Adsorption and Desorption Mechanisms of Rare Earth Elements (REEs) by Layered Double Hydroxide (LDH) Modified with Chelating Agents

Round 1

Reviewer 1 Report

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Job review: Adsorption and desorption mechanisms of rare earth 2 elements (REEs) by layered double hydroxide (LDH) 3 modified with chelating agents

I have read the work and I have the following comments:

It was not written how lanthanides were determined in the aqueous phase Quantum calculations of forming complexes are not complete. The structure of the complexes being formed is not given, i.e. the length of the bonds, angles, etc. The paper talks about the potential of zeta, but there are no drawings about the potential of zeta Literature is very poor, there are no recent publications on sorption of lanthanides

Author Response

Thank you for your suggestion. According to your comment, I clearly know that there are still many shortcomings in the manuscript. We checked the manuscript again and modified some parts which you point out.

 

Comment: It was not written how lanthanides were determined in the aqueous phase.

Response: Thank you for your comment. The suspension containing the adsorbent and each of the above metallic solution was filtered through a 0.45 μm membrane filter to remove each metallic ion that have been adsorbed into the adsorbent. Then, The amount of Ln(III) ions in the solutions were determined by inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The description is added in the revised manuscript (Section 2.3, Line 120-122 in the revised manuscript, page 3).

 

Comment: Quantum calculations of forming complexes are not complete. The structure of the complexes being formed is not given, i.e. the length of the bonds, angles, etc.

Response: Thank you for your comment. Because the structures of the complexes themselves and those of the complexes adsorbed on hydrotalcites are not necessarily essential, the structural (in particular, the bond lengths) data and discussions on them are shown in Supplementary Information (Figure S3 and Table S1-S4).

 

Comment: The paper talks about the potential of zeta, but there are no drawings about the potential of zeta

Response: Thank you for your suggestion. Based on your comment, the results of zeta potential including numerical values were denoted in the revised manuscript (page 5 including Table 1 and Figure 2).

 

Comment: Literature is very poor, there are no recent publications on sorption of lanthanides.

Response: Thanks very much for the valuable comments. As suggested by the reviewer, we modified the Introduction. Some recent literatures on sorption of lanthanides were inserted in the revised manuscript. For instance, Colloid. Surface A 2019, 570, 127-140 [1]; Chem. Eng. J. 2018, 347: 398-406 [5]; Chem. Geol. 2019, 525: 210-217 [6]; Chem. Eng. J. 2018, 341, 75-82 [10] were inserted in our revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

As regards the concept of the work: in which aqueous solution La and Eu can be commonly found? Generally they are solubilized at low pH from solid residues: but a lot of other ions can be solubilized as well.  So, when applying this technology to real wastewater containing La and Eu, considering their low concentration with respect to other ions, can competitive effects hinder process application?

Figure 1: at what pH the tests figure 1 reports have been carried out?

Line 134: positive value (specify: of zeta potential)

Did the authors observe any release of compounds (elements) from the adsorbent? A SEM or RX analysis after adsorption could be useful to assess process mechanism.

Regeneration: 20 mg adsorbent require 30 mL acid. It results from a preliminary optimization study?

Author Response

Thank you for your suggestion. According to your comment, I clearly know that there are still many shortcomings in the manuscript. We checked the manuscript again and modified some parts which you point out.

 

Comment: As regards the concept of the work: in which aqueous solution La and Eu can be commonly found? Generally they are solubilized at low pH from solid residues: but a lot of other ions can be solubilized as well. So, when applying this technology to real wastewater containing La and Eu, considering their low concentration with respect to other ions, can competitive effects hinder process application?

Response: Thank you for your suggestion. For your first comment, Rare earth elements ions are generally found in rare earth refining industrial wastewater. It is well known that there are more common ions (i.e., Na⁺, K⁺, Ca²⁺, Mg²⁺ etc.) than REEs ions in natural water. In our previous study [8], adsorption experiments of lanthanides were carried out under the presence of common ions. The remarkable decrease of sorption capacity of lanthanides was not observed (as shown in Figure S1 in Supplementary Information). Particularly, chelating agents such as EDTA were used in this work. We judged that the inhibition from common ions on the adsorption of lanthanides may be small, although the kind of adsorbent is different. However it is important, and adsorption experiments of lanthanides under the presence of common ions will be performed in future manuscript as needed.

On the other hand, several kinds of REEs ions may coexist together to a large extent. Therefore, we did the adsorption experiment both in the single La(III) (or Eu(III)) ions and in multiple REEs (16 elements) simulated wastewater solutions, and the results are shown in Figure S2 in Supplementary Information. Different adsorption capacities between La(III) (or Eu(III)) ion and other REEs ions were observed. This may be attributed to their stability constants (The stability constants of La-EDTA and Eu-EDTA were 15.5 and 17.4 respectively).

 

Comment: Figure 1: at what pH the tests figure 1 reports have been carried out?

Response: Thanks very much for your comment. The adsorption experiments were carried out at pH 4 which was obtained from the effect of pH at the batch adsorption experiments stage. The description is added in the revised manuscript (Section 3.1, Line 170 in the revised manuscript, page 4).

 

Comment: Line 134: positive value (specify: of zeta potential)

Response: Thanks very much for your comment. The zeta potential of L1 and L2 at pH 4 are 35.3 mV and 20.2 mV respectively. The description is added in the revised manuscript (Section 3.1, Line 185 in the revised manuscript, page 5).

 

Comment: Did the authors observe any release of compounds (elements) from the adsorbent? A SEM or RX analysis after adsorption could be useful to assess process mechanism.

Response: Thank you for your suggestion. According to your comment, we added the XRD, FT-IR and SEM-EDS results of L1 and L2 before Ln(III) adsorption and L2 after the Ln(III) adsorption as Section 3.2 in the revised manuscript. The evolution of the samples along the exchange reaction were observed from the adding part of SEM image. We also added the elemental mapping results of L2-Eu in Figure 3 (e) to obtain the composition of the samples (Section 3.2, Figure 3 in the revised manuscript, page 6). Moreover, the XRD pattern and FT-IR results of L1, L2 or L2-Eu(III) were shown in Figure 4 (page 6), and Figure 5 (page 7) in the revised manuscript. Results and discussion for these characterization has also been denoted in Section 3.2 in the revised manuscript (page 5-7).

 

Comment: Regeneration: 20 mg adsorbent require 30 mL acid. It results from a preliminary optimization study?

Response: Thank you for your suggestion. As the reviewer mentioned, 20 mg adsorbent required 30 mL acid solution in our regeneration study. The value is obtained from the former optimization study as you mentioned.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript entitled “Adsorption and desorption mechanisms of rare earth elements (REEs) by layered double hydroxide (LDH) modified with chelating agents” aims the LDH modification through EDTA to improve the Ln(III) adsorption.

Beyond the idea is to use EDTA to improve the Ln(III) adsorption is clever, the experimental information provided by the authors is really scarce.

Hence, I don’t find the manuscript suitable to be published in Applied Science.

 

Comments:

The introduction is very poor.

References about LDH in general and, moreover of exchange reaction in LDH are required. Comparison with state-of-the-art or the last approaches are missed. For instance: J. Am. Chem. Soc. 2019, 141, 531−540

 

There is any information about the characterization of the LDH and LDH-EDTA phases (L1 and L2, respectively). For instance: PXRD, FT-IR, TGA, elemental analysis. These experimental characterizations will provide very important information such as:

-Crystallinity and phase of the samples.

-Interlayer distance of the L1 and L2 samples before and after the Ln(III) adsorption.

-Evolution of the samples along the exchange reaction.

-Composition of the samples in term of the amount of EDTA is L2 samples. Especially, there is not information of the intercalated anion in L1 sample.

 

Moreover, no information (“Materials and Methods” or “Results and Discussion”) about the determination of Ln(III) in the solutions and the Z-potential measurements are available.

 

In “2.5 Calculation of adsorption and desorption rates”, there is any description of each variable in the equations. The reading process of the manuscript is quite complicated. Most of the details are no present (They are in other references).

 

The exchange reactions were carried out at pH=4. At this pH the LDH phase is not stable. The pH value of MgAl LDH is equilibrium with water is around pH=8 (Chem. Mater. 1999, 11, 298-302). Probably, at pH=4 the solid phase is Al(OH)3 instead of LDH (Journal of Colloid and Interface Science 57(1) (1976); Journal of Colloid and Interface Science 51(3), 449{458 (1975); Coordination Chemistry Reviews 248(5-6), 441-455 (2004)). PXRD measurement can provide information about the identity of the solid samples. In addition, PXRD measurement can clarify the affirmation given at lines 159-161: “The hydrogen ions may destroy the structure during the desorption process, which results in the decrease of the removal efficiency after the fourth or fifth cycle”.

Beyond they claimed the adsorption, it is important to define in which kind of solid phase the reaction takes places in order to compare with simulation.

 

In the case of “Quantum Chemistry Calculation” there is any description about how the calculation were made. No information about the structure optimisation in both molecular cluster and complexes are presented.

The authors compare the stability of the molecular cluster with Eu(H₂O)₈³⁺ and [Eu(EDTA)(H₂O)]^- (Figure 4).

 

There are some questions which could be interesting evaluate:

Evaluation of the interaction of the molecular cluster with [Ln(III)(EDTA)(H₂O)]^- in order to compare with the experimental results. Evaluation of the molecular cluster with EDTA with Ln(H₂O)₈³⁺. In this case the comparison should be more close to the real experiment.

Author Response

Thank you for your suggestion. According to your comment, I clearly know that there are still many shortcomings in the manuscript. We checked the manuscript again and modified some parts which you point out.

 

Comment: The introduction is very poor. References about LDH in general and, moreover of exchange reaction in LDH are required. Comparison with state-of-the-art or the last approaches are missed. For instance: J. Am. Chem. Soc. 2019, 141, 531−540

Response: Thanks very much for the valuable comments. As suggested by the reviewer, we modified the introduction. Some state-of-the-art literatures on sorption of lanthanides were inserted in the revised manuscript. For instance, Colloid. Surface A 2019, 570, 127-140 [1]; Chem. Eng. J. 2018, 347: 398-406 [5]; Chem. Geol. 2019, 525: 210-217 [6]; Chem. Eng. J. 2018, 341, 75-82 [10]; J. Am. Chem. Soc. 2019, 141(1), 531-540 [11] ;Chem. Commun. 2019, 55, 7824-7827 [12]; Appl. Clay Sci. 2019, 180, 105193 [13]; Chem. Eng. J. 2015, 279, 597-640 [14] were inserted in our revised manuscript.

 

Comment: There is any information about the characterization of the LDH and LDH-EDTA phases (L1 and L2, respectively). For instance: PXRD, FT-IR, TGA, elemental analysis. These experimental characterizations will provide very important information such as: -Crystallinity and phase of the samples. -Interlayer distance of the L1 and L2 samples before and after the Ln(III) adsorption. -Evolution of the samples along the exchange reaction. -Composition of the samples in term of the amount of EDTA is L2 samples. Especially, there is not information of the intercalated anion in L1 sample.

Response: Thank you for your suggestion. According to your comment, we added the XRD, FT-IR and SEM-EDS results of L1 and L2 before Ln(III) adsorption and L2 after the Ln(III) adsorption as Section 3.2 in the revised manuscript. The evolution of the samples along the exchange reaction were observed from the adding part of SEM image. We also added the elemental mapping results of L2-Eu in Figure 3 (e) to obtain the composition of the samples (Section 3.2, Figure 3 in the revised manuscript, page 6). Moreover, the XRD pattern and FT-IR results of L1, L2 or L2-Eu(III) were shown in Figure 4 (page 6), and Figure 5 (page 7) in the revised manuscript. Results and discussion for these characterization has also been denoted in Section 3.2 in the revised manuscript (page 5-7).

 

Comment: there is not information of the intercalated anion in L1 sample.

Response: The intercalated anion of L1 is NO₃^-. The description is added in the revised manuscript (Section 2.1, Line 102 in the revised manuscript, page 3). The ability of exchange with layers anions mostly depends on the number of charges they carry, and the order in which they are easily exchanged is NO₃^-＞OH^-＞F^-＞Cl^-＞Br^-＞I^-＞CO₃ ^2-.

 

Comment: Moreover, no information (“Materials and Methods” or “Results and Discussion”) about the determination of Ln(III) in the solutions and the Z-potential measurements are available.

Response: The amount of Ln(III) ions in the solutions were determined by inductively coupled plasma atomic emission spectrophotometer (ICP-AES) (Seiko Instrument Inc., SPS 1500). The description is added in the revised manuscript (Section 2.3, Line 120-122 in the revised manuscript, page 3). The Z-potential was measured by Electrophoretic light scattering method (Otsuka, ELSZ-2000ZS). The description is added in the revised manuscript (Section 2.2, Line 111-113 in the revised manuscript, page 3).

 

Comment: In “2.5 Calculation of adsorption and desorption rates”, there is any description of each variable in the equations. The reading process of the manuscript is quite complicated. Most of the details are no present (They are in other references).

Response: Thank you for the invaluable comment. In the revised manuscript, we have described the definitions of the variables and the details on calculation procedures to ease the reading process as shown in Section 2.6 (page 4).

 

Comment: The exchange reactions were carried out at pH=4. At this pH the LDH phase is not stable. The pH value of MgAl LDH is equilibrium with water is around pH=8 (Chem. Mater. 1999, 11, 298-302). Probably, at pH=4 the solid phase is Al(OH)3 instead of LDH (Journal of Colloid and Interface Science 57(1) (1976); Journal of Colloid and Interface Science 51(3), 449-458 (1975); Coordination Chemistry Reviews 248(5-6), 441-455 (2004)). PXRD measurement can provide information about the identity of the solid samples. In addition, PXRD measurement can clarify the affirmation given at lines 159-161: “The hydrogen ions may destroy the structure during the desorption process, which results in the decrease of the removal efficiency after the fourth or fifth cycle”. Beyond they claimed the adsorption, it is important to define in which kind of solid phase the reaction takes places in order to compare with simulation.

Response: Thank you for your comment. According to the comment, the XRD pattern of (a) L1, (b) L2 and (c) L2-Eu were shown in Figure 4 in the revised manuscript (page 6). The FT-IR results of L1, L2 and L2-Eu were also shown in Figure 5 in the manuscript (page 7). Moreover, results and discussion for these characterization has been denoted in Section 3.2 in the revised manuscript (page 6-7).

 

Comment: In the case of “Quantum Chemistry Calculation” there is any description about how the calculation were made. No information about the structure optimization in both molecular cluster and complexes are presented. The authors compare the stability of the molecular cluster with Eu(H₂O)₈³⁺ and [Eu(EDTA)(H₂O)]- (Figure 4).

Response: Thank you for your comment. We have added the sentence, “The interlayer distance was kept 9.3 Å to imitate the experimentally observed one [28]” (Section 2.5, Line 139-140 in the revised manuscript, page 4) to give a more detailed information on the calculation method. Because the structures of the complexes themselves and those of the complexes adsorbed on hydrotalcites are not necessarily essential, the structural (in particular, the bond lengths) data and discussions on them are shown in Supplementary Information (Figure S3 and Table S1-S4).

 

Comment: There are some questions which could be interesting evaluate: Evaluation of the interaction of the molecular cluster with [Ln(III)(EDTA)(H₂O)]- in order to compare with the experimental results. Evaluation of the molecular cluster with EDTA with Ln(H₂O)₈³⁺. In this case the comparison should be more close to the real experiment.

Response: Thank you for your invaluable suggestions. We are willing to investigate them in our upcoming works. Although they will be interesting, we will not show them in the present manuscript because they are not the focus in this work.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I accept the manuscript in the present form.

Author Response

Comment: I accept the manuscript in the present form.

Response: Thank you for your comment. We wish to express our appreciation for your judgement. We were encouraged.

Reviewer 2 Report

The paper has been substantially improved after authors' revision.

Author Response

Comment: The paper has been substantially improved after authors' revision.

Response: Thank you for your comment. We wish to express our appreciation for your judgement. We were encouraged.

Reviewer 3 Report

The author of the manuscript entitled “Adsorption and desorption mechanisms of rare earth elements (REEs) by layered double hydroxide (LDH) modified with chelating agents” have provided new experimental information such as PXRD, FT-IR, SEM-EDS to support they results. However, no information about the Zn:Al ratio, nitrate and EDTA, and Ln amount in the LDH phase are present.

Experimental details are still not presented.

Some section in the manuscript must be carefully revised.

 

Hence, I don’t find the manuscript suitable to be published in Applied Science at the moment.

 

Comments:

In the abstract the authors claimed:

“the calculation results of adsorption and desorption rates show that adsorption rates are larger for Eu(III) than for La(III), which agrees with the experimental result that Eu(III) has a higher adsorption ability under the same conditions”

However, no explanation about this fact is presented.

In the case of anion-exchange reaction different physicochemical models were developed to rationalize the anion-exchange constants (Langmuir 2014, 30, 8408−8415, Dalton Trans., 2014,43, 11587-11596, among others).

And also:

“The LDHs synthesized in this work have a high affinity for removing REEs ions”

However, the authors did not provide the retention amount of Ln(III) in the LDH (e.g.: mmol/mg), and neither compared with the other materials described in the literature (e.g.: seaweed biomasses, and shell biomass, carbon).

 

 

2.1 Synthesis of the adsorbent

Any description of the synthesis is presented: concentration, ratio Zn:Al, reaction volume, time temperature. Neither in the case of the anion-exchange reaction.

The reference 18, provided by the authors any explanation regarding to the concentration ratio of NaOH/NaNO₃ and the anion exchange reaction is presented.

 

 

2.3 Adsorption experiments using L1 and L2 as the adsorbents

The authors employed the same amount of mg with both samples, L1 and L2. However, the molecular formula of L1 and L2 need to be different, due to the molecular mass of EDTA and NO3 is markedly different (290 g/mol vs 62 g/mol, respectively).

This will affect the amount of LDH, in terms of mmol, in the suspension. The anion exchange will depend on the amount of positive charges, Al(III) mmol.

The authors should provide Elemental Analysis to quantify the amount of nitrate, and EDTA (before and after the adsorption experiments) in order to provide a molecular formula of the solid phases.

Does this information match with the Zn:Al ratio? Which is the amount of Ln absorbed?

E.g.: Zn:Al ratio, EDTA:Al ratio, EDTA:Ln ratio

 

 

2.5 Quantum chemistry calculation

The authors described the chemical structure by a Mg₁₂Al₇(OH)₃₆⁹⁺ cluster.

In the case in which Mg is replaced by Zn, is there any change to point out?

During the simulation the authors kept the interlayer distance in 0.93 nm, however in the case on L2 and L2(Eu) the interlayer distance increases until 1.47 nm (more than 55%).

The interaction between the charged positively layers and the anions depend on the distance between them (Barrer and Townsend - J. Chem. Soc., Faraday Trans. 1 1976, 72, 2650−2660 and J. Chem. Soc., Faraday Trans. 1 1972, 68, 1956−1963. -, or Eisenman- Biophys. J. 1962, 2, 259−323 - models based on clays).

Have the authors evaluated the role of the interlayer distance in the relative energies?

An interlayer distance of 0.93 nm could be unrealistic for the L2 samples.

 

The information from the structure relaxation could be interesting, specially to evaluate the typical metal to metal distances and the charge density.

 

Comment:

The affinity between LDH layers and anions depends on charge and size. The correlation provided by the authors is not correct. More information could be found in: Applied Clay Science 54 (2011) 132–137

 

 

Comment:

Line 144: Should C_Cu be C_Ln?

Line 148: B, D, M/(AV) are no defined. Are these constant related with some physicochemical parameter or are they dimiensionlees?

 

Comment:

A solution containing 200 mmol/L of HCl or HNO₃ presents a pH lower than 1. At this pH both cations Zn and Al are soluble, also the LDH phase (Applied Clay Science 51 (2011) 366–369).

From my personal point, the regeneration process is a dissolution process. For that reason, the Recovery efficiency decrease.

A regeneration process could be an anion-exchange reaction employing a anion with high affinity by the interlayer space (Applied Clay Science 54 (2011) 132–137).

 

Lines 190-192: “Therefore, the intercalation of EDTA (i.e., LDH-EDTA) leads to a lower zeta potential, which leads to the adsorption ability of REEs ions (i.e., cation, having positive charge) larger than LDH.”

Does the adsorption of Ln take place by the lower zeta potential of the surface or by Ln-EDTA complex formation?

This tense must be reviewed

 

At the same time, information of the EDTA amount, and the Ln amount in the final phase is require to analyse the process’s nature: absorption vs complex formation and intercalation.

 

Lines 211-212: The tense “These figures show that the layer structure was destroyed by the intercalating process for neither EDTA nor EDTA-Ln(III)” is confusing. Figure 4 depicts LDH structure.

 

Conclusions section:

Form my personal point of view the authors should make explicit they have worked with ZnAl LDH. The phrase “The current study discusses layered double hydroxides (LDHs) modified with EDTA” is too ambitious.

 

The author claimed:

“The intercalation of EDTA (i.e., L2) exhibited a lower zeta potential, which indicates that the intercalation leads to the larger adsorption capability of REEs ions. Moreover, quantum chemistry calculations were performed using the GAUSSIAN09 program package. In the calculations, the molecular locally stable state structures were optimized by density functional theory (DFT).”

This tense, repeated in the manuscript, suggest physisorption instead of complex formation in the interlayer space. The authors provide DFT calculation to explain the experiments (second option). The phrase is confusing and could cause misunderstandings.

 

Minor comments:

-It could be better if the authors indicate that they have worked with a ZnAl LDH.

-In materials and method EDTA is not mentioned.

-Speciation diagram of EDTA in order to provide information about the moieties present in the LDH. At pH=4.0, EDTA should by EDTA^2-.

-Lines 211: “Figures 4 (b) and (c), These”: dot or comma?

Lines 211: “Figures 4 (b) and (c), These”: dot or comma?

Lines 216-218: Typically, the bibliography use interbasal space, interlayer distance instead of “gallery height”. The use of this phrase could be confusing.

Author Response

Comment: The author of the manuscript entitled “Adsorption and desorption mechanisms of rare earth elements (REEs) by layered double hydroxide (LDH) modified with chelating agents” have provided new experimental information such as PXRD, FT-IR, SEM-EDS to support they results. However, no information about the Zn:Al ratio, nitrate and EDTA, and Ln amount in the LDH phase are present.

Response: Thank you for your comment. We have dissolved the sample and measured the concentration of Zn, Al and Ln ions by an ICP-MS (Agilent HP4500, USA). The operating conditions of ICP-MS was added as Table 1 in the re-revised manuscript (Section2.3, page 4). Moreover, the elemental chemical analyses of C, H and N in LDHs were carried out using an elemental analyzer instrument (JMC10, J-SCIECE LAB CO., Ltd.). These component analysis results were added as shown in Table 3 (Section 3.2, page 6) in the re-revised manuscript.

 

Comment: In the abstract the authors claimed:“the calculation results of adsorption and desorption rates show that adsorption rates are larger for Eu(III) than for La(III), which agrees with the experimental result that Eu(III) has a higher adsorption ability under the same conditions” However, no explanation about this fact is presented.

Response: Thank you for the invaluable comment. As we claimed in the manuscript, our works have been mainly focused on the potential application of materials for removal of lanthanides from wastewater. Along with some pieces of our experimental information published in our other papers (Refs. 20 and 21), we expect that the results of calculation and analysis shown in this manuscript might further clarify the adsorption experiments. In this re-submission, some analyses of the experimental results are added in the Supplementary information (Figure S1-S8 and Table S1-S6), although we added some results in the revised manuscript last time.

.

 

Comment: In the case of anion-exchange reaction different physicochemical models were developed to rationalize the anion-exchange constants (Langmuir 2014, 30, 8408−8415, Dalton Trans., 2014,43, 11587-11596, among others).

Response: Thank you for your comment. According to the suggestion, we have introduced these papers from the 66^th to 70^th lines in the re-revised manuscript.

 

Comment: “The LDHs synthesized in this work have a high affinity for removing REEs ions”However, the authors did not provide the retention amount of Ln(III) in the LDH (e.g.: mmol/mg), and neither compared with the other materials described in the literature (e.g.: seaweed biomasses, and shell biomass, carbon).

Response: Thank you for your comment. As your suggestion, the adsorption capacities of L1 and L2 are compared with the other materials described in the literature. The result is shown in Table S7 in the Supplementary Information.

 

Comment: Any description of the synthesis is presented: concentration, ratio Zn:Al, reaction volume, time temperature. Neither in the case of the anion-exchange reaction.

The reference 18, provided by the authors any explanation regarding to the concentration ratio of NaOH/NaNO₃ and the anion exchange reaction is presented.

Response: Thank you for the invaluable comment. Based on your comment, we have added the description of the synthesis process in the re-revised manuscript (Section 2.1, Line 106-121, page 3). The concentration ratio of NaOH/NaNO₃ was 1:1.

 

Comment: The authors employed the same amount of mg with both samples, L1 and L2. However, the molecular formula of L1 and L2 need to be different, due to the molecular mass of EDTA and NO₃ is markedly different (290 g/mol vs 62 g/mol, respectively). This will affect the amount of LDH, in terms of mmol, in the suspension. The anion exchange will depend on the amount of positive charges, Al(III) mmol.

Response: Thank you for your comment. The dosage of adsorbent is an important factor when it comes to the adsorption process. Therefore, we put the same amount of L1 and L2 into the suspension to clarify the effect of the dosage on adsorption. In terms of mmol, the effect is quite different between the two.

 

Comment: The authors should provide Elemental Analysis to quantify the amount of nitrate, and EDTA (before and after the adsorption experiments) in order to provide a molecular formula of the solid phases. Does this information match with the Zn:Al ratio? Which is the amount of Ln absorbed? E.g.: Zn:Al ratio, EDTA:Al ratio, EDTA:Ln ratio

Response: Thank you for your comment. We have dissolved the sample and measured the concentration of Ln ions by an ICP-MS (Agilent HP4500, USA). The Zn:Al ratio and the amount of Ln absorbed are shown in Table 3 (Section 3.2, page 6) in the re-revised manuscript. The analytical results in Table 3 indicate that the molar ratio of Zn/Al is approximately equal to that in the preparing solution, which shows that the dosage of reagents is reasonable.

 

Comment: The authors described the chemical structure by a Mg₁₂Al₇(OH)₃₆⁹⁺ cluster. In the case in which Mg is replaced by Zn, is there any change to point out?

Response: Thank you for your invaluable suggestions. We have added the reference [15], and the change to point out is described from the 310^th to 317^th lines (Section 3.5, page 11) of the re-revised manuscript.

 

9. Comment: During the simulation the authors kept the interlayer distance in 0.93 nm, however in the case on L2 and L2(Eu) the interlayer distance increases until 47 nm (more than 55%).The interaction between the charged positively layers and the anions depend on the distance between them (Barrer and Townsend - J. Chem. Soc., Faraday Trans. 1 1976, 72, 2650−2660 and J. Chem. Soc., Faraday Trans. 1 1972, 68, 1956−1963. -, or Eisenman- Biophys. J. 1962, 2, 259−323 - models based on clays).

Have the authors evaluated the role of the interlayer distance in the relative energies?

An interlayer distance of 0.93 nm could be unrealistic for the L2 samples. The information from the structure relaxation could be interesting, specially to evaluate the typical metal to metal distances and the charge density.

Response: Thank you for your invaluable suggestions. As you suggested, the interlayer distance increases by intercalating EDTA. Indeed, the interlayer distance of L2 and L2(Eu) is estimated to be 0.99 nm (subtracting from basal spacing :1.47 nm to layer width :0.48nm), although that of L1 is 0.43 nm (basal spacing :0.91 nm) in our experimental work of XRD analyses. However, in our calculation this time, roughly comparison of stability between L1 and L2 have been mainly focused. More accurate calculation considering the interlayer distance will be presented elsewhere in the future paper.

 

10. Comment: The affinity between LDH layers and anions depends on charge and size. The correlation provided by the authors is not correct. More information could be found in: Applied Clay Science 54 (2011) 132–137

Response: Thank you for your comment. As you suggest, we think that the affinity between LDH layers and anions depends on charge and size. Therefore, the anion with a larger charge and a smaller size has a higher affinity to intercalate into interlayer. However, the stronger the affinity is, the more difficult it becomes exchanged. Therefore, we conclude that the order of increasing exchangeability is NO₃^-＞OH^-＞F^-＞Cl^-＞Br^-＞I^-＞CO₃ ^2-.

 

Line 144: Should C_Cu be C_Ln?

Response: Thank you for your invaluable suggestion. According to this suggestion, C_Cu and C_Cu,0 in the previous manuscript have appropriately been changed to C_Ln and C_Ln,0 in the re-revised manuscript.

 

12.Line 148: B, D, M/(AV) are no defined. Are these constant related with some physicochemical parameter or are they dimensionless?

Response: Thank you for your comment. B, D, and M/(AV) were introduced to make clear the formulas to define the appropriate number of the parameters to be fitted by genetic algorithm. The description of B, D, M/(AV) has been shown from the166^th to 170^th lines (Section 2.6, page 4) of the re-revised manuscript.

 

A solution containing 200 mmol/L of HCl or HNO3 presents a pH lower than 1. At this pH both cations Zn and Al are soluble, also the LDH phase (Applied Clay Science 51 (2011) 366–369). From my personal point, the regeneration process is a dissolution process. For that reason, the Recovery efficiency decrease. A regeneration process could be an anion-exchange reaction employing an anion with high affinity by the interlayer space (Applied Clay Science 54 (2011) 132–137).

Response: Thank you for your suggestions. The solution we used for regeneration process has a lower pH. The lower pH may affect the adsorbent and recovery efficiency. However, the adsorbent was not dissolved during the process in our work. What is different from the literature is that the anion in the interlayer of L1 and L2 is nitrate ions and EDTA^2-. In contrast, the interlayer anion of LDH in the literature is carbonate ions. As your comment, the regeneration progress is an anion-exchange reaction. Moreover, this chelating reaction between the Ln ions and EDTA is a process which releases hydrogen ion. Desorbing the adsorbent by acid solution indicates that chelation and complexation play a significant role in the adsorption.

 

Lines 190-192: “Therefore, the intercalation of EDTA (i.e., LDH-EDTA) leads to a lower zeta potential, which leads to the adsorption ability of REEs ions (i.e., cation, having positive charge) larger than LDH.” Does the adsorption of Ln take place by the lower zeta potential of the surface or by Ln-EDTA complex formation? This tense must be reviewed. At the same time, information of the EDTA amount, and the Ln amount in the final phase is require to analyse the process’s nature: absorption vs complex formation and intercalation.

Response: Thank you for your comment. The adsorption mainly takes place by the formation of Ln-EDTA complex, based on the stability constants of Ln-EDTA corresponding to the adsorption capacity. However, the zeta potential also has an impact on it. In order to verify it, adsorption isotherms of adsorption were studied by fitting adsorption models. The Langmuir adsorption isotherm and Freundlich adsorption isotherm were applied to the data obtained in this work. Furthermore, in order to describe the adsorbate-adsorbent interaction, the kinetic isotherms were analyzed by fitting experimental data to pseudo-first-order, pseudo-second-order, intra particle diffusion, and Elovich equation to find out the suitable model that may be used for design consideration. The Gibb’s free energy change(∆G^o), standard enthalpy (∆H^o) and standard entropy (∆S^o) are also discussed. These results are shown in Supplementary Information (Figure S1-S8 and Table S1-S6). Table S1-S6 (obtained from Figures S1-S8) and the adsorption isotherm data are well fitted by Freundlich isotherms which indicate that the adsorption progress is not only a homogeneous adsorption but also a multilayer adsorption of the adsorbate onto the adsorbent. That is to say, both the lower zeta potential of the surface and the formation of Ln-EDTA affect the adsorption capacity. The adsorption kinetics data are well described by a pseudo-second-order kinetic model, showing that the adsorption process is controlled by the identical chemical process. The results of the adsorption thermodynamics suggested that the reaction of REEs adsorbed on LDHs is spontaneous and endothermic.

Lines 211-212: The tense “These figures show that the layer structure was destroyed by the intercalating process for neither EDTA nor EDTA-Ln(III)” is confusing. Figure 4 depicts LDH structure.

Response: Thank you for your comment. The sentence was changed to “These figures show that the layer structure was destroyed by the intercalating process neither for EDTA nor for EDTA-Ln(III).” in the re-revised manuscript. (Section 3.2, from the 235^th to 236^th lines in the re-revised manuscript).

 

Form my personal point of view the authors should make explicit they have worked with ZnAl LDH. The phrase “The current study discusses layered double hydroxides (LDHs) modified with EDTA” is too ambitious.

Response: Thank you for your comment. In our previous study, the Zn-Al LDH was synthesized and used for heavy metallic ions. Moreover, the adsorption process of Ln(III) was also discussed. The part of materials and methods was revised. It may be that the expression is not suitable here. We have deleted it in the re-revised manuscript.

 

The author claimed:

“The intercalation of EDTA (i.e., L2) exhibited a lower zeta potential, which indicates that the intercalation leads to the larger adsorption capability of REEs ions. Moreover, quantum chemistry calculations were performed using the GAUSSIAN09 program package. In the calculations, the molecular locally stable state structures were optimized by density functional theory (DFT).”

This tense, repeated in the manuscript, suggest physisorption instead of complex formation in the interlayer space. The authors provide DFT calculation to explain the experiments (second option). The phrase is confusing and could cause misunderstandings.

Response: Thank you for your comment. Our main conclusion is that the complex formation plays the role more dominant than the physisorption indicated by the lower zeta potential. Therefore, the corresponding parts have been changed as shown by the sentence at the 208^th to 210^th lines (Section 3.1, page 6) and that from the 329^th to 333^th (Conclusion, page 12) in the re-revised manuscript.

 

It could be better if the authors indicate that they have worked with a ZnAl LDH.

-In materials and method EDTA is not mentioned.

-Speciation diagram of EDTA in order to provide information about the moieties present in the LDH. At pH=4.0, EDTA should by EDTA^2-.

Response: Thank you for your comment. We have added the information on EDTA in the re-revised manuscript. EDTA has seven forms in aqueous solution. When pH is 2.67-6.16, EDTA mainly exists in the form of H₂Y^2-. Therefore, EDTA changes to EDTA^2- (divalent) when the pH of solution is 4.0. The synthesis processes of L1 and L2 were added in the re-revised manuscript (Section 2.1, page 3, from 106^th to 121^st lines in the re-revised manuscript).

 

Lines 211: “Figures 4 (b) and (c), These”: dot or comma?

Response: Thank you for your comment, we have changed the comma to dot in the revised manuscript.

 

Lines 216-218: Typically, the bibliography use inter basal space, interlayer distance instead of “gallery height”. The use of this phrase could be confusing.

Response: Thank you for your comment. As your suggestion, we have changed the “gallery height” to “interlayer distance”.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

The authors have improved dramatically the quality of the manuscript, in both description of the results and providing new experiments.

Author Response

Thank you for your comment. We wish to express our appreciation for your judgement. We were much encouraged.