Interaction of a synthesized Na-montmorillonite compacted and percolated with pollutants (Pb, Zn): application to subsurface waste landfill
Résumé
Due to their low permeability and their capacity to retain pollutants, smectites are of great importance for industrial applications, in particular as engineered barrier in landfills. One of the main challenges is to predict the long-term behaviour of these clays, especially the evolution of their properties at different scales depending on interactions with pollutant metals. The originality of this study was to reproduce at a laboratory scale the in-situ compaction of clays soaked by metallic solutions coupled with the use of synthesized smectites whose chemical composition and charge location could be fixed. For this purpose, hydrothermal syntheses were carried out at 350°C and 120 MPa for 28 days, using the gelling method of Hamilton (1968) (Lantenois et al., 2008). As expected, the synthesized product contained one well-crystallized dioctahedral smectite phase (montmorillonite), as shown by X-ray diffraction, and had the following structural formulae, confirmed by ICP-ES and microprobe analyses: Na0.66[Al3.34Mg0.66][Si8]O20(OH)4.nH2O. Percolation experiments were performed with œdometer cells equipped with an injection system (Jullien et al., 2002). Clay samples were compacted up to 0.5 MPa then percolated under a constant pressure of 0.3 MPa, either with water or with metallic (Zn2+ or Pb2+) nitrate solutions at 2.10-3 mol/L. The hydraulic conductivity, k, was calculated from the measured injected volume and using the Darcy’s law. Low k values in the order of 10-12 m/s were obtained, either with water or with Pb2+ or Zn2+ solutions. After a percolation of nine months, neither Pb2+ nor Zn2+ was detected in leachates, which show their total retention in the compacted montmorillonite. Moreover, the measured Na+ contents in leachates coupled with the geochemical modelling (Phreeqc2) highlight the cationic exchange as the main mechanism for this pollutant retention. At the end of these percolation experiments, the compacted clay samples will be investigated along a vertical profile by using X-ray diffraction, scanning and transmission electron microscopy, and electron microprobe.
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Jullien A., Proust C., Le Forestier L., Baillif P. (2002). Hydro-chemio-mechanical coupling effects on permeability and swelling behaviour of a Ca smectite soaked by Cu solutions. Applied Clay Science, 21, 143-153.
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.