Tc(IV) complexation with organic ligands
Boggs, Mark Antony
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99Technetium poses a difficult problem at multiple Department of Energy sites, where there are currently vast quantities in storage. 99Tc has a half-life of 2.3 x 105 years, thus it will have to be accounted for a very long time. This problem is compounded by past releases of significant amounts of Tc into the environment. At the Hanford Site, in Washington State, it is projected that 30 Ci of Tc have been released into the vadose zone, while 30,000 Ci remains in underground tanks. It is known that in the +7 oxidation state Tc is highly mobile in the environment, and can travel at up to 90% the rate of ground water. Tc can be found, to a lesser extent, in the environment and waste in +4 oxidation state. Very little is currently known about how this lower oxidation state interacts with organic ligands in the environment and may also be found in many of the different waste forms. In particular how these interactions may influence its environmental migration rates. The interaction of Tc(IV) with the organic ligands acetate and EDTA were determined using solvent extraction. Both ligands formed complexes with Tc(IV), as expected the complexes formed with EDTA were much stronger than those of acetate. Using these findings the solubility of Tc(IV) in the presence of these ligands was modeled. It was shown that EDTA could cause a two order of magnitude increase in solubility of Tc(IV), which could have a great impact on its environmental mobility. The interaction of Tc(IV) and a variety of humic substance was determined using a solvent extraction system. Large binding constants were also determined for Tc(IV) with both humic and fulvic acids. In order to include the dynamic nature of humic substances in the complexation of Tc(IV) ultracentrifugation was used to selectively sediment humic acid size fractions. From these studies it was found that independent of humic acid source, ionic strength, pH, and background electrolyte there was a constant ratio of Tc(IV) to humic acid. These studies pave the way for future work on the characterization of Tc(IV) interactions in the environment.