Interaction of Corroding Iron with Eight Bentonites in the Alternative Buffer Materials Field Experiment (ABM2)

• Published in: Minerals (Basel) Vol. 11; no. 8; p. 907
• Main Authors: Wersin, Paul, Hadi, Jebril, Jenni, Andreas, Svensson, Daniel, Grenèche, Jean-Marc, Sellin, Patrik, Leupin, Olivier X.
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.08.2021; MDPI
• Subjects: Analytical methods; Bentonite; Boreholes; Buffers; Carbon steel; Cementation; Chemical interactions; Chemical reactions; Clay; Corrosion; Electron microscopy; Experiments; Fluorescence; Galvanic corrosion; Grain size; High level radioactive wastes; in situ experiment; interface; Interfaces; Iron; Iron 57; Laboratories; Nuclear fuels; Oxides; Oxidoreductions; Physics; Quartz; Radioactive wastes; Raman spectroscopy; Redox reactions; Scanning electron microscopy; Scientific imaging; Smectites; Spatial discrimination; Spatial resolution; Spectrometry; Waste management; X rays; X-ray diffraction; X-ray fluorescence; X-ray spectroscopy; Sweden; iron; bentonite; in situ experiment; interface
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min11080907

Bentonite, a common smectite-rich buffer material, is in direct contact with corroding steel in many high-level radioactive waste repository designs. The interaction of iron with the smectite-rich clay may affect its swelling and sealing properties by processes such as alteration, redox reactions and cementation. The chemical interactions were investigated by analysing the Fe/clay interfaces of eight bentonite blocks which had been exposed to temperatures up to 130 °C for five years in the ABM2 borehole at the Äspö Hard Rock Laboratory managed by the Swedish Nuclear Fuel and Waste Management Co (SKB). Eleven interface samples were characterised by high spatial resolution methods, including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and μ-Raman spectroscopy as well as by “bulk” methods X-ray diffraction, X-ray fluorescence and 57Fe Mössbauer spectrometry. Corrosion induced an iron front of 5–20 mm into the bentonite, except for the high-Fe bentonite where no Fe increase was detected. This Fe front consisted mainly of ferric (oxyhydr)oxides in addition to the structural Fe in the smectite fraction which had been partially reduced by the interaction process. Fe(II) was also found to extend further into the clay, but its nature could not be identified. The consistent behaviour is explained by the redox evolution, which shifts from oxidising to reducing conditions during the experiment. No indication of smectite alteration was found.