Restoration Insights Gained from a Field Deployment of Dithionite and Acetate at a Uranium In Situ Recovery Mine

Round 1

Reviewer 1 Report

This manuscript lays out the results of an ISR restoration experiment in nice detail and discusses reasons why reduction of uranium to U(IV) does not seem to occur (along with future suggestions). Discussions on the single-well push-pull tests vs. the manuscript multiple well testing were a bit confusing in the introduction on "where one ended and the other started" from the interpretation standpoint. In addition, it is easy to "get lost" in the results section with Fig. 3-8 and text intermixed and no subheadings. In the attached pdf I have commented directly in the text to highlight suggestions for improvement. Other main suggestions in the commented pdf are renaming the discussion section to the conclusion section with the addition of subheadings and/or a bulleted list summary to highlight important findings. This might help guide readers through all of the detailed geochemistry. 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript describes the results of an extensive field study dedicated to the remediation of former uranium sites that utilized in-situ recovery (ISR). The idea of this investigation was to inject sodium dithionite into the aquifer to reduce U(VI) to less soluble U(IV). The dithionite treatment was followed by injection of sodium acetate to ensure reducing conditions via biostimulation. The field experiment took place at the Smith Ranch Highland (SRH) mine located at the Wyoming`s Powder River basin. Six five-spot well patterns were involved in these multi-well reductive amendments experiment that lasted ten months in total. The samples collected periodically from each of the six productions wells were analyzed for cations by ICP-OES. Uranium was determined by ICP-MS and anions by ion chromatography. TOC, pH, and oxidation-reduction potential (ORP) were measured as well.

The results obtained are clearly presented in the text and the accompanying figures and tables. It was observed that uranium and iron concentrations increased throughout the reductive treatment and remained higher that before this study. The increase of iron in the well water has been rationalized by reductive dissolution of Fe(III) minerals. As consequence, U(VI) sorbed on iron(III) minerals is released into the aquiver. Due to the presence of Ca²⁺ and carbonate, U(VI) forms stable carbonate complexes with and without Ca that are more difficult to reduce to U(IV) than UO₂²⁺. Based on their results, the authors suggest several strategies to improve the remediation of ISR mining sites. Without any doubt, the results are very interesting and show that the reductive treatment of sandstone deposits of former ISR facilities can be very complex, involving several interlinked processes like ion exchange, reductive mineral dissolution, complexation, precipitation etc.

The manuscript can be recommended for publication in Minerals with minor revision. I suggest the authors consider the following in the revision:

1. In Table 2 the calculated uranium speciation in a typical background water and a typical post-mining ore zone water of the SRH facility are cited from literature (from [41]). The importance of uranium speciation cannot be overemphasized! Please perform similar calculations of the uranium speciation for important steps of the field study, for example at the end of dithionite injection or shortly after acetate injection. Without such calculations it is not clear to the reader which role sulfate or acetate complexation of U(VI) plays in the overall speciation of uranium. A few of such calculations would support the authors’ statement about the importance of Ca-U(VI)-CO₃
2. It is good that Table 1 reports values on pH and ORP for six productions wells at various times during the test. It would be possible to calculate the stability fields of Fe(II)/Fe(III), U(IV)/U(VI), and also of Se(IV)/Se(VI) in a pH-Eh diagram. Such Pourbaix diagrams would allow to verify that the conclusions about the dominance of dissolved Fe(II) and U(VI) in the well samples agrees with measured pH-ORP conditions. Also, the drop of Se concentration, that is rationalized by reduction of Se(VI) to Se(IV) (or Se(0)), could be discussed in the light of the corresponding Pourbaix diagram.
3. Please add some information about the mineralogical composition of the sandstone deposit of the former ISR facility. What are the minerals capable of significant ion exchange?
4. Page 7, line 269: Subscript in HNO₃

Author Response

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Author Response File: Author Response.docx