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Water dynamics in water-clay systems

Applicant Juranyi Fanni
Number 122092
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Condensed Matter Physics
Start/End 01.09.2009 - 31.08.2012
Approved amount 172'275.00
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All Disciplines (2)

Discipline
Condensed Matter Physics
Geochemistry

Keywords (10)

water diffusion; confinement; compacted clays; neutron scattering; tracer through-diffusion; nuclear waste management; clay; quasielastic neutron scattering; anisotropy; radioactive waste management

Lay Summary (English)

Lead
Lay summary
BackgroundDiffusion in porous - and/or in confined media plays an important role in many different materials, like in membranes for fuel cells, in hydrogen storage materials, in micro- and nanoporous filters, in biological systems, etc.Water diffusion in clays is important in the context of radioactive waste repositories, because the pore water is the most likely transport medium for radioactive particles through the clay barrier into the environment.Clays are build up from disc shaped particles, and may contain water either in the larger pores between these particles and/or in a two dimensional space between crystalline sheets of these particles, in the so called interlayer. In the water, near to internal and external surfaces charge compensating cations may also be located.AimThe aim of this project is to describe the process of water diffusion in swelling clays.Main focuses are to understand the coupling between cation and water dynamics, to describe the anisotropy in thediffusion, and finally to investigate the effect of different parameters, like bulk dry density and particle size.Our approachIn porous materials the determining process for diffusion depends on the lengh-and timescale. We measure the water diffusion at the atomic level by quasielasticneutron scattering. These data will be evalutated according to realistic models,suggested by computer simulation. Furthermore, water diffusion is measured in nearly identical samples by tracer through-diffusion method over distances of centimeters. We could show in a previous project, that from such a datasetgeometrical and chemical effects can be separated quantitatively.Newly, also cation diffusion should be measured by tracer through-diffusion.SignificanceUnderstanding the process of water diffusion in clays is beneficial for the safety of radioactive waste repositories, and thus the protection of the environment. Furthermore,the quantitative separation of geometrical and chemical effects on the water diffusion can be transferred to other porous systems, being interesting for other reasons.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

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Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
European Conference on Neutron Scattering 17.07.2011 Prag, Tschechien
EUROCLAY Conference 26.06.2011 Antalya, Tuerkei


Associated projects

Number Title Start Funding scheme
140847 Water dynamics in water-clay systems 01.09.2012 Project funding (Div. I-III)

Abstract

This project is a continuation of the successful PhD work of Fatima Gonzalez Sanchez at the University of Berne, which was financed by the FoKo at PSI. The aim of this work was to study the water dynamics in highly compacted clays, which are planned to be used as sealing material for the underground disposal of radioactive waste. Understanding the water dynamics in these systems is important, because molecular diffusion through the confined water is the dominant transport mechanism for released radionuclides. Clays are complex, layered, porous materials, therefore in this PhD work the translational diffusion of water has been compared in highly compacted purified natural clays a) having systematic differences in the clay structure b) at the macro- and microscopic scales using tracer through-diffusion experiments and quasielastic neutron scattering (QENS). According to our knowledge this is the first time, where diffusion in a porous medium has been compared on these very different time scales. Since QENS has an observation time of the order of picoseconds, the obtained translational diffusion coefficients are determined by local effects. Tracer through-diffusion experiments are influenced additionally by the mesoscopic - macroscopic pore structure of the clays. From these measurements the 'geometrical' and 'chemical' effects of the clay on the water diffusion could be quantified.The activation energies (which represents the temperature dependence of the diffusion) for clays with larger pores were near identical at the two scales, and the obtained G and q factors represented the geometrical and chemical similarities and differences. In montmorillonite however, where the majority of the water molecules are located - for the given bulk density - in a form of a double-layer between the clay sheets, the activation energy obtained from tracer through-diffusion measurements was much larger.At present, the reason for this difference is not clear. This difference not only affects the characterisation of the water dynamics in clays, but possibly also questions the validity limits of the different methods. This needs further investigations, which gives the basefor the continuation of the project and the new PhD work.Our motivations to continue this project are:- to understand the water dynamics in natural clays.Beside their importance for long term radioactive waste disposals, clays are also used in many industrial processes, such as pottery, paper making, cement production and chemical filtering.- to understand the diffusion in narrow pores measured by neutron scattering and tracer diffusion. It is a basic research interest, how these two can be linked. Different approaches give complementary informations, which is especially advantageous for complex systems.- to characterize the dynamics in 2D confinement.Diffusion in a confinement is different from that in the bulk because of geometrical restrictions and surface interactions, and is not fully understood. State of the art material science use composite materials which often provides a confinement for a diffusing component.Water diffusion in clays is widely studied by different methods, however due to the variety and complexity of the clay-water system, there are still a lot of open questions. We aim to use this variety to obtain general statements about water dynamics, which can later beused to predict properties and behavior in a new system or for a system under different conditions. The use of several different techniques helps to get a complete picture by describing the motion over broad time- and spatial scale. Collaboration between the Laboratories for Neutron Scattering (ETHZ&PSI) and Waste Management at PSI and the Institute of Geological Sciences at the University of Berne provides the necessary know-how, experimental techniques and access to the Swiss National Supercomputing Center. The main methods are quasielastic neutron scattering, tracer through diffusion and molecular dynamics simulations, and special attention is payed to the characterisation of the samples.In the first part of the work the diffusion in the montmorillonite has to be understood.- Neutron scattering experiments with better energy resolution will be performed. Present measurements could not distinguish between 2 and 3 dimensional motion, but they indicate, that it is possible to use a setup where the 2D signature becomes visible.- Tracer-diffusion experiments will be performed simultaneously in different manners from the same starting material, and structural characterisation will be done before and after the measurements.- Molecular Dynamics simulations will be used to support the experimental work: find a suitable, but simple model to describe the motion of the water. Especially the geometry of different motions and the corresponding time scales should be defined based on these simulations.- If necessary, samples with higher degree of orientation of the clay platelets will be prepared, characterised and the water diffusion will be measured by the two different techniques.The second part is based on the results of the first and will deepen the understanding of the link between clay structure and water properties. Experiments with optimisedconditions should be done for:- different clay particle sizes- different degrees of compaction, i.e. dry densities- different smectites, different compensating cations in the interlayer.The PhD student will be a member of a competent research team. He or she will be located at the PSI and make his/her PhD under the supervision of Prof. Dr. Larryn Diamond (Institute of Geological sciences, University of Berne).
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