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Evolution des propriétés macroscopiques d'argiles compactées et soumises à l'infiltration de polluants métalliques – METALCLAY

Submission summary

Nowadays, subsurface landfill is the main final destination of household and industrial wastes and residues obtained after incineration (bottom and fly ashes). According to the European legislation, engineered clay barriers are put on the sides and bottom of each pit, in order to prevent any pollution of soils and water tables by waste leachates. For this reason, clay materials must have a low hydraulic conductivity (less than 10-9 m.s-1) and a high cation exchange capacity for the retention of pollutants. However, even if clay barriers are initially in accordance with legislation, no information is known concerning their long term behaviour. The METALCLAY project aims to determine the evolution of macroscopic clay properties, when submitting to metallic solutions. Some experiments were already performed at ISTO on natural compacted clay materials, percolated with copper or lead solution. These first studies have shown that clays rapidly and significantly lose their swelling and cohesive properties, and also became more permeable. The METALCLAY project intends to better understand mechanisms responsible for these dramatic evolutions. Besides, smectites, one of the most abundant phyllosilicates in soils and sediments, have specific characteristics (high specific area, high cation exchange capacity, good hydration properties), which give to this clay mineral family a high industrial interest for waste confinement. For this reason, the METALCLAY team has chosen to deal with smectites. Two important aspects of METALCLAY project have to be stressed. Firstly, in order to better apprehend mechanisms of interaction between clays and metal cations, it is vital to use pure clays. For this reason, the METALCLAY team plans to work, not only with reference natural smectites, but also with synthetic smectites. Indeed, natural smectites also contain minor phases, such as calcite for the hectorite of Hector, quartz and feldspar for the montmorillonite Swy-2 from Wyoming, which can have an influence on the interactions with metals. In contrast, synthetic smectites are pure, and their local tetrahedral or octahedral charge deficit can be fixed. Hydrothermal synthesis requires three autoclaves (type 'Tuttle'), in which pressure (up to 3000 bars) and temperature (up to 750°C) are controlled separately. This specific equipment will be set up, tested and used by METALCLAY team, using the experimental facilities and expertise already at work at ISTO. Secondly, one of main technical skill of the METALCLAY team is its ability to reproduce at the laboratory scale the subsurface landfill conditions. Previous work done at the University of Orleans (Jullien and Lecomte, 2000), has lead to the development of an œdometric cell equipped with a separated injection system characterized by a controlled mechanical pressure. Moreover, experimental parameters have been defined and optimized, to reproduce the hydromechanical conditions of subsurface landfill as well (Pothier, 2001; Pothier et al., 2003). Natural and synthetic smectite samples will be compacted then percolated under constant pressure, with pure water or with solutions having a fixed metal concentration. Copper, nickel, zinc, and chromium will be investigated; these metals are all present in landfill leachates. Hydraulic conductivity will be calculated from measured parameters using Darcy's law. Moreover, chemical analyses (cations and anions) and pH values of leachates will give us information about the evolution of clay-metal exchanges. Samples obtained at the end of each œdometer experiment will be characterized in detail, by several methods: ICP-ES and ICP-MS for chemical analyses, cryo-scanning electron microscopy and transmission electron microscopy for structural pictures, X-ray diffraction for mineralogy, X-ray absorption spectrocopy (EXAFS) for the determination of metal speciation. X-ray diffractograms will be simulated, to have information about the evolution of interfoliar space and about possible modifications in the perpendicular plan ab. In addition, the transport/chemical modelling aspect will be investigated, in terms of hydrological aspects, major species equilibrium in clayey media, and sorption and cation exchange processes. This geochemical modelling at the interface between clays and solution will be performed with the PHREEQC2 software and PHAST coupled code, and will be enhanced thanks to the obtained experimental results. The METALCLAY project aims to combine an experimental approach with a fine characterization and a geochemical modelling, in order to understand the main mechanisms of metal-smectite interactions (sorption either on surface or in interlayer smectites) and to better define the evolution of clay macroscopic properties. The main objective is to contribute to the elaboration of a tool for predicting the long term behaviour of clay barriers.

Project coordination

Lydie LE FORESTIER (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 150,393 euros
Beginning and duration of the scientific project: - 48 Months

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