Round 1

Reviewer 1 Report

Authors should provide better analysis of material - I suggest SEM if it will be enough. There should be also added reference commercial materials tested in proposed analysis method to compare obtaing results. Article is very interesting and I really like that there is practical aspect of science.

Author Response

1        Comments from reviewer 1

Authors should provide better analysis of material - I suggest SEM if it will be enough. There should be also added reference commercial materials tested in proposed analysis method to compare obtaing results. Article is very interesting and I really like that there is practical aspect of science.

Answer: The recommendation is helpful to guide us for future research. The following SEM micrographs of virgin adsorbents have been added in the manuscript as Figure 2 with description of “Figure 2 are the SEM micrographs of virgin adsorbents which show the morphology of the adsorbent particles. Figure 2 (a) and (b) are the SEM micrographs of virgin WDC adsorbents, the particles have cracking features. Figure 2 (c) and (d) are the SEM micrographs of virgin SDC that presenting as smaller particles supported on the larger particles. Figure 2 (e) and (f) are the SEM micrographs of virgin AA which demonstrate the large cluster of fine power.”

Figure 2 SEM micrographs of virgin WDC (a) and (b); virgin SDC (c) and (d); virgin AA (e) and (f).

Author Response File: Author Response.docx

Reviewer 2 Report

The subject of research is very interesting. The manuscript is well-written and certainly contributes to the field. The work described is sufficient in respect of novelty and quality.  However, the manuscript has some minor weaknesses, which should be explained before publication:

1) References. Please, re-check again all cited literature. Some citations do not match the references, for example: page 7, line 233 – “De Boer in references [54,56,57]”

-54. ALOthman, Z. A review: Fundamental aspects of silicate mesoporous materials. Materials 2012, 5, 2874- 2902.

-56. Wang, R.; Sang, S.; Zhu, D.; Liu, S.; Yu, K. Pore characteristics and controlling factors of the lower cambrian hetang formation shale in northeast jiangxi, china. Energy Exploration & Exploitation 2018, 36, 43-65.

-57. Shao, L.; Liu, L.; Wen, H.; Li, Y.; Zhang, W.; Li, M. Characteristics and influencing factors of nanopores in the middle jurassic shimengou shale in well yq-1 of the northern qaidam basin. Earth Sci. Front. 2016, 23, 164-173.

 

2) Table 2 and line 181  “ pH value was slightly increased from 7.2 to 7.4” - The pH value does not agree with the value given in the Table 2 (7.3).

3) page 5, line 181: Authors claims that slightly increasing pH from 7.2 to 7.4 (7.3 in table) indicates the H2S removal is mainly based on physical adsorption and the pH values for spent SDC and AA are slightly lower than the virgin adsorbents which indicate the chemical reaction occurred between the H2S and the alkaline compositions. Could you please add some convincing arguments to justify the type of adsorption (physical-chemical) based on such a small change in pH?

4) I suggest small modification of the manuscript title “Study of H2S removal capability from simulated biogas by using waste-derived adsorbent materials”. Authors used very simple gas mixture, which isn’t even close to composition of biogas. Therefore, the small change in the title would be more reliable for readers.

Author Response

1        Comments from reviewer 2

The subject of research is very interesting. The manuscript is well-written and certainly contributes to the field. The work described is sufficient in respect of novelty and quality.  However, the manuscript has some minor weaknesses, which should be explained before publication:

1) References. Please, re-check again all cited literature. Some citations do not match the references, for example: page 7, line 233 – “De Boer in references [54,56,57]”

-54. ALOthman, Z. A review: Fundamental aspects of silicate mesoporous materials. Materials 2012, 5, 2874- 2902.

-56. Wang, R.; Sang, S.; Zhu, D.; Liu, S.; Yu, K. Pore characteristics and controlling factors of the lower cambrian hetang formation shale in northeast jiangxi, china. Energy Exploration & Exploitation 2018, 36, 43-65.

-57. Shao, L.; Liu, L.; Wen, H.; Li, Y.; Zhang, W.; Li, M. Characteristics and influencing factors of nanopores in the middle jurassic shimengou shale in well yq-1 of the northern qaidam basin. Earth Sci. Front. 2016, 23, 164-173.

Answer: Thanks for the reviewer to point out the confusing interpretation.

The original sentence is “The shape of the adsorption hysteresis loop exhibited similar as H₃ shape indicates the porous material could has slit-shaped pores with two ends open which has been classified by De Boer [54,56,57]”.

I meant the classification of the pore shapes were firstly classified by De Boer, the references 54, 56 and 57 were the research results from other researchers who have the similar results with me. I have revised the sentence as below:

“The shape of the adsorption hysteresis loop exhibited similar as H₃ shape indicates the porous material could has slit-shaped pores with two ends open, which can be supported by other researchers’ finding [54, 56, 57].”

 

2) Table 2 and line 181 “ pH value was slightly increased from 7.2 to 7.4” - The pH value does not agree with the value given in the Table 2 (7.3).

Answer: Thanks for the reviewer to correct my mistake. I have revised it to the correct number 7.3 same as the number in the table. I will make sure to check the manuscript more carefully before submission in the future.

3) page 5, line 181: Authors claims that slightly increasing pH from 7.2 to 7.4 (7.3 in table) indicates the H2S removal is mainly based on physical adsorption and the pH values for spent SDC and AA are slightly lower than the virgin adsorbents which indicate the chemical reaction occurred between the H2S and the alkaline compositions. Could you please add some convincing arguments to justify the type of adsorption (physical-chemical) based on such a small change in pH?

Answer: I agree there should be more supportive results. The statement should be more accurate and I need to be more critical to draw the statement. So, the following revision has been made:

 “Table 2 listed the elemental composition, pH values for the virgin and spent adsorbent materials. The CHNO compositions for virgin and spent WDC adsorbents are similar, and the pH value was slightly increased from 7.2 to 7.3 which indicates chemical reaction is mostly not happened during the adsorption of the H2S removal is mainly based on physical adsorption by WDC. The pH values for spent SDC and AA are slightly lower than the virgin adsorbents which that indicate the chemical reaction occurred between the H2S and the alkaline compositions. In other words, the chemical adsorption of H2S by SDC and AA are more dominated than WDC”.

4) I suggest small modification of the manuscript title “Study of H2S removal capability from simulated biogas by using waste-derived adsorbent materials”. Authors used very simple gas mixture, which isn’t even close to composition of biogas. Therefore, the small change in the title would be more reliable for readers.

Answer: I have revised the title as “Study of H₂S removal capability from simulated biogas by using waste-derived adsorbent materials”.

Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors 

It was a pleasure for me to read Your paper, because it presents the good level of scientific. I would suggest only a correction of the figures 1-5 style for a more professional style.

   Yours sincerely

Mirosław Kwiatkowski

 

Author Response

It was a pleasure for me to read Your paper, because it presents the good level of scientific. I would suggest only a correction of the figures 1-5 style for a more professional style.

Answer: I have updated the Figure1-5.

Author Response File: Author Response.docx

Reviewer 4 Report

Paper Processes-888470: Study of H₂S removal capability from biogas by using waste-derived adsorbent materials

 

Comments

The comments I include below show that, in my opinion, the paper has not enough quality as to be published in Processes and, consequently, I think that it should be rejected.

 

The main results of the paper are included in Table 1, where is shown the adsorption capacity of the three adsorbents used in the study. Results included in that table are:

Adorbent WDC shows a removal capacity of 2.2 mgH₂S/g adsorbent

Adorbent SDC shows a removal capacity of 0.1 mgH₂S/g adsorbent

Adorbent AA shows a removal capacity of 3.22 mgH₂S/g adsorbent

 

● In page 4, lines 160 to 167, authors say: “The results show the ash contents ranking of the three adsorbents is AA > SDC > WDC and the ranking of organic components is in a reverse direction. The results are consistent with the elemental analysis results listed in Table 2 where the carbon compositions for three raw adsorbents is ranking as WDC > SDC > AA. This result could explain the H₂S removal capacity of WDC is higher than SDC in Table 1, because the porous carbon content in WDC is 88.93% which is much higher than the content in SDC is 33.94%. The porosity of carbon could also be supported by the nitrogen adsorption and desorption analysis shown in Table 4, where the WDC has the highest surface area and total pore volume that are 436.38 m² g^-1 and 0.64 cm³ g^-1, respectively”.

○ The fact that the carbon content of the three adsorbents ranking as WDC>SDC>AA could explain that the adsorption capacity of SH₂ is higher for WDC (carbon content 88.93%) than for SDC (carbon content 33.94%), but it cannot explain that the adsorption capacity for WDC (carbon content 88.93%) is lower than for AAS (carbon content 7.42%).

○ With reference to the data shown in Table 4, two aspects to consider.  First, the data in Table 4 does not correspond to the data included on page 4 (line 167) of the text. Surface area and total pore volume of WDC shown in Table 4 are 210 (m²·g^-1) and 0.50 (cm³·g^-1), respectively, while those shown in line 167 of page 4 are 436.38 (m²·g^-1) and 0.64 (cm³·g^-1), respectively.                                          

Second, the greater surface area and total pore volume of WDC (210 m²·g^-1, 0.50 cm³·g^-1, respectively), may explain the higher adsorption capacity of WDC than SDC (1.89 m²·g^-1, 0.004 cm³·g^-1, respectively), but it cannot explain the lower adsorption capacity of WDC (210 m²·g^-1, 0.50 cm³·g^-1, respectively) than AA (9.38 m²·g^-1, 0.011 cm³·g^-1, respectively).

 

● In page 4, lines 168 to 174, authors say: “Although the SDC contains a higher amount of carbon (33.94 wt.%) compare with AA (7.42 wt.%) in Table 2, the H₂S removal capacity of SDC (0.1 mgH₂Sg^-1) is lower than AA (3.22 mgH₂Sg^-1) and this is related to the high ash content in AA (73.21 wt.%), as shown in Table 1. It has been reported the incineration residues contain many compounds would form sulphides as H₂S-trap materials which can react with H₂S at ambient temperature, such as manganese, zinc, copper and iron [37]. The porosity analysis results shown in Table 4 can also prove there is no significant difference between SDC and AA in terms of surface area and total pore volume”.

○ Explanation given by the authors is no valid. Table 2 shows that AA ash content (73.21 wt.%) is higher than SDC ash content (32.68 wt.%), but Table 3 shows that zinc, copper and iron content of AA (0.13%, nd, 1.91%, respectively) are much lower than zinc, copper and iron content of SDC (1.62%, 0.63%, 45.15%, respectively). I mean, 0.13% of 73.21 is 0.095 but 1.62% of 32.68 is 0.520 (Zn), or 1.91% of 73.21 is 1.398 but 45.15% of 32.68 is 14.755 (Fe). That is, presence of zinc, copper and iron is much higher in SDC than in AA, although total ash content of AA is higher than SDC.

○ The term “Table 1” should be change by “Table 2”.

 

● In page 5, lines 179 to 183, authors say: “Table 2 listed the elemental composition, pH values for the virgin and spent adsorbent materials. The CHNO compositions for virgin and spent WDC adsorbents are similar, and the pH value was slightly increased from 7.2 to 7.4 which indicates the H₂S removal is mainly based on physical adsorption. The pH values for spent SDC and AA are slightly lower than the virgin adsorbents which indicate the chemical reaction occurred between the H₂S and the alkaline compositions”.

○ I am not sure that a minimum pH increase from 7.2 to 7.3 (7.3 in table 2, 7.4 in the text) indicates physical adsorption and a small pH decreases from 7.9 to 7.6 or from 8.8 to 8.4 indicate chemical adsorption (described by the reactions at the end of page 5 and beginning of page 6), especially when (as I will comment below) the iron content (implied in those reactions) of the two adsorbents with the highest adsorption capacity (AA and WDC) is much lower than that of the adsorbent with the lowest adsorption capacity (SDC).

 

● In page 5, lines 184 to 194, and in page 6, lines 195 to 201, authors say: “Table 3 presents the ash composition of the pristine and spent adsorbent materials by XRF which prove the existence of alkaline compositions, such as Fe, Cu, Mn. Calcium is the main mineral species in WDC and AA adsorbents, and iron is the main mineral species in SDC adsorbents. Juarez et al. [37] reported the alkaline or pH-neutral waste contain sulfites or a mixture of Fe-II, Fe-III, Mn-II, Cu-I, Cu-II, Zn-II which play the role as a potential H₂S-trap material. The possible reaction for the H₂S removal by SDC and AA adsorbents could relate to the “primary” and “secondary” reactions [37]. The primary dissociation reactions include reaction (2) and (3), the suggested secondary sulfidation reactions could be (3)-(11) [37].

OH⁻ + H₂S = HS⁻ + H₂O (2)

HS⁻ + OH⁻ = S²⁻ + H₂O (3)

FeO + H₂S = FeS + H₂O (4)

2Fe(OH)₃·H₂O + 3H₂S = Fe₂S₃ + (6+x)H₂O (6)

2FeOOH + 3H₂S = Fe₂S₃ + 4H₂O (7)

2HS⁻ + 4HSO₃⁻ = 3S₂O₃⁻² + 3 H₂O (8)

2HS⁻ + HSO₃⁻ = 0375S₈ + 3OH⁻ (9)

S²⁻ + nS = S_n+1²⁻ (10)

MnO + H₂S = MnS + H₂O (same with Cu, Ca and Ni) (11)”

 

○ I include again the removal capacities and Fe content of the different adsorbents

Adorbent WDC:  removal capacity of 2.2 mgH₂S/g adsorbent; 3.27% Fe

Adorbent SDC shows a removal capacity of 0.1 mgH₂S/g adsorbent; 45.15% Fe

Adorbent AA shows a removal capacity of 3.22 mgH₂S/g adsorbent; 1.91% Fe

Why are so significant and important to the adsorption process the reactions 1 to 10 if the iron contents are very low in the adsorbents with the highest adsorption capacity and only in the adsorbent with the lowest adsorption capacity those iron contents are very high?.

○ Regarding equation 11, it should be noted that in reference 37 the following appears:

MnO + H2S = MnS + H2O (+ same with Cu, Ni)

As can be seen, there is no reference to calcium (Ca) in reference 37.

What bibliographic reference has been consulted by the authors to support the inclusion of calcium in this equation 11?.

○ Equation 6. What does x mean?.

○ Equation 9. Does 0375 mean 0.375?

 

● In page 6, lines 211 to 217, and in page 7, lines 218 to 221, authors say: “Table 4 shows the porosity of the virgin and spent waste-derived adsorbent materials. The surface area of WDC decreased significantly from 210.18 to 96.95 m² g^-1 after the H₂S removal. This could explain as the large amount of H₂S was adsorbed by the porosity of WDC. However, there is no significant change of surface area and total pore volume for SDC and AA would also support the conclusion drawn earlier that the H₂S removal capacity of WDC and AA were dominated by the chemical reactions rather than the physical adsorption. Although the AA adsorbent is not porous as WDC, the surface area and total pore volume also decreased slightly which indicate the physical adsorption also happened. In Table 4, the average pore diameter for virgin WDC is 19.64 Å, which indicate the presence of mesopores. According to the BJH pore size distribution illustrated Figure 4(a), that can confirm the presence of mesopore distributions in WDC sample. The mesopore has been reported as the active site for H₂S adsorption [33,52]”.

○ Why the significant decrease of WDC surface area from 210.18 to 96.95 m2 g^-1 after the H₂S removal (large amount of H₂S adsorbed, according to authors, 2.2 mgH₂S/g) is explained by the porosity of WDC, if AA shows higher adsorption (3.22 mgH₂S/g) having lower decrease of surface are and lower porosity.

○ The affirmation of the authors saying that H₂S adsorption by WDC and AA adsorbents is dominated by chemical reactions rather than physical adsorption is hardly acceptable and is in contradiction with the results showed in some of the preceding tables.

As indicated above, the low iron, copper and nickel content of the adsorbents with the highest adsorption capacity (WDC and AA) compared to the very higher iron content (and higher copper and nickel content) of the adsorbent with the lowest adsorption capacity prevents accepting that the equations described above are the main responsible for adsorption and that the adsorption is fundamentally chemical rather than physical.

○ About the last sentence of the analyzed paragraph (“The mesopore has been reported as the active site for H₂S adsorption [33,52]”), I include the last sentence of Results and discussion section (just before Conclusions sections) (page 168) of reference 52:

“H₂S adsorption capacity showed a better relationship with micropore surface area than with mesopore surface area. Consequently, it was suggested that H₂S adsorption on the activated carbon, prepared from petcoke with KOH chemical activation, is influenced by the pore structure, especially with micropore surface area”.

The affirmation of the authors related to reference 52 is not fully true.

Moreover, it is described that mesoporous materials have pores sizes between 2 and 50nm, that is 20 and 500 Å, and the three adsorbents studies in the paper are inside that size range.

 

● Some of the previous comments can also be applied to not fully accept as valid the explanations given by the authors in other parts of the paper:

○ Page 8, lines 242 to 248: “Figure 4 (a) and (b) are illustrating the pore size distributions of virgin and spent WDC adsorbents. According to the peak, A diminished significantly for the spent WDC in Figure 4 (b) compare with the virgin WDC shown in Figure 4 (a), and the total pore volume decreased from 0.50 to 0.14 cm³ g^-1 as shown in Table 4 indicate the mesopore size ~ 38 Å contribute to H₂S adsorption. The peak A in Figure 4 (c) and (d) has no obvious change after the H₂S adsorption which could support the assumption made early that the chemical reaction is more dominated to the H₂S removal from the simulated biogas stream by using AA adsorbent material”.

○ Page 10, lines 271 to 274, and page 11, lines 275 to 283: “The MS result shows the AA adsorbent has the highest removal capacity at 3.22 mgH₂Sg^-1 and WDC has slightly lower H2S removal capability is 2.2 mgH₂Sg^-1. The TGA result shows the ash residues composition of each adsorbent materials is ranking as AA (73.21 wt.%) > SDC (67 wt.%) > WDC (7.94 wt.%). The ash content results are also supported by the elemental analysis which the carbon content of each adsorbent materials is ranking as WDC (88.93 wt.%) > SDC (33.97 wt.%) > AA (7.42 wt.%). The ash composition was measured by XRF which show the existence of alkaline mineral species in all adsorbent materials, such as Fe, Cu and Mn. The main mineral species in WDC and AA adsorbents is Ca, and the main mineral species in SDC adsorbent is Fe. The WDC has relatively higher H2S removal capability (2.2 mgH₂Sg^-1) compare with SDC (0.1 mgH₂Sg^-1) is due to the physical adsorption by high porosity, that the surface area of WDC is 210.18 m² g^-1. The mesopores play the most important role to adsorb H₂S. The AA has the highest capability to remove H₂S is due to the high content of ash residues which contain alkaline mineral species is more dominated to the H₂S removal rather than physical adsorption”.

SDC ash content is not 67 wt.% but 37.68 wt.%

 

● References should be checked, especially regarding the need for capital letters in some journal names and in the formula of some chemical compounds.

Author Response

Dear Editor:

I have answered all of the reviewers' comments very carefully. Please accept my revised version of manuscript.

 

Yours sincerely.

Yeshui Zhang

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

The paper has been greatly improved by the authors. Small mistakes and contradictions have been corrected and new explanations (more complete and scientifically convincing) of the obtained results have been included, also supported by new bibliographic references.

Consequently, the article can be accepted.