Diffusion of Colloids in Compacted Bentonite
Compacted saturated bentonite has been seen as a complete barrier for filtering out colloids. In a deep bedrock repository different classes of colloids can be present such as bentonite colloids, mineral oxides, organic colloids such as humic and fulvic acid or micro organisms. Since colloids have a large specific area and are often negatively charged in natural pH, they have a tendency to bind positively charged cations. The microstructure of compacted saturated bentonite describes the structure as a porous medium. The dominating part comprises pores in nm size between the montmorillonite sheets, and a very small part consists of pores in between aggregates which are gel filled. If this is a correct way of describing the microstructure is a reasonable assumption that mobile colloids will be filtered out. However there are contradictory studies out where gold colloids have been shown to be filtered out in the micro structure whereas humic colloids do diffuse through without any larger hindrance. The difference in diffusion behaviour can probably be explained to some extent of the differences in conformation of these two classes of colloids. However, to be able to understand the diametric differences it is of great importance to understand how the microstructure is build up. Colloid diffusion studies are continuing with different size classes and different types of colloids to get information about the compacted bentonite microstructure.