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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

View larger version (33K): [in a new window]   Figure 1. XRD patterns obtained on unreacted and reacted smectite samples (upper and lower patterns, respectively). Montmorillonite, beidellite, nontronite and saponite samples were reacted for 45 days with metal iron at 80°C (see text for details). (a) Sample SWy-2, (b) sample SAz-1, (c) sample SbId, (d) sample CP4, (e) sample Garfield, (f) sample SapCa-2, and (g) sample SapFe08. Scale factor (x3) over the 20–35°2 CoK range. Patterns were normalized by pairs to the integrated intensity of the corundum 012 reflection (solid line, S). The 001 reflection and the 02,11 reflection of the clay samples are labeled. Q indicates the presence of quartz impurities in some samples (dashed lines). Dotted lines indicate the reflections of newly-formed phases. These phases are labeled M, L and P for magnetite (Fe3O4), lepidocrocite (-FeOOH), and a newly-formed 1:1 phyllosilicate, respectively.

View larger version (18K): [in a new window]   Figure 2. Method for quantifying the relative proportion of destabilized smectite using IR spectroscopy. (a) IR spectrum of sample SWy-2 (unreacted) before subtraction of the baseline. Si-O indicates the Si–O vibrations in phyllosilicates whereas Al–Al–OH, Al–Fe–OH and Al–Mg–OH refer to the OH-bending mode of the respective hydroxyl groups. Q denotes the two vibrations bands of quartz. (b) Decomposition assuming three Gaussian-shaped contributions of the OH-bending zone after baseline subtraction (sample SWy-2 unreacted).

View larger version (101K): [in a new window]   Figure 3. SEMimages of unreacted and reacted samples. Metal Fe particles in unreacted and reacted SWy-2 sample are shown in (a) and (b), respectively. (c,d) Smectite particles in the reacted sample SWy-2. (e) Smectite particles in the initial sample SWy-2. (f) Smectite particles in the reacted Garfield sample. (g) Smectite particles in the initial sample SapCa-2. (h) Smectite particles in the reacted sample SapCa-2.

View larger version (23K): [in a new window]   Figure 4. Relative proportion of destabilized smectite calculated by XRD and CEC methods (a) and by XRD and IR methods (b). Solid square: sample SapCa-2; solid triangle: sample SAz-1; solid circle: sample SWy-2; solid diamond: Garfield sample; open diamond: Drayton sample. The (1:1) line is shown as a solid line

View larger version (23K): [in a new window]   Figure 5. XRD patterns obtained for unreacted SbId sample (a) and for reacted SbId sample (b–e). (b, c, d, e) correspond to 5, 15, 30 and 45 day experiments, respectively. Scale factor (x3) over the 20–35°2 CoK range. Patterns were normalized as in Figure 1. Other labels and patterns as in Figure 1.

View larger version (18K): [in a new window]   Figure 6. Evolution of the relative proportion of destabilized smectite as a function of reaction time. Symbols as in Figure 4. Open triangles: sample SbId.

View larger version (20K): [in a new window]   Figure 7. XRD patterns obtained on unreacted and reacted beidellite samples (upper and lower patterns, respectively). Patterns are ranked as a function of their structural Fe3+ content (Table 1). Samples were reacted for 45 days with metal Fe at 80°C without controlling the initial pH. (a) Sample SbS-1, (b) Drayton sample, and (c) sample SWa-1. Scale factor (x3) over the 20–35°2 CoK range. Patterns were normalized as in Figure 1. Other labels and patterns as in Figure 1.

View larger version (15K): [in a new window]   Figure 8. Relative proportion of destabilized smectite after reaction with metal Fe as a function of their content of structural Fe3+ (Table 1). The relative proportions of destabilized smectite were estimated using XRD in 45 day experiments.

View larger version (21K): [in a new window]   Figure 9. XRD patterns recorded on sample SWy-2 reacted with metal Fe after K, Ca and Na saturation (b, c and d, respectively). Unreacted Na-saturated sample corresponds to the raw SWy-2 sample (a). Scale factor (x3) over the 20–35°2 CoK range. Patterns were normalized as in Figure 1. Other labels and patterns as in Figure 1.

View larger version (93K): [in a new window]   Figure 10. Transmission electron micrograph of smectite particles from sample CP4. ‘Rolled’ and ‘flat’ particles are labeled 1 and 2, respectively.

View larger version (24K): [in a new window]   Figure 11. Conceptual model leading to the destabilization of dioctahedral smectites as a result of their interaction with metal Fe. (a) Deprotonation of MeFe3+OH groups. (b) Oxidation of metal Fe as a result of its interaction with released protons. (c) Sorption of Fe2+ cations on the edges of smectite particles and reduction of structural Fe3+ cations. (d) Migration of solution Fe2+ cations in smectite interlayers to compensate for the layer charge deficit. Part of this migration results from an Fe2+-for-Na+ exchange. With time, interlayer Fe2+ cations migrate to the di-trigonal cavity and further to the octahedral sheet to compensate locally for the charge deficit. (e) Coexistence of di- and trioctahedral domains in the octahedral sheet of smectite leads to its destabilization.