Long-term iron-bentonite interaction in a water-saturated room-temperature environment: Advancing for safety of metal waste containment in the geosphere

Abstract

Corrosion of C-steel containers may affect the long-term safety of deep geological repositories. To address uncertainties, the FeMo experiment was initiated in 2007 to evaluate iron mobility and bentonite alteration under aerobic conditions, granite-type water saturation, high corrosion rates, and room temperature. After 15 years, the experiment was dismantled for comprehensive analysis. The Fe-bentonite interface exhibited a multi-layered structure, with a black iron stripe and a brown-stained bentonite zone (2–5 mm thick). Altered iron comprised magnetite (50%), goethite (25%), siderite (5%), and ≈20% wt. metallic iron. The brown zone contained montmorillonite, cristobalite, feldspar, quartz, magnetite, goethite, lepidocrocite, and minor siderite. Chemical zonation of Fe, Mg, and Ca was observed within the first 0.5 mm from the interface. Iron penetrated up to 2–3 mm into the bentonite. Evidence of a trioctahedral clay component mixed with montmorillonite was found. Magnetite, carbonates, and Mg-rich clays precipitated under alkaline reducing conditions during the oxic-anoxic transient at the interface (<1 mm). This study's results are not only relevant for guaranteeing radioactive waste repository safety but also to assess other heavy metal containment strategies in other environmental applications, such as mines and landfills.