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 di-trigonal 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.

Because of their isolating capacity, smectite-rich clays have been proposed as buffer and backfill materials in high-level radioactive waste repositories. These repositories have to guarantee long-term safety for ~1 million years. Thermodynamics and kinetics of possible alteration processes of bentonite determine its long-term performance as a barrier material. Smectites in 25 different clays and bentonites were investigated in order to identify possible differences in their rates of alteration. These samples were saturated for 30 days in 1 M NaCl solution and deionized water, and then overhead rotated at speeds of 20 rpm and 60 rpm. Depending on the octahedral and interlayer composition, each of the smectites studied had specific rate of alteration, a so-called specific dissolution potential of smectite. The bentonites were classed as ‘slow-reacting bentonite’, ‘moderate-reacting bentonite’, or ‘fast-reacting bentonite’ corresponding to a relatively low (ΔP specific dissolution potential — <-5%), moderate (-5% < ΔP < -20%), or high specific dissolution potential (ΔP > -20%), respectively. The larger the amount of octahedral Fe and Mg compared to octahedral Al, the greater the specific dissolution potential. The present study found that the interlayer composition has a discernible impact on the rate of alteration. In experiments with rotation speeds of 60 rpm and a 1 M NaCl solution, Na+ was found to be the stabilizing cation in the interlayers of all the smectites. The Na-stabilizing mechanism was identified in only some of the smectites (type A) in experiments with 20 rpm (1 M NaCl solution). A second stabilization mechanism (by interlayer cations; Ca and Mg) was identified for other smectites (type B). Each bentonite has a specific rate of alteration. ‘Slow-reacting bentonite’ and clay with smectite-illite interstratifications are recommended as potential clay barriers in HLW repositories. The experimental and analytical procedures described here could be applied to potential barrier materials to identify ‘slow-reacting bentonite’.