Actinides sorption mechanisms

The results of this preliminary investigation (particularly the general behavior of the distribution coefficients and the Identification of multiple sorption mechanisms) will provide a basis from which plans can be developed for studying the sorption and complex chemistry of the actinides. The results will also provide a basis from which plans can be developed for studying sorption equilibria involving several competing nuclides. 

The differences in sorptive behavior of Th, Pu, U, and Np are evident by examining Table II. Plutonium and thorium isotopes at tracer concentrations (parts per billion, element mass/clay mass) were equilibrated for 24 hours with the < 2-pm fraction (clay) of a silt loam soil. The pH of the equilibration solutions was 6.5 and the aqueous phase contained Ca at a concentration of 5 mM. Both tetravalent actinides failed to remain 1n solution. Whether this is a direct function of sorption mechanisms or simply related to the solubility of the ions in solution is not distinguished by the results. Uranyl ion was not removed to the same extent as the tetravalent species. Neptunium(V) sorbed very poorly. It should be noted that while Np(V) is a mono-charged cation, Np02+ does not sorb like Na+. 

Such a mechanism would require modifications to some concepts of solubility controlled release. This report concerns a preliminary effort to determine some of the sorption properties of colloidal species representative of those formed during waste form/waste package interaction tests. Sorption of actinides and Tc on those colloids as a function of pH at 25°C was studied. 

After precipitation, adsorption is the most important sink for U in natural systems. Uranium sorption in soils is primarily controlled by pH and carbonate levels (see Davis et al., 2002, this publication). At high pHs, where anionic uranyl-car-bonate complexes predominate, U is only weakly sorbed due to electrostatic repulsion by negatively charged mineral surfaces. When carbonate concentrations are low or absent uranyl-hydroxy surface complexes are observed to form. Minor irreversible sorption of U typically occurs when Fe and Mn oxides are present. Colloidal transport appears to be a less important transport mechanism for U relative to other actinides (see review of Jove Colon et al., 2001). 

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