The effect of desferrioxamine B on the desorption of U(VI) from Georgia kaolinite KGa-1b and its ligand-promoted dissolution at pH 6 and 25 °C

The pathway of actinides through the soil column hinges on many intricate processes with soil constituents. These processes often involve the bioactivity of the microenvironment conferring the biosphere a crucial role in the natural attenuation of radionuclide contaminants in soils. In order to investigate the bioavailability of uranium, sorption experiments with U(VI) on kaolinite in the presence of an important organic chelator, DFO-B, at pH 6 and 25 degrees C were performed. Under these conditions uranyl adsorption reached a constant similar to 70% of which virtually all was subsequently desorbed in the presence of the siderophore. It is reasoned that the almost quantitative desorption is a result of both direct displacement of uranyl surface species by DFO-B adsorption onto the same kaolinite Al edge sites and indirect desorption via thermodynamic disequilibrium, between free and surface-complexed U(VI). Estimates suggest that indirect desorption prevails over direct exchange processes in the detachment of uranyl surface complexes. Yet, direct desorption and the concept of ligands competing for the same adsorption sites is corroborated by experiments where DFO-B and oxalate were employed to assess a potential synergistic effect on kaolinite dissolution. However, no such effect was observed. Species calculations as well as experiments indicate that DFO-B was preferentially adsorbed. In separate systems, DFO-B and oxalate each caused a ligand-promoted dissolution of kaolinite. This enhancement of dissolution rates is consistent with a mechanism involving ligand adsorption rather than chemical affinity or aluminum inhibitory effects. Further support for this hypothesis comes from the observation that siderophore-promoted dissolution of kaolinite showed the same relationship with aqueous ligand concentration as does the surface coverage: At low aqueous DFO-B concentrations < 100 mu M increases in surface coverage and dissolution rates are more pronounced than at aqueous DFO-B concentrations > 100 mu M, where increase in these parameters is only marginal. The results show that DFO-B is a very efficient U-desorbent and affects uranium mobility in clay containing soils. Moreover, the ligand-promoted desorption results principally in a stable neutral aqueous species (UO2DFOBH) not likely to re-adsorb on soil particles to a great extent. Hence, the siderophore effect on the bioavailability of U(VI) is not exclusively determined by its desorptive strength but also encompasses the aqueous speciation that follows desorption. (c) 2007 Elsevier B.V. All rights reserved.