Bentonite Alteration in Batch Reactor Experiments with and without Organic Supplements: Implications for the Disposal of Radioactive Waste

Round 1

Reviewer 1 Report

GENERAL COMMENTS

The paper studies the alteration of two bentonites, subjected to temperature (25 and 90ºC) and hydration with different synthetic groundwaters. The characterization included the analyses of microbial diversity.

The study is of interest in the field of nuclear waste management, where bentonites are considered as engineered barriers. The paper fits well with the journal scope and audience. The paper is well written and easy to follow.

Significant part of the paper is devoted to characterization of the initial and final clays. But this is not the case for the microbial analyses. The inclusion of the microbial analyses of the initial samples is mandatory! The analysis of the final samples is irrelevant if it is not compared with the initial stage.

However, the representativeness of this study to a particular repository scenario is arguable. Bentonites are mixed with synthetic ground waters and not compacted, and the study last two years under very specific conditions. The study is rather specific, with two particular bentonites (beidellite and Fe-rich), not fully representative of a real repository, subjected to very specific conditions.

In general, discussion of the implications of the observed changes (or lack thereof) is missed. In the results section, the authors simply say that at 25º they find whatever change, and not with 90ºC and that in the sample subjected to OPA water they showed uptake of Na⁺, or not,….…… In the discussion section, the findings are treated in a general way, but little discussion on the implications in a real repository. Please include some discussion about the relevance of the conditions analysed, and on the possible extrapolation of your findings to a real repository scenario.

MATERIALS AND METHODS

• Please give some details on the relevance of selecting a beidellite (SD80) and Fe-rich (B36) bentonites for the study, in comparison of other most-commonly analysed Na- or Mg-Ca bentonites. Dis you expect any particular effect compared to a Na or Ca-Mg bentonite? Please discuss.
• Page 3, line 102. Why you have not sterilized the bentonites and substrates? I agree that bentonite in a repository will not be sterilized, but the lack of sterilization may have an impact on “scientific” microbial analyses. Please explain.

Water solutions.

• Was the Opalinus synthetic water prepared under oxygen-free atmosphere? If not, I highly doubt the balance has been maintained. Please indicate.
• Table 1. Please provide the chemical analyses of selected synthetic waters and not the receipts.
• Table 1: There is no need to provide the oxide composition both in g/L and mol/L
• Page 4, line 121. Why do you dialysed the bentonite? With what electrolyte? Did you dialysed the samples before and after the complete experiments? Please indicate.

RESULTS

• Are the initial characterisation values representative (average values) of studied bentonites or are inhomogeneous deposits? Please comment.
• Please compare your results with previous studies on the same clays.

Microbial analyses.

• It is mandatory to include the microbiological analysis of the initial samples. Please include this information. As you do not sterilize, the origin of the detected microorganisms is debatable.
• Where the reaction experiments done in duplicate or triplicate? The microbial results could be “exotic” and therefore hardly extrapolated to a repository case!
• Page 10 line 320. Authors mentioned that both bentonites have abundance of “unknown genera” Please explain why you cannot identify them.
• The results including “donors” (acetate, lactate,…) have little relevance. Where the values very different form the raw cases? Please discuss!
• No discussion is made on the findings!! Please comment.

Charge distribution

• No comparison is made on the findings. Please comment.

Solution chemistry

• Do acetate or lactate,.. influence chemical conditions? Please indicate.

• Table 8, why you do not have any pH data? Have you tried to include the chemical composition in a geochemical code to estimate the value?
• Table 9 and Page 16, line 470. How do you explain the low pH values you reach in some experiments at 90ºC with B36?? In line 531, authors said that reported decrease in pH by cation exchange (CE), but I doubt CE is responsible for such lower decrease!! Please comment

Charge distribution.

• Page 20, line 580. Authors mentioned that that it was likely that smectite in each material acted differently. Did you carry out the same experiments with other bentonites? I don’t see why a smectite should act differently, but rather it may be differently affected by the different environments.

• I missed in the discussion a paragraph devoted to express the relevance of your specific experimental conditions in a real repository scenario. Please include.

Conclusions

• Please, introduce the conclusions with a paragraph summarising the specific experimental conditions to which the bentonites where subjected.

Author Response

Dear Reviewer,

We thank you for reviewing our manuscript and the many constructive comments.

Please find attached a point-by-point response to your comments in which we highlight the improvements made in the text.

We look forward to hearing from you.

Sincerely,
Carolin Podlech

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript of Podlech et al. is original and very well written.
It details with precision and clarity the objectives pursued, the methods used and the results obtained. I recommend its publication without changes.

The use of bentonites as a barrier for the disposal of radioactive waste is well known, and numerous publications have been made about it. The novelty of this manuscript lies in the thoroughness of the analysis carried out, particularly in relation to the variation of the structural formula and layer charge distribution of the smectites present in the two studied sets of bentonite samples, as a consequence of variations in temperature and /or chemical disequilibrium caused by the contact with different solutions. Likewise, the potential influence of long-term microbial activity in environments within the repository setting is discussed in depth.

The authors present numerous data on the possible mineralogical, chemical and crystallinity variations observed in smectites to determine the alteration mechanisms of bentonites subjected to two types of solution (19 gL-1 and 155 gL-1) and two temperatures (25 ° C and 90 ° C), after 1 and 2 years of experimentation. The conclusions are consistent with the arguments presented. 

Author Response

Dear Reviewer,

Thank you for reviewing our manuscript. We revised the manuscript according to the comments of all the reviewers. We have added additional information regarding the relevance of the added organic substrates as well as more discussion of our results in terms of their relevance to a real repository site.

We look forward to hearing from you.

Sincerely,
Carolin Podlech