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Provides a powerful examination of complicated mixtures of elements

XAFS spectroscopy as a probe of actinide speciation with oxygen ligation

One of the more fascinating characteristics of the coordination chemistry of the light actinide elements is the wide range of oxidation states (or valence) that they exhibit, coupled to a correspondingly broad span of structure, bonding, and reactivity. In its typical location at the extreme of behavior, plutonium in aqueous solution can be found in oxidation states from III to as high as VII, and under the appropriate conditions at low pH, oxidation states III, IV, V, and VI can coexist.

One of the most incisive methods that has been used for probing the structures associated with this chemistry is XAFS spectroscopy. X-ray absorption techniques have a variety of advantages for the study of plutonium in diverse matrices. Every element absorbs x-rays at a different, characteristic energy that increases with atomic number (Z), giving XAFS a high degree of element selectivity. In some samples x-ray fluorescence from other elements can interfere with the signal of interest. In that case, one can move from examination of the LIII edge to another edge (or characteristic absorption energy) in the L shell, such as LII, or LI. This flexibility makes XAFS very powerful for examination of complicated mixtures of elements.

With third-generation sources, XAFS can be measured to the parts per million concentrations of plutonium typical of many residues. For standards, only a few milligrams of plutonium yield optimum spectra. Such small sample sizes are beneficial when dealing with radioactive materials both to limit radiation exposure to the research scientist and to limit the hazard to the environment and the public.

Furthermore, the x-rays can be focused, allowing spot sizes of several microns in diameter instead of the typical 1.5 by 12 mm size of the native beam. This focusing, in addition to allowing microquantities of plutonium to be probed, can be used to do spectromicroscopy that allows pinpointing or rastering selected areas and imaging versus averaging of the chemical environment across a sample.

Plutonium has a high affinity for oxygen-containing molecules and it is therefore important to understanding the fundamental coordination chemistry and properties of plutonium-oxygen bonds. From this perspective, the plutonium aquo ions are the “baseline” plutonium species in aqueous solution because they only have water molecules as ligands to the central plutonium ion. Other species form as different ligands substitute for one or more of the water molecules in the coordination sphere of the plutonium ion.

Knowing the characteristics of the aquo ions in detail can provide a starting point for understanding other plutonium complexes. The aquo ions of plutonium in the (III) and (IV) oxidation states have the general formula Pu(H2O)n3+ and Pu(H2O)n4+, respectively, which are often designated simply as Pu(aq) and Pu4+(aq). But plutonium in the (V) or (VI) oxidation state has such a large positive charge that, in aqueous solution, it extracts the O from water to form the transdioxo (plutonyl) cations PuO2+ and PuO22+. The corresponding aquo ions therefore have the general formulas PuO2(H2O)n+ and PuO2(H2O)n2+, which are conveniently designated as PuO2+(aq) and PuO22+(aq).

Because the chemical environment surrounding the plutonium ion in each solution is very consistent (the only ligands are water molecules), we could look for behavioral or structural trends among the four oxidation states. Plutonium in the (VII) oxidation state is a powerful oxidant and can only be stabilized for study in alkaline solutions, where its reaction with water is thought to produce a tetraoxo anion of formula PuO4(OH)23-.

Identifying oxidation states

The XANES of an XAFS spectrum, consisting of the absorption edge, the absorption peak, and fine structure on (or before for some transition metals) these two primary features, can be used to determine the oxidation state of the target (x-ray absorbing) element in solution or the solid state. The exact energy at which these features appear depends on the ionization energy of the ion.

This ionization energy increases with the ion's actual charge, so in general the XANES shifts toward higher energy with increasing valence. We have observed this shift in the plutonium aquo ions. The figure below shows a detailed view of the XANES spectra for a plutonium alloy, Pu(0); the aquo ions of Pu(III), (IV), (V), and (VI) all in perchloric acid solution; and Pu(VII) in a lithium hydroxide solution. The shift in energy between successive oxidation states is clearly visible, with the exception of the spectra of the Pu(IV)–(V) complexes where there is actually a small negative shift.

In this XANES spectra of Pu(0) and the aqueous species of Pu(III)–Pu(VII), the absorption edge shifts toward higher energy and increases as a function of oxidation state. The change in energy between Pu4+(aq) and PuO2+(aq) is quite small or even negative, reflecting a decrease in the actual charge on the plutonium in the plutonyl(V) species, but the two oxidation states can easily be distinguished by other features, such as the "-yl" shoulder in the XANES spectrum.

Other parts of the XANES, however, can be used to help correlate a spectrum with an oxidation state. For example, the shoulder that appears just after the main absorption peak in the spectra of Pu(V) or (VI) complexes in solution--the "actinyl" (or "-yl") shoulder--can be used to distinguish those oxidation states from Pu(IV). We have observed nearly identical plots for other plutonium species with oxygen ligand environments--including plutonium nitrates, carbonates, carboxylates, and oxides--in solution and the solid state. (In fact, other than the ~450 eV shift in the ionization energy as Z changes, we observe nearly identical spectra for uranium and neptunium compounds as well.)

Because the edge energies are independent of the chemical form of the plutonium, they can be used to identify the oxidation state of plutonium complexes in unknown chemical matrices. There is still speculation about the underlying reason behind the small shift between the (IV) and (V) oxidation states. The shift toward higher energies depends on the actual charge of the ion, rather than its "formal" valence, and evidently the actual charge does not vary much between the two states.

One possible explanation is linked to the formation of the plutonyl cations. The plutonyl cations have a linear structure, O=Pu=O. Recent calculations have shown that bonding between the plutonium and oxygen atoms in the plutonyl has a substantial covalent character, reducing the actual charge of the central plutonium ion. The actual charge of the plutonium ion in the Pu(V) complex PuO2+(aq) may therefore be very similar to the actual charge of the plutonium ion in Pu4+(aq).

In the image to the left, the Fourier transforms of the EXAFS of Pu3+(aq) and Pu4+(aq--the top two blue lines--show only one major peak, which indicates that all the oxygen atoms of the water ligands lie in a single coordination shell. The amplitude of the peak indicates between eight and nine oxygen atoms per shell. The bond length between (III) and (IV) complexes contracts by 4% with the increase in charge, whereas the number of ligands stays roughly the same. The molecule in the upper right shows a possible structure for n = 9, the tricapped trigonal prism. The Fourier transforms of the EXAFS of PuO2+(aq) and PuO22+(aq)—the bottom two blue lines—show two large peaks, which indicate two well-defined coordination shells. The first peak corresponds to the two oxygen atoms of the plutonyl moiety, which are located at 1.74 Å from the central plutonium ion. The second peak corresponds to the oxygen atoms of the water ligands, which are located about 2.4Å from the central plutonium ion. The equatorial coordination of the Pu(V) complex compared with the Pu(VI) complex shows a significantly smaller number of water ligands, which are located at a longer distance. The molecule in the lower right shows a possible structure for n = 5 water ligands, the pentagonal bipyramid.

The structure of plutonium aquo ions

While changes in the local chemical environment surrounding plutonium have little effect on the XANES, they do have a significant effect on the oscillations in the absorbance that occur at energies beyond the edge. In fact, the local molecular structure can be determined from these oscillations, known as EXAFS. Data from the EXAFS region provide information about the local atomic-scale environment.

For simple structures with well-separated shells of neighbor atoms, a Fourier transform of the data results in a (phase-shifted and inverse distance-squared weighted) radial distribution function that can be interpreted as shells of near-neighbor atoms surrounding the central metal ion. The position and intensity of the peaks in the Fourier transform are related to the absorber-scatterer distance and the number of atoms in each shell.

The figure on page 38 shows the Fourier transforms of EXAFS data for plutonium aquo ions in oxidation states (III)–(VI). For Pu3+(aq) and Pu4+(aq), the data show only a single large peak, which indicates that all the nearest-neighbor atoms (the oxygen atoms of the water ligands) lie evenly dispersed at the same distance from the plutonium ion. All of these results were confirmed by curve-fitting analysis, which also showed a Pu–O distance of approximately 2.49 Å for the (III) state and 2.39 Å for the (IV) state. The shorter bond lengths for the (IV) state are associated with the higher charge of the central ion. The number of water molecules bound to the plutonium is similar for both species, with n = 8 or 9. (The Pu3+(aq) ion with n = 9 has been isolated and characterized in the solid state, see the article on page 7.)

The transform of PuO2+(aq) and PuO22+(aq) data shows two peaks. The first corresponds to the two oxygen atoms in the plutonyl ion. Analysis of the data indicates a Pu=O distance of 1.74 Å. The second peak in the Fourier transform corresponds to the oxygen atoms of the water ligands. It is known from other studies that all these oxygen atoms bond in the equatorial plane of the plutonyl moiety, and we deduce Pu–O distances that range from 2.4 to 2.5 Å.

A significant result of our research is the finding that the PuO2+(aq) has a lower number of water ligands (n = 4 to 5), all at a longer bond distance, than PuO22+(aq), where n = 5 to 6. This finding confirms a trend that was seen in studies of the actinyl ions of uranium and neptunium, namely, that An(V) species appeared to coordinate fewer ligands than An(VI) species. Because plutonium exhibits aquo ions in four oxidation states (and is the only actinide to do so), our experiments are the first to observe this trend directly.

To overcome the problem of samples having to be prepared at Los Alamos but studied at a synchrotron located a thousand miles away, researchers developed an electrochemical cell (left) where they could prepare samples in situ at the beam line. The series of XANES measurements (right) show the successful conversion of Np(VI)–(VII).

When other information is taken into account, the XAFS data are consistent with a bipyramidal coordination geometry for PuO2+(aq) and PuO22+(aq). The plutonyl moiety forms the axis of the bipyramid, and, depending on conditions and the ligand used, the geometry may be a tetragonal bipyramid (four ligands in the equatorial plane), a pentagonal bipyramid (five ligands), or a hexagonal biypramid (six ligands with bidentate species such as CO32–). We have used the plutonium aquo ions to establish the baseline data and oxidation state trends necessary to determine the oxidation states of plutonium complexes in matrices of unknown composition. This background data and understanding have allowed us to apply XAFS spectroscopy to characterize plutonium and other actinide species in a wide variety of chemical environments ranging from contaminated soils (see ARQ 1st Quarter, 2002) to process solutions used in our weapons production mission.

Unusual oxidation states--spectroelectrochemistry

It is difficult to study the unusual oxidation state (VII) at a synchrotron facility due to the length of time between sample preparation at Los Alamos and the actual measurement of its spectrum at a synchrotron facility a thousand miles away. To overcome this obstacle, we developed an electrochemical cell wherein we could prepare Np(VII) and Pu(VII) in situ at the beam line. (For a discussion of related structural studies of Np(VII), see the article on page 15.)

In alkaline solution, Np(VI) exists as a dioxo ion, while Np(VII) displays a tetraoxo unit. These changes in molecular geometry alter the spectral shape of the XANES, which shows two distinct peaks of comparable amplitude for the tetraoxo species instead of the single primary peak and shoulder characteristic of the dioxo structure. A 0.8 eV shift is observed in the edge between Np(VI) and (VII) complexes, consistent with the higher charge of the latter.

This article was contributed by Los Alamos researchers Steven D. Conradson of the Materials Science and Technology Division and David L. Clark of the Nuclear Materials Technology Division.


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