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Heavy
Element and Separation Science
Separations
Science and Coordination Chemistry
Overview
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.
Designing
Metal-Ion-Specific Extractants
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.
Selectivity
vs. Power in Aqueous Metal Complexes
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.
Ion-Specific
Complexants in Solid Matrices
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.
Selected
Publications
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)
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Glassblowing
Interfacial
Processes
Radiation
and Photochemistry
Photosynthesis
Biological
Materials Growth Facility
Cluster
Studies
Chemical
Dynamics
Atomic Physics
Nanophotonics
Heavy
Elements
Coordination
Chemistry
f-Electron
Interactions
Actinide
Facility
Computational
Materials and Electrochemical Processes
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