Dissolution and Solubility of the Calcite–Otavite Solid Solutions [(Ca1−xCdx)CO3] at 25 °C
, Hongqu Yang 2, Peijie Nong 2, Zhiqiang Kang 1, Shen Tang 2, Lihao Zhang 2 and Yinian Zhu 4,*
Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript represents potentially interesting results to advance in the knowledge of SS-AS systems. I consider that the manuscript should be published in this journal with Major Modifications, but they must make some structural modifications and other more specific that I explain below:
General aspects:
In several parts of the manuscript are exposed jointly the data from the experiments with N2-degassed water and CO2-saturated water, generating confusion. For example, in lines 24-26, 29-30, 284-285, 292-293. I think that the authors should better separate these two experiments, either in different sections within the results, one specific for the results of N2 and another for those of CO2, or in the same section but clearly separating the results of one and the other.
A better differentiation between the proportions of Cd in solution and solids is needed. For both are used the same terminology, XCd that in some cases induces to misunderstanding.
Material and Methods section:
The methodology deserves a better explanation of the following aspects:
- How the N2-degassed and CO2-saturated waters were prepared? How are maintained that conditions during the experiments?
- Missing important x-ray diffraction parameters, sample/detector rotation 2-THETA/min, peak position, full width half máximum (FWHM), etc.
- Lack of SEM parameters, pre-treatments of the sample, coated or uncoated…
- Characteristics of the water used.
- Specific detection limits or error limits of the measurements must be included for the different techniques, especially for ICP-OES.
Results and Discussion section:
The amount of Ca1-xCdxCO3 precipitates obtained for the different fractions is not mentioned. Could that information be included?
In line 141 the authors say “…the solid residues increase a little, suggesting a preferential partition of Cd ions into the solid phases…”. In my opinion, this fact is not supported by the data because the difference between the theoretical data and the obtained differs only in 0.01 units. This difference could be explained better if the uncertainty of the measurements were known, but that uncertainty has not been calculated.
It is not clear how the XCd of the synthetic solids has been calculated. This fact needs an explanation.
The authors affirm in line 182 that “…the solids after dissolution [] presented the same morphologic characters as before, i.e., the solid changed from coarse crystals to spherical aggregates of very fine small crystals with the XCd increase”, but are there significant changes inside the same XCd? Do the solids of a given XCd show the same external characteristics before dissolution than after dissolution?
Terminology:
There is a confusion between increase and decrease (lines 306-308 and abstract) when the authors talk about log IAP and Gibbs free energies. I recommend using increase when the numbers are more positive and decrease the opposite.
It must be done a unification with the terms “pure water” or “ultrapure water” and explain what the water characteristics are in the methodology section. Also, with the terms “Mole ratios, molar ratios, mol ratios”, for example in lines 461-462-493.
It is recommended, although not essential, to separate of a clearer way Cd concentration from other aqueous species (Ca2+, CO32-) due to the great difference of concentration between them (3 orders of magnitude). I suggest using μmol/L for Cd concentration and mmol/L for Ca2+ and CO32-. This recommendation would avoid the use of long numbers (with 6 decimal positions) for Cd concentration.
Specific comments
Line 20 – Data has not units. It must be included.
Line 114-117. This paragraph can be re-written more clearly.
Lines 138-139 - The paragraph refers to table 1 but not to which specific column on it. Also, table 1, it is recommended a clearer separation of the columns, volume of the precursors and XCd, and what sub-columns are included within them.
Line 179-180 – metasTable Snd , fix.
Figure 5 and 6 - The scale used is too small to appreciate any change for Cd concentration, another scale must be used. In addition, it is not clear what the Note (*1/10) means within the Cd concentration chart (Figure 5). It is recommended (Figures 5 and 6) to use another scale for the pH graphs, more adjusted to the values obtained. The data legend could be removed from the graph for clarity.
Lines 294-300 – Data could be better represented in a table for clarity (maybe in the supplementary).
Tables 2 and 3. In the column of Cd and Ca, there are data that are not represented. Have they not been measured or were they below detection limits?
Line 361 - Lippmann (the last n is missing).
Line 439 - theCO2-saturated, fix.
Line 447 - Remove italics from word “curve”.
Figure 7 and 8 - Low resolution. The graph on the left is not well explained, what it is? Is it a zoom of the one on the right?
Formula 18 - The “ [ “ symbol after “ = “ is missing.
Figure S4 - Unappropriated scale to represent the data, maybe it should be changed to logarithmic scale.
Figure S5 - Time of the represented data must be specified into the text.
Figure S7 - Include measurement error in the graph, instead of using a fat point.
Figure S9 - Change “si” to “SI” in uppercase and include it in the figure text. It is not clear what the graphs want to represent. What does it mean each point represented for a specific XCdCO3? Are they different times? What time does each point correspond to?
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
This paper helps understanding the environmental cycle of Cd in natural water. The extreme low solubility of otavite compared to calcite solubility indicates the preferential partitioning of Cd in carbonate solids suggesting that carbonate matrices can be a potential tool for cadmium reducing from natural waters.
The study offers a laboratory experimental determination of the apparent solubility and dissolution behaviour of the Calcite-Otavite solid solution. The authors suggest that Cd-poor aqueous phases might reflect a quasi- equilibrium state between aqueous solution and otavite, which is the end-member of the solid-solution.
Despite this work is not very original, the experimental data may clarify some specific point of the process (i.e the quasi stationary state) and I recommend the publication.
Some point may be takes into account during the revision of the manuscript.
Lin121: The total HCO3/CO3 concentrations were measured by….. You mean alkalinity? If yes please, specify.
Lin 177. The decline in the crystal size was expected owing the lower solubility of Cd-rich carbonates…..This is not clear to me. Generally, at the microscopic crystal size, solubility is different from larger size crystals. Previous works have already discussed this point. Does the observed increase of solubility product (by 1 order of magnitude) correspond to this phenomena ? If yes please explain in the text.
Could you discuss in the text if in the experiment as well under natural conditions, CdCO3 coatings form on calcite large crystals without formation of Cd-Ca- carbonate solid solution ?
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
No more comments.
Reviewer 3 Report
The manuscript can be published in the present form
