EXAMINING THE ORDER OF ADDITION IN TERTIARY SORPTION EXPERIMENTS OF PLUTONIUM, ORGANIC MATTER, AND COLLOIDAL MONTMORILLONITE

Understanding the interaction of plutonium (Pu) with clay is necessary to predict its migration at contaminated sites (e.g. Nevada National Security Site, Nevada) and to assess the performance of nuclear waste storage sites, as clays are expected to be used as backfill at a number of proposed sites. Clays such as montmorillonite have previously been shown to strongly sorb radionuclides, which coupled with their ability to be transported as colloids, can lead to an increase in the environmental mobility. Little is currently known about how the various reactive components of natural organic matter will affect the sorption characteristics of Pu(IV) in the presence of montmorillonite. We have studied Pu(IV) adsorption in the presence of montmorillonite with three organic ligands: humic acid, fulvic acid, and the siderophore desferrioxamine B (DFOB). Our experiments were designed to determine if the order in which the colloidal mineral, metal, and ligand were added altered the sorption characteristics of Pu(IV). We used three batch experimental setups to probe the tertiary system; 1) complexing Pu(IV) with a ligand prior to addition of montmorillonite 2) equilibrating the ligand and montmorillonite prior to addition of Pu(IV) 3) equilibrating Pu(IV) to montmorillonite prior to the addition of a ligand. Mineral ligand and metal concentrations were held constant at 0.1 g/L, 0.8 ppm carbon and 10-10 M respectively, while the pH ranged from 4 to 10. The presence of humic or fulvic acid decreased sorption of Pu relative to a ligand free system in almost every instance and limited sorption of the humic or fulvic acid was observed. Therefore, there is a rather straightforward competition for Pu between the organic ligand and the colloidal mineral. In contrast, sorption of Pu increased in the presence of DFOB. This increase is due to strong interactions of DFOB with montmorillonite, which was confirmed using XRD and carbon analysis. The current study highlights the need to both understand how organic matter influences radionuclide interaction in simple binary inorganic interactions, as well as direct organic-inorganic interactions. This work was supported by the Subsurface Biogeochemical Research Program of the U.S. D.O.E office of Biological and Environmental Research. Prepared by LLNL under Contract DE-AC52-07NA27344.