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

DOI: 10.1180/minmag.2015.079.6.11 DOI Help

Authors: J. Quirke (University of Manchester) , C. M. B. Henderson (University of Manchester) , R. A. D. Pattrick (University of Manchester) , K. M. Rosso (University of Manchester) , A Dent (Diamond Light Source) , J. W. Sharples (University of Manchester) , C. I. Pearce (University of Manchester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Mineralogical Magazine , VOL 79 (6) , PAGES 1353 - 1367

State: Published (Approved)
Published: December 2015
Diamond Proposal Number(s): 4941

Open Access Open Access

Abstract: 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.

Journal Keywords: geosphere; iron-bearing minerals; Fe(II)/Fe(III) ratio; reducing potential; X-ray absorption

Subject Areas: Environment, Earth Science, Chemistry

Instruments: B18-Core EXAFS