Quantitative Analysis of Ferrate(VI) and Its Degradation Products in Electrochemically Produced Potassium Ferrate for Waste Water Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents mainly Mossbauer spectroscopy results for the quantitative analysis of ferrite (VI) and its degradation products. It is a very deep analysis that could be of interest to a potential reader. There are no questions or doubts from my side.

The only suggestion is addressed to figure 7, where DSC studies are presented:

string 368-369 says "As shown in Fig.7, a transformation occurs at around 180 K in this sample". However, the units used in this figure are Celsius degrees, not Kelvin. The figure should be consistent with the text. Moreover, no sense to show such a precision containing thousandths of a unit for temperature transformation.

Author Response

Comments 1: The manuscript presents mainly Mossbauer spectroscopy results for the quantitative analysis of ferrite (VI) and its degradation products. It is a very deep analysis that could be of interest to a potential reader. There are no questions or doubts from my side.
The only suggestion is addressed to figure 7, where DSC studies are presented:
string 368-369 says "As shown in Fig.7, a transformation occurs at around 180 K in this sample". However, the units used in this figure are Celsius degrees, not Kelvin. The figure should be consistent with the text. Moreover, no sense to show such a precision containing thousandths of a unit for temperature transformation.

Response 1: We are very thankful for the comments from Reviewer 1. We have modified Fig. 7 according to his/her suggestion.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

I found Your work very inetresting. The number of experiments undertaken and the amount of data gathered during Your work is really impressive. Also the analysis seems to be very meticulous.

I have some important comments:

1. Please include a table with the mossbaure paramaters for all the componentes of Your spectra for different temperatures.

2. Please explain the why the "noramlized area" ranges from more than 3 to less tnan 0.1 and do not sum to 1. It is the most issue important I detected.

3. Maybe changing the figures to one panel (spectra arranged one below te other not in separate sub-figures) coud improve the visibility of the differences bettween temperatures. 

Author Response

Comments 1: Please include a table with the mossbaure paramaters for all the componentes of Your spectra for different temperatures.

Response 1: Thank you for this comment. Presenting all the Mössbauer parameters measured probably would be too much for the readers (data for more than 50 spectra). We included two tables for Experiments 4 and 8 where regular trends were observed for the intensities of ferrate(VI) as well as of FeIII(1) and FeIII(2).

Comments 2: Please explain the why the "noramlized area" ranges from more than 3 to less tnan 0.1 and do not sum to 1. It is the most issue important I detected.

Response 2: The normalization of the area is only normalization to the baseline (more exactly: to the product „baseline value” x „velocity range” – so that area is compared to area) of the spectra to account for the different line areas solely due to the different durations of the measurements. This normalized area is proportional to the Mössbauer-Lamb factor. The proportionality factor largely depends on several other parameters like the matrix of the iron atoms in the absorber, the non-resonant background (setting the energy window for the 14.4 keV gamma rays), etc. Thus the normalized area values may be spread in a wide range.
This allows calculating the Debye-temperature correctly, but affects significantly its calculated error, which can be very high when the measurement conditions cannot be made optimal.

Comments 3: Maybe changing the figures to one panel (spectra arranged one below te other not in separate sub-figures) coud improve the visibility of the differences bettween temperatures.

Response 3: We did consider vertical arrangement of the spectra within a figure, but it would be suitable mostly for a two-column typesetting. However, Applied Sciences uses single column pages. Thus we would suggest keeping the present internal arrangements of the figures.

Reviewer 3 Report

Comments and Suggestions for Authors

Article entitled Quantitative analysis of ferrate(VI) and its degradation products in electrochemically produced potassium ferrate for wastewater treatment written by Zoltán Homonnay, Sándor Stichleutner, Ernő Kuzmann, Miklós Kuti, Győző G. Láng, Kende Attila Béres, László Trif, Dániel J. Nagy, Gyula Záray, József Lendvai and submitted to Applied Sciences journal as a draft no applsci-3181493 deals with an important issue of ferrate technology.

Article is in journal’s scope. Therefore, it could be considered for publication in Applied Sciences journal. As English is not my native language, I am not able to assess language correctness. However, while reading, I found some statements missing, confusing or unclear. Below, I enclose list of my comments.

The authors refer to their earlier publication [4] in several places in a way that suggests a very high similarity of the research. I would suggest slightly reworking the indicated fragments, at the same time showing the difference between the publications and emphasizing the novelty of the current publication.

Fig 7. I would suggest giving the temperature, as elsewhere in the article in K.

The manuscript gives the impression of a research report. The discussion of the results with the literature is relatively small, I would suggest expanding it.

The manuscript cites only 22 literature items, some of which are old or even very old. Only 9 cited publications are less than 10 years old. I don't know if citing a book on Mossbauer spectroscopy from over 50 years ago is a good idea. Maybe it's worth looking for something newer. It is therefore difficult to state that the publication presents the current state of knowledge. This may be the reason why the authors often cite their earlier publication [4].

It is worth improving the technical side of the article - the figures could be more legible and unified, contain units, etc. The panels could be described with letters. The temperature unit [K] could appear on the last panel - it is currently missing in figs 5,6, 8, 9.

The authors write in the abstract that the proposed method allows for obtaining significant stability of the material for "long term storage". I have not found any research results regarding the long term stability of the material. How exactly do the authors understand a long term?

Based on my comments and overall impression I suggest major revision.

Author Response

Comments 1: The authors refer to their earlier publication [4] in several places in a way that suggests a very high similarity of the research. I would suggest slightly reworking the indicated fragments, at the same time showing the difference between the publications and emphasizing the novelty of the current publication.

Response 1: Thank you for this comment. Yes, in our previous work, we reported on numerous Mössbauer measurements of ferrate(VI) products at  ~90 K only, approximating the ferrate(VI) content solely on the basis of the spectral areas. In this work, we performed several more series of experiments measuring Mössbauer spectra of samples as a function of temperature in order to calculate the Debye temperatures from which the Mössbauer-Lamb factors can be deduced and the ratio of spectral areas can be converted into molar ratios. (This results in quantitative chemical determination of the iron species. These calculations were successful for the ferrate(VI) and for the most abundant FeIII(1) species.)

We have modified two sentences on pages 2 and 3 to make this distinction clearer (lines 92 and 120, in yellow).

Comments 2: Fig 7. I would suggest giving the temperature, as elsewhere in the article in K.

Response 2: Fig. 7 has been corrected accordingly.

Comments 3: The manuscript gives the impression of a research report. The discussion of the results with the literature is relatively small, I would suggest expanding it.
The manuscript cites only 22 literature items, some of which are old or even very old. Only 9 cited publications are less than 10 years old. I don't know if citing a book on Mossbauer spectroscopy from over 50 years ago is a good idea. Maybe it's worth looking for something newer. It is therefore difficult to state that the publication presents the current state of knowledge. This may be the reason why the authors often cite their earlier publication [4].

Response 3: The focus of our study is rather restricted to the improvement of the quantitative determination of the ferrate(VI) samples by Mössbauer Spectroscopy through Debye temperature measurements. Thus the literature background is limited. We have expanded the literature part mostly by adding more references (plus 21) connected mostly to the application of the ferrate(VI) for various water treatment procedures.
The very old reference (Greenwood) refers to a basic formula which is very clearly explained in that book, the perhaps mostly respected fundamental work in Mössbauer Spectroscopy for chemists.

Comments 4: It is worth improving the technical side of the article - the figures could be more legible and unified, contain units, etc. The panels could be described with letters. The temperature unit [K] could appear on the last panel - it is currently missing in figs 5,6, 8, 9.

Response 4: We apologize for our mistake not noticing the missing units (K) in some figures. This is now corrected.
For better visibility, we changed the Excel default option „grey” for the axes labels to black. The temperature values are printed now in black bold. (In the MS version edited by us, the figures are simply inserted as screenshots with poor resolution. Original figures with high resolution are provided in separate files for the Journal.) 
We believe that writing temperature values into the figures instead of a), b), c), etc. helps the reader to understand the content more quickly.

Comments 5: The authors write in the abstract that the proposed method allows for obtaining significant stability of the material for "long term storage". I have not found any research results regarding the long term stability of the material. How exactly do the authors understand a long term?

Response 5: We have explained in the text that long term storage means insignificant decomposition over several months. The long term stability was studied in our previous publication [17] we refer to often in the text.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I suggest the publication of the manuscript in its present form.

Comments on the Quality of English Language

As usual please check spelling etc. before the final publication.

Reviewer 3 Report

Comments and Suggestions for Authors

This is my second reviev of this manuscript. Corrected version is better than the first one. The Authors answered my comments, suggested corrections have been applied. I suggest to accept this manuscript in its currenc form.