Solubility Product of Vanadinite Pb₅(VO₄)₃Cl at 25 °C—A Comprehensive Approach to Incongruent Dissolution Modeling

Round 1

Reviewer 1 Report

Review Manuscript Ref.: MINERALS-1088182

Review: The manuscript is well structured and present interesting information on vanadinite and its re-precipitation as chervetite under specific experimental conditions. However, there are some few aspects that require further scientific explanation. Additional comments/questions: 

Abstract:

Lines 10 and 19: Please write the chemical formula with the proper subscript.

Line 20: I suppose that “KspV” must contain also some terms in subscript. Please re-write.

Section 1:

Line 36: I suggest to replace the word “Totally” by “Overall”.  

Lines 83-84: I suggest to replace the word “publication” by “work”.

Section 2:

Line 89: Can the authors explain why they decided to use specific conditions, such as 90 °C and pH=3.5? (The explanation about the pH is found later in the manuscript though)

Line 114: Why the authors digested the solid material in EDTA? Why not mineral acids? In case the authors have considered a standardised method, it should be included in the manuscript as a reference.

Lines 122-123: Why the authors considered the utilisation of KNO3 as lixiviant and why specifically at a concentration of 0.05 M?

Section 3:

Table 1: The text below the table should be aligned to the left.

Fig 2: By mere comparison of Fig 1 and Fig 2, there is a significant difference in the type of crystals. Can the authors explain such change? What’s the fundamental mechanism behind?

Section 4:

Line 211: Need to be re-phrase.

Line 222: How the authors have confirmed the polymeriation of vanadates?

Conclusions:

Certainly, the authors have obtained empirically the solubility product of synthetic vanadite over a very long experimental period. However, such constant has been obtained under specific experimental conditions, such as a constant pH-value, temperature and an electrolyte with a specific concentration. In fact, it is very well known that the utilisation of different dissolution reagents allows different speciation and kinetics.

Although the methodology is quite interesting and can be applied to other systems, the results showed in this work do not allow to state that the method is, as this stage, universal.

Author Response

We would like to thank you for taking the time to review our publication. We are very grateful for sharing your remarks with us. We found them very helpful in improving this paper.

You wrote:

Review: The manuscript is well structured and present interesting information on vanadinite and its re-precipitation as chervetite under specific experimental conditions. However, there are some few aspects that require further scientific explanation. Additional comments/questions: 

We are very grateful for your appreciation of our work. We are also very grateful for all remarks suggested by you to improve our paper. Below we present our responses to your detailed remarks (line numbers refer to the simple adjustment, in the attachment there is a manuscript with indicated changes).

Remarks #1 - 4

Abstract:

Lines 10 and 19: Please write the chemical formula with the proper subscript.

Line 20: I suppose that “KspV” must contain also some terms in subscript. Please re-write.

Section 1:

Line 36: I suggest to replace the word “Totally” by “Overall”.  

Lines 83-84: I suggest to replace the word “publication” by “work”.”

Response to the remarks #1 - 4:

According to your suggestion, the subscripts were carefully edited and the indicated words replaced.

Remark #5

Section 2: Line 89: Can the authors explain why they decided to use specific conditions, such as 90 °C and pH=3.5? (The explanation about the pH is found later in the manuscript though).”

 Response to the remark #5:

We introduced following explanation in the manuscript.

Lines 96-98: “The temperature and the pH of the synthesis were indicated by preliminary geochemical modeling [64,65] as the conditions favorable for the VO­₄^3- presence in the solution, thus favorable for vanadinite precipitation.”

Remark #6

Section 2: Line 114: Why the authors digested the solid material in EDTA? Why not mineral acids? In case the authors have considered a standardised method, it should be included in the manuscript as a reference.

 Response to the remark #6:

Mineralization of Pb-apatites in mineral acids was not effective (quick & complete dissolution), thus once we have decided to use EDTA, which turned out to be an effective solvent in this case. According to your suggestion, we presented the references supporting that the method was used previously with good results as in the case of this study.

Lines 111-113: Compared to other reagents e.g.: HNO₃ the EDTA is an effective solvent for mineralization of Pb-apatites yielding relatively quick and complete dissolution of the solid [14,36,37,39,55].

Remark #7:

Section 2: Lines 122-123: Why the authors considered the utilisation of KNO3 as lixiviant and why specifically at a concentration of 0.05 M?

 Response to the remark #7:

The following explanation was introduced in the Manuscript lines: 130-136.

The KNO₃ (POCH, pure for analysis) was used as the background electrolyte because both, K⁺ and NO₃^- ions are not involved in the reactions in the studied experimental system. At this concentration of the background electrolyte, the increase in ion concentration caused by the dissolution of the mineral during the experiment does not cause significant changes in the total ionic strength of the solution, and thus does not affect the experimental conditions. At the same time, the ionic strength is low enough that the Debye-Hückel equation can be used to calculate ion activity. 

Remark #8:

Table 1: The text below the table should be aligned to the left.

Response to the remark #8:

The text was aligned to the left as suggested.

Remark #9:

Fig 2: By mere comparison of Fig 1 and Fig 2, there is a significant difference in the type of crystals. Can the authors explain such change? What’s the fundamental mechanism behind?

Response to the remark #9:

Yes, as the reviewer observed, during the dissolution in the solid residue the additional crystals of chervetite appeared. We rephrased and enhanced the paragraph in the results section introducing description of the possible mechanism as following:

Lines 186-199: The SEM image of the dissolution residue after 12 years of aging in the suspension is presented in Figure 2. During the experiments, apart from the very fine, hexagonal particles of primary vanadinite (the experimental material subject to dissolution; please compare with Figure 1), notably larger (> 10 µm) prismatic crystals with the monoclinic symmetry appeared in the solid fraction. The EDS (data not shown) and the XRD analysis indicated that the prismatic crystals of the secondary mineral phase are chervetite Pb₂V₂O₇ [69–70]. All X-ray diffraction peaks were assigned to either vanadinite or chervetite, confirming beyond doubt the incongruent nature of the dissolution process in which vanadinite re-precipitates partially into chervetite (Table A1). The possible mechanism behind the appearance of chervetite in the solid residue was that dissolving vanadinite released Pb and V ions in concentrations exceeding the solubility product of chervetite inducing its precipitation as the secondary mineral. However, the recrystallization of vanadinite into chervetite due to adsorption of polymerized vanadates or their direct formation on the vanadinite surface cannot be excluded

Remark #10:

Line 211: Need to be re-phrase.

Response to the remark #10:

According to the reviewer’s suggestion the sentence was rephrased as following:

Lines 230- 231: In this study, the geochemical speciation model PHREEQC [64] was used to support the calculations of thermodynamic parameters

Remark #11:

Line 222: How the authors have confirmed the polymeriation of vanadates?

Response to the remark #11:

Thank you for the remark. All we meant was just to point out, that the conditions were favorable for polymerization (as indicated by Phreeqc), we haven’t carried out the measurements. We rephrased the sentence as following:

Lines 240-242: Even though, the log SI values only slightly exceeded 0, the experimental conditions were favorable for vanadates polymerization as indicated by the PHREEQC modeling [64] and the precipitation of chervetite occurred as confirmed by the SEM/EDS and XRD analysis.

Remark #12:

Conclusions:

Certainly, the authors have obtained empirically the solubility product of synthetic vanadite over a very long experimental period. However, such constant has been obtained under specific experimental conditions, such as a constant pH-value, temperature and an electrolyte with a specific concentration. In fact, it is very well known that the utilisation of different dissolution reagents allows different speciation and kinetics.

Although the methodology is quite interesting and can be applied to other systems, the results showed in this work do not allow to state that the method is, as this stage, universal.

Response to the remark #12:

Thank you for the remark indicating that our text was not precise enough. As “universal” we find only the modeling method with a use of the “inverse modelling”, which has not been presented to date by anyone, not the dissolution experiment which, as the reviewer indicated ,can be carried out in many different ways. We rephrased the conclusions as following:

Lines 303-310: For the first time, the experimentally determined solubility product of vanadinite equal to K_spV,298 = 10 ^{-91.89 ± 0.05} is presented. Vanadinite is an exceptional mineral among Pb-apatite series; it dissolves incongruently inducing precipitation of chervetite as the sole secondary phase. Incongruent mechanism of vanadinite’s dissolution poses the challenge on direct experimental determination of the solubility constant, however, a new calculation approach along with the unique, long term experiments proposed here allowed to overcome this difficulty. The approach to modeling with a use of the inverse method is novel, universal and can be applied to other systems.

Author Response File: Author Response.docx

Reviewer 2 Report

Add the term "vanadinite" to the keywords

Excellent text

As far as I understand, long-term formation of chervinite from vanadinite takes place under acid oxidizing conditions. Maybe you can give a hint in one sentence, which happens in carbonaceous resp. reducing (formation of VO²⁺ ) environments

Author Response

We would like to thank you for taking the time to review our publication. We are very grateful for sharing your remarks with us.

You wrote:

“Excellent text.

As far as I understand, long-term formation of chervetite from vanadinite takes place under acid oxidizing conditions. Maybe you can give a hint in one sentence, which happens in carbonaceous resp. reducing (formation of VO²⁺ ) environments.”

We are very grateful for your appreciation of our work. We introduced the information you has asked for in the introduction section as indicated below (line numbers refer to the simple adjustment and there is a manuscript with indicated changes in the attachment):

Lines 74-77: In the carbonaceous environments, the amount of adsorbed (or possibly precipitated as chervetite) vanadium is greatly affected by the dissolved organic compounds (humic substances, organic acids) because of reduction of vanadate V(V) to vanadyl (V(IV)) VO²⁺ ions (Breit and Wanty 1991; Lu et al., 1998; Bruyere et al., 2001;  Gustafsson 2019).

Breit, G. N., & Wanty, R. B. (1991). Vanadium accumulation in carbonaceous rocks: a review of geochemical controls during deposition and diagenesis. Chemical Geology, 91(2), 83-97.

Lu, X., Johnson, W. D., & Hook, J. (1998). Reaction of vanadate with aquatic humic substances: an ESR and 51V NMR study. Environmental science & technology, 32(15), 2257-2263.

Bruyère, V. I., Rodenas, L. A. G., Morando, P. J., & Blesa, M. A. (2001). Reduction of vanadium (V) by oxalic acid in aqueous acid solutions. Journal of the Chemical Society, Dalton Transactions, (24), 3593-3597.

Gustafsson, J. P. (2019). Vanadium geochemistry in the biogeosphere–speciation, solid-solution interactions, and ecotoxicity. Applied Geochemistry, 102, 1-25.

Author Response File: Author Response.docx

Reviewer 3 Report

The main purpose of the article is to determine experimentally the thermodynamic data for vanadinite, a secondary mineral of the weathering and oxidation zones of primary vanadium minerals. In the course of experiments, the authors determined the solubility product of vanidinite, which can be used in physicochemical modeling of processes occurring in natural and technological systems. The main difficulty of definition of solubility product of vanadinite was its incongruent dissolution. Using the data obtained, the authors constructed a method that is quite universal for application to other minerals with incongruent solubility. The data interpretations seem to have been done well, also the experimental data obtained. So, the paper adds valuable new information and can recommend for publication in Minerals but with minor revisions.

Comments.

1. What time is needed to reach equilibrium between solid phases ( chervetite and vanadinite) and the solution? There is no such information in table 1. More visual graphs would be the concentrations of dissolved particles (Pb, V, Cl) as a function of time.
2. Equation 1 should be corrected it is written with an error. Correctly write like this

PbVO_3.5 + 5H⁺ = Pb²⁺ + VO³⁺ + 2.5H₂O.

3. Correct the place of the table 2’ title.

Author Response

We would like to thank you for taking the time to review our publication. We are very grateful for sharing your remarks with us. We found them very helpful in improving this paper.

You wrote:

“Comments and Suggestions for Authors

The main purpose of the article is to determine experimentally the thermodynamic data for vanadinite, a secondary mineral of the weathering and oxidation zones of primary vanadium minerals. In the course of experiments, the authors determined the solubility product of vanidinite, which can be used in physicochemical modeling of processes occurring in natural and technological systems. The main difficulty of definition of solubility product of vanadinite was its incongruent dissolution. Using the data obtained, the authors constructed a method that is quite universal for application to other minerals with incongruent solubility. The data interpretations seem to have been done well, also the experimental data obtained. So, the paper adds valuable new information and can recommend for publication in Minerals but with minor revisions.”

We are very grateful for your appreciation of our work and your recommendation to publish it in the Minerals. We are also very grateful for all minor remarks suggested by you to improve our paper. Below we present our responses to your detailed remarks (line numbers refer to the simple adjustment and there is a manuscript with indicated changes in the attachment):

Remark #1:

What time is needed to reach equilibrium between solid phases (chervetite and vanadinite) and the solution? There is no such information in table 1. More visual graphs would be the concentrations of dissolved particles (Pb, V, Cl) as a function of time.

Response to the remark #1:

Thank you for this question. The kinetic of the reactions was not our aim, so we did not check the solutions parameters frequently enough to describe this processes reasonably. All that we can suggest is the time between 8 moths to 8 years. However, we agree that this phenomena could be interested for the readers that would like to further explore the kinetic of the reactions so as an inspiration we include in the attachment the graph proposed in your remark (Figure A1) and the information about it in the manuscript line: 206. We also find, that the differences in sampling time as well as in the concentration ranges cause the data insufficiently readable in the graph, while the data presented in a table allows the reader to repeat the modeling as well to evaluate the statistics. Therefore, we would like to keep presenting the data in the form of a table and support it additionally in the appendix by the graph as you sugessted.

Remark #2:

Equation 1 should be corrected it is written with an error. Correctly write like this

PbVO_3.5 + 5H⁺ = Pb²⁺ + VO³⁺ + 2.5H₂O.

Response to the remark #2:

Thank you for indicating the error; of course in the presented form, the reaction was not balanced. We made an error during rewriting the reaction from the Minteq data base. We corrected the mistake to be exactly as in the database and now it is in the text as following:

PbVO_3.5 + 3H⁺ = Pb²⁺ + VO₂⁺ + 1.5H₂O

Remark #3:

Correct the place of the table 2’ title.

Response to the remark #3:

The title was placed above the table as it should be.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The Authors have properly addressed all my comments/suggestions.