Silicas Chemically Modified with Sulfur-Containing Groups for Separation and Preconcentration of Precious Metals Followed by Spectrometric Determination

Round 1

Reviewer 1 Report

Major comments: 1) Authors are encouraged to consider the article category: in the presented version the manuscript is more related to a full-length article. But it should be arrange with the corresponding sections, for example, the description of synthesis detailes hardly need to be described in introduction. The final table summarizes three self-cited works. There are much more publications to be mentioned in this field in the case of review article.

2) Heating and adding a labilising agent are the well known approaches for enhancing the efficiency of kinetically inert complexes of platinum metals. What features of the developed sorbents could distinguished them from other sorbent materials for platinum metals?

Minor comments: 1) There are several phrases not in English, for example, in Table 1 and 3.

2) The sructure of BTUS in Table 1 containes an error.

3) The line 204 need to be checked: Au or Ag?

4) The abbreviation CMS is used only once, while the full definition is used frequently.

Author Response

We are deeply appreciated to the reviewer for careful reading and valuable comments.

Response to the major comments:

1. We agree with the comments of the reviewer and changed the submission type from "review" to "original article" and, accordingly, corrected the structure of the manuscript.
2. We agree that such approaches as heating and adding SnCl₂ for labilizing complexes of platinum metals are well known. But the combination of such approaches with the proposed sulfur-containing adsorbents makes it possible to achieve better intragroup separation of platinum metals and their separation from accompanying non-ferrous metals.

Response to the minor comments:

1. We have made corrections to Table 1 and Table 3.
2. We have fixed an error in the BTUS structure in Table 1.
3. Line 204 (now line 270) meant exactly Au.
4. The abbreviation CMS is used more frequently in the revised manuscript.

Reviewer 2 Report

This manuscript describes some interesting studies on extraction of precious metals using sulfur ligands immobilized on silica supports. The paper contains a very substantial amount of research work, however, in my opinion some major revision of the manuscript is required in order for the work to be acceptable for publication.

The manuscript is rather long.  The Introduction especially is very long (about 3.5 pages), and I suggest that the authors revise this to remove any unnecessary material. In addition, in the Introduction and elsewhere in the manuscript there are quite a few paragraphs that are one sentence long.  This makes the text not flow as well as it could, so I suggest the authors incorporate such sentences into longer paragraphs.

Until I got quite a long way into the manuscript, it reads more like a review than an original research article. The long Introduction is partly responsible for this, but mainly the absence of a clearly defined Experimental section. Assuming this is indeed an original research article, an Experimental section is essential, in order that others could repeat the work if necessary.  I note that on page 10, experimental details are provided for the extraction of metals from geological samples, but not for the preparation of the immobilized ligands themselves.  I note that some materials have been made previously, and appropriate acknowledgement of this must be made, otherwise the authors are, by default, claiming these as new substances. For example, mercaptopropyl silica has been made and characterized by Khatib and Parish (Journal of Organometallic Chemistry, 1989, vol 369, 9-16) and a reference for this or an equivalent alternative should be cited.

Some minor points:

Table 1 BTUS is a thiourea not a urea in the diagram. Also in Table 1, there are gaps where aromatic rings have a bond attached...this should be fixed

Page 5 gold(III) etc - no gap between element name and oxidation state

Figure 1.  This shows extraction plots for non-PGMs, but this information is rather supplementary to the main point of the research, so to shorten the paper I suggest it is removed and put as supplementary data.

Page 7 paragraph 3 "As you can see from the table..."  please use passive voice, i.e. "It can be seen from table 1 that...."

Page 8 paragraph 2. Platinum metal exist in the form of tin chloride complexes.  A reference should be given for this information, since it is not determined in the present study.

Page 8 extracteded should be extracted

Page 9. It is not clear how the various precious metals were analysed; again, the inclusion of an Experimental section should describe this.

Author Response

We are deeply appreciated to the reviewer for careful reading and valuable comments.

Response to the major comments:

We changed the structure of the manuscript, shortened the Introduction, added Materials and Methods, and changed the Results and Discussion. We have added the reviewer's suggested reference “Khatib and Parish (Journal of Organometallic Chemistry, 1989, vol 369, 9-16)” when describing the synthesis of silica gel with mercapto groups.

Response to the minor comments:

1. We have fixed an error in the BTUS structure in Table 1, and also fixed bonds with aromatic rings.
2. The typo has been fixed.
3. Figure 1 has been moved to Supplementary Material.
4. We have changed the phrase and used the passive voice.
5. The sentence has been deleted.
6. We fixed "extracteded".
7. Instrument descriptions have been added to the section “Materials and Methods”.

Round 2

Reviewer 1 Report

The manuscript has been revised regarding its structure, but there are some questions to be discussed (see major comments below). Also I would like to ask the authors for more careful reading the text prepared for submitting - for example, the authors have confirmed the mistake (minor comment #3 in the previous report), but the mistake is still in the text; the abbreviation ATMS has been changed in the table, but it is still presented in the text (line 335); there is 'modifie' instead of 'modified' (line 394) and so on.

Major comments: A lot of sorbents were synthesized, a huge experimental work was carried out, but the results are presented chaotically, without the explanation of the differences between the functional groups of sorbents and their affinity to metals and selectivity. The different ranges of HCl concentration is mentioned without explanation: 0.5-4M HCl in the experimental part, 0.2-6M HCl in Results and Discussion, 1-4M HCl in Conclusions. At the same time the recovery of some heavy metals (e.g., Pb, Cu) is remarcable at 1M HCl. From this point of veiw the composition of the certificed referenced materials (the CRM) would be helpful to be described (maybe in the Supplementary file). Concerning the CRM and mentioned "high accuracy and reproducibility" of the methods is not obvious because of the absence of any confirming analytical data. Table 3 should not be named as "Determination...", it is just the list of the CRM, metals and instrumental methods. Finaly, it is hardly worth mentioning only one oxidation state for iridium, ruthenium and osmium ions, when referring to a wide range of hydrochloric acid concentration, especcially less than 1M.

Author Response

We are grateful to the reviewer for careful reading the manuscript and for providing valuable comments.

1. We have made a number of clarifications in the text of the manuscript regarding the difference in functional groups when interacting with various precious metals.
2.  We have adjusted the lower limit of the concentration of hydrochloric acid in order to unify the presented results throughout the text of the manuscript and in the table. The authors believe that the difference in the concentration of hydrochloric acid 0.2 M and 0.5 M is not significant.
3. We have given the composition of the reference materials used. The content of precious metals is presented in Tables 3 and 4, the content of a number of accompaning components in Table S4.
4. . For osmium, ruthenium, iridium and platinum the most stable oxidation states are +4, for gold and rhodium +3, for silver +1. In these oxidation states precious metals are present in solutions after liquid-phase oxidative degestion of geological and other samples. The reduction of metals in these solutions of hydrochloric acid under the conditions of their preconcentration and determination is unlikely. In the text of the manuscript, there is a mention of the fact that in dilute solutions of hydrochloric acid, chloride complexes of precious metals are aquated with the formation of aquachloride complexes of various compositions, as well as references are given in which this phenomenon and the compositions of aquachloride complexes are described.
5. Minor comments have been corrected.

Reviewer 2 Report

The authors have made substantial revisions to this manuscript, and it is now significantly improved. While I would personally like to see some of the very short, one-sentence paragraphs combined into more substantial paragraphs, this is a minor point. I therefore consider that the paper is now acceptable for publication, but if the authors were to address these short paragraphs, it would definitely help improve the readability of the paper.

Author Response

We are grateful to the reviewer for careful reading the manuscript and for providing valuable comments.

We combined one-sentence paragraphs into more substantial ones.

Round 3

Reviewer 1 Report

All of the comments were intended to improve the manuscript. The most of them have been taken into account. It would be useful to study the solution chemistry of iridium-group metals after sample digestion and dilution with HCl. Not only aquation, but redox processes are possible in the mentioned media [for example, Fine D.A. Inorg. Chem. 1969. 8(4):1014-1016]. But maybe this question along with the mechanism of sorption, and especially quantitative desorption of metals using thiourea in the case of the sorbent with thiourea fuctional groups, is in the upcoming studies by the authors.