Are Colloids Important for Transport Rate Assessment of Radionuclides? A Microscopic Modeling Approach

by J. van der Lee, Cent d'Informatique Geologique et, d'Hydrogeologie Mathematique, Fontainebleau, France,
G. de Marsily, Cent d'Informatique Geologique et, d'Hydrogeologie Mathematique, Fontainebleau, France,
E. Ledoux, Cent d'Informatique Geologique et, d'Hydrogeologie Mathematique, Fontainebleau, France,

Abstract: Understanding the transport phenomena of radionuclides migrating through weakly-permeable rocks forms an essential part of far-field safety assessment studies. Colloidal matter can be important in view of the solubility of radioactive waste, and as a potential rapid carrier of the radionuclides in far-field migration. When estimating the influence of colloids on the fate of radionuclides a key aspect is the understanding of the interaction mechanisms of the phases involved, i.e. aqueous, solute and colloidal phase and the solid medium, because they define the retardation and hence the arrival time of the pollutant in the human environment. The present study is an attempt to quantify interaction phenomena according to a microscopic modeling approach, based on a combination of chemical and electrophysical considerations. In order to define the state of the surface of colloids and medium, a model based upon the surface complexation theory was developed. Migration from the inner pore-space towards the surface is governed by diffusion, gravitation and, very close to the surface, electrostatic forces. Colloid fixation is electrochemically as well as physically bounded by a maximum adsorption capacity and it is expected that electrostatical surface properties change when the surface becomes progressively saturated by colloids. Therefore, the model includes surface reactions with parameters depending on the maximum concentration at the surface. The result is a microscopic model which includes chemical speciation, electrochemical characteristics of the medium and colloids, pH, ionic strength and surface boundary-layer phenomena. The model can be used for predictive purposes in laboratory experiments, using simple hydrogeological geometries and organic colloids. Effects of radionuclide concentration (e.g. Am(III)), pH and surface potential are quantified.

Subject Headings: Colloids | Surface properties | Radioactive materials | Radioactive wastes | Terrain models | Mathematical models | Transportation safety | Model analysis

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