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EXPERIMENTAL STUDY OF SMECTITE INTERACTION WITH METAL Fe AT LOW TEMPERATURE: 1. SMECTITE DESTABILIZATION

¹ Institut des Sciences de la Terre d’Orléans (ISTO), CNRS – Université d’Orléans, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France
² Environmental Geochemistry Group, LGIT, Maison des GéoSciences, Université J. Fourier – CNRS, BP 53, 38041 Grenoble Cedex 9, France
³ Institut für Nukleare Entsorgung, Forschungzentrum Karlsruhe, PO Box 3640, 76021 Karlsruhe Germany
⁴ Commissariat à l’Energie Atomique (CEA), Centre d’Etude de Cadarache DEN/DTN/SMTM/Laboratoire de Modélisation des Transferts dans l’Environnement, Bat 307, 13108 Saint Paul Lez Durance Cedex, France

^* E-mail address of corresponding author: sebastien.lantenois{at}univ-orleans.fr

Interaction between metal Fe and a variety of natural and synthetic smectite samples with contrasting crystal chemistry was studied by scanning electron microscopy and X-ray diffraction from experiments conducted at 80°C. These experiments demonstrate an important reactivity contrast as a function of smectite crystal chemistry. An XRD method involving the use of an internal standard allowed quantification of the relative proportion of smectite destabilized as a function of initial pH conditions as well as of smectite structural parameters. In mildly acidic to neutral pH conditions, a significant proportion of metal Fe is corroded to form magnetite without smectite destabilization. Under basic pH conditions, smectite and metal Fe are partly destabilized to form magnetite and newly-formed 1:1 phyllosilicate phases (odinite and crondstedtite). More specifically, systematic destabilization of both metal Fe and smectite is observed for dioctahedral smectites while trioctahedral smectites are essentially unaffected under similar experimental conditions. In addition, smectite reactivity is enhanced with increasing Fe³⁺ content and with the presence of Na⁺ cations in smectite interlayers. A conceptual model for smectite destabilization is proposed. This model involves first the release of protons from smectite structure, MeFe³⁺OH groups being deprotonated preferentially and metal Fe acting as proton acceptor. Corrosion of metal Fe results from its interaction with these protons. The Fe²⁺ cations resulting from this corrosion process sorb on the edges of smectite particles to induce the reduction of structural Fe³⁺ and migrate into smectite interlayers to compensate for the increased layer-charge deficit. Interlayer Fe²⁺ cations subsequently migrate to the octahedral sheet of smectite because of the extremely large layer-charge deficit. At low temperature, this migration is favored by the reaction time and by the absence of protons within the ditrigonal cavity. Smectite destabilization results from the inability of the tetrahedral sheets to accommodate the larger dimensions of the newly formed trioctahedral domains resulting from the migration of Fe²⁺ cations.

Key Words: Clay Barrier • Clay Stability • Engineered Barrier • Fe–clay Interactions • Fe Corrosion • Nuclear Waste Disposal • Smectite • X-ray Diffraction

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