Modelling of geochemical reactions and experimental cation exchange in MX80 bentonite

Bentonites are widely used for waste repository systems because of their hydrodynamic, surface and chemical-retention properties. MX80 bentonite (bentonite of Wyoming) contains approximately 85% Na/Ca-montmorillonite and 15% accessory minerals. The dominant presence of Na/Ca-montmorillonite in this clay mineral could cause it to perform exceptionally well as an engineered barrier for a radioactive waste repository because this buffer material is expected to fill up by swelling the void between canisters containing waste and the surrounding ground. However, the Na/Ca-montmorillonite could be transformed to other clay minerals as a function of time under repository conditions. Previous modelling studies based on the hydrolysis reactions have shown that the Na/Ca-montmorillonite-to-Ca-montmorillonite conversion is the most significant chemical transformation. In fact, this chemical process appears to be a simple cation exchange into the engineered barrier. The purpose of the present study was two-fold. Firstly, it was hoped to predict the newly formed products of bentonite-fluid reactions under repository conditions by applying a thermokinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass Transport). The system modelled herein was considered to consist of a 1-m thick zone of water-saturated engineered barrier. This non-equilibrated system was placed in contact with a geological fluid on one side, which was then allowed to diffuse into the barrier, while the other side was kept in contact with iron-charged water. Reducing initial conditions (Po-2 congruent to 0; Eh = - 200 mV) and a constant reaction temperature (100 degrees C) were considered. Secondly, it was hoped to estimate the influence of interlayer cations (Ca and Na) on the swelling behaviour of the MX80 bentonite by using an isothermal system of water vapour adsorption and an environmental scanning electron microscope (ESEM) coupled with a digital image analysis (DIA) program. Here, the MX80 bentonite was previously treated with concentrated solutions (1 N) of calcium and sodium chlorides. The results confirmed that the Na/Ca-montmorillonite-to-Ca-inontmorillonite conversion was the main chemical transformation in the bentonite barrier under repository conditions. A simplified method (based on volume balance) has shown that the swelling capacity of the engineered barrier would be slightly affected after 1000 years of diffusion-reaction because the volume of neo-formed swelling clays is almost directly proportional to the volume of transformed initial-montmorillonite. Minimal neo-formation of saponites, vermiculites and chlorites was also observed. In addition, an isothermal system of water adsorption and ESEM-DIA methods showed that in the raw-bentonite-to-Ca-bentonite exchange there is a small decrease in the amount of adsorbed water and the swelling potential. (c) 2005 Elsevier Ltd. All rights reserved.