Carbon dioxide adsorption in open nanospaces formed by overlap of saponite clay nanosheets

Nanoscale open spaces formed by partial overlap of two-dimensional nanosheets in clays, abundantly and ubiquitously available, possess reactive molecular sites such as nanosheet edges in their interior. Here, the capture and storage of CO2 molecules in open spaces within saponite clay are explored by solid-state nuclear magnetic resonance coupled with open space analysis using positronium. CO2 physisorption occurs on the nanosheet surfaces inside the open spaces under ambient conditions. Thereby, CO2 molecules are activated by picking off weakly-bound oxygen from octahedral sites at the nanosheet edges and carbonate species are stabilized on the nanosheet surfaces. This instantaneous mineral carbonation and CO2 physisorption occurs in the absence of an energy-consumption process or chemical solution enhancement. This finding is of potential significance for CO2 capture and storage and presents an approach of environmentally friendly recycling of low contaminated soil in Fukushima.

The work entitles "Carbon dioxide adsorption in open nanospaces formed by overlap among clay nanosheets" deals with sorption of CO2 onto two saponite clays. The results are interesting ones, but I have some specials issues before it can be published on Communications Chemistry.
-The used clay must be evidenced into the title and abstract. Maybe you can change clay by saponite.
Abstract: -include the used clay -Regarding Fukushima contaminated soils, it contains CO2 in higher concentrations? Why? -Introduction title is missing -Lines 57-58. Authors mention: "Clay, a silicate mineral abundantly and ubiquitously available in nature, is structured by a stack of 2D nanosheets with a thickness of a few nm, which is a minimum structural unit". The authors must be more specific, there are not a single clay in nature, there are many clay minerals on nature, each one with their characteristics. -Again, in reference to the sentence "The 2D nanosheets with a variety of size cannot be perfectly stacked being partially overlapped as schematically illustrated in the left-hand side of Fig. 1, which has been observed in the images of field-emission type scanning electron microscopy (FE-SEM)12", said specifically for Na commercial saponite.
-Lines 64-65, it is not clear why authors mention Cs adsorption on nanosheet edges, introduction lost the conductor line.
-Line 67, please define Ps -Line 102, regarding the sentence "This demonstrates that the CO2 molecules adsorb at Cs+ cations on the nanosheet surfaces and the concentration is reasonably assumed to be ~ 0.03 mmol/g as estimated above", what about X-Ray diffraction technique? Probably a change on the reflection peak supports this fact.
Physisorption and chemisorption of CO2 molecules -Why 13C MAS NMR spectrum of Na type saponite is not include? There are not signal as that of Cs-type clay?? Maybe a sentence could be included.
-Regarding the sentence: "Upon 13CO2 loading, an intense peak and broad hump arising from CO2 adsorption appeared at around the chemical shifts of 125 and 170 ppm (see (b) in Fig. 2). This together with the above result of 133Cs MAS NMR indicates that CO2 adsorption occurs at Na+ cations on the surface of 112 2D nanosheet." Figure b is the spectrum of Na-Saponite, then, why do you joint it with results of Cs-saponite? It is confusing.

Discussion
-Add some references for the sentence "According to a number of earlier works on zeolite materials with CO2 gas, the polarizing power of exchangeable alkali cations is one of the decisive factors for the capacity of CO2 gas adsorption" -Regarding the sentences "the open space with the size of ~ 9 Å for the Cs-type saponite probed by Ps is much higher than that of Na-type one (see Table 2)", is confusing, do you mean that the intensity of this site is much higher for Cs-saponite than for Na-saponite? We are grateful to you for thorough reading and constructive comments, including polishing sentences.
All the criticisms made by you have been carefully considered. The manuscript was revised following your suggestions. Below, our responses to your comments. In addition, the comments suggested by Reviewer 2 have been considered. Note that revised parts in the manuscript are highlighted in red.
> One thing I wish they had looked at was reversibility of the physiosorbed CO2. This is a critical need to understand the stability of the sorbed CO2 for the timeframes necessary to duplicate geologic storage. I think the manuscript would be improved if they added a discussion on this.

Author reply
The detailed investigation on the reversibility of CO2 physisorption is one of our future topics, but have not yet studied. In light of the fact that the peak of CO2 physisorption in the 13C MAS NMR spectrum completely disappeared upon heat treatment (see Fig. 3), the open space formed by nanosheet overlap could again offer large enough surface area to accommodate CO2 molecules. It is thus expected that the ability of CO2 physisorption reversibly occurs when the sample is heated under mild conditions as 200 ºC. This is stated in the revised manuscript (line 12 from the bottom, page 7). > This isn't true for basalt reservoirs. The authors need to cite the work from the CarbFix project in Iceland and BigSky project in Washington State.

Author reply
We appreciate your suggestion. This part was deleted and relevant works on CarbFix project in Iceland and Big Sky project in Washington State were alternatively cited as refs 10 and 11, respectively. > It is unclear what you are trying to say here.

Author reply
We agree with you that this part is confusing. The phrase "characteristic for 2D structure" was deleted in the revised manuscript (see line 12, page 3).

> Concentration of what?
Author reply We mean "site concentration". But this part was deleted to cope with the comment from Reviewer #2. > Can you be certain there is no contribution from organic carbon?
Author reply A series of works relevant to CHN analysis are completely free from organic carbon. All the materials including sample holder are not associated with organic carbon. In addition, the samples were isolated from the air during CO2 loading. We thus believe that the C contents detected in the CHN analysis are solely associated with CO2.
> Additional details on the equipment and analysis need to be provided.

Author reply
It is now described in the revised manuscript (line 9, page 10).
> Did a physical transfer occur, or did you change the pressure in the vessel holding the sample? If a physical transfer occurred, how did you eliminate atmospheric contact?

Author reply
Here, we first evacuate the samples inside the quartz tube and then successively replace with CO2 gas without any contact with air. This was stated in the revised manuscript (line 9, page 8) > Again, please include information on the instrument used.

Author reply
We added the detailed information of instrument used in the present work in the revised manuscript (line 10 from the bottom, page 8).
We are grateful to you for thorough reading and constructive comments. All the criticisms made by you have been considered. The manuscript was revised following your suggestions. In addition, the comments suggested by Reviewer 1 have been carefully considered. Note that revised parts in the manuscript are highlighted in red.
> -The used clay must be evidenced into the title and abstract. Maybe you can change clay by saponite.

Author reply
The local structural unit causing CO2 adsorption demonstrated in this work is a layered silicate, which is a main component of clay. We thus expect that any types of clay minerals can be applied for CO2 sorbents and therefore maintained "clay" in the title. From the reason mentioned above, we maintained "clay" in the revised manuscript.
> -Regarding Fukushima contaminated soils, it contains CO2 in higher concentrations? Why?

Author reply
Probably, our English is not good. We do not mean that contaminated soils in Fukushima contain high concentration of CO2. We are looking for an approach of environmentally-friendly recycling for low contaminated soil in Fukushima. Thus, Cs-type saponite was studied in addition to Na-type one in the present work. This was stated in the introductory part of the revised manuscript (line 10 from the bottom, page 3).

> -Introduction title is missing
Author reply This is our careless mistake.
"Introduction" is now placed in the revised manuscript.
> -Lines 57-58. Authors mention: "Clay, a silicate mineral abundantly and ubiquitously available in nature, is structured by a stack of 2D nanosheets with a thickness of a few nm, which is a minimum structural unit". The authors must be more specific, there are not a single clay in nature, there are many clay minerals on nature, each one with their characteristics.

Author reply
We agree with you that a number of clay minerals are available in soil environment. Any types of clay minerals including saponite can be the target of the present CO2 research because a layered silicate structure is of significance as mentioned above.
> -Again, in reference to the sentence "The 2D nanosheets with a variety of size cannot be perfectly stacked being partially overlapped as schematically illustrated in the left-hand side of Fig. 1, which has been observed in the images of field-emission type scanning electron microscopy (FE-SEM)12", said specifically for Na commercial saponite.

Author reply
We want to emphasize again that any types of clay minerals including saponite can form the local structures partially overlapped.

> -Lines 64-65, it is not clear why authors mention Cs adsorption on nanosheet edges, introduction
lost the conductor line.

Author reply
We agree with you that this part is not clear and difficult to follow. This sentence was thus deleted (see line 13, page 3).

> -Line 67, please define Ps
Author reply This is our careless mistake.
"Ps" is now defined in the revised manuscript.

Author reply
We agree with you that this part is difficult to follow. We added "after CO2 loading" in the text of the revised manuscript (line 4, page 4).
> -Please, add the associated errors to the determined R1 and R2 values (Table 2) Author reply Following your suggestion, standard deviations (error bars) of R1 and R2 were added in the caption of Table 2. > -Line 102, regarding the sentence "This demonstrates that the CO2 molecules adsorb at Cs+ cations on the nanosheet surfaces and the concentration is reasonably assumed to be ~ 0.03 mmol/g as estimated above", what about X-Ray diffraction technique? Probably a change on the reflection peak supports this fact.

Author reply
You are right. XRD peaks are shifted toward lower diffraction angles for Cs-type saponite because of the expansion of basal spacing. We have checked XRD data, but did not employ them in the present paper due to out of the present research scope.
> Physisorption and chemisorption of CO2 molecules -Why 13C MAS NMR spectrum of Na type saponite is not include? There are not signal as that of Cs-type clay?? Maybe a sentence could be included.

Author reply
Yes, no signal appears in the NMR spectrum. This was stated in the revised manuscript (line 5, page 5).
> -Regarding the sentence: "Upon 13CO2 loading, an intense peak and broad hump arising from CO2 adsorption appeared at around the chemical shifts of 125 and 170 ppm (see (b) in Fig. 2). This together with the above result of 133Cs MAS NMR indicates that CO2 adsorption occurs at Na+ cations on the surface of 112 2D nanosheet." Figure b is the spectrum of Na-Saponite, then, why do you joint it with results of Cs-saponite? It is confusing.

Author reply
Here, we want to compared the data between Na-and Cs-types. Note that Cs-type corresponds to (c). Ps is much higher than that of Na-type one (see Table 2)", is confusing, do you mean that the intensity of this site is much higher for Cs-saponite than for Na-saponite?
Author reply Yes, we mean "intensity". This part could be confusing and was modified to "the fraction of open space …" and "see relative intensity I2 in Table 2 This was stated in the caption of Fig. 4 as well.
> -The reason why the red-shift of FTIR peaks of CO2 -saponite it is not clear.

Author reply
We agree with you that this party is confusing. The paragraph was revised so that reader can see an influence of atom-missing in the octahedron on the chemical bond (see line 6 from the bottom, page 6). In addition, defective sites in the octahedron was clearly stated in the caption of Fig In this work, Cs-type saponite was studied in addition to Na-type one to find out an approach of environmentally-friendly recycling for low contaminated soil in Fukushima. This is stated in the introductory part in the revised manuscript following your suggestion (line 10 from the bottom, page 3).

> -How where performed the CO2 sorption experiments?
Author reply Here, we first evacuate the samples inside the quartz tube and then successively replace with CO2 gas without any contact with air. This was stated in in the revised manuscript (line 9, page 8) I have reviewed the revised manuscript and have found that the authors have satisfactorily addressed my comments and proposed edits. I appreciate their earnest effort to improve the readability and quality of the manuscript. I have no further edits or recommendations for them.
Reviewer #2 (Remarks to the Author): -Regarding the previous comments about clay type must be included on title and abstract, author said: " The local structural unit causing CO2 adsorption demonstrated in this work is a layered silicate, which is a main component of clay. We thus expect that any types of clay minerals can be applied for CO2 sorbents and therefore maintained "clay" in the title. It is true, the CO2 could be then adsorbed into the interlayer space on any clay. But, it could, you are not sure, and the works deals with saponite clay. Proposing the title "Carbon dioxide adsorption in open nanospaces formed by overlap among clay nanosheets" does not show work done, because authors only use a single type of clay for CO2 adsorption: Saponite. The lack of this world on title will make it difficult for interested readers to find the article. If find, the no mention in the abstract, where a short explanation about the work must be presented, does not invite to read the paper.
-Regarding the previous comments about clay type must be included on title and abstract, author said: " The local structural unit causing CO2 adsorption demonstrated in this work is a layered silicate, which is a main component of clay. We thus expect that any types of clay minerals can be applied for CO2 sorbents and therefore maintained "clay" in the title. -On introduction, authors mention clays in general, without mention saponite characteristics, the used clay in this work. -Author said that "any types of clay minerals including saponite can form the local structures partially overlapped." It cannot affirm that, works are necessary, it could be and hypothesis but not an affirmation.
-Experimental errors must be presented with significant digits, i.e., 0.22 represent an experimental error of 0.3 (Table 2).