Application of Response Surface Methodology for Preparation of ZnAC₂/CAC Adsorbents for Hydrogen Sulfide (H₂S) Capture

Round 1

Reviewer 1 Report

The manuscript should be improved according to the following recommendations:

• The title should specify that a ZnAc₂/AC adsorbent is prepared.
  The title should be changed to Application of Response Surface Methodology for Preparation of ZnAc₂/AC Adsorbents for Hydrogen Sulfide Capture
• Include as keyword the term "ZnAc₂/AC".

• In the Introduction:

  Develop in a summary table the different most relevant technological alternatives from the literature, emphasising the most relevant results including the approximate economic cost.

• In the Materials and Methods section:

  You should use uniform nomenclature throughout the manuscript. Please review it. 
  i.e. "with granular activated carbon (CAC) in the range of 2-36 mm".

  Delete "as recommended by Zulkefli".

  It should be specified the procedure used for the regeneration of the adsorbent material.

• In the results and discussion section:

  The statistical analysis is correct. However, it should be discussed the results obtained in Table 2 and compared with other results from literature. For example, it is not discussed that at lower specific surface area the adsorption capacity of H₂S is higher.

  In page 11 It should be explained or give some argument for the adsorption capacity of the material that explains the deviation from the model.

         It should be rewritten this paragraph. It is not clear.

"The surface morphology and chemical composition of the catalyst particles were as-sessed to visualize the details of the adsorbent properties in terms of structural particles and the percentage of elements present on the adsorbent surfaces as shown in Figure 10. An SEM-EDX study was conducted for fresh adsorbents and exhausted adsorbents. Fresh adsorbents were new adsorbents that have not yet been used for real H2S adsorption test-ing, whereas the exhausted adsorbent has been used for the H2S adsorption test."

The reference to Table 6 is wrong. 

"The EDX analysis was conducted on elements C, Ca, Zn, O, K, Na (Table 6)."

In page 12, the nomenclature of these two adsorbents is not defined. Please define it.

"As a results, the ZnAc₂/CAC_O had higher oxygen presence shows better performance compared to ZnAc₂/CAC_N."

The analysis in Table 11 is incomplete. Major elements such as Fe or Ti are missing. It is suggested to complete the chemical analysis by XRF technique with analytical standards.

In page 12 Rewrite this sentence. It is wrong. It should say: "surface area calculated using the BET equation".

"The surface area was obtained via a BET isotherm calculation, while pore vol-ume and average pore volume were calculated at P/P₀ of 0.98 through the N₂ adsorption isotherm."

It should be defined the meaning of the terminology F and E in Table 12.

In the last paragraph of page 13 it should be provided a reference to support its hypothesis.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

 

I have read the article “Application of Response Surface Methodology for Preparation of Catalytic Adsorbents for Hydrogen Sulfide Capture”.

My recommendation is rejection for several reasons:

1.This manuscript contains plenty of spelling and grammar errors making reading and understanding very difficult.

2. References 1-7 are not found in the manuscript.
3. Statistical parameters must be clearly presented and more comprehensible (if necessary in Supporting Information).
4. Unclear formulations and statements throughout the manuscript.
5. Many abbreviations are introduced without explanation. A list of abbreviations is required.

A non-exhaustive but representative list of comments and questions:

Abstract

In the abstract authors are given two series of optimised conditions for preparation of the catalytic adsorbent: “with optimal preparation conditions (molarity 0.2–1.0 M, soak temperature 30–100°C, and soak time 30–180 min).” and “The optimum conditions for the catalytic adsorbents were obtained at 95.08°C soak temperature, 0.22 M of ZnAc2, and soak time of 48.82 min.” Please comment   

What is a real test adsorption? Please comment.

 “Then, the comparison data between real test adsorption of optimize and non-optimize adsorbents shows the adsorbents can be enhance up to 33%.”What does this sentence mean??? Please comment.

Comments in detail:

1. Introduction

p.1: references 1-7 are not found in the manuscript.

p.1: low concentration is related with > 1 ppm? Please comment.

p.1: Limited adsorption capacity of H2S? Adsorption capacity is a characteristic of the adsorbent, not of the adsorbate. Please comment.

p.2: Cu is an abbreviation of carbonates? Zn, Fe and Cu is the abbreviation of transition metal oxides? Zn is the abbreviation of metal acetic acid? Please comment.

p.2: In this manuscript the description “real surface area” is used. In adsorbents such as AC research, surface areas are determined. Please comment.

p.2: Heterogeneous catalysts… catalyst? Should be rephrased.

p.2: what is meant by ”increase in percent utilization”. Please comment.

p.2: what are interaction results and how can they be determined? Please comment.

p.2: What is a “dropping path” and  a “real H2S adsorption test”? Please comment.

 

2. Materials and Methods

p.2: Particle size of GAC within 2-36 mm looks very broad to obtain a reproducible impregnation. Please comment.

p.2: There is confusion when one speaks of adsorbent material (2.1) and adsorbents preparation(2.2). Please comment. 

p.2: Which volume of solvent (?) was used to impregnate 350 g of AC? Please comment.

p.3: Real H2S adsorption testing??? Please comment.

p.3: What is a DCM adsorbent? Please comment.

p.3: what is a breakthrough adsorption power”? Please comment.

p.3: Figure 3 (a) is not clear.

p.4: What is the difference between a “center point” and a “middle point” ? Please comment.

p.4:in equation (2) α values are used, in the manuscript a values are used. Please comment.

p.4: when using equation(3) how can one obtain 17 runs for k= 3 and C_o= 4? Please comment.

p.5: What is a real H2S test? Please comment.

 

3. Results and Discussion

p.5: All the adsorbents synthesis as suggested by the Box-Behnken models?? The each of run of experiments was testing with real H2S adsorption at 0.2L adsorber with 5.5 L/min feed flowrate?? What is meant by the sentences?? Please comment.

p.6: “These tables” which tables?

p.6: DF??

p.6: “As in Table 4, the model significant F-value are suggested 1.34 which compared to the noise are shown as the model is not important. ???

p.7: the model reduction may enhance the  model??? Please comment.

p.7:, Adj. R-Squared, P_red R-Squared, PRESS, A_deq not defined in the manuscript.

p.7: what is the design area?

p.8: “As Table 10 shows the model reliability analysis with Pred R-Squared were 0.2080 is in reasonable agreement with the Adj R-Squared were 0.3512. While, Adeq Precision measured the signal to noise ratio. A ratio greater than 4 is desirable and the model proposed were 5.096 where indicates an adequate signal. This model can be used to navigate the design space”. Cannot be understand without explanation of “Items”. Please comment.

p.8: Wat is meant by “relative concentration of H2S” and adsorbent’s capability”? Please comment.

p.9: 95.08°C. Is the temperature experimentally determined with an accuracy of 0.01 °C?

p.10: What is meant by “the adsorption potential of H2S” and “adsorption reaction”? Please comment

p.10: no units in Figure 7.

p.11: What is meant by “real test”?

p.11: What are structural particles? Please comment.

p.12: Key source of adsorbents was extracted from the activated carbon families??? Please comment.

p.12:  “The adsorbents tend to carry the S atoms on the adsorbents “??? Please comment.

p.12: “Besides, the S atom tend to bond on adsorbent surface even after desorption process”. We are dealing with H2S adsorption not with S atoms. Please comment.

p.12: “optimized and non-optimized adsorbents”, not optimize adsorbents.

p.12: “adsorb” instead of “adsorp”

p.12: no units are found in Table 12.

p.12: “sorbents show” instead of “sorbents shows”

p.12: “The performance of adsorbents had investigated through adsorption degradation in adsorption-desorption cycles”???

p.13: what is meant by “The adsorption capacity calculated through the reaction time”? Please comment.

p.13: “Besides, chemisorption also takes place when functional group effectively weight on adsorbents surfaces”. Which functional group? Please comment.

p.13: “As the results… 6-18.3%.” This paragraph requires rephrasing.

p.13: the present of adsorbate (S elements)????

p.13: A strong chemisorption bond????

p.13: “optimized and non-optimized adsorbents”, not optimize adsorbents.

4. Conclusion

It is necessary to review conclusion. Real H2S adsorption test?? Presence of S elements on adsorbents?? As the active sites lessen due to the S elements bound on the surfaces???

References

Reference 46 is not complete.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper was revised according to the journal rules. The topic treated is crucial and deserves to be considered for publication.

Few revisions are required and they are reported below:

• please try to reduce acronyms in the abstract
• add a space between numbers and unit of measure
• please use ppm(v) instead of ppm
• please add more references in (14) such as (10.1016/j.fuproc.2016.06.006, 10.1016/j.biombioe.2020.105754,10.1016/j.wasman.2019.05.048)
• please add for all instruments used the details
• quality of figure 1 should be improved, written parts should be enlarged
• in table 2 standard deviation should be considered and discussed for the results part
• numbers of digits should be reduced properly, for example in table 10 there are too many decimals
• figure 4 should be improved
• figure 11 should be improved
• nomenclature section should be added using also SI as unit of measure
• please add a graphical abstract

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors improved the manuscript according to the recommendations. However, these minor changes should be considered before final acceptance:

- References to equations in the main text should be removed from the conclusions section.

- In the scheme of Figure 1 should be included the following reference in the caption: https://doi.org/10.1371/journal.pone.0211713.

- Check the typographical errors of the experimental setup in Figure 1, please.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Editor,

I have read the revised version of the article “Application of Response Surface Methodology for Preparation ZnAC₂/AC Adsorbents for Hydrogen Sulfide (H₂S) Capture”.

Authors have responded to the comments and suggestions of the reviewer and made considerable efforts to implement changes where needed (especially the presentation of statistical parameters and including a list of abbreviations).

However English grammar should be checked by a native speaker (also in the new implemented paragraphs) as there are still too many errors making reading very difficult. Additional there are still unclear formulations and statements throughout the manuscript left which require additional revision.  

A non-exhaustive list of grammar errors is included together with comments for clarifications. Rephrasing of statements is required.

In the graphical abstract and manuscript:

“mins” should be “min”

Consistent use of “soak time (temperature, period)” or “soaked time (temperature, period)”

 

Graphical abstract

Axes should include units

 

List of abbreviations

C₀ is used as well for center point as well for concentration in regeneration experiment (C/C₀). Please comment.

 

1. Introduction

p.3: What is anaerobic ingestion? Please comment. 

p.3: among the elements of anaerobic digestion for the era of heat and energy??  Please rephrase.

p.3: Besides, the research on liquid-based and membrane techniques for H₂S removal found as the method proposed still non-economically or energetically viable?? Please rephrase.

p.3: “Adsorption is” instead of “adsorption methods is”

p.3: “alternative technology” instead of “alternative technological”

p.4: “Costs is minimal since existing units are available” instead of “Costs is minimal since have existing units”

p.4: where required high H2S concentration? Please comment. 

p.4: “The sulfur bound on the mesoporous materials are strong and might affect to exhaust irreversible?” In that case no regeneration by desorption will be possible. Please comment.

p.4: New study on ionic liquid for H2S still not widely exposed?? Please comment.

p.4: What are “pure environmental based materials”? Please comment.

p.4: What is meant by “The purification and separation studies are difficult”? Please comment.  

p.4: ”in terms of CO2 separation” instead of “in term CO2 separation”

p.4: “can lead towards an overall better economical process” instead of “can lead overall process towards better economical.”

p.4: “that are also widely known” instead of “that also widely known”

p.4: “better surface area” instead of “better adsorbents surface”

p.4: “as active sites to capture” instead of “ as active site that used to capture”

p.5: “through ZnO impregnated performance”?? Please comment.

p.5: what are “some sort of parameters”? Please comment.

p.5: “Then, the gathered information can be used for interaction study for determination of optimization study purposed”?? Please rephrase.

p.5: “The interaction parameters indicate as condition variables which able to analyze by two types of methods, univariate and multivariate optimization”. Please rephrase.

p.5: “by evaluation and controlling the Zn acetate CAC impregnation” instead of “by evaluated and controlled level used for catalytic condition through zinc acetate CAC impregnation”.

 

2. Materials and Methods

p.5: “Effigen Carbon Sdn. Bhd, Malaysia supplied granular commercial coconut activated carbon (CAC) which was sieved to obtain a particle size in the range of 3–5 mm”  instead of “Effigen Carbon Sdn. Bhd, Malaysia was supplied with granular commercial coconut activated carbon (CAC) and sieved in the range of 3–5 mm “

 p.5: “350 g of CAC was placed into the solvent prepared as molarity proposed in 600 ml of distilled water”. Not clear to me, please rephrase.

p.5: DOE (Design of experiments) not explained when first introduced.

p.5: “for optimized and non-optimized adsorbents” instead of  “for adsorbents of optimized and non-optimized adsorbents“

p.5: “was used to visualize” instead of “used to visualize”

p.6: “has loaded” instead of “loaded”

p.6: ”Customization of the portable H2S analyzer recognized the outlet gas (model GC310) was directly imported into the computer program”. Please rephrase.

p.6: “were followed by set-up in previous study” instead of “were followed the set-up preparation in previous study”

p.6:”without temperature”??? Please comment.

p.6: what are surface adsorbents? Please comment.

p.6: “several cycles” instead of “several cycle”

p.6: (A) “solution molarity” should be ZnAc₂ solution molarity.

p.7: “0” should be included as value for center point.

p.7: “This series of equations were solved using the Least Squares approach (2).” Should be deleted since it is a duplicate of the previous sentence.

p.7: “response variables” instead of “response variable”

p.8: it is suggested to include the step numbers in Figure 2.

p.8: “and the BET surface was determined for each of the 17 runs” instead of “and determined the BET surface area for each 17-run of adsorbents.”

3. Results and Discussion

p.9: In table 3 “Molarity, M” should include “ZnAc₂”

p.9:”The results…… 400 m²/g” should be rephrased and should include ZnAc₂

p.9 and p.10: In “min” instead of mins”, also in Figure 3.

p.11: “common statistical method in different applications” instead of “usual statistical method in the different fields”

p.11: on experimental data” instead of “on data from experiments”

p.12: “coefficient of determination” instead of “co-efficient of determination”

p.13:  “Values of prob> F below 0.05 signify the model terms are significant “As seen statistical studies, the value of prob> F below than 0.05 signify the model term are significant“.

p.13: “coefficient of determination” instead of “co-efficient of determination”

p.13: The sentence “It shows…experimental data” can be deleted.

p.14:”However…good fitness”. Please rephrase.

p.14: In this BET surface …level of significance.” Please rephrase.

p.14: The sentence “The higher F is the more significant effect on the response” is a duplicate of the sentence on p.12.

p.15: According to Table 8 the calculated F-value (F0.05,(6 ,10)=2.44) and not 1.34. Please comment.

p.15: “which is 13.68%” instead of “which are 13.68%”

p.15: “which the ratios are more than 4 that indicate the model are adequate to navigate the design space.”??? Please rephrase.

p.15: “These plots are also useful in the study of the effect of process variables on the H₂S adsorption capacity and BET surface area in wider range of preparation conditions of adsorbents.” instead of “This plots also useful in the study of effect process variables on the H₂S adsorption capacity and BET surface area in wider range of preparation adsorbents parameter conditions.”

p.16: “At constant soaked temperature….in captured H2S gas” Please rephrase.

p.18: “min” instead of “mins”

p.18 Figure 8: too many significant numbers for adsorption capacity and BET surface area.

p.19: “BET surface area was” instead of "BET surface area was“

p.19: “error between experimental an theoretical values was” instead of "error between experimental and theoretical were”

p.19: “optimized and non-optimized adsorbents” instead of “optimized adsorbents and non-optimized adsorbents”

p.19:  “While, for non-optimized are taken through previous study that prepared based on 0.2 M with 30 mins of soaked period at 65°C. Both samples synthesis was tested with commercial mixed gas up to exhausted point and analyzed the samples". Please rephrase.

p.19: “and the percentage of weight percentage (wt.%) for elements” ???

p.19: 0.2M ZnAc₂.

p.20 Table 10: results for Na and K content are not reported in Table 10. Please comment. Sum of elements for ZnAc₂/CAC_O=100.32wt.%? Please comment.

p.20 Table 10: What is the reason for the higher O content in the non-optimized sample? What is the experimental error on this concentration? Please comment.

p.20: What are S elements? Please comment.

p.20: H₂S particles (H₂S is a gas)? Please comment.

p.20: “The retaining of S elements (?)….. regeneration study”: Not clear to me. Is it chemisorption or physisorption? Which bonds are formed? What is the role of ZnAc₂ ?

p.20: “The higher presence of element O in the catalyst also enhanced the capability of adsorbent to adsorb H₂S gas.” What is the reason for the positive effect of an enhanced O content on adsorption capacity of H₂S? Please comment.

p.20: “The surface area ….macropore volumes”. if surface area and pore volume are involved in H₂S removal then physisorption is involved rather then chemisorption. However this would be in conflict with previous paragraphs. Please comment. How can adsorption of H₂S increase the surface area of this adsorbent? One would expect the reverse? What are H₂S components? Please comment.

p.21: The performance….inlet stream”??? Please rephrase.

p.21:”However the high H₂S adsorption capacity in the physical adsorption”. If there was physical adsorption the non-optimized adsorbent would show a better performance since the BET surface area is higher than for the optimized adsorbent (Table 11). Moreover from Table 11 it was demonstrated that an exhaustive adsorbent as well for optimized and non-optimized adsorbents has a higher BET surface area. Please comment.

p.21: “The H₂S adsorption capacity was” instead of “The H₂S adsorption capacity were”

p.21: “Comparison” instead of “comparison” and “adsorption-desorption” instead of “Adsorption-desorption”.

p.21: “the presence” instead of “the present”

p.21: “Moreover…. next cycle of regeneration”. What is meant by this statement? What is the S concentration on the adsorbents after the first cycle of regeneration (adsorption-desorption)? If chemisorption occurs by irreversible bonding there will be no desorption and thus no regeneration. This statement should be supported by experimental data. Please comment.

p.21: “As in cycle of regeneration… optimized adsorbents” The statements within the paragraph are not clear and in conflict with experimental data. Please comment.

4. Conclusions

It is necessary to review conclusions especially regarding mechanism of H2S capture and the role of ZnAc₂ (physisorption versus chemisorption) and adsorption-desorption regeneration cycle.

 

References

Ref.10 is identical with ref. 27

Reference 57 is not complete.

 

  

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

I have read the 2^nd revised version of the article “Application of Response Surface Methodology for Preparation ZnAC₂/AC Adsorbents for Hydrogen Sulfide (H₂S) Capture”.

Authors have responded to the comments and suggestions of the reviewer and made considerable efforts to implement changes where needed.

Especially English grammar has been checked and corrected; however in the new implemented paragraphs there are still errors which need correction.

A major issue is the discussion of the mechanism of H₂S removal by non-optimised and optimised AC adsorbents and their regeneration by thermal desorption. ZnAc2 is considered as a catalyst but it is consumed because of the reaction ZnAc₂+ H₂S → ZnS + 2HAc which will have an effect on surface charge (protonation) and local pH (decrease). Therefore an important mechanism is precipitation coinciding with a decrease of the amount of available ZnAc₂. As a result, after several regeneration cycles, the amount of ZnAc₂ will continuously decrease and the adsorption capacity will also decrease. This is the case for both non-optimised and optimised adsorbents. It is also not clear how thermal regeneration at 150°C can remove ZnS?

It should be explained why after regeneration an increase of the BET surface is observed.

Comments and questions regarding these issues are repeated with dedicated paragraphs in the manuscript.

Comments and questions 

Abstract

“0.2-1.0M ZnAc₂ solution” and “0.22M ZnAc₂ solution”.

Graphical abstract

“mins” should be “min”

2. Materials and Methods

p.5: “a particle size in the range of 3–5 mm”  instead of “a particle size in in the range of 3–5 mm “

p.6: “was followed by set-up in previous study” instead of “ followed by set-up in previous study”

3. Results and Discussion

p.16: “the constant soaked temperature shows” instead of “the constant soaked temperature showws”.

p.16: “Hence, the reduction in the BET surface area led to better adsorbent capabilities in the captured H2S gas” If an increase of BET surface area will lead to an increase of absorption capacity then physical adsorption plying an important role in H₂S removal.

p.18: “using the suggested optimum parameter conditions” instead of “using the suggested of optimum parameter conditions”

p.19: “The experimental data were collected” instead of “The experimental data collected”  

p.19: “a 0.2 M ZnAc₂ solution””

p.19: “both sample syntheses”” instead of “both the sample synthesis”

p.20:  “The presence of the Ca elements in the DCM adsorbents also proved that the mesoporous materials used were from the activated carbon family”. Nor clear, please rephrase.

p.20 Table 10: results for Na and K content are not reported in Table 10. Please comment. Sum of elements for ZnAc₂/CAC_0 =100.32wt.%? Please comment.

p.20: As observed in this study, the metal sulfide, zinc sulfide (ZnS), formed on the adsorbent’s surface”. In this case we cannot speak of a catalyst since ZnAc₂ is consumed. Please comment.

 p.20:  The statements “The formation of ZnS had decrease the number of active sites on the adsorbent’s surface. However, to develop high performance adsorbents, both mecha-nisms of chemisorption and physisorption promoting better adsorption-catalytic oxida-tion mechanism for H₂S through morphology (high surface area, micro/meso pore vol-ume) and surface chemistry with presence of oxygen even at low temperature. Further-more, the degradation in the H2S adsorption capacity after several cycles of the adsorp-tion–desorption process has also been reported in a previous study by Zulkefli et al. [32] and may have an effect because of the presence of ZnS, which would also lead to a de-crease in the number of active sites.” Is not clear to me and is in conflict with statement on p.16:  “Hence, the reduction in the BET surface area led to better adsorbent capabilities in the captured H2S gas” . So, a reduction of BET surface area is not promoting adsorption of H₂S. Please comment. Additionally no information is given on difference in  micro/meso pore volume, only total pore volume is given (Table 111). Please comment.

p.20: “Meanwhile, based on this study, the difference of concentration between ZnAc2 for optimized and non-optimized adsorbents were 0.22 M and 0.2 M, respectively, which about 10% difference. Hence, it was expected the Zn element would increase as shown in Table 10.” There is only 10 % difference in molarity of used ZnAc₂ while a 5-fold increase of Zn content is observed The other parameters (temperature and contact time) should also be considered . Please comment. Only 10% more S is found in the optimised ZnAc₂/CAC is found. Please comment.

p.20: “The retaining of S elements (?)….. regeneration study”: Not clear to me. Is it chemisorption or physisorption? Which bonds are formed? What is the role of ZnAc₂ ?

p.20: “The higher presence of element O in the catalyst also enhanced the capability of adsorbent to adsorb H₂S gas.” What is the reason for the positive effect of an enhanced O content on adsorption capacity of H₂S? What is the reason for an enhanced O comment. In my opinion this increase of O content is coinciding with enhance ZnAc₂ content.  Please comment. Why S and Zn content is not determined after thermal desorption? Please comment.

p.20: “However, by applying a higher temperature for desorption can removed more ZnS on the surface compared to that at room temperature?”  Can ZnS be thermally desorbed at 150°C? Please comment.

p.21 Table 11: BET surface area, in m²/g (not in mg H₂S/g)

p.21: ”However the high H₂S adsorption capacity in the physical adsorption”. If there was physical adsorption the non-optimised adsorbent would show a better performance since the BET surface area is higher than for the optimised adsorbent (Table 11). Moreover from Table 11 it was demonstrated that an exhaustive adsorbent as well for optimized and non-optimized adsorbents has a higher BET surface area. Please comment.

p.21: “Upon the adsorption–desorption of H2S, the BET surface area was influenced by the impregnation of ZnAc2 as chemical compound in CAC and the presence of H2S and its elements on the adsorbent which cause pores blocking by the H2S components as previ-ously observed”. When H₂S is causing blocking of the pores, how can pore volume and BET then increase as well for optimised and non-optimised adsorbents? I would expect the reverse. Please comment

p.21: “The high H2S adsorption capacity in the physical adsorption mostly depended on the least number of BET surface area, which indicated that the adsorbents had a highly microporous structure”. I did not find an experimental proof for such statement. Please comment.

p.21: “Furthermore, the increase in the BET surface area value, as shown in Table 11, for the exhausted adsorbents was caused by the composition blocking (ZnS) on the adsorbent surface.” See previous comment.

p.21: “ Basically, the presence of S composition as reported in EDX analysis indicated remaining of ZnS bound on the adsorbent surface had blocked the active site for H2S capture. The pore blockage can lead to a degradation of the H2S adsorption capabilities in the next cycle of adsorption–desorption”. Why S and Zn content are not determined after thermal desorption? Please comment.

p.21: However, the adsorbent’s surface was not fully covered or impregnated with chemicals, hence, the adsorbents still can reversible”??? Please comment. “This data can be confirmed through EDX analysis in Table 10 that shows the wt.% of elemental composition for C (70.5%) are higher than Zn (10.14%) and O (12.37%).” Why S and Zn content are not determined after thermal desorption? Please comment.

The degradation happens still consider at low level for each adsorption-desorption cycle. “This formation of ZnS also increased value of the BET surface area, which might limit the H2S adsorption capacity in the next cycle.”  Why? Please comment.

4. Conclusions

It is necessary to review conclusions especially regarding mechanism of H₂S capture and the role of ZnAc₂ including adsorption-desorption regenerationby thermal desorption at 150°C.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 4

Reviewer 2 Report

 

I have read the 3rd revised version of the article “Application of Response Surface Methodology for Preparation ZnAC₂/AC Adsorbents for Hydrogen Sulfide (H₂S) Capture”.

Authors have responded to the comments, questions and suggestions of the reviewer one by one and made considerable efforts to implement changes and additional data where needed.

I am convinced the overall quality of the article has improved considerably. I also hope the authors will proceed to unravel the mechanism of H2S adsorption and desorption in their further research.

I support and recommend publication of this revised manuscript. 

Minor comment: In table  I would replace "0.00" by "ND" (not detected).