Facile Synthesis, Characterization, and Adsorption Insights of Lanthanum Oxide Nanorods
, Janardhan Reddy Koduru *, Yoon-Young Chang, Seon-Hong Kang and Jae-Kyu Yang *
Round 1
Reviewer 1 Report
This paper reported La2O3 nanorods and analyzed their physical and chemical properties. The maximum adsorptive removal capacity of La2O3 for As(V) has been found to be comparable or more than the reported some of hybrid materials. This manuscript should be accepted with some minor changes.
- The introduction part is a little weak
- The figures need to be clearer for publication.
- The language needs to be polished
- The format of reference is not unified.
Author Response
Responses for Reviewers Comments
We appreciate the valuable comments of the all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with text with yellow background color in the revised manuscript.
Reply to Reviewer
Comment 1. The introduction part is a little weak
Response: Thank you for the valuable suggestion. As suggested, the introduction section is improved, keeping intact the flow and objective of the study.
Comment 2. The language needs to be polished
Response: Thank you for your clean observation and suggestion. We polished the English Language with help of Professional English Proof Reader.
Comment 3.The format of reference is not unified.
Response: The format of all references is unified in the revised manuscript.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript describes the synthesis, characterization and application as an adsorption material of one batch of La2O3 nanosized rods, to be used for the removal of As(V) species from contaminated aqueous solutions.
First, by reading the theme of the text, it is not clear if the submission is adequate for this Journal. Indeed, this manuscript does not deal with metals (in their metallic form), alloys or metallurgy issues. Rather, it is more directed to a readership of environmental and/or analytical chemists. I would therefore suggest to re-direct the submission towards a more chemistry-oriented journal, such as Molecules, Materials or (for the environmental remediation aspects) Environments.
After this main point of concern, other import issues need to be carefully addressed, before the text could be positively evaluated for publication. In particular:
1) The use of lanthanum oxides for the removal of As species is a well-known application and some valuable works date back to the end of the 1990s (cf. J. Colloid Interf. Sci., 188, 1997, 340 or Water Env. Res., 68, 1996, 295, not cited here) as well as commercial patents (cf. US Patent 5,603,838; 1997). It is therefore necessary to stress the points of novelty of the present type of material with respect to other similar ones, that have already appeared on the previous literature. Clearly, the present system showed a very high As uptake (260 mg/g), but the material displays also a rather poor specific surface area (19 m2/g). Can the surface area (and thus the adsorption capability) be improved by finely tuning the synthesis conditions? No comments are present in the text whether the synthesis procedure has been optimized or not. Please, improve this part.
2) The section 2 is very short. Some important experimental details about the preparation of the material were not described. For instance: was the material prepared in only one batch? how reproducible is the synthesis (in terms of size and morphology of the final materials)? how was the PZC value measured? the description of the analytical methods (IR, XPS, SEM-TEM, PZC analysis) should be moved to the main text, as it is essential for the comprehension of the results.
3) The IR band at 1462 cm-1 is attributed to the CN triple bond. How can a nitrile group be formed under these conditions? In addition, the presence of nitrogen on the material suggests that the calcination step at the end of the preparation was not enough to obtain a pure oxide material. Did the Authors perform an elemental C,H,N analysis? Or a EDX mapping of the observed material? How much N is present on the final solids?
4) The actual nature of the surface species has been only marginally discussed. The presence of nitrate or carbonate species (line 84 and 86) are not fully consistent with a purely oxidic surface as depicted in Fig. 6. Are the Authors sure that the nitrate or carbonate species do not play a role in the As removal? Does the carbonate species come from a surface carbonation of the oxide due to the presence of atmospheric CO2? Are the solids kept under inert atmosphere after synthesis and calcination? Or are they exposed to open air, with moisture and natural CO2? Then, the use of HCl to adjust pH value may lead to a partial chlorination of the surface lanthanum oxide and the surface Cl species can modify the surface environment of the material. Can the Authors exclude the presence of chloride species? Did they observe the material at HR-TEM after the contact with HCl-rich acidic solutions? All these points must be carefully clarified, in order to understand which is the exact chemical nature of the species on the materials’ surface.
5) Line 96. The rods are 18 nm wide. How narrow is the distribution of these nanorods? On how many objects was this measurement carried out?
6) Section 3.2. The meaning of “significant” has to be clarified (lines 118). Actually, <30% removal at pH 10 is not truly significant. Moreover, the As uptake is described as “slowly” increasing, but actually it is an abrupt increase (from 10% to 99% in only 1 pH unit).
7) The experimental conditions under which the tests in Fig. 3 were carried out should be described more extensively. What is the As(V) concentration for Fig. 3a and 3b? Actually, a range from 10 to 100 mg/L is reported. Fig. 3b. Does the “dosage” refer to As(V) or to the La2O3?
8) Line 188. the concept of “better adsorption affinity” is not clear. Please, rephrase the meaning of the sentence. in addition, a deeper comment on the detrimental role of carbonate and phosphate species is needed. If carbonate moieties negatively affect the adsorption properties (Fig. 4b), can the formation of surface carbonates (revealed by IR; line 86) be avoided after the synthesis? Can a fresh calcination lead back the oxide to a carbonate-free condition?
9) Line 175. It is stated that other reports show “comparable” As(V) removal performances. However, as seen in Table 3, the performance of other systems is sensibly lower.
10) Lines 205-207. The increase in degree of freedom at the solid-liquid interface is not clear. One might expect the opposite. The degree of freedom of As(V) species should be higher in free aqueous solution than at the surface during the adsorption process. Please, clarify this doubtful point.
11) Is there any release of La(III) soluble species in solution, especially under acid conditions (due to the addition of HCl to rise the pH to 3 or 4)? Did the Authors check this point?
12) Fig. 7. Was the oxide pretreated (through drying, calcination, etc.) before recording the IR spectrum? Actually, the ratio between isolated and H-bonded La-OH (ca. 3570 cm-1 and 3450 cm-1, respectively) can largely change according to the moisture and hydration on the oxide surface. So, invoking the change in the intensity ratio of these two bands as a confirmation of the surface adsorption process can be misleading. Please, revise carefully the comments on these spectra. Moreover, a TEM image of one sample after AS(V) adsorption might be very interesting as well.
13) Fig. 6. If a surface protonation occurs, how can La-OH2+ and La-O- species coexist in aqueous solution? Please, explain the meaning of the scheme.
In summary, after a preliminary evaluation by the Editor whether this submission is consistent with the topics of the Journal or not, I would suggest a careful major revision of all of the above mentioned points, before the manuscript can be positively considered for publication.
Author Response
Responses for Reviewers Comments
We appreciate the valuable comments of the all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with text with yellow background color in the revised manuscript.
Reply to Reviewer
Material of one batch of La2O3 nanosized rods, to be used for the removal of As(V) species from contaminated aqueous solutions.
First, by reading the theme of the text, it is not clear if the submission is adequate for this Journal. Indeed, this manuscript does not deal with metals (in their metallic form), alloys or metallurgy issues. Rather, it is more directed to a readership of environmental and/or analytical chemists. I would therefore suggest to re-direct the submission towards a more chemistry-oriented journal, such as Molecules, Materials or (for the environmental remediation aspects) Environments.
Responses: Thank you constructive and valuable suggestions. This manuscript deals with metal oxide which is used for adsorptive removal of heavy metals. That means this manuscript deals with metals and its metallurgical application (for removal of Heavy metal) that are the aim of this journal. Moreover, there are many publication published on adsorption using metal based and its derivative composites in this journal.
After this main point of concern, other import issues need to be carefully addressed, before the text could be positively evaluated for publication. In particular:
1) The use of lanthanum oxides for the removal of As species is a well-known application and some valuable works date back to the end of the 1990s (cf. J. Colloid Interf. Sci., 188, 1997, 340 or Water Env. Res., 68, 1996, 295, not cited here) as well as commercial patents (cf. US Patent 5,603,838; 1997). It is therefore necessary to stress the points of novelty of the present type of material with respect to other similar ones that have already appeared on the previous literature. Clearly, the present system showed a very high As uptake (260 mg/g), but the material displays also a rather poor specific surface area (19 m2/g). Can the surface area (and thus the adsorption capability) be improved by finely tuning the synthesis conditions? No comments are present in the text whether the synthesis procedure has been optimized or not. Please, improve this part.
Response: The present lanthanum oxide was adsorbed the As(V) predominately by chemical complexation rather than electrostatic interaction. That means the adsorption capacity is mainly depend on the surface functional groups rather than the surface area. Moreover, the present material adsorption capacity was more or near to the literature reported materials that is may be due to the nano size rods morphology having highly active surface oxygen functions. These results shows that the novelty of present material. The same was included in the revised manuscript at end of introduction, experimental and conclusions section at lines, 49-52, 75-79, 214-245, 277-278.
2) The section 2 is very short. Some important experimental details about the preparation of the material were not described. For instance: was the material prepared in only one batch? how reproducible is the synthesis (in terms of size and morphology of the final materials)? how was the PZC value measured? the description of the analytical methods (IR, XPS, SEM-TEM, PZC analysis) should be moved to the main text, as it is essential for the comprehension of the results.
Response: Thank you for constructive comment. According to your comments, we further carefully checked and revised the manuscript. The changes were highlighted with yellow color background text in the revised manuscript at section 2.
3) The IR band at 1462 cm-1 is attributed to the CN triple bond. How can a nitrile group be formed under these conditions? In addition, the presence of nitrogen on the material suggests that the calcination step at the end of the preparation was not enough to obtain a pure oxide material. Did the Authors perform an elemental C,H,N analysis? Or a EDX mapping of the observed material? How much N is present on the final solids?
4) The actual nature of the surface species has been only marginally discussed. The presence of nitrate or carbonate species (line 84 and 86) are not fully consistent with a purely oxidic surface as depicted in Fig. 6. Are the Authors sure that the nitrate or carbonate species do not play a role in the As removal? Does the carbonate species come from a surface carbonation of the oxide due to the presence of atmospheric CO2? Are the solids kept under inert atmosphere after synthesis and calcination? Or are they exposed to open air, with moisture and natural CO2? Then, the use of HCl to adjust pH value may lead to a partial chlorination of the surface lanthanum oxide and the surface Cl species can modify the surface environment of the material. Can the Authors exclude the presence of chloride species? Did they observe the material at HR-TEM after the contact with HCl-rich acidic solutions? All these points must be carefully clarified, in order to understand which is the exact chemical nature of the species on the materials’ surface.
Responses for 3 and 4 comments: We really ashamed for wrong interpretation. I agree with reviewer comments. We revised the interpretation of FT-IR spectrum results with suitable explanation as well intact with suitable references. We also provided SEM-EDX results at Fig.S1 that are indicating the existing of traces carbonate along with La2O3. This EDX results are agreement with FT-IR results. This details included at revised manuscript 115-139. Hence Hope now you may reconsider your decision. The HR-TEM of HCl-interact lanthanum oxide, we did not analyzed due to lack of availability of instrument in the short time reviewing.
5) Line 96. The rods are 18 nm wide. How narrow is the distribution of these nanorods? On how many objects was this measurement carried out?
Response: Thank you for your valuable suggestion. But really, I didn’t get what’s your mean. The size of particle was measured using TEM and XRD results. The resultant particle size (18 nm) measured from TEM was close agreement Scherer’s crystalline size (19.31 nm).
6) Section 3.2. The meaning of “significant” has to be clarified (lines 118). Actually, <30% removal at pH 10 is not truly significant. Moreover, the As uptake is described as “slowly” increasing, but actually it is an abrupt increase (from 10% to 99% in only 1 pH unit).
Response: Thank you for your constructive suggestion, we rectified them in the revised manuscript.
7) The experimental conditions under which the tests in Fig. 3 were carried out should be described more extensively. What is the As(V) concentration for Fig. 3a and 3b? Actually, a range from 10 to 100 mg/L is reported. Fig. 3b. Does the “dosage” refer to As(V) or to the La2O3?
Response: Thank you for your constructive comment. According to your comments, we further carefully checked and rectified at Fig. 3b. As(V) dosage effect did at 20 mg/L As(V).
8) Line 188. the concept of “better adsorption affinity” is not clear. Please, rephrase the meaning of the sentence. in addition, a deeper comment on the detrimental role of carbonate and phosphate species is needed. If carbonate moieties negatively affect the adsorption properties (Fig. 4b), can the formation of surface carbonates (revealed by IR; line 86) be avoided after the synthesis? Can a fresh calcination lead back the oxide to a carbonate-free condition?
Response: Thank you for your constructive comment that are useful for improve our article. We revised the section 3.3 of manuscript according to your suggestion. The IR details also revised according suggestions. We did not checked either the calcination lead to a carbonate free oxide. We will consider it in our future work.
9) Line 175. It is stated that other reports show “comparable” As(V) removal performances. However, as seen in Table 3, the performance of other systems is sensibly lower.
Response: We rectified that sentence in the revised manuscript at Lines 215-217.
10) Lines 205-207. The increase in degree of freedom at the solid-liquid interface is not clear. One might expect the opposite. The degree of freedom of As(V) species should be higher in free aqueous solution than at the surface during the adsorption process. Please, clarify this doubtful point.
Response: We agree with reviewer and rectified it as “The positive value of ΔS0 suggests increased randomness at the solid–liquid interface during the process of As(V) adsorptive removal by La2O3”.
11) Is there any release of La(III) soluble species in solution, especially under acid conditions (due to the addition of HCl to rise the pH to 3 or 4)? Did the Authors check this point?
Response: Thank you for the good question. We checked the leaching of La(III) at lower pH, there is no any significant amount of leaching of La(III) into aqueous solution was observed.
12) Fig. 7. Was the oxide pretreated (through drying, calcination, etc.) before recording the IR spectrum? Actually, the ratio between isolated and H-bonded La-OH (ca. 3570 cm-1 and 3450 cm-1, respectively) can largely change according to the moisture and hydration on the oxide surface. So, invoking the change in the intensity ratio of these two bands as a confirmation of the surface adsorption process can be misleading. Please, revise carefully the comments on these spectra. Moreover, a TEM image of one sample after AS(V) adsorption might be very interesting as well.
Response: It is interesting comment, we rectified it in the revised manuscript as discussed at responses Comment 3 and 4. The same was discussed in the revised manuscript.
13) Fig. 6. If a surface protonation occurs, how can La-OH2+ and La-O- species coexist in aqueous solution? Please, explain the meaning of the scheme.
Response: Thank you for your valuable comment. As we know, lanthanum oxide exist as O=La-O-La=O, when it was protonated it may exist as +H2O-La-O-La-OH2+ which can interact with As(V) as shown in Fig.6. It was authenticated from the following references showed structures,
According to Xavier Oudet, DOI. 10.13140/RG.2.1.3880.8802
http://www.chemspider.com/Chemical-Structure.2529886.html
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript is focused on removal of arsenium from water via adsorption on La2O3 nanorods.
I have many remarks to the manuscript:
What is mechanism of As(V) removal? Adsorption? Compound is formed?
Is this paper in the focus of the Metals manuscript as the material is about ceramics?
Line 61 Lanthanum nitrate – IUPAC name should be applied. Otherwise it is not clear. Lanthanum(III) nitrate(V)? * ???H2O?
Line “Figure1a” – space is missing
FTIR seems to add no value to manuscript.
I miss that methods do not provide details about equipment (FTIR, XRD, BET, SEM…) . XRD gives no details about equipment, kind of radiation etc. The values of 2 theta are meaningless without kind of radiation. XRD spectrum add little value to manuscript. Later on Authors mention that the L2O3 is nanomaterial. Please use Scherrer formula to evaluate average crystallite size. Needle-like material should give possibility to measure two reflexions in order to measure two dimensions of nanometric material. After that please compare TEM results with results derived from Scherrer formula.
XPS spectrum should have the X-axis beginning from high values and then decreasing – this is the standard presentation.
Line 94 about Fig. 2 b, c – these particles are not “one-dimensional”. They are elongated, but do have other dimensions. E.g. graphene is 2D material, because one of dimensions is one atom thick. Here the thickness is 18 nm (Author claims). It is several dozens of atoms! Not one dimensional for sure.
Line 121 As confirmed by FT-IR and XPS, there are active surface functional groups… Authors just gives spectra. I am not convinced that there is prove for functional groups. Usually it requires eg. High resolution XPS spectrum for given elements and proper deconvolution. There is nothing like this in the manuscript.
The comparison of As(V) adsorption capacities is valuable part of the manuscript.
Table 3 – round up the values to one place after decimal point. It should be uniform. Proper way should be estimation of uncertainty.
Summing up, the manuscript should be improved. My impression is that Authors used several available techniques without squeezing the useful information about material. On the other hand this material can be useful for other researchers.
Author Response
Responses for Reviewers Comments
We appreciate the valuable comments of the all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with text with yellow background color in the revised manuscript.
Reply to Reviewer
The manuscript is focused on removal of arsenium from water via adsorption on La2O3 nanorods.
Comment 1. What is mechanism of As(V) removal? Adsorption? Compound is formed?
Response: As(V) adsorption possibility in chemical surface completion and electrostatic interaction. Adsorptive removal of As(V) with La2O3 involves electrostatic interaction and surface complexation. From the pH experiment, high adsorption was obtained between pH 3.0–7.0, suggesting an electrostatic attraction between anionic arsenic species and positive surface charge of the nanorods. However, the arsenic removal decreased when pH was increased to 7.0–12.0, implying an electrostatic repulsion between the negative surface charge of the adsorbent and anionic species of As(V). Regardless, there is still significant adsorptive removal, revealing that the adsorption process involved chemical surface complexation predominantly at higher pH values (3.0–7.0).Thus, both electrostatic interaction and chemical surface complexation led to the adsorption process of As(V) at pH 3.0–7.0. In Figure .6 shows clearly the formation final compound after interaction with La2O3.
Comment 2. Is this paper in the focus of the Metals manuscript as the material is about ceramics?
Response: In this manuscript, we focused preparation metal oxide nanorods (La2O3) and were used for the removal of Arsenic metals. Hence, the content of this manuscript was well fit to the Metals journal which is focusing on metals and derivatives properties and their applications.
Comment 3. Line 61 Lanthanum nitrate – IUPAC name should be applied. Otherwise it is not clear. Lanthanum(III) nitrate(V)? * ???H2O?
Response: Thank you for your constructive suggestion, we rectified it in the revised manuscript at Line 61.
Comment: Line “Figure1a” – space is missing
Response: Thank for your constructive comment, we rectified it in the revised manuscript.
Comment: FTIR seems to add no value to manuscript.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The Authors have addressed some of the critical points. However, some important flaws were not satisfactorily covered. In detail:
1) I agree with the Authors that the As(V) removal mainly depends on the availability of surface functional groups. However, the higher the specific surface area, the higher the number of exposed sites and therefore of surface functional group. So, the role of specific surface area cannot be underestimated in the path towards the optimization of these materials. If the Authors agree, a note on this could be welcome.
3, 4 and 8) The nature of the chemical species at the surface of the nanorods is a key point and should not be underestimated. It is affirmed that the Authors “provided SEM-EDX results at Fig.S1”, but in the Supplementary Files only the PZC graph can be found at Fig. S1. Then, the presence of HCl, under acid conditions, can give rise to LaCl3 through the attach of HCl onto La2O3 and surface chloride species are likely to be formed. In addition, the presence and the characterization of carbonate species, which showed to possess a detrimental role in the As(V) adsorption process, should be considered in the present work, rather than “in a future work”, as stated by the Authors in the Response Letter to reviewers. I would invite the Authors to try if a carbonate-free surface (by putting the material under inert dry atmosphere, just after the calcination step) can provide a better adsorption performance than the previously tested materials. A more comprehensive explanation of the surface species, as mentioned in Reviewer’s points 3,4 and 8, should be also added.
13) At the surface of the La2O3 nanorods, La-OH2+ moieties will predominate under acid conditions, whereas La-O- moieties will predominate under basic conditions. It is really unlikely that both of them will be present at the same pH value. In addition, the O=La-O-La=O formula reported by ChemSpider is a pure formality and it should exist only in vapour phase (under extreme conditions). Under ordinary conditions, La2O3 has an A-M2O3 hexagonal crystal structure, with La3+ ions surrounded by six O2− ions. Only at the boundary of the crystal, terminal La-OH (either protonated or deprotonated) are present. Moreover, it is unlikely that doubly negatively charged arsenate species can approach La-O- moieties via electrostatic interactions as depicted in the bottom section of Fig. 6. In such a situation, arsenate anions would be repelled by the surface negative species. Please, take into careful account all these points in the revision of Fig. 6 and of the related discussion.
After these points are all thoroughly covered, the submission could be positively evaluated for publication.
Author Response
Responses for Reviewers Comments in Round 2
We appreciate the valuable comments of the all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with text with red color text in the revised manuscript.
Reviewer 2:
The Authors have addressed some of the critical points. However, some important flaws were not satisfactorily covered. In detail:
1) I agree with the Authors that the As(V) removal mainly depends on the availability of surface functional groups. However, the higher the specific surface area, the higher the number of exposed sites and therefore of surface functional group. So, the role of specific surface area cannot be underestimated in the path towards the optimization of these materials. If the Authors agree, a note on this could be welcome.
Responses: We are very much thankful for the reviewer for giving valuable comment. We agree with reviewers regarding surface area which can be also alter the adsorption capacity along with surface functional groups. But in this present case the adsorption capacity is high even at low surface area which is mainly due to the surface functional groups.
3, 4 and 8) The nature of the chemical species at the surface of the nanorods is a key point and should not be underestimated. It is affirmed that the Authors “provided SEM-EDX results at Fig.S1”, but in the Supplementary Files only the PZC graph can be found at Fig. S1. Then, the presence of HCl, under acid conditions, can give rise to LaCl3 through the attach of HCl onto La2O3 and surface chloride species are likely to be formed. In addition, the presence and the characterization of carbonate species, which showed to possess a detrimental role in the As(V) adsorption process, should be considered in the present work, rather than “in a future work”, as stated by the Authors in the Response Letter to reviewers. I would invite the Authors to try if a carbonate-free surface (by putting the material under inert dry atmosphere, just after the calcination step) can provide a better adsorption performance than the previously tested materials. A more comprehensive explanation of the surface species, as mentioned in Reviewer’s points 3,4 and 8, should be also added.
Responses: Comment 3 & 4. We are very sorry in the attached file the SEM-EDX was missed. We provide the SEM-EDX of present material in the revised supplementary file that shows clearly the of traces carbonate along with La2O3. As you comment is absolutely right that the presence of atmospheric CO2 caused for traces of CO2 which are authenticated with the previous reports [34, 35]. This details included at revised manuscript 136-139. The HR-TEM of HCl-interact lanthanum oxide, we did not analyzed due to lack of availability of instrument in the short time of reviewing.
Figure S1. SEM-EDX spectrum of La2O3 nanorods.
Response to Comment 8: Thank you for good comment and constructive suggestion. We agree with reviewers comments and it’s a good suggestion. But at this moment, we are not able prepare and characterize La2O3 at inert conditions due to short period of revision schedule as well as considering the length of article. We are sorry for this, we must consider it in our future study and make as new material at new conditions in the considering present observations. Moreover, at the present conditions, the prepared material are discussed as it.
13) At the surface of the La2O3 nanorods, La-OH2+ moieties will predominate under acid conditions, whereas La-O- moieties will predominate under basic conditions. It is really unlikely that both of them will be present at the same pH value. In addition, the O=La-O-La=O formula reported by ChemSpider is a pure formality and it should exist only in vapour phase (under extreme conditions). Under ordinary conditions, La2O3 has an A-M2O3 hexagonal crystal structure, with La3+ ions surrounded by six O2− ions. Only at the boundary of the crystal, terminal La-OH (either protonated or deprotonated) are present. Moreover, it is unlikely that doubly negatively charged arsenate species can approach La-O- moieties via electrostatic interactions as depicted in the bottom section of Fig. 6. In such a situation, arsenate anions would be repelled by the surface negative species. Please, take into careful account all these points in the revision of Fig. 6 and of the related discussion. After these points are all thoroughly covered, the submission could be positively evaluated for publication.
Responses: We agree with reviewer and modified the proposed mechanism representation at Fig.6 as per discussion in text at section 3.5, to avoid the misinterpretation of readers. For your reference, I am here with providing the same Figure 6 and details as “The overall results adsorption process suggests that the electrostatic attraction of arsenic species through protonated surface of La2O3 and surface complexation are predominate at acidic conditions. While electrostatic repulsion and surface complexation of arsenic species with La2O3 surface are predominate at basic conditions. Those representations are clearly showed at Figure 6 for better understanding.”
Author Response File: Author Response.pdf
Reviewer 3 Report
As the Authors addressed most of my remarks I do not have the reasons for advising against its publication. Therefore I recommend the manuscript for further evaluation.
One small remark:
Line 27 – 19.31 nm. The precision is way to high. The Authors should at least evaluate the standard deviation for several measurement of the same sample to have a feeling of accuracy. In this range +-0.01 degree uncertainty results +-0.5 nm of accuracy. Therefore it should be rounded at least to 19.3 nm.
Author Response
Responses for Reviewers Comments in Round 2
We appreciate the valuable comments of the all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with text with red color text in the revised manuscript.
Reviewer 3:
Line 127 – 19.31 nm. The precision is way to high. The Authors should at least evaluate the standard deviation for several measurement of the same sample to have a feeling of accuracy. In this range +-0.01 degree uncertainty results +-0.5 nm of accuracy. Therefore it should be rounded at least to 19.3 nm.
Responses: Thank you for your constructive comment. We agree with reviewer comment, we rectified it in the revised manuscript.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
Since surface carbonate species have so an important role in influencing (and worsening) the adsorption/removal capabilities, I would find an additional test carried out under conditions that do not lead to surface carbonation as an essential point of the revision. Otherwise, all the results in the present manuscript might be biased by the (uncontrolled) exposure of the freshly calcined and synthesized La2O3 material to atmospheric CO2 (therefore, longer exposure, longer carbonation, poorer performance, and so on...).
I agree with the Authors that this additional needs some time. However, it is typically possble to ask for an extension of the deadline when additional time for further experimental evidence collection is needed.
Author Response
We appreciate the valuable comments of all reviewers on our manuscript. The followings are the explanations presented in reply to each reviewers’ comment. The critical comments and useful suggestions have been helped us to improve our paper considerably. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with red color text in the revised manuscript.
Reviewer 2:
Since surface carbonate species have so an important role in influencing (and worsening) the adsorption/removal capabilities, I would find an additional test carried out under conditions that do not lead to surface carbonation as an essential point of the revision. Otherwise, all the results in the present manuscript might be biased by the (uncontrolled) exposure of the freshly calcined and synthesized La2O3 material to atmospheric CO2 (therefore, longer exposure, longer carbonation, poorer performance, and so on...).
Response to Comment: Thank you for your good comment and constructive suggestion. We agree with reviewers' comments and it’s a good suggestion. We provide the SEM-EDX of N2 atmospheric La2O3 material in the revised supplementary file where we found that the absence of carbon peak which is responsible for (CO3). That means the surface of prepared La2O3 is the absence of Carbonates. We also tested the adsorption capacity of La2O3-without Carbonate for As(V) and was found that be 272.12 mg/g. The details are also included in the revised manuscript Lines 137-142, 226-228, and 248-250.
For details see Figure. La2O3 nanorods with N2 atmosphere (Absence of carbonate surface).
Author Response File: Author Response.pdf
Round 4
Reviewer 2 Report
I thank the Authors, as they have carried out the additional tests required by the Reviewers. The additional results shows that the As(V) adsorption capacity can be event slightly increased, by carefully removing surface carbonate species.
The manuscript can now be positively accepted for publication.
Only, some revision of the English language is necessary, especially for the newly added sentences in red.
For instance, lines 248-250:
"the results of the adsorption capacity of La2O3, which is prepared at N2 atmosphere has been confirmed the carbonate surface is predominately influenced the adsorptive removal of La2O3 for As(V) (Fig.S1 (b))."
could be changed into:
"the results of the adsorption capacity of La2O3, prepared under N2 atmosphere, confirmed that surface carbonate species predominately influence the removal of As(V) by adsorption (Fig.S1 (b))."
Author Response
Comment:
The manuscript can now be positively accepted for publication.
Only, some revision of the English language is necessary, especially for the newly added sentences in red.
For instance, lines 248-250:
"The results of the adsorption capacity of La2O3, which is prepared at N2 atmosphere has been confirmed the carbonate surface is predominately influenced the adsorptive removal of La2O3 for As(V) (Fig.S1 (b))."
could be changed into:
"The results of the adsorption capacity of La2O3, prepared under N2 atmosphere, confirmed that surface carbonate species predominately influence the removal of As(V) by adsorption (Fig.S1 (b))."
Response: We are very much thankful to the reviewers for careful observation and suggestions. As indicated in the reply’s that follow, we have taken these comments and suggestions into account in the revised version of our manuscript and marked with red color text in the revised manuscript at Lines, 247-249.
