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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

Svensson, Per Daniel and Hansen, Staffan LU (2013) In Clays and Clay Minerals 61(6). p.566-579
Abstract
Excavated bentonite from two large iron-bentonite field experiments at Aspo 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... (More)
Excavated bentonite from two large iron-bentonite field experiments at Aspo 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 similar to 130 degrees C (ABM1) or similar to 150 degrees 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 similar to 10 cm (ABM1) and similar to 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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
ABM, Alteration, Alternative Buffer Material, Aspo, Bentonite, Iron, Montmorillonite, Redox, Temperature Buffer Test (TBT), XANES, XRD
in
Clays and Clay Minerals
volume
61
issue
6
pages
566 - 579
publisher
The Clay Minerals Society
external identifiers
  • wos:000339789500009
  • scopus:84902160696
ISSN
0009-8604
DOI
10.1346/CCMN.2013.0610609
language
English
LU publication?
yes
id
f9d3ea5d-90ce-4b5e-b6ba-ac4841926704 (old id 4668167)
date added to LUP
2014-09-24 14:42:44
date last changed
2019-02-20 01:08:09
@article{f9d3ea5d-90ce-4b5e-b6ba-ac4841926704,
  abstract     = {Excavated bentonite from two large iron-bentonite field experiments at Aspo 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 similar to 130 degrees C (ABM1) or similar to 150 degrees 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 similar to 10 cm (ABM1) and similar to 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.},
  author       = {Svensson, Per Daniel and Hansen, Staffan},
  issn         = {0009-8604},
  keyword      = {ABM,Alteration,Alternative Buffer Material,Aspo,Bentonite,Iron,Montmorillonite,Redox,Temperature Buffer Test (TBT),XANES,XRD},
  language     = {eng},
  number       = {6},
  pages        = {566--579},
  publisher    = {The Clay Minerals Society},
  series       = {Clays and Clay Minerals},
  title        = {Redox Chemistry in Two Iron-Bentonite Field Experiments at Äspö Hard Rock Laboratory, Sweden: An XRD and Fe K-Edge XANES Study},
  url          = {http://dx.doi.org/10.1346/CCMN.2013.0610609},
  volume       = {61},
  year         = {2013},
}