Heavy Element and Separation Science
Separations Science and Coordination Chemistry
The focus of the activities of this research group is the design, synthesis, characterization, and application of chelating agents for metals separations and recovery. The chelating agents include lipophilic ligands that function as extractants, water-soluble species having use in separations and water treatment, and chelating agents immobilized in a polymeric matrix or adsorbed on an inert support for chromatographic applications. The expertise includes not only the ability to design, synthesize, and characterize chelating agents and their complexes, but also to implement the science in real-world applications. This ability is born of a long-standing intimate involvement in the DOE programs dedicated to the separation and isolation of the transuranium elements. Operating in such an environment, it is clear that the success of a scientific breakthrough can be best measured by the successful application of that breakthrough in solving a problem of practical interest.
The critical parameters for the design of a new chelating agent are complex strength, solubility of the ligand and its complexes, selectivity for particular metals or classes of metals, and the thermal stability of the chelated complexes. The relative importance of these parameters is determined by the practical objective. For example, for scale prevention in neutral, low ionic strength process waters, complex stability and solubility are often the key parameters. To complex a particular metal ion in a mixture of similar metal ions, selectivity is the key characteristic. For biomedical applications, for example, carrier ligands for magnetic resonance imaging, metal complex stability, and kinetics of complex dissociation are most important. The most desirable characteristics are also shaped by the conditions under which the substances will be used. These design parameters apply equally to both water-soluble chelating agents and to lipophilic chelating agents used as extractants.
Historically, Argonne's research has centered on the separation of radioactive materials, in particular, actinides. However, the expertise accumulated over years of research has produced broad understanding of separations science and coordination chemistry of all heavy metals. The demonstrated ability to translate basic science into practical solutions also reflects the results-oriented approach to scientific research. Clearly, these talents and abilities have broad relevance for many issues related to waste isolation and resource recovery beyond the arena of radioactive waste treatment.
How could a process be designed to selectively complex alkali and alkaline earth elements and extract them from acidic solutions? The radioactive isotopes 90Sr and 137Cs constitute a major contribution to the total heat and radiation from irradiated fuel for the first 300 years after the fuel is removed from the reactor. Their separation has the dual benefit of reducing the volume of high-level waste that must be disposed of in a geological repository, and producing a high-purity sample of radioactive materials suitable for use as remote thermal generators (90Sr) or as a g-irradiation source (137Cs). The Chemical Separations Science Group is developing solvent extraction processes to remove both of these elements from radioactive wastes. Each process is based on the use of crown ethers.
In the SREX (strontium extraction) process, the crown ether ligand di-t-butylcyclohexano-18-crown-6 (1) is used to accomplish phase transfer for Sr. The t-butyl substituents improve the solubility of both the free extractant and the Sr complex in the organic diluent. This complexant selectively binds Sr2+ over most of the fission products, transuranic elements, and nonradioactive background elements present in radioactive wastes. High-extraction factors for Sr from nitric acid solutions are achieved through the use of an organic solvent that is somewhat hydrophilic, extracting several percent of water. This modification increases the steady-state concentration of NO3- in the organic phase, thus increasing the extraction of Sr(NO3)2.
Cesium presents a more difficult separations problem. Its larger size and lower charge make Cs an order of magnitude harder target than Sr. Two promising reagents that may be suitable extractants for Cs are under development. The first is di-t-butylcyclohexano-18-crown-6 (1), which is successful for Sr. Physical measurements indicate that a perched Cs sandwich complex is formed in the extraction of Cs. An alternate extractant that possesses a cavity of the appropriate size and shape for Cs complexation is dibenzo-21-crown-7 (2). The dibenzo derivative is favored for Cs because of the added stiffness created in the large 21-crown-7 ring. Molecular modeling designs have led to modifications of this extractant with important successes.
Complex strength or cation selectivity: which is most important? The answer to this question lies in the nature of the problem to be solved. For example, if a valuable constituent is to be isolated from a complex matrix, selectivity is the primary design criterion. On the other hand, to prevent scale buildup in process waters, complexing strength with many metals is critical.
To illustrate, it has recently been found that a polycarboxylic acid chelating agent, tetrahydrofuran-2,3,4,5-tetracarboxylic acid (3), will enable highly selective partitioning of uranium away from other transuranic elements in the TRUEX process for radioactive waste. This ligand forms moderately strong complexes with spherical trivalent and tetravalent actinide and lanthanide cations but unusually weak complexes with the hexavalent dioxouranyl cation. The highly radioactive transuranic elements can therefore be readily isolated from uranium. Investigations of the basic chemistry of the complexation of lanthanide and uranyl ions with (3) strongly suggest electrostatic repulsion between the partial negative charge of the uranyl oxygens with the ether oxygen of the ligand as the likely cause of the unusually weak uranyl complexes.
When general complexing power is required, few reagents exceed the capabilities of the derivatives of methane diphosphonic acid (4).
These ligands strongly bind polyvalent cations even in very acidic environments wherein ligands like EDTA are ineffective.
This basic structure is amenable to many modifications that confer unique properties on the complexant without disturbing its complexing strength. For example, 1,2-dihydroxyethane-1,1-diphosphonic acid (5) combines complex strength with an inherent thermodynamic instability. Certain amine substituents, for example morpholine (6), confer much greater solubility on its metal complexes without adversely affecting complex stability. The vinylidene derivative (7) can be readily copolymerized to immobilize this important chelating moiety on an inert support for ion exchange.Argonne is exploring other modifications to enhance the specificity of the complexant for selected metal ions as well as preparing derivatives for extraction work in unique environments ranging from wastewater treatment to biomedical applications.
For enhanced chemical separations, what advantages are gained if the chelating agent is immobilized in a solid matrix? Metal separations procedures based on solvent extraction are capable of high throughputs in efficiently designed processes based on countercurrent flow of reagents. Such processes generally require significant inventories of materials and often a substantial investment in hardware. The complementary technique of column chromatography offers higher selectivity and simpler operating equipment when high throughput is not required. Materials suitable for chromatographic applications include both conventional extractants immobilized on inert supports and polymeric materials containing appropriate functional groups. This research has addressed the use of both materials.
The powerful chelating agent vinylidene-1,1-diphosphonic acid (7) can be copolymerized with acetamide and styrene to produce a chelating ion exchange resin. Several experimental techniques have elucidated the representative structure of the monomer. Metal ions are bound primarily to the chelating methylenediphosphonate moiety. The sulfonic acid groups satisfy the remaining cation charge neutralization requirements. Under different conditions, this resin can exhibit high selectivity for certain classes of metal ions or function as an indiscriminant powerful adsorbent for many polyvalent metal ions.
An alternative approach to covalently bound chelating agents is to immobilize an organic extractant solution on an inert support. The molecular forces holding the extractant on the support are weaker than those in the resin, but are of adequate strength to provide a useful chromatographic material, and several specific solvent extraction processes can be made to operate in an extraction chromatographic mode.
To date, we have immobilized extractants selective for Sr (Sr-Spec), Pb (Pb-Spec), trivalent lanthanides and actinides (Ln-Spec), hexavalent actinides (U-Spec), actinides in all oxidation states (TRU-Spec), and technetium. Each of these materials is based on a well-known solvent extraction process. These materials have already found numerous applications in analytical chemistry and water treatment.
Uptake of Metal Ions by a New Chelating Ion-Exchange Resin. Part 1: Acid Dependencies of Actinide Ions, E. P. Horwitz, R. Chiarizia, H. Diamond, R. C. Gatrone, S. D. Alexandratos, A. Q. Trochimczuk, and D. W. Crick, Solvent. Extr. Ion Exch. 11, 943-966 (1993)
Separation and Preconcentration of Actinides from Acidic Media by Extraction Chromatography, E. P. Horwitz, R. Chiarizia, M. L. Dietz, H. Diamond, and D. M. Nelson, Analyt. Chim. Acta 281, 361-372 (1993)
A Lead-Selective Extraction Chromatographic Resin and Its Application to the Isolation of Lead from Geological Samples, E. P. Horwitz, M. L. Dietz, S. Rhoads, C. Felinto, N. H. Gale, and J. Houghton, Anal. Chim. Acta 292, 263-273 (1994)