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Characterizing mineralogy and redox reactivity in potential host rocks for a UK geological disposal facility

Published online by Cambridge University Press:  02 January 2018

J. Quirke ,
C. M. B. Henderson ,
R. A. D. Pattrick ,
K. M. Rosso ,
A. Dent ,
J. W. Sharples  and
C. I. Pearce
J. Quirke
Affiliation:
School of Chemistry and Dalton Nuclear Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK
C. M. B. Henderson
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK ASTeC, STFC Daresbury Laboratory, Warrington WA4 4AD, UK
R. A. D. Pattrick
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
K. M. Rosso
Affiliation:
School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK Pacific Northwest National Laboratory, Richland, Washington, 99352, USA
A. Dent
Affiliation:
Diamond Light Source, Didcot, Oxfordshire OX11 0DE, UK
J. W. Sharples
Affiliation:
Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK
C. I. Pearce*
Affiliation:
School of Chemistry and Dalton Nuclear Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK
*
*E-mail: Carolyn.Pearce@manchester.ac.uk
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Abstract

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Geological disposal facilities (GDF) are intended to isolate and contain radioactive waste within multiple protective barriers, deep underground, to ensure that no harmful quantities of radioactivity reach the surface environment. The last line of defense in a multi-barrier GDF is the geosphere, where iron is present in the host rock mineralogy as either Fe(II) or Fe(III), and in groundwater as Fe(II) under reducing conditions. The mobility of risk-driving radionuclides, including uranium and technetium, in the environment is affected significantly by their valence state. Due to its low redox potential, Fe(II) can mediate reduction of these radionuclides from their oxidized, highly mobile, soluble state to their reduced, insoluble state, preventing them from reaching the biosphere. Here a study of five types of potential host rocks, two granitoids, an andesite, a mudstone and a clay-rich carbonate, is reported. The bulk rocks and their minerals were analysed for iron content, Fe(II/III) ratio, and for the speciation and fine-grained nature of alteration product minerals that might have important controls on groundwater interaction. Total iron content varies between 0.9% in clays to 5.6% in the andesite. X-ray absorption spectroscopy reveals that Fe in the granitoids and andesite is predominantly Fe(II), and in mudstones, argillaceous limestone and terrestrial sandstone is predominantly Fe(III). The redox reactivity of the potential host rocks both in the presence and absence of Fe(II)-containing 'model' groundwater was investigated using an azo dye as a probe molecule. Reduction rates as determined by reactivity with the azo dye were correlated with the ability of the rocks to uptake Fe(II) from groundwater rather than with initial Fe(II) content. Potential GDF host rocks must be characterized in terms of mineralogy, texture, grain size and bulk geochemistry to assess how they might interact with groundwater. This study highlights the importance of redox reactivity, not just total iron and Fe(II)/(III) ratio, when considering the host rock performance as a barrier material to limit transport of radionuclides from the GDF.

Keywords

geosphereiron-bearing mineralsFE(II)/FE(iii) ratioreducing potentialX-ray absorption spectroscopygranitoidsandesitemudstoneclay-rich carbonate
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 6 , November 2015 , pp. 1353 - 1367
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2015. This is an open access article, distributed under the terms of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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