Diamides have been studied extensively as agents for selective extraction of trivalent f-block metal ions from aqueous solutions. They form the basis for separation processes designed to partition the minor actinides from spent nuclear fuel for subsequent transmutation (with concomitant closure of the nuclear fuel cycle) or immobilization (for disposal).
One possible drawback associated with the current generation of diamide extractants concerns the relatively modest actinide distribution coefficients obtained even at high (greater than 1M) extractant concentrations. In a recently published report in the Journal of the American Chemical Society, we described the extraction characteristics for a new type of diamide molecule This new diamide (structure 1b, below is one million to 10 million times more efficient at extracting trivalent lanthanides and actinides from aqueous solution than previously examined diamides such as tetraalkylmalonamides.
Sergei Sinkov of Pacific Northwest National Laboratory (PNNL) presented results of research on the development a new diamide ligand with some truly amazing extraction capabilities. Compared to conventional diamide ligands, the new ligand molecule is one million to 10 million times more efficient at removing f-block metals from process waste.
It was assumed that this extraction enhancement reflects superior ligand-metal binding of the bicyclic diamide as compared to acyclic analogs such as tetraalkylmalonamides. A fuller understanding of such structure-function aspects of actinide extraction by amides is needed for the intelligent design of extractants with superior characteristics to those currently available.
A new bicyclic diamide ligand architecture optimized for bidentate lanthanide/actinide binding. R = methyl (1a), R = octyl (1b).
One of the possible approaches to quantify the magnitude of ligand-to-metal binding is the application of ultraviolet-visible (UV-Vis optical absorbance spectroscopy to monitor spectral changes induced by 1a, left in the absorption spectra of actinide ions in aqueous solution. The spectral information obtained by varying the ligand-to-metal ratio not only allows estimation of the number of light-absorbing species in solution, but can be used for measurement of the binding (or formation) constants of a ligand with metal ions.
The following sections compare the formation constants for the dimethyl bicyclic diamide (1a) and N,N,N',N'-tetramethylmalonamide(TMMA) with plutonium(IV), plutonium(VI) and uranium(VI) as measured in the aqueous phase at 1.0-molar ionic strength (HNO3).
Results with plutonium(IV)
In this experiment, we conducted a spectrophotometric titration by exposing a single portion of a plutonium-239, -240(IV) stock solution (better than 99.5 percent valence purity) to successive introduction and dissolution of either solid crystals (in the case of 1a) or tiny aliquots of a concentrated stock solution (in the case of TMMA). Neither precipitation nor perturbations in the oxidation state of plutonium(IV) were observed with either ligand throughout the titration procedure at a metal concentration of 8 millimolar (mM).
With 1a, the most significant spectral changes observed with increasing ligand concentration were found in the region of the main plutonium(IV) peak (a 476 arrow 497 nanometer [nm] shift) and in the red part of the plutonium(IV) spectrum (a 660 arrow 684 nm shift and a 2.4-times intensification of the absorption band).
Three-dimensional structure of the new bicyclic diamide. Carbon atoms are black, nitrogen atoms are blue, and oxygen atoms are red. The vectors on each oxygen atom, which indicate the direction required for optimal interaction with a metal ion, converge in the computer-designed structure, in contrast to conventional malonamide with diverging vectors.
Although the spectral changes for TMMA were similar, we found that such changes were less pronounced and not identical to those observed with 1a. Moreover, these changes occurred at much higher ligand-to-metal ratios. Using the Singular Value Decomposition procedure, we found no less than four complexed species for the plutonium(IV)-1a system and three major complexed species (and perhaps even a fourth minor species) for the TMMA-plutonium(IV) system.
Results with plutonium(VI)
Not only was the most intense and sharp absorption peak of plutonium(VI) at 833.6 nm monitored for expected complexation effects, but a wider range (330-950 nm) was examined to evaluate possible changes in the oxidation state of the plutonium (initially better than 99.5 percent hexavalent plutonium).
As with plutonium(IV), the 1a ligand produced pronounced spectral changes in the plutonium(VI) spectrum, with two, new, well-resolved peaks at 840.6 nm and 846.3 nm emerging with even a moderate excess of ligand. After reaching a ligand-to-metal molar ratio of 80, all three peaks of plutonium(VI) gradually disappeared. Moreover, several new spectral features emerged as shown in the figure on page 26. For example, the visible range of the spectrum showed new major peak maxima at 497 nm and 684 nm. This new spectral signature clearly indicated a reduction of plutonium(VI) to plutonium(IV) in the presence of 1a.
The spectrophotometric titration of plutonium(VI) by 1a in 1.0-molar HNO3. Spectra shown in dark pink correspond to seven successive additions
of the ligand up to L/M =80. Red spectra are kinetics of the plutonium(VI)L+plutonium(VI)L2 reduction to plutonium(IV)L4. Black trace shows the reference spectrum of plutonium(IV) with no ligand present
at the same acidity and metal concentration as the initial spectrum of plutonium(VI). The kinetic series spectra 08, 09, 10, 11, 12, 13, 14, 15,
and 16 were taken at zero minutes, 10 minutes, 20 minutes, 30 minutes,
40 minutes, 52 minutes, 63 minutes, 150 minutes, and 24.5 hours, respectively, after the last addition of the ligand.
A careful redox speciation analysis of the plutonium(VI) and (IV) spectra showed that this plutonium(VI) reduction was accompanied by the formation of plutonium(V)Ña weak spectral feature of variable intensity at 568 nm. Once we accounted for the presence of plutonium(IV) and (V), we used the nonlinear spectra processing routine known as SQUAD to process the portion of the spectral set in which plutonium(VI) remains predominant in terms of concentration. The formation constants refined from this analysis are shown in the table on page 26.
In contrast to 1a, the complexation of plutonium(VI) with TMMA did not induce any redox reaction (even after an overnight contact time). Complexation effects were significantly weaker, with the 1:2 complex seen not as a separate peak but rather as a weak shoulder in the 842Ð846 nm range.
A summary of all the refined formation constants resulting from this work. Conditional formation constants of plutonium(IV), plutonium(VI), and uranium(VI) complexes with 1a and tetramethylmalonamide in 1.0-molar HNO3 at 23±0.5o C. Values in brackets represent one sigma standard deviations.
Results with uranium(VI)
Both ligands induced significant spectral changes in the UO22+ spectrum, accompanied by a bathochromic shift and intensification of the absorption bands. Subsequent analysis indicated that 1a again acted as a much stronger binding agent than did TMMA. In both cases no spectral evidence was found for formation of higher complexes (1:3 and 1:4 stoichiometry).
Conclusion Using UV-visible spectroscopy, we have quantified binding constants for both bicyclic and acyclic diamide ligands with plutonium(IV), plutonium(VI), and uranium(VI) in acidic aqueous solution.
This work has yielded three key results. First, we showed that alternating the diamide structure from acyclic to bicyclic increased the overall formation constants with plutonium(IV) by as much as seven orders of magnitude. Second, we found that comparing actinide(VI)-ligand binding affinities reveals enhanced binding to uranium(VI) versus plutonium(VI). And third, the plutonium(VI), but not uranium(VI), in the bicyclic diamide triggers reduction of plutonium(VI) to plutonium(V) and (IV).
We are already at work evaluating the formation constants of TMMA and 1a with actinides in the +3 and +5 oxidation states.
This article was contributed by Sergei Sinkov, Brian Rapko, and Gregg Lumetta of Pacific Northwest National Laboratory (PNNL); and James Hutchison and Bevin Parks of the Department of Chemistry, University of Oregon, Eugene.
NMT |
LANL |
DOE
Phone Book |
Search |
Help/Info
L O S A L A M O S
N A T I O N A L
L A B O R A T O R Y
Operated by the University of California for the US Department of
Energy
Questions? -
Copyright © UC
1998-2000
-
For conditions of use, see Disclaimer