Dynamics of confined water in compacted clay systems

Compacted clays and argillaceous rocks are used worldwide as barriers around disposal sites of hazardous wastes. The clays have well-suited physico-chemical and hydrological properties (i.e., large retention capacities and low hydraulic conductivities). Water plays a central role in the transport of radionuclides through clay rocks. Solutes can move via molecular diffusion through the thin water films that wet the clay particle surfaces and interfaces. The physical and chemical properties of water films in clays may be different as compared to bulk liquid water, which affects also the diffusion of solutes. In order to predict the rates of diffusive transport and design effective barriers, it is essential to understand the characteristic properties of pore water in clays. Our aim was to characterise the following properties: the self-diffusion of pore water, the activation energy of this process, and the freezing behaviour of pore water in well-defined compacted clays. All of these properties are very likely influenced by interactions of the pore water with the clay surfaces and the cations and thus probably differ from the corresponding properties of bulk water. Self-diffusion is, in addition, also influenced by the geometrical arrangement of primary particles in the compacted clays, and thus depends on the scale of observation. In this contribution, we present first results of this ongoing work. Three different clays were investigated because of their different behaviour in contact with water: Montmorillonite, illite, and kaolinite. In the case of montmorillonite, water is located in between particles and in the interlayer space. In illite, water is found only in between particles, because the interlayer surfaces are tightly linked by potassium cations. The layers of kaolinite are uncharged and, consequently, water is also located only in between particles. (authors)