Oxidized and Non-Oxidized Multiwalled Carbon Nanotubes as Materials for Adsorption of Lanthanum(III) Aqueous Solutions

Round 1

Reviewer 1 Report

The authors studied the adsorption of lanthanum from aqueous synthetic solutions on multiwalled carbon nanotubes. The topic is of great interest for the readers due the rare earths are critical materials and are used in serveral industrial sectors.

The manuscript is well-arranged and the quality of paper is good. Anyway the following a list of suggestions and questions.

• The introduction can be improved by adding more in detail from which industrial applications can be produced lanthanum solutions. In addition I suggest to insert more citations regarding lanthanum adsorption and to explain better those that you already reported.
• According to which criteria were choosen the investigated initial lanthanum concentrations?
• A statement regarding the possibility of using this technology in real solutions in which in addition to lanthanum also other rare earths could be present, the authour think that the adsorption could be selective for lanthanum recovery? 
• Regarding the regeneration of saturated material, the authors tried to reuse for new tests the regenerated materials? I think that this paper could be useful to explain better the mechanism of regeneration (doi: 10.3303/CET1647037)

Author Response

• The introduction can be improved by adding more in detail from which industrial applications can be produced lanthanum solutions. In addition I suggest to insert more citations regarding lanthanum adsorption and to explain better those that you already reported.

The introduction has been modified following the reviewer's criteria.

• According to which criteria were choosen the investigated initial lanthanum concentrations?

The range of lanthanum concentrations, used in the present work, was chosen in accordance with the range of metals concentrations which normally we used in our investigations on metals adsorption using carbon nanotubes or activated carbons produced by various sources.   

• A statement regarding the possibility of using this technology in real solutions in which in addition to lanthanum also other rare earths could be present, the authour think that the adsorption could be selective for lanthanum recovery? 

Selectivity using non-oxidized MWCNTs must be difficult to asses, since they do not contain any functionality to help to this selectivity. In the case of ox-MWCNTs , and since they contained functionalized (carboxylic) groups, they selectivity, on the adsorption of the REE, might be improve; this improvement is based in the variation of the pH of the solution. However, with both oxidized and non-oxidized materials, exists the possibility of perform the adsorption of the REE without any control, i.e. the REE are adsorbed freely, and then performed a selective elution step of the REE involved in the process. In any case, and to fix the conditions for any case, the above must be experimentally proved.

• Regarding the regeneration of saturated material, the authors tried to reuse for new tests the regenerated materials? I think that this paper could be useful to explain better the mechanism of regeneration (doi: 10.3303/CET1647037)

In the case of the ox-MWCNTs, they are regenerated because of the use, as eluent, of an acidic solution which shifted to the left the equilibrium shown in eq,(6) of the manuscript, regenerating the functional group. In the case of non-oxidized MWCNTs and since we used mild acid conditions, the adsorption sites must be regenerated also.

Prior to further use in another adsorption step, both types of carbons, must be wased with water to eliminate residual acidity moisture contained in them.

Please note that we include the next paragraph in the revised manuscript (Section 3.3.):

In the case of non-oxidized MWCNT, probably, the use of a mild acidic solution activates the adsorption sites present in the carbon matrix, without destroying this matrix, though different behaviour between initial and eluted carbon material might be observed under continuous use, i.e. columns. (Ippolito, N.M., Maffei, G., Medici, F., Piga, L. Adsorption and regeneration of fluoride ion on a high alumina content bauxite. Chem. Eng. Trans. (2016), 47, 217-222. DOI: 10.3303/CET1647037 

Reviewer 2 Report

Dear Authors

three major comments to your manuscript:

1) more should be written witin the introduction concerning commercial REE & La recovey, for example from Ni-MH, some SoA technologies should be shortly given, as well as types of resins, which are typically used for lanthanum sorption from recycled solutions. there is a lot of references in your manuscript but given without comments.

2) at the end you gave information about elution, the data of CNT and oxCNT after elution should be given (for example BET, active species), please define whether material can be directly re-used or reoxidation of oxCNT is required.

3) I can see too many auto-citations of F.J. Alguacil (10 if well remember)!!!!!

within minor changes:

Z potential should be canged into Zeta potential or use Greek Z letter here

page 2: "The concentration of activated carbon..." where it comes from?

Author Response

1) more should be written witin the introduction concerning commercial REE & La recovey, for example from Ni-MH, some SoA technologies should be shortly given, as well as types of resins, which are typically used for lanthanum sorption from recycled solutions. there is a lot of references in your manuscript but given without comments.

We have carried out a review of the most relevant literature on the recycling of Ni-mH batteries and a paragraph has been included in the corrected manuscript as well as the citations that we have considered most important

2) at the end you gave information about elution, the data of CNT and oxCNT after elution should be given (for example BET, active species), please define whether material can be directly re-used or reoxidation of oxCNT is required

In the present investigation, we did not perform these studies on the eluted carbon nanotubes. As you can read in subsection 3.3.Elution, we add a paragraph about that you mentioned. Please note that in the case of ox-MWCNTs, the active groups are regenerated, via the shift to the left of eq.(6), as a consequence of the elution process, thus, re-oxidation is not necessary.

3) I can see too many auto-citations of F.J. Alguacil (10 if well remember)!!!!!

All the references included in the manuscript are directly related to its content. Our Research Group has a long history and experience in this field

4) Z potential should be changed into Zeta potential or use Greek Z letter here

the error has been modified in the corrected manuscript

5) page 2: "The concentration of activated carbon..." where it comes from?

The concentration of activated carbon: Please note that the use of “activated” was an error by us, we changed the paragraph to:

On both adsorbents, Zeta potential was measured using a Zetasizer Malvern Nano ZS (Malvern, UK) at 25º C.. Aqueous suspensions were prepared at pH values between 1 and 13 using solutions of HCl and NaOH, whereas the carbon dosage was adjusted to 0.1 g L^-1. The suspensions were dispersed with a Bandelin Electronic Sonopuls HD 3100 sonicator (Bandelin, Berlín, Germany), using amplitudes of 60% for 150 s.

Round 2

Reviewer 2 Report

Thanks for your corrections, especially in the introduction part. Now it's much better to read it.

I have still some allegations concerning too many self-citations, which is not an error rather the ethical problem in science. No matter how good the research team is. However, if it's accepatble by MDPi I can congrat well done paper.

Author Response

Dear Reviewer,
We have deleted 5 references from the co-author Alguacil et al. in the corrected manuscript. They were very specific references that we cannot replace with others. I hope that in this way the possible ethical problems disappear. Thank you very much for your help and collaboration.