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Redox Chemistry in Two Iron-Bentonite Field Experiments at Äspö Hard Rock Laboratory, Sweden: An XRD and Fe K-Edge Xanes Study

Published online by Cambridge University Press:  01 January 2024

Per Daniel Svensson  and
Staffan Hansen
Per Daniel Svensson*
Affiliation:
Swedish Nuclear Fuel and Waste Management Co, Oskarshamn, Sweden
Staffan Hansen
Affiliation:
Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Sweden
*
*E-mail address of corresponding author: Daniel.Svensson@skb.se
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Abstract

Excavated bentonite from two large iron-bentonite field experiments at Äspö Hard Rock Laboratory in Sweden was investigated with respect to iron redox chemistry and mineralogy. The iron redox chemistry was studied by Fe K-edge X-ray absorption near edge structure spectroscopy and the mineral phases were studied using X-ray diffraction. Bentonite is to be used as a buffer material in high-level radioactive waste repositories to protect the waste containers from their surroundings. Montmorillonite, which is responsible for the sealing properties in the bentonite, is susceptible to redox reactions. A change in the montmorillonite iron redox chemistry may affect its layer charge and hence its properties. The experiments included are the first Alternative Buffer Material test (ABM1) and the Temperature Buffer Test (TBT). The clays were heated to a maximum of ~130°C (ABM1) or ~150°C (TBT) for 2.5 and 7 y, respectively. In the central part of the compacted clay blocks was placed an iron heater and the distance from the heater to the rock was ~10 cm (ABM1) and ~50 cm (TBT), respectively. Eleven different clay materials were included in the ABM1 experiment and five were analyzed here. In the ABM1 experiment, the Fe(II)/Fe(III) ratio was increased in several samples from the vicinity of the heater. Kinetic data were collected and showed that most of the Fe(II)-rich samples oxidized rapidly when exposed to atmospheric oxygen. In the TBT experiment the corrosion products were dominated by Fe(III) and no significant increase in Fe(II) was seen. In ABM1, reducing conditions were achieved, at least in parts of the experiment; in TBT, reducing conditions were not achieved. The difference was attributed to the larger scale of the TBT experiment, providing more oxygen after the installation, and to the longer time taken for water saturation; oxidation of the samples during excavation cannot be ruled out. Minor changes in the bentonite mineral phases were found in some cases where direct contact was made with the iron heater but no significant impact on the bentonite performance in high-level radioactive waste applications was expected as a result.

Keywords

ABMAlterationAlternative Buffer MaterialÄspöBentoniteIronMontmorilloniteRedoxTemperature Buffer Test (TBT)XANESXRD
Type
Research Article
Information
Clays and Clay Minerals , Volume 61 , Issue 6 , December 2013 , pp. 566 - 579
Copyright
Copyright © European Higher Education Society 2013

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