Etude multi-échelle des propriétés structurales et dynamiques de l’eau et des ions dans les milieux poreux de smectite – POROUSCLAY
Numerous processes located at the Earth surface or at its sub-surface imply interactions between fluids and minerals and lead to important modifications of the initial geological material. Such interactions require taking into account the mineralogy and the texture of the material, the chemical reaction occurring in the system, etc' For most minerals the interactions occur at particle surfaces. Consequently lamellar clay minerals showing small particle sizes and thus high specific surface area are known to be one of the most reactive components. Among them, swelling clay minerals (smectite) present in addition the peculiar property to sorb water and ions also inside the crystals. Water sorption induces a swelling of the structure leading to a complete modification of the pore network and thus impacts the fluid circulation pathways. For these reasons, smectite minerals play a major role in the control of the physical and chemical behaviour of the environments/rocks where they are found (soils, sedimentary rocks and engineered barrier for waste confinement). In the scope of constraining reactive transport models of water and elements in smectite porous medium, the present project aims at providing a full characterization and understanding of the different processes occurring in a finite volume of the system for volume scale lower than the cubic millimetre for which the spatial and dynamical properties can be extended to larger scales. For smectite such a volume has to consider the presence of different sorption sites and reservoirs for water and cations (crystalline sites, micro- to macro-pores) and has to take into account the presence of heterogeneities at each scale of investigation. This can be achieved only through a multi-technique approach. The experimental program will consider a series of 4 smectite samples with different crystal-chemistry (amount and location of layer charge). Two interlayer cations, extremely abundant in natural waters, will be considered: Na and Ca, because of (i) their extremely contrasted behavior in term of hydration and swelling properties and (ii) their contrasted effect on water mobility. The first task will focus on the determination of the location of the different types of water (in interlayer spaces, at the crystal edges or in the meso and macropores) as a function of relative humidity. Such balance of water distribution is a fundamental prerequisite to determine the degree of hydration at which the main changes in organization and dynamics of water and ions are expected to occur. The objective of the second task will be to determine the parameters controlling the organization and mobility of the interlayer species, representing the main location of water molecules for relative humidity below 80%. These parameters will be obtained through a coupling (i) between information concerning the organization of water and ion organization in the interlayer space and their dynamics properties and (ii) between information derived from numerical procedures and those obtained experimentally. The third task will focus on the organization of the system at the mesosopic scale i.e., the organization in an aggregate of crystals separated by mesopores. The objective will be to establish a coherent model for the organization of the stacks of crystals as a function of relative humidity taking into account the information obtained at the crystal and the molecular scales. The objective of the fourth task will be to investigate the largest scale of the smectite porous medium upon hydration, i.e., the macroscopic swelling properties taking into account results obtained from the previous tasks at lower length scales. This will be used to quantify the variation of the macro-aggregates volumes and the resulting modification of their total pore space volume available for water filling during hydration. Finally the fifth task will focus on the influence of the organization of the system (water/smectite ratio) on the local exchange process of Na and Ca cations between the medium and the fluid at its contact. The obtained ion-exchange model will be used to predict the Ca/Na ratio as a function of the available pore spaces of the smectite porous medium. The reactive transport processes leading to the migration of different chemical species are governed by elementary phenomena occurring at short length scale that are rarely taken into account in reactive transport models or through adjusted variables not measured experimentally. The results determined in the present project will then be tested experimentally through a sixth task (not financed in the present ANR program) to improve exiting reactive transport models. Such models, refined on the basis of a reduced number of characteristic parameters of the medium (e.g., crystal chemistry of layers and relative humidity), could thus be used for the prediction of fluids and chemical elements migration in sedimentary rocks, engineered barriers and soils.
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.
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