Sorption in organo-clays at varying thermo-chemical conditions ...

The main goal of the project is the characterization of basic principles of sorption processes and sorption stability of persistent anionic radionuclides, cations and non-polar molecules in organically modified clays by diffusion experiments and molecular simulation. In aquatic systems persistent radionuclides such as 79Se, 99Tc, 129 I exist in a form of anions. There is an urgent requirement to find a material with advanced sorption capacities for this type of pollutants especially in geotechnical multibarrier systems of planned high level waste repositories (HLWR). Specific requirement on barrier material beside multiplex sorption of ions is long-term stability under expected environmental conditions such as temperatures higher than 100C and hanging hydrostatic pressure. Organo-clays, as perspective materials, have a capability to sorb high amounts of cations, anions, and non-polar molecules simultaneously. This research project is proposed to improve the understanding of sorption processes in organo-clays. Additionally, the modification of material properties under varying chemical and thermal conditions will be determined. Therefore, diffusion and advection experiments will be performed using newly constructed measured cell with a temperature and pressure control. Changes by sorption and diffusion will be analyzed by applying various experimental techniques (e.g. IR-spectroscopy, PCD-analysis or contact angle). Molecular simulations on models of organo-clay structures will be conducted in an accord with experiments with the aim to understand and analyze experimental achievements. Molecular simulation studies represent an important tool for the validation and the interpretation of obtained results, for the characterization of the structure and diffusive transport properties at molecular level as well as for planning of further experiments. Based on the results of the simulations, the existing molecular models of the structure of organo-clays shall be improved or enhanced, respectively. During the working schedule coordinated experiments and computer simulations will be conducted to determine sorption capacity, diffusive transport, and changes of molecular structure under temperature, pressure and pH variation for organically modified clays, particularly HDPy + - and HDTMA+ -montmorillonite. Based on previous investigations, diffusion experiments with untreated and thermally pre-stressed organo-clays are planned in the 1st step. Subsequently the effect of competing ion on the diffusive transport will be characterized. Following probes will be investigated: anions ReO 4 - , NO 3 - , SeO3 2- , and I - , the cation ammonium (NH 4 + ), and the non-polar organic molecule phenanthrene. Some combined mixtures of the ions will be investigated as well. Within the duration of the project it is planned to develop a new bigger diffusion cell connected with the multi-reflectance ATR-unit having a temperature control. In the field of computer simulation, Monte Carlo simulation will be conducted to maintain representative structural models for sorption and interaction of the studied ions with organically modified montmorillonite. The 2nd step comprises three basic parts: (i) characterization of structural changes upon sorption processes and during the diffusive transport by using methods such as particle-charge-detector-analyses (PCD) (to quantify the surface charge) or Wilhelmy-plate-method (WPM) (to determine wettability). Additionally, based on the experimental achievements, validation of the results obtained by simulations in the step 1 will be accomplished. (ii) Molecular dynamic simulations will be conducted to calculate the dynamic properties, especially diffusion coefficients, and to support the interpretation of the experimental results from the step 1. (iii) Diffusion experiments will be performed at several temperatures (20, 40, 60, and 80C) and pH (7-8 and 4), and varying ion concentrations. Additionally, advection experiments will be conducted. In the 3rd step, the validation of the steps 1 and 2 resulting in models of the diffusion-, advection- and sorption processes will be carried out. The validation of the influence of the thermal stress and the diffusive processes on the sorption capacity of the organo-clays will be performed. The proposed project will benefit from a long-term experience in an application of various techniques for a characterization of clays and their organic modifications, and from the expertise in molecular simulations. It is expected that by combination of advanced experimental and molecular simulation methods new detailed characterization of adsorption and diffusion processes of studied species on organically modified clays will be achieved.

The project Sorption, diffusion and advection of anions, cations and non-ionic molecules in organoclays at varying thermo-chemical conditions validation by analytic methods and molecular simulation represented a bilateral DACH cooperation between the group from Austria (Institute for Soil Research, BOKU, Vienna, PI Dr. Daniel Tunega, FWF project No. I880-N21) and the group from Germany (Institute of Soil Science, Leibniz Universität Hannover, PI Dr. Birgit Schampera, DFG project No. SCHA 1732/1-1). The project was focused on the experimental (Germany group) and molecular modelling (Austrian group) investigations of the structure and physical-chemical properties of organoclays, materials prepared from alkylammonium cations and clays. The combination of the experimental and modelling approach was shown as very effective tool for the detailed molecular-scale characterization of the structure and arrangement of the organic cations in the interlayer space and on the surface of the clay mineral montmorillonite. The changes of the surface properties (e.g. wettability or surface charge) were directly related to the varying amount of organic cations at the clay surface. The diffusion experiments with selected ions and nonpolar molecules showed a variable retention capability of the organoclays with respect to various chemical species and the applied amount and type of organic cations. Molecular simulations provided detailed description of the interaction mechanism of the selected species at the organoclay-water interface. The predicted diffusion coefficients from simulations correlated very well with the experimental data. It was shown that the prepared organoclays are very suitable materials with desired properties that can be used in various technological applications such as a treatment of contaminated soils or in water purification systems.