XANES and EXAFS Measurements of Pu(III) and Pu(VI) Hydrates

The hydrate is the simplest chemical form of plutonium in aqueous solution. The knowledge of the structural parameters of the hydration sphere is important for the interpretation of EXAFS results on complicated aqueous plutonium complexes where the water molecules are partly or fully replaced by other ligands. Plutonium(VI) hydrate was prepared by dissolution of PuO2 (Pu-242, AEA Technology, QSA GmbH) and electrochemical oxidation. Part of this solution was reduced to Pu(III) in an electrochemical cell. The Pu(III) and Pu(VI) hydrates were in perchloric and nitric media (1 M acidic solution), respectively. The final Pu concentration was 50 mol/L. The Pu oxidation states were confirmed by UV/Vis spectroscopy. For the measurements 4.7 mL of solution (7.5 MBq)
was filled and sealed in polyethylene cuvettes. The samples were measured at the Rossendorf Beamline ROBL at the European Synchrotron Radiation Facility (ESRF), Grenoble, France within 48 hours after their preparation.

Multiple scans of the Pu LIII-edge EXAFS were collected in transmission mode at room temperature using the Si(111) double-crystal monochromator in fixed-exit mode /1/. The energy scale was calibrated using the first inflection point of the absorption spectrum of a Zr foil (17998 eV). The scattering phases and amplitudes where calculated for hypothetical clusters of PuO8, and PuO2O5 using FEFF6.

Results
As one can see from the XANES spectra given in Fig. 1, the LIII absorption edge of Pu(VI) is shifted by 4 eV toward higher energy as compared to that of Pu(III). The energy shift and the distinct XANES features of these two Pu hydrates can be used for the determination of the Pu oxidation states /2/.

The different electronic and molecular structures of Pu(III) and Pu(VI) hydrates are also reflected in the EXAFS shown in Fig. 2. The coordination sphere of Pu(III) hydrate can be written as Pu(H2O)83+ with an average Pu-O bond distance of 2.48 Å. The Fourier transform corresponding to the EXAFS of Pu(VI) hydrate shows two coordination shells. The Pu(VI) forms a plutonyl ion PuO2(H2O)4-52+. The axial and equatorial Pu-O bond distances are 1.74 and 2.42 Å, respectively. The structural parameters of Pu(VI) hydrate are nearly identical to those of U(VI) and Np(VI) hydrates (see Tab. 1), which were measured recently at ROBL /3/.

Fig. 2: Raw Pu LIII-edge k3-weighted EXAFS spectra (left) and corresponding Fourier transforms (right) of 50 mMol/L Pu hydrates. Solid line - theoretical fit; dots - experiment.

Tab. 1: EXAFS structural parameters for 50 mMol/L Pu hydrates and comparison with 50 mMol/L Np(VI) and U(VI) hydrates.
Sample Shell R(Å) N s2 a)
Pu(III) Pu-O 2.48 7.6(2) 1.02
Pu(VI) Pu -Oax 1.74 1.9 0.12
Pu -Oeq 2.42 4.4(2) 0.50
Np(VI) Np-Oax 1.75 2.0 0.15
Np-Oeq 2.42 4.6(2) 0.56
U(VI) U-Oax 1.76 1.9 0.12
U-Oeq 2.41 4.9(2) 0.61
a) s2 in units of 10-2 Å2