Collaborators Determine the Structure of Plutonium Nitrate Complexes in Nitric Acid

The first determination of the structure of a plutonium complex in solution has been carried out by scientists in NMT Division in collaboration with the Materials Science and Technology (MST) Division and the Glenn T. Seaborg Institute at Lawrence Livermore National Laboratory (LLNL). The experiments were conducted at the Stanford Synchrotron Radiation Laboratory (SSRL) in Palo Alto, California in the summer of 1992. The geometry of a series of plutonium-nitrate complexes that form in nitric acid has been studied. The experiments were enormously successful in demonstrating that radioactive substances such as plutonium can be studied safely at SSRL and in establishing the struc-ture of plutonium nitrate complexes in nitric acid.

Figure 2.a. A monodentate plutonium nitrate. b. A bidentate plutonium nitrate.

An immediate benefit of the present study is its application to plutonium processing. The nitric acid anion exchange process used to purify plutonium at TA-55 is based on the capability of Pu(IV) to form both anionic (negatively charged) and cationic (positively charged) nitrate complexes. An anionic nitrate complex is found on anion exchange resins in contact with 8 M nitric acid. A cationic nitrate complex is eluted from the resin with 0.4 M nitric acid. Understanding the speciation of the Pu(IV) ion in such nitric acid solutions is fundamental to understanding this process. The questions we wish to answer about these chemical species are 1) How many water and nitrate ligands are coordinated to the central plutonium ion in each of the six possible species that can form in nitric acid? and 2) Are the nitrates coordinated bidentate (i.e., two of the nitrate oxygen atoms bound to the plutonium) or monodentate (i.e., one nitrate oxygen bound to the plutonium)? Obtaining a detailed structure for the complexes involved could, for example, allow new ion exchange resins to be designed to provide more efficient means of processing nuclear materials. New bifunctional anion exchange resins based in part on these results have been synthesized at Texas Tech University and tested at Los Alamos. These new resins, covered by a recent patent application, have shown much higher efficiencies in the nitric acid anion exchange separation of plutonium. The structures of plutonium nitrate complexes in solution had not been accurately characterized previously even though the nitric acid anion exchange process has been used and studied for nearly forty years.

Extended x-ray absorption fine structure (EXAFS) is a modern experimental technique used to obtain the distances and the numbers of atoms that surround a central x-ray-absorbing atom, in this case plutonium. This is the technique that was used at SSRL. It is based on interferences between light rays scattered off of the surrounding atoms and those passing directly to the central atom. These distances are small, on the order of Angstroms, so the wavelength of light needs to be small in order to see the interferences, thus x-rays are used. The effect is also quite weak so a bright x-ray source, such as found at modern synchrotron sources, is required. The EXAFS technique is element-specific because it uses the natural resonance absorption edge of the element of interest. The medium, in this case water and nitric acid, does not give rise to any signal, and data analysis is relatively straightforward.

We chose to study the species formed in 3 M, 8 M, and 13 M nitric acid. We knew from earlier studies that the principal species in 3 M nitric acid contains two nitrate ligands and an unknown number of water ligands. We also knew that in 13 M nitric acid the principal species contained six nitrate ligands and no water ligands. The structure of this plutonium hexanitrato complex was reasonably well known because 1) the structures of the pluto-nium hexanitrato anion in solution and in single crystals are the same and 2) single crystal structures of thorium and neptunium hexanitrato compounds have been determined and are thought to be the same as the structure of plutonium hexanitrato compounds. We chose to study the complexes in 8 M nitric acid because 8 M nitric acid is used in the anion exchange process and because we knew there are almost equal concentrations of the dinitrato complex, the hexanitrato complex, and a third complex tentatively assigned as the tetranitrato complex (contains four nitrate ligands and an unknown number of water ligands). We wanted to study the tetranitrato complex because the efficiency of sorption of Pu(IV) onto anion exchange resin is proportional to the concentration of this species.

Examination of the three curves in Figure 3 allows us to answer our questions. There are three principal peaks in the spectra at 1.8 Å, 2.5 Å, and 3.6 Å. (The smaller wiggles are artifacts of the data analysis.) The peak positions and intensities correspond to the position and the number of atomic nearest-neighbors of Pu respectively. The largest peak represents the nearest-neighbor oxygen atoms (or first-shell oxygen atoms), which are part of the nitrate and water ligands. The peaks at 2.5 Å and 3.6 Å arise from second-shell nitrogen atoms and third-shell noncoordi-nated oxygen atoms of the nitrate ligands.

Figure 3: For the first time in nearly 50 years, scientists have determined the structure of a plutonium complex in solution. Qualitative information can be obtained from Fourier-transform analysis of raw EXAFS data as shown here.

The positions of these peaks indicate that the nitrate ligands are bound bidentate as shown in Figure 2b. If the nitrates were monodentate, then the positions of the peaks would have been different. The intensities of the outer two peaks are proportional to the number of nitrates bound to the plutonium. Systematic increases in the intensities of the nitrogen peak and the noncoordinated oxygen peak clearly demonstrate the trend of increasing nitrate ligation as a function of increasing nitric acid concentration. At the same time the intensity of the first peak does not appear to change substantially indicating that the nitrate ligands are replacing the nearest-neighbor water ligands. A quantitative analysis indicates that the number of first-shell oxygen atoms is either 11 or 12. Therefore, the dinitrato complex which has 4 first-shell nitrate oxygen atoms must also have 7 or 8 nearest-neighbor water ligands to bring the number of first-shell oxygen atoms to 11 or 12 and the tetranitrato complex must have 3 or 4 nearest-neighbor water ligands.

The complete text of a journal article describing this work and the quantitative analysis can be found on the World Wide Web at http:/www.lanl.gov/people/dsanchez/public/public.html or on the NMT-6 Home Page when its construction is complete.

Contributors to the project include Pat Allan of the Glenn T. Seaborg Institute for Transactinium Science at LLNL, Kirk Veirs and Coleman Smith of NMT Division, Steve Conradson of MST Division, and Fred Marsh of Sandia National Laboratory. The experiment required the help of a large number of TA-55 groups and individuals. Bill Haag in NMT-4 handled the requirements for proper transportation containers and made sure that the samples were received at Stanford. Radiation Control Technicians (RTCs) from TA-55 made the trip to Stanford to provide radiation protection as Stanford does not have properly trained RCTs. Experiments were conducted day and night. For our samples Arch Nixon provided the RCT coverage.

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