Uranium(VI), Thorium(IV), and Lanthanides(III) Extraction from the Eudialyte Concentrate Using the N,O-Hybrid Heterocyclic Reagents

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the extraction of Th, U, and rare earth elements (REEs) from eudialyte concentrate using N,O-hybrid heterocyclic compounds. While the separation of U and Th from REEs was achieved to some extent, the extraction of REEs—highlighted in the title—is not adequately addressed in the final results. Moreover, the work is limited to preliminary extraction experiments and lacks detailed and systematic analysis. Therefore, I do not recommend this manuscript for publication in its current form. Below are my specific comments and suggestions for improvement:

1. The introduction should provide a clearer rationale for selecting phenanthroline derivatives as extractants. The chemical or structural advantages of these compounds for selective extraction should be explained in more depth.

2. There is a noticeable mismatch between the manuscript title and its content. For example, Table 1 does not support the broader scope suggested by the title.

3. The concentrations of U and Th in the original eudialyte concentrate are not reported in Table 1. This information is essential for evaluating extraction efficiency and should be included.

4. In Section 3.3, slope analysis is used to determine the solvation number. However, the metal ions are coordinated with the extractant in the organic phase. Can this coordination truly be defined as a "solvation number"? The terminology and interpretation should be clarified.

5. The manuscript suggests an intention to explore the stripping conditions for U and Th. Why not directly investigate the stripping of Th and U from the loaded organic phase? This approach would be more relevant and informative.

6. The experimental conditions for extracting metals from the eudialyte nitric acid solution are not clearly described. Key parameters such as acidity, phase ratio, extractant concentration, and metal concentrations should be explicitly stated to ensure reproducibility and to allow proper assessment of the results.

Comments on the Quality of English Language

The manuscript requires moderate revision for English language and grammar to meet the standards of academic publication. While the general meaning of the text is understandable, there are issues with sentence structure, punctuation, and word choice that hinder clarity and fluency. Specific concerns include: run-on sentences, verb usage and tense consistency, word order and phrasing, and terminology clarity.

Author Response

Thank you very much for the kind attention to our work. We provided some changes and expansion of the manuscript.

Comment 1. The extraction of some actinides in (IV) and (VI) valence states are already known for ligand L1. The corresponding reference inserted into the text. The phenanthroline derivatives possess tetradentate coordination core and con be considered as analogs of ligand L1.

Comments 2 and 3. The corresponding uranium and thorium content included into the table 1.

Comment 4. The term “solvation number” wildly used in extractive metallurgy for describing the mean composition of metal complexes in organic phase. The salvation number is crucial for technology as a measure of potential extractive efficiency of the extractant. This term only describe the  composition of complexes but not to types of metal to ligand coordination. At the manuscript we discuss the mean composition of complexes but not the coordination.

Comment 5. We describe the stripping conditions for effective U and Th separation from Zr, Hf and Ti by washing the loaded organic phase by nitric acid solution (for fourth group elements stripping) followed by carbonate stripping of U and Th.

Comment 6. The extraction experimental condition doubled in figure titles together with experimental section.

Reviewer 2 Report

Comments and Suggestions for Authors

This document presents the extraction and separation of U, Th and Ln from the leaching of eudialyte by N,O-hybrid ligands. The efficiency was compared for three ligands. High separation efficiency was obtained for the separation of U(Th)/Ln. The compositions of the extracted complexes were also deduced. This work might give a feasible way for the recovery of f-elements from the eudialyte concentrate. However, there are some Zr, Hf and Ti in the leaching of eudialyte which was not discussed in detail. Although Figure 6 illustrates the extraction of Zr, Hf and Ti, how to separate them is not given, which is suggested to be added.

Author Response

Thank you very much for the kind attention to our work. We provided some changes and expansion of the manuscript in the spripping section discussion. The fourth group elements can be stripped by dilute nitric acid followed by carbonate washing.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors conducted a study on the solvent extraction of actinides and lanthanides from eudialyte minerals. While the subject of the manuscript is potentially interesting, it lacks to address the structural aspects of the complexes formed within the system, a deficiency that makes the manuscript extremely difficult to comprehend. For instance, a subtle change in the structure of the ligands from L2 to L3 results in a substantial change in extraction behavior. However, this discrepancy remains unexplained with respect to the complex formed and the reason for the enhanced extraction of complex L3 remains untouched. The term "solvation number" is widely used in the field of coordination chemistry to denote the number of solvent molecules coordinated to ions, and therefore should be avoided in the present context. In the present manuscript, the term "solvation number" is used to denote the number of ligands that are coordinated to the metal, thereby referring to the stoichiometric ratio between metal and ligand. It is observed that these numbers deviate significantly from integer values, although they should theoretically be integer. Notably, the values for Th(IV) and U(VI) remain constant for all extractants (L1/L2/L3). This behavior is puzzling, given the substantial deviation in their distribution coefficients. The authors' exclusive focus on presenting data, without undertaking a thorough analysis and interpretation, further complicates the interpretation of these findings. Consequently, readers are left confused by the presentation of "raw data" without any interpretation. This type of studies must be augmented by additional data sets, including calorimetry, spectroscopy (NMR, fluorescence, etc.), and potentially quantum chemical calculations. The mere presentation of distribution ratios, in the absence of underlying interpretation and analysis, is considered a mere phenomenological approach. In my view, this manuscript is more like a technical report than a scientific article. On a minor note: there are two captions for Table 1. The initial caption, "Distribution ratios for Am and Eu …" appears to have been remained from their previous manuscript. While this may be considered a mere typographical error, there is considerable doubt concerning whether all authors thoroughly reviewed the manuscript prior to its submission. Such negligence is indicative of a lack of professionalism, casting doubt on the contributions of the seven authors.

Author Response

Thank you very much for the kind attention to our work. But we couldn’t agree with some remarks.

The reviewer wrote: “The term "solvation number" is widely used in the field of coordination chemistry to denote the number of solvent molecules coordinated to ions, and therefore should be avoided in the present context.”

The term “solvation number” wildly used in extractive metallurgy for describing the mean composition of metal complexes in organic phase. The salvation number is crucial for technology as a measure of potential extractive efficiency of the extractant.

The reviewer wrote: “it is observed that these numbers deviate significantly from integer values, although they should theoretically be integer.”

This term only describe the averaged composition of complexes, as a median value in nature the solvation numbers often (in vast majority of cases) are non-integer.

The reviewer wrote: “Notably, the values for Th(IV) and U(VI) remain constant for all extractants (L1/L2/L3). This behaior is puzzling, given the substantial deviation in their distribution coefficients.”

It is not surprising due to exhaustive extraction of those metals into organic phase by all three ligands, so the metals content in water phase are on the detection limits.

The reviewer wrote: “This type of studies must be augmented by additional data sets, including calorimetry, spectroscopy (NMR, fluorescence, etc.), and potentially quantum chemical calculations.”

The manuscript deals with the processing of a new type of ore (eudialite mineral): leaching and subsequent separation of hazardous, requiring special regulation, radioactive metals (U, Th), rare metals of the 4th group, and raffinate, which contain lanthanides. The types of complexes forming by the ligand L1, investigated in [35], L2 – [38]; a light insight into the properties of L3 complexes with lanthanides can be found here: DOI:10.1039/C6DT04681A. As the manuscript devoted to hydrometallurgy but not coordination chemistry of f-elements, 4th group elements etc. the composition of leaching solution, further, distribution coefficients and stripping procedures are discussed (together with solvent loadening potential for which the average salvation numbers determined for all of the compounds).

Thank you very much for checking the mistypes! The Scheme 1 title migrated with the template of our two years ago article concerning the actinide separation due to non user-friendly template. I had some difficulties with de-novo creation of the tables satisfying the Journal format.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I would like to thank the authors for their responses to the previous review comments. The revisions have addressed all major concerns raised in the earlier round. The manuscript is suitable for publication in its current form.