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

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 (2-5mm 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 0.5mm from the interface. Iron penetrated up to 2-3mm 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 (