DIFFUSION OF ANIONIC SPECIES IN COMPACTED BENTONITE

Abstract

Compacted bentonite, planned as an engineered barrier in deep geological repositories for radioactive waste in many countries, is frequently conceptualised as a multi-porous material for geochemical and transport modelling. Three water types are generally assumed in these models, free water, diffuse double layer water and interlayer water. In the latter water, anions are typically assumed completely excluded. Several diffusion experiments with chloride, iodide, pertechnetate, selenate, sulphate, and perchlorate were performed in compacted bentonite saturated with solution of ionic strength of 0.1 mol·L-1. The main finding of this research is that evaluated volumes of free and diffuse double layer water differs for different anions. Under identical conditions, bentonite pore solution was demonstrated to contain significantly more sulphate and selenate than iodide or chloride. The data obtained strongly suggest that restricting anions from the interlayer pores is improper assumption in the modelling. The data obtained are in line with predictions of model conceptualizing bentonite as a homogeneous mixture of water and montmorillonite.