Calorimétries aux Interfaces Solide-Liquide : Adsorptions, Mécanismes et Energies de Surface – CILSAMES

Scientific background and objectives French policy on nuclear waste, based on the national law of 1991 (actualised in 2006), has been implemented within the framework of scientific research on 'Physico-chemistry of radionuclides at the solid-solution interface' included in the CNRS interdisciplinary programme PACE, and more recently in the programme proposed by the Research Network GdR Practis/ Paris. On this occasion, it was possible to define the most essential needs and indications related to the various thermodynamic approaches applied to the adsorption of ions at the Solid-Liquid Interface. Nowadays, the emphasis is placed on the improvement of the knowledge of interactions operating between the heavy metal cations and the soils or minerals, both natural and synthetic. The aim of this project is to propose solutions going well beyond the state of the art (e.g. macroscopic modelling and spectroscopy studies). The scope is to develop an integrated calorimetry-based approach to ions adsorption onto ordered solids characterised by various types of surface reactivity in order to reach a substantial progress in thermodynamic comprehension of the interactions and surface energies involved. From the experimental standpoint, this approach will be supplemented by the speciation studies at the surface, in the solution and in the suspension, as well as by the measurements of ion-water interactions. The long-term objective is also to develop appropriate modelling approach in view of the simulation and prediction of the phenomena studied. Description of project, methodology A complete comprehensive study of all physical factors governing the adsorption phenomena (i.e., pH, ionic strength, solid surface chemistry and morphology, etc.) is almost impossible. The originality of the project lies in the systematic methodology applied to the study of the carefully selected 'adsorbate/adsorbent' systems. The solids matrices include nanostructured silicate materials with tuneable structural and textural properties afforded through the controlled synthesis routes (clay minerals with different charge ratio, heteroatom-doped silica-based materials possessing uniformly sized pores). Much effort will be put on the solids characterisation stage, the crystallochemistry, textural and surface reactivity aspects being taken into consideration. To quantify the ion hydration effects, two series of alkaline and earth-alkaline cations have been selected for further studies. Furthermore, such heavy-metal cations as Zn, Mn, Cd, Pb, Co, Ni, Cu, and Eu will be studied to check for the various speciation and hydrolysis behaviours. The systematic measurements of the adsorption extent and the related thermal effects will be performed by following a two-entry table scheme (i.e., solid matrix × cation). For some selected systems, the complementary study will aim at quantifying the influence of such parameters as the aqueous phase composition, the ionic strength, pH, or the nature of the interactions involved in function of the probing molecule used. With Eu(III) retained onto different solids, an additional SLRT study will allow the bulk solution and interfacial complexes to be characterised. In the case of clay minerals possessing various compensating cations, the dielectric relaxation experiments will yield the cation-matrix interaction energies and the precise loci of ionic charge. The modelling stage will be structured around two research axes. The phenomenological approach will provide the correlations between the observables issued from the experimental studies and such ions characteristics as electronegativity and hardness-softness. Then the molecular simulation approach will be developed to determine the cation-matrix interaction energies. Expected results An essential improvement in the understanding of ions adsorption mechanisms is expected to be achieved through the combination of the expertise offered by the participating groups in the field of materials characterisation, analytical techniques, calorimetry and modelling/simulation. The main scope is to gain, at the end of the project, the quantitative prediction and optimisation scheme to propose efficient solutions for management and treatment of industrial wastes or any contamination of aqueous ecosystems. Besides the scientific objectives, the present project satisfies the need for consolidation of ideas and strategies, as well as for integration of research in the field by bringing together young researchers and experimented researchers accepting common goals and team-work spirit. The integration and clustering among the groups working on similar topics are also identified as high-level objectives of the ICG which emerged in the beginning of this year. The present project meets the local requirements for integration and clustering and provides the appropriate tools to attain these objectives.