Textural and hydration properties of a synthetic montmorillonite compared with the natural Na-SWy2 clay analogue
Résumé
Smectite, the main clay mineral in bentonites, is responsible for their low permeability, their high specific surface and their capacity to retain pollutants, properties of great importance in industrial applications. When looking at improvement of processes or at understanding the major mechanisms, it appears judicious to choose synthetic clays instead of natural reference clay samples, due to their homogeneity and their reproducibility. Whereas the dioctahedral smectites are predominant in soils and widely used in industry, these clays are rarely synthesized and used in systematic studies. The objective of this study is to synthesize a montmorillonite (Na-MMT), and to compare its textural and hydration properties to the natural Na-exchanged Wyoming montmorillonite (Na-SWy2).
Hydrothermal syntheses were carried out at 350°C and 120 MPa for 28 days, by using the gelling method of Hamilton (1968) (Lantenois et al., 2008). The chemical analyses using ICP-OES coupled with electron microprobe demonstrate that the obtained structural formulae is near from the theoretical one: Na0.66[Al3.34Mg0.66][Si8]O20(OH)4.nH2O. Moreover, X-ray diffraction reveals that smectite is the only crystalline phase in Na-MMT sample, with the mean position of the (06.33) band at 1.49 nm, confirming the presence of dioctahedral domains in the octahedral sheets. Na-MMT has a CEC value of 83 meq/100g and a specific surface area of 764 m2/g, characteristic of swollen smectites. N2 adsorption-desorption isotherms coupled with the t-plot method reveal a specific surface of 87 m2/g and also give the pore size distribution. In addition, low-pressure argon adsorption coupled with the Derivative Isotherm Method (Villiéras, 1992) allowed to give information about adsorption energy distributions on basal and edge surfaces. Hydration properties were obtained at once by continuous water gravimetry and by X-ray diffraction along a water adsorption-desorption cycle. Percolation experiments using œdometer cells were carried out with water through compacted clay samples, in order to simulate subsurface waste landfill. Low hydraulic conductivities in the order of 10-12 m/s were obtained for Na-MMT and Na-SWy2.
Hamilton D.L., Henderson C.M.B. (1968). The preparation of silicate compositions by a gelling method. Mineralogical Magazine, 36, 832-838.
Villiéras F., Cases J.M., François M., Michot L.J. & Thomas F. (1992) Texture and surface energetic heterogeneity of solids from modeling of low pressure gas adsorption isotherms. Langmuir, 8, 1789-1795.
Lantenois S., Champallier R., Bény J.M., Muller F. (2008). Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series. Applied Clay Science, 38, 165-178.
Keywords: Hydrothermal synthesis, Adsorption isotherm, Oedometer cell.