Presence and mobility of colloidal particles

Colloids are commonly defined as small particles with dimensions roughly between 1 nm and 1 μm. They represent important sorbents for environmental contaminants keeping them into suspension over long periods of time. Colloidal material is ubiquitous in nature, and includes biocolloids (viruses, bacteria, and protozoa), silicates, clays, oxides and dissolved organic carbon. Within the context of geological disposal of radioactive waste in a clay sediment (Boom Clay), the presence of colloids would strongly impact the migration behaviour of radionuclides. It is therefore required to elucidate the role of colloids in the transport process to assess the suitability of Boom Clay to retain contaminants for time-frames in excess of thousands of years. This report provides an overview on the current state of knowledge with respect to colloid facilitated transport under undisturbed Boom Clay conditions currently prevailing at the Mol site (Belgium) which is intensively studied in Belgium (SCK•CEN & ONDRAF/NIRAS) as a potential host formation for the geological disposal of radioactive waste. An overview about different types of colloids, their generation, composition, and properties (e.g. stability) is given. In more detail, organic carrier colloids are discussed because of their predominance and special significance in Boom Clay. The transport of colloids, the governing mechanism and processes, and the role of colloid filtration are discussed. A careful extrapolation is made of the conclusions drawn for Boom Clay conditions at Mol site to the conditions prevailing in the Netherlands.