0.01 cm³/g, respectively, which are lower than the pristine FZA (28.46 m²/g and 0.01 cm³/g). It could be because of the CTAB units congregate on the exterior surface of the FZA in the form of a bilayer or admicelle, occluding several of the FZA's major channels by the CTAB and inhibiting N₂ diffusion through these channels. Furthermore, after CTAB modification, the average pore diameter value rose. The most plausible scenario is that the CTAB blocked the smallest pore sizes in a higher proportion.
We have added the BET surface area results in the revised manuscript for the reader's better understanding.

what is the purpose of using a zeolite rather than some other low internal-surface area particle?

Reply: The FZA has been selected as an adsorbent for its relatively mild surface area and extremely high selective capacity of ion exchange and low cost. FZA is synthesized by waste flyash which is an economically feasible way and using waste flyash can prevent its secondary pollution. These reasons inspired us to use FZA for this study.

• Again, the CTAB is an important factor in driving adsorption. There are several articles in the literature that discuss CTAB-modified particles and enhanced Cr(VI) adsorption. Yet the authors do not mention any of these in their paper. 

Reply: Thank you for your valuable suggestion. We have added some CTAB-modified composite as references in the revised manuscript.

We feel, the revised version could fulfill the reviewers suggestions and may reconsider their decision in positive way. Moreover, we provided suggested characterization results to the revised manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have investigated and discussed the mix of the cationic surfactant CTAB with the synthesized zeolite Na-A from fly ash in order to increase Cr(VI) removal from aqueous solutions.

I suggest some revision before accepted this manuscript.

1# Why authors are used sometimes “flyash” and “fly ash” ? What is difference ?

2# (Line 244 245) Can authors provide the values of pH of the wastewater from the Banwol and Sihwa industrial sites in the Republic of Korea ?

3# Coexisting cations have no effect but sodium seems slightly favored the adsorption of Cr(VI) Can the authors comment on this point?

4# What could be the effect of coexisting ions at pH higher than 3 ?

5# The legends of figures (3, 5, 6 and 8) are blurred and difficult to read.

6# Line 101 :  C₀  (subscript)

Author Response

The authors have investigated and discussed the mix of the cationic surfactant CTAB with the synthesized zeolite Na-A from fly ash in order to increase Cr(VI) removal from aqueous solutions. I suggest some revisions before accepting this manuscript.

Reply: The authors thanking to the reviewer for spending their valuable time and  giving feedback/comments towards improve our article quality. We consider the reviewers suggesation and improved the revised version accordingly.

1# Why do authors are used sometimes “flyash” and “fly ash”? What is the difference ?
Reply: Thank you for your valuable comment. We mentioned, “fly ash” instead of “flyash” throughout the revised manuscript.

2# (Line 244 245) Can authors provide the values of pH of the wastewater from the Banwol and Sihwa industrial sites in the Republic of Korea ?
Reply: Thank you for your valuable comment. The wastewater from the Banwol and Sihwa industrial sites in the Republic of Korea has a pH of 1.5. We mentioned it in the revised manuscript (Section 3.3, line 344).

3# Coexisting cations have no effect but sodium seems slightly favored the adsorption of Cr(VI) Can the authors comment on this point?
Reply: Thank you for you valuable comment. But, we did not observed any significant  influence with all cations including Sodium ion. Even in the presence of sodium ion, the adsorption of Cr(VI) was the same as with alone. However, it may be the un avoidable errors during experiment.

4# What could be the effect of coexisting ions at pH higher than 3?
Reply: Thank you for your question. There could be an negative effect on adsorptive removal of Cr(VI) when increasing the soultion pH beyond 3 which may be the repulsoion between anionionc species of HCrO₄^- and Cr₂O₇^2- ions with negative surface of adsorbnet that occurred during the pH increasing. Moreover, it was clearly observed according to pH effect findings (figure 4), pH 3 would be optimized for the removal of Cr(VI) from water. Hence, we conducted all studies at pH 3.

5# The legends of figures (3, 5, 6 and 8) are blurred and difficult to read.
Reply: Our apologies for that. We changed the figure legends to make it clear without blur. Thank you for your valuable suggestion.

6# Line 101 :  C0  (subscript)
Reply: Our apologies for that. We changed the manuscript according to your suggestion (in the Section 2.2, line 101). Thank you for your valuable suggestion.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors performed adequate experimentation in  response to the reviewer comments. In my estimation, the analysis/discussion needs to be updated before considering acceptance. 

The following are my specific critiques

Page 2 line 71/72 - Discuss CTAB on exterior surface of Na-A. Clear from BET data that CTAB is adsorbed to interior surface as well. 

This is the crux of my initial review as well. All data indicate that CTAB is everywhere. The authors need to be upfront about this. 

Page 3 line 129 - Same issue as above

Page 5 line 162 - CTAB in extended conformation is 2nm. The pore size of NaA (5.3 nm) is clearly large enough for CTAB to be incorporated into the pores. 

Discussion: Authors need to discuss their results in context of other CTAB-modified particles. Why use fly ash generated NaA? If there is a good reason, the community should know. If there isn’t, the community should also know that. It is the authors’ role to give their readers this critical insight. 

Author Response

Comment: Page 2-line 71/72, page 3 line 129, and page 5 line 162-Discuss the CTAB on the exterior surface of the Na-A.

Reply: We apologize for those mistakes. Thank you for your insightful and informative suggestions. We have rectified the mistakes in corresponding places.

Comment: Authors need to discuss their results in the comntext of other CTAB modified particles. Why use flyash generated Na-A?

Reply: Thank you for your valuable comment.

So far, some researchers used CTAB-modified composites for the successful removal of Cr(VI) from an aqueous solution. For example, Zhang Yu et al 2019 achieved 76.33 mg/g by surface-modified leaves with CTAB, CTAB functionalized double-shelled hollow microspheres were synthesized by Cai et al 2019 and their maximum adsorption capacity was 202.02 mg/g, Cai et al 2020 were developed MoS₂/CTAB and obtained the adsorp-tion capacity 88.3 mg/g, and Li Na et al 2017 were prepared MnFe₂O₄@SiO₂−CTAB, and their adsorption capacity was 25.04 mg/g towards Cr(VI) removal from the water. These findings suggested that the adsorption capacity of CTAB@FZA is comparable and relatively higher than other CTAB-modified composites. Additionally, the solid waste flyash-derived zeolite is an economically feasible material as well as could skip the problems of its disposal issues. (page-8, line 223-233)

Synthetic zeolites such as ZSM-5, Na-X, Na-A, and Na–Y are often utilized in water reme-diation. Na-A has a high cation concentration due to its Si/Al ratio being 1, which re-sults in a greater ion exchange capacity. Because of its unique surface area, porosity, high ion exchange capacity, and potent reactivity for hazardous metals, Na-A has been the subject of a several of prior studies. (page-2, line 52-55)

Reviewer 2 Report

I  am satisfied with the authors' responses to my questions and I thank them to the changes they have made to the manuscript to improve the quality of their work. 

I recommend that the authors check and confirm the title of Figure 4

I invite them to reread the manuscript to correct the small typos that are still there (Capital letter after comma (line 130) ; lack of space between some references and the words that precede them (line 40 and so one , ...) 

Author Response

Comments:  I am satisfied with the authors' responses to my questions and I thank them to the changes they have made to the manuscript to improve the quality of their work. 

I recommend that the authors check and confirm the title of Figure 4

I invite them to reread the manuscript to correct the small typos that are still there (Capital letter after comma (line 130) ; lack of space between some references and the words that precede them (line 40 and so one , ...) 

Reply: Thank you for your valuable and constructive comments. We have done modifications and have rectified the errors as per your suggestions.

Round 3

Reviewer 1 Report

The authors have made most of the necessary changes to this manuscript. Prior to publication, they must change the following: 

On line 230, they state: "These findings suggested that the adsorption capacity of CTAB@FZA is comparable and relatively higher than other CTAB-modified composites."

However, this is not true. They cite an example (CTAB hollow microspheres) that have twice the capacity of the FZA-CTAB composite described in this paper. Additionally, that this composite has twice the capacity is likely due to the fact that it can absorb chromium on the inner and outer surface of the sphere. The FZA-CTAB may likely only (or primarily) be absorbing Cr on the external surface. This gets into my concern about using FZA. If there is no internal adsorption, why use FZA over other microparticles. Wouldn't a non-porous micropoarticle work just as well. As with my previous comments, I don't believe the authors have to have "best-ever results" to justify publication. But it would benefit them to be open about the limitations of FZA as a platform for this.

Author Response

Reviewer # 1

Comment: The authors have made most of the necessary changes to this manuscript. Prior to publication, they must change the following: 

On line 230, they state: "These findings suggested that the adsorption capacity of CTAB@FZA is comparable and relatively higher than other CTAB-modified composites."

However, this is not true. They cite an example (CTAB hollow microspheres) that have twice the capacity of the FZA-CTAB composite described in this paper. Additionally, that this composite has twice the capacity is likely due to the fact that it can absorb chromium on the inner and outer surface of the sphere. The FZA-CTAB may likely only (or primarily) be absorbing Cr on the external surface. This gets into my concern about using FZA. If there is no internal adsorption, why use FZA over other microparticles. Wouldn't a non-porous micropoarticle work just as well. As with my previous comments, I don't believe the authors have to have "best-ever results" to justify publication. But it would benefit them to be open about the limitations of FZA as a platform for this.

Response: Dear Reviewer, I am very grateful for your worthwhile guidance and I appreciate your valuable comments. According to your comments, we revised our manuscript. I hope it may be reaching your standards.

We agree with you. Sorry for the misleading of our statement. The line 230 and the followed para was modified for more clarity in the revised version. Also, clearly mentioned the limitation of FZA and advances of present developed material in the revised version of manuscript conclusions.

For your reference clear clarification given here:

Flyash is a byproduct of coal-fired power stations, and a substantial portion of flyash is created globally as waste material each year across the world. The leaching of harmful compounds from spilled flyash into groundwater has made the disposal of this material an urgent environmental concern. For example, flyash has Al and Si, which are the major elements of natural zeolites coupled with O atoms. As a consequence, an economically feasible and widely obtainable alumino-silicate crystal derived from waste flyash can be used as the raw goods for the synthesis of zeolite; nevertheless, zeolites have some drawbacks. Owing to the negatively charged framework structure of the zeolite, there is very little affinity toward anionic heavy metals which leads to the low adsorption capacity of FZA (0.42 mg/g) alone for Cr(VI). Even though CTAB shows better adsorptive removal for Cr(VI), it cannot use directly for water treatment because of it soluble in water. That is reason many researchers CTAB used for water treatment after modification. Moreover, the literature survey (discussed in manuscript at line 230 and introduction) reveals that the modification of CTAB and FZA can enhances their ability for adsorption of water pollutants. These survey suggest that to prepare the present material CTAB@FZA in order to enhances adsorption ability of FZA for Cr(VI) by modifying with CTAB, which can provide positive surface (based on literature) for enhance adsorptive removal of Cr(VI) is the main goal of this study. As we expected, the modified CTAB@FZA shows higher adsorption capacity for Cr(VI) than that FZA. Moreover, CTAB@FZA shows higher adsorption capacity compared to some of FZA-modified materials [reported in Table 4]. Additionally, the solid waste FZA is an economically feasible material as well as could skip the problems of its disposal issues of flyash. However, the modification leads to lower surface area CTAB@FZA than FZA alone. If improve the surface area or porous of composite (CTAB@FZA) may increase adsorption capacity furthermore that may need further or alternative modifications.

Author Response File: Author Response.docx