Coupled Chemical and Diffusion Model for Compacted Bentonite
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COUPLED CHEMICAL AND DIFFUSION MODEL FOR COMPACTED BENTONITE
M. OLIN, J. LEHIKOINEN and A. MUURINEN
VTT Chemical Technology, P.O.Box 1404 (Otakaari 3A, Espoo), FIN-02044 VTFl, Finland
ABSTRACT
A chemical equilibrium model has been developed for ion-exchange and to a limited extent for other reactions, such as precipitation or dissolution of calcite or gypsum, in compacted bentonite water systems. The model was successfully applied to some bentonite experiments, especially as far as monovalent ions were concerned. The fitted log-binding constants for the exchange of sodium for potassium, magnesium, and calcium were 0.27, 1.50, and 2.10, respectively. In addition, a coupled chemical and diffusion model has been developed to take account of diffusion in pore water, surface diffusion and ion-exchange. The model was applied to the same experiments as the chemical equilibrium model, and its validation was found partly successful. The above values for binding constants were used also in the coupled model. The apparent (both for anions and cations) and surface diffusion (only for cations) constants yielding the best agreement between calculated and experimental data were 3.0xl010 m2/s and 6.0x10- 12 m2/s, respectively. These values are questionable, however, as experimental results good enough for fitting are currently not available. INTRODUCTION
Bentonitic clays are planned to be used as a buffer material in final repositories for spent fuel in many countries. The major transport mechanism in compacted bentonite is diffusion. On the other hand, bentonite is a chemically active medium, the diffusion properties of which are closely bound up with its chemical and structural properties. The main component of bentonite, montmorillonite, has a high ion-exchange capacity, which gives rise to interactive chemical and diffusional phenomena in bentonite. Our objective was to develop a consistent coupled chemical and diffusion model for compacted bentonite type clays subject to the assumptions of ion-exchange', anion exclusion 2 and surface diffusion 3. Many interesting reactions, which were of no major importance, however, (like sorption of anions), were omitted in order to maintain consistence between adopted reactions and mechanisms. These reactions are planned to be added in our model in the future. THEORY
Basic assumptions The most important assumptions and facts regarding compacted bentonite in our model are: 0 The model concerns bentonite clay, the major component of which is montmorillonite (about 70 %). 0 The compound dominating the transport and the ion-exchange processes is montmorillonite.
Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
254
0 Compounds other than montmorillonite may influence some other chemical properties (like
mineral dissolution or precipitation, or sorption reactions). 0 Montmorillonite has two types of pores: external ones between particles and internal ones between individual clay platelets. 0 Anions can move in a fraction of the external pore water outside the mont
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