Formation of Colloids by the Weathering of Rock Material

Colloids can either stimulate or retard the transport of actinides. Environmental colloids are formed by a variety of mechanisms. The most important one is the precipitation of secondary phases. There have already been several publications on colloid generation within water bod-ies due to chemical reactions after the mixing of waters of different chemical composition or after access of air. Much less information exists about a different mechanism of colloid for-mation in the nature – the direct production of secondary mineral colloids at the water-rock interface by the weathering of rock material.

The formation of colloids during the weathering of ground phyllite from an abandoned ura-nium mine and the sorption of uranium(VI) onto these colloids were studied. Centrifugation and ultrafiltration were applied. A method to differentiate between the primary mineral col-loids of the phyllite and the precipitated secondary mineral colloids was developed.

Significant amounts of colloids of the 101 to 102 nm size class were found in the suspension of ground phyllite after weathering times of 87 h to 792 h. Considerable fractions (up to 10 mg/L) of these 101 to 102 nm colloids consisted of secondary minerals. The following compo-sition of the freshly precipitated colloids is to be expected: Ferihydrite, aluminosilicates, amorphous Al(OH)3, gibbsite, possibly iron silicates, possibly iron-alumino silicates. It should be emphasized that a significant fraction of the secondary mineral colloids was composed of iron(III) compounds which is of relevance for the sorption behaviour of these colloids. A more detailed mineralogical characterization of the colloids is underway. The 101 to 102 nm secondary mineral colloids were relatively stable and remained in colloidal suspension over longer periods of time.

The direct formation of secondary mineral colloids at the mineral-water interface by the weathering of rock material is an alternative to the well-known mechanism of colloid forma-tion by the mixture of different water bodies or by aeration of a water body. This direct mechanism of colloid production occurs during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in mine dumps. Colloids produced by this mechanism can influence the transport of actinides since they have a large specific surface area and a high sorption affinity for heavy metals. This was demonstrated for uranium(VI).