Diffusion and Sorption Studies of Cs, Sr and Co in Intact Crystalline Rock

Round 1

Reviewer 1 Report

The paper is devoted to the very important question: the experimental investigations of the diffusion and sorption of three cationic tracers, Sr2+, Co2+ and Cs+  ions in studied Grimsel granodiorite which is serving for the safety assessment of a nuclear waste repository. Of course, this is necessary to estimate the potential availability of nuclear pollution from the depositary.

The authors used different interesting and effective method.  Of course, C-14-PMMA autoradiography and scanning electron microscopy together give the conclusion that the diffusion within the unfractured rocks aare not very effective to produce the real migration of the radioactive elements.

The approaches with the electromagnetic fields show the real very strong influence on the migration of the studied isotopes.  This approach is very interesting from a theoretical point of view.  It seems that the natural electric fields have no such intensities. Such processes in nature could take place in some sulfide ore deposits where the ion migration may realise the natural electric battery.

 The authors demonstrate bothvery good theoretical knowledge and investigations and experimental technics as well as usage of their obtained results for the construction of theories.

 For the aplicatuion, it is clear that migration of the radioactive ions in the unfractured rock media is not dangerous for radioactive pollution.

It is necessary to control mainly the structure and fracturing of the rocks.  But what is necessary to estimate if the radioactive elements could produce the destruction of the feldspars and thus the channels for the migration in the hydrous solutions.  But this is a bit different type of problem which also should be checked.

It seems better to give some comparison with the similar works in the introduction and discussion and to widen a bit the list of the references.

The paper is an example of the very good experimental and theoretical investigation.  It is written in a good manner with clear language and it may be accepted after minor revision.

 

Author Response

Thanks very much for your recognition of our work. Your suggestions are very nice. We agree that some parts of the paper should be expanded. In the revised manuscript, we have added new discussions with other researchers’ work, e.g. the discussion of Co2+ diffusivities, and the sorption properties of all the tracers. All the changes are listed in the revised manuscript with change markers. Please check them if you are interested.  

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The proposed article is both scientifically and practically interesting because it addresses the issue of assessing the safety of proposed radioactive waste disposal in geological environments. Based on electrochemical experiments, the authors propose a method for estimating parameters such as radionuclide sorption distribution coefficient and radionuclide diffusion coefficient in granodiorite samples (from Grimsel URL).

Minor comments

1. In the text, references are formatted differently; for example, they are like Hellmuth et al., 1994 in one place and square brackets in another ([1], [2], etc.). Authors must standardize the reference format.

2. Equations and some symbols are inserted as images in the text. Authors must use an equation editor to insert equations.

3. Estimated sorption distribution coefficient values must be compared to values found in the literature.

4. I inserted some editorial and grammatical comments directly in the copy  of the submitted pdf manuscript (see attachment).

Major comments

My main concern with the paper is the doubts about the correctness of using equation (7) written in the Laplace-domain for experimental data processing. In this solution authors ignore the effect of dilution of the tracers in a receiving chamber with a certain volume, V.  This characteristic is present in the earlier equations (4)/(5) (plug-flow model), but is omitted in equations (7)/(8) (advection-dispersion model). Moreover, the authors did not show the correspondence of the dimensionless parameters used in equation (7) to the dimensional characteristics in the original equation (6). Therefore, there is no way to check the correctness of mathematical operations.

In general, the section devoted to the mathematical formulation of the problem, is written rather casually. For example, in equation (6), the concentration is denoted by the symbol C, but in solution (7), a new symbol for concentration appears, C_R, the question arises, if C and C_R  are the same ?  (it is clear that the upper bar over C_R denotes the Laplace transform). I do not understand the physical meaning of the convection velocity in equation (6) which requires the presence of the hydraulic gradient. Some more questions: does coefficient D in equation (6) differ from the coefficients D_e and D_d (line 369)? what can parameter R in (8) mean? There are a lot of more questions related to the extremely careless design of the manuscript.

According to the breakthrough curves (Figs 5, 6, and 7), the rise of the concentration in the receiver chamber does not exceed 1-3% from the boundary concentration, С₀, that seems to be too low to diagnose somehow the mass transport regime.    

Two below self-citations look a little strange with viewpoint of the topic of the reviewed manuscript.
 
28. Li, X.; Puhakka, E.; Liu, L.; Zhang, W.; Ikonen, J.; Lindberg, A.; Siitari-Kauppi, M. Multi-Site Surface Complexation 572 Modelling of Se(IV) Sorption on Biotite. Chemical Geology 2020, 533, 119433, doi:10.1016/j.chemgeo.2019.119433.
35. Li, X.; Puhakka, E.; Ikonen, J.; Söderlund, M.; Lindberg, A.; Holgersson, S.; Martin, A.; Siitari-Kauppi, M. Sorption of Se 586 Species on Mineral Surfaces, Part I: Batch Sorption and Multi-Site Modelling. Applied Geochemistry 2018, 95, 147–157, 587 doi:10.1016/j.apgeochem.2018.05.024. 

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thanks very much for your patient reading and precious comments. The sentences and the results will be more clarified and better presented after addressing these comments. The quality of the paper will be improved.

Please see the attachment for detailed answers.

Author Response File: Author Response.docx

Reviewer 3 Report

See attached pdf

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thanks very much for your patient reading and precious comments. The sentences and the results will be more clarified and better presented after addressing these comments. The quality of the paper will be improved.

Please see the attachment for detailed answers. 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I have one comment left regarding the omission of the definitions of dimensionless coefficients in equation (7).

Author Response

Dear reviewer,

Thanks very much for your patient reading again. Please see the attachment for the replies of your comments.

Author Response File: Author Response.docx

Reviewer 3 Report

The revision of the manuscript satisfactorily addresses my previous review comments. 

However the additional information provided describing the experimental system (Figure 2) indicates that the filters are comprised of rock material. This adds further complexity and uncertainty to the design since the metals studied could additionally sorb onto these surfaces. The paper should discuss this possibility and should also note in the methods section, the purpose of the filters and why this rock material is used rather than a more inert material. Similarly, the consideration of sorption of the metals onto other surfaces in the experiment and the use of a control blank experiment  requires discussion.

Overall the electromigration and petrographic results merit publication, but the conclusions regarding sorption processes are rather weak. 

Some minor comments are highlighted in the attached pdf

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thanks very much for your patient reading again.

Overall, after more careful consideration, I do agree that the sorption onto the filter surface might have caused errors in the measurements of breakthrough curves. This comment gives us some hints to improve our future experimental plans. Thanks very much for your great comments.

Please see the attachment for my replies. 

Author Response File: Author Response.docx