428 EDTA and Mixed –Ligands Complexes of Trivalent and Tetravalent Actinides at High Ionic Strength

Friday, November 6, 2009: 4:00 PM
Hereford (Camino Real Hotel)
P. Thakur , Carlsbad Environmental Monitoring & Research Center, Carlsbad, NM
J.L Conca , Carlsbad Environmental Monitoring & Research Center, Carlsbad, NM
G.R. Choppin , Department of Chemistry & Biochemistry, Florida State University, Tallahassee
Large quantities of hazardous and radioactive waste generated during over forty years of manufacturing and testing of nuclear weapons are stored in several U.S. Department of Energy (DOE) generator sites. The chemical nature of the waste is quite varied as it consists of radionuclides, metals, organic complexing agents and other organic substances. The interaction of organic ligands with radionuclides can affect the solubility of actinides and, hence, their migration from a radioactive waste repository to the environment. To assess the potential release of radioactive waste, knowledge of the

thermodynamics and kinetics data of the interactions of the radionuclides with the specific ligands are necessary.

Ethylenediaminetetraacetic acid, EDTA, which is present in significant amounts in mixed radioactive/hazardous chemical waste tanks forms stable complexes with tri- and tetravalent actinides. The WIPP (Waste Isolation Pilot Plant) inventory contains 7.5 metric tons of citrate (Cit) and ~270 kg of EDTA. In this article, the complexation thermodynamics of trivalent Am, Cm and Eu and tetravalent Pu with Cit and EDTA alone and their mixed ligands complexation with EDTA+Cit at I = 5.0 M (NaClO4) have been measured by the solvent extraction. Cit forms 1:1 and 1:2 complexes with these cations, while EDTA forms only 1:1 complex. The formation of a mixed ligand complex, M(EDTA)(Cit)4- (M3+ = Pu, Am, Cm and Eu) has been observed in the presence of EDTA+Cit, which have higher stability constant than the corresponding binary complexes. Stochiometry of the complex species has been established by Eu/Cm-luminescence lifetime measurements. These studies are of fundamental interest as they assess the steric interactions between ligands, the binding preference of various ligand donor groups and the expansion of coordination number of cations.