Incorporation of Radionuclides into Mineral Phases via a Thermally Unstable Complexant Ligand
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INCORPORATION OF RADIONUCLIDES INTO MINERAL PHASES VIA A THERMALLY UNSTABLE COMPLEXANT LIGAND ALLEN W. APBLETT*, GALINA D. GEORGIEVA, and JOEL T. MAGUE Tulane University, Department of Chemistry, New Orleans, LA 70118
ABSTRACT
The complexation and thermal behavior of pyruvic acid oxime (PAO) with representative cations associated with nuclear reprocessing waste has been investigated. This ligand has the dual advantage that it precipitates most of the cations from solution and these complexes decompose at ca. 150 0 C to CO 2 , acetonitrile, and, initially, the metal hydroxide. Introduction of the appropriate elements required for Synroc phases to a simulated waste stream followed by NaPAO and disodium fumarate leads to precipitation of a metalorganic Synroc precursor in which the components have been intimately mixed at the molecular level. This approach circumvents incorporation of alkali metals in the final ceramic. INTRODUCTION Synrocl, 2 , a crystalline titanate ceramic based on naturally occurring materials (hollandite, BaAl 2Ti 6O 16 , perovskite, CaTiO 3 , and zirconolite, CaZrTi 20 7 ) has previously been developed on a laboratory scale for the immobilization and encapsulation of several high-level radioactive waste streams including Barnwell waste 3' 4 , Rockwell-Hanford waste 5 , thermal oxide reprocessing waste 6, and unreprocessed used reactor fuel rods 7 . Although the compositions of these waste streams vary widely, a durable, leach-resistant
ceramic waste form could be prepared in all cases. The general approach to incorporation of radwaste into Synroc involves prior synthesis of a powder which contains those elements necessary for formation of the ceramic matrix (i.e. Ti, Zr, Ca, Ba, Al). The preceramic powder may be prepared by oxide routes whereby fine powders of the individual element oxides and carbonates are mixed or by hydrolysis of a metal alkoxide mixture 8. Typically, the radioactive waste is then added as an aqueous solution to the Synroc precursor. Subsequent drying, calcining, and hot pressing then yields a ceramic pellet. An alternative method of more homogeneously mixing the radionuclides with the matrix elements could possibly lead to lower sintering temperatures and a more leach-resistant waste form. One viable option of achieving this goal is metal organic deposition (MOD) 1 . Of the many methods of producing ceramics, MOD, is outstanding in its simplicity, versatility, and inexpensiveness. MOD utilizes metal organic precursors which upon pyrolysis are converted into their constituent metallic elements, oxides, nitrides, or other compounds. Suitable metal organic precursors are covalent compounds with the metal atom bonded to an organic group via oxygen, sulfur, nitrogen, or phosphorus. Metal carboxylates are the best candidates for the preparation of ceramic oxides, and MOD processes for the generation of titania-based materials have already been developed 9 . The versatility of the MOD process arises from two factors; firstly, the ligands can be easily varied to adjust their phy
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