URANIUM RETENTION BY WEATHERED SCHISTS - THE ROLE OF IRON MINERALS

Uranium sorption experiments were carried out on weathered schist samp les which had been treated with citrate/dithionite/bicarbonate (CDB) r eagent to remove iron oxides, and the results were compared to those o f similar experiments with untreated samples. Uranium sorption was gre atly decreased by the CDB extraction, which reduced the surface area o f the samples by about 30-40%. Chemical analysis and XRD confirmed tha t iron oxides were the major mineral phase extracted by the CDB reagen t. To further elucidate the role of iron minerals in the natural envir onment, we carried out transformation (aging) experiments with synthet ic ferrihydrite which contained adsorbed natural uranium (predominantl y U-238). In these experiments, the ferrihydrite was partially convert ed to crystalline forms such as hematite and goethite. The uptake of a n artificial uranium isotope (U-236) and the leaching of U-238 from th e samples were then studied in adsorption/desorption experiments. The results showed that the transformation of ferrihydrite to crystalline minerals substantially reduced the ability of the samples to adsorb U- 236 from solution. The desorption data for U-238 showed that some of t he U-238 which was adsorbed to the samples prior to the transformation step was irreversibly incorporated within the mineral structure durin g the transformation process. These experiments highlight the potentia l importance of iron minerals both in the initial sorption of radionuc lides and in isolating them from interactions with the aqueous phase. Transformation of iron minerals from amorphous to crystalline forms pr ovides a possible mechanism for uranium immobilisation. In considering the overall effect on U migration, this must be balanced against the reduced ability of the transformed iron oxide to adsorb U from solutio n.