Raman spectroscopy of neptunyl and plutonyl in aqueous solutions: hydrolysis of Np(VI) and Pu(VI) and disproportionation of Pu(V)

Abstract

Abstract While the infrared and Raman spectra of the uranyl ion and its complexes have been the subject of considerable attention, there have been very few publications concerning the vibrational spectra of the plutonyl and neptunyl ions. For example Basile et at. [1, 2] have published values of the symmetrical stretching vibration, v1, of the PuO2+2 and NpO2+2 ions and of their carbonato complexes in 0.1 M Na2CO3 solutions. Madic et al. [3] published recently t001 . (cm−1). Plutonium v1 (F.W.H.M.) 1 Neptunium v1 (F.W.H.M.) 1 Mo2+2 833(9) 854(34) 1st hydroxo complex 817(14) 854(36) 2nd hydroxo complex 826(10) 805(25) 3rd hydroxo complex 794(43) 1 Full width at half maximum. Raman spectra of actinide(VI) complexes in 2.0 M Na2CO3 solutions. Recently Toth and Begun [4] demonstrated the usefulness of Raman spectroscopy for studying the hydrolytic behavior of the uranyl ion. The purpose of the present work was to use Raman Spectroscopy to study the hydrolysis of PuO2+2 and NpO2+2 ions in slightly complexing aqueous solutions. This paper also presents the Raman spectrum of the PuO+2 ion as well as evidence for the disproportionation of PuO+2 as a function of time. Figure 1 presents the modifications of the Raman spectra in the range 700 to 900 cm−1 of 0.1 M Pu(VI) solution (μ = 1.0) as a function of increasing pH. For pH = 1.62 and 3.77, the 833 cm−1 Raman band is found to be unchanged; consequently hydrolysis of PuO2+2 at a concentration of 0.1 M does not occur for pH = 3.77. At pH = 4.19 and 4.42 only two bands are observed (833 and 817 cm−1). For higher pH values the spectrum becomes more complex. Between pH = 4.66 and 5.31 two new bands appear at 826 and 805 cm−1 (shoulder). At pH = 6.12 and 7.51 a large increase of the intensity near 800 cm−1 is observed which is due to the new peak observed at 794 cm−1 at high pH values; this peak shows a resonance Raman effect. The modifications of the Raman spectrum of a 0.1 M Np(VI) solution (μ = 1.0) in the range 750 to 950 cm−1 caused by an increase in pH were studied. At pH = 2.96 the Raman band was found to be essentially that of NpO2+2 ion (854 cm−1). For higher pH values only a broadening of the 854 cm−1 band is observed; this effect corresponds to the appearance of a Np(VI) hydroxo complex in solution. Digitally recorded spectra for both Pu(VI) and Np(VI) were analysed using a computer peak fitting program. Table I presents the main characteristics of PuO2+2 and NpO2+2 ions and their hydroxo complexes. Normalization of the height of the Raman bands was made using v1 stretching bands of ClO−4 ion (for Pu(VI)). The variations of the relative intensities of the different Raman bands as a function of pH have been used to derive the concentrations of the free ions and those of the hydroxo complexes and the results have been compared with calculations done using hydrolytic concentration quotients published in the literature [5]. Reasonable agreement was found, thus the following assignments can be made: 833 cm−1, PuO2+2; 817 cm−1, (PuO2)2(OH)2+2; 826 and 805 cm−1, (PuO2)4(OH)+7; 854 cm−1, NpO2+2; 834 cm−1, (NpO2)2(OH)2+2. A plutonyl(V) solution PuO+2 was prepared by electrochemical reduction of PuO2+2. Raman spectra obtained at different steps of the electrolysis are shown in Fig. 2. During the electrolysis the 833 cm−1 PuO2+2 band disappeared and a new band centered at 748 cm−1 with a F.W.H.M. equal to 20 cm−1 grew in intensity. The variation of the relative intensity of the 748 cm−1 band versus the relative intensity of the 833 cm−1 band (both normalized to the intensity of the 934 cm−1v1 band of the ClO−4 ion) was found to be linear. The 748 cm−1 Raman can be assigned as the v1 stretching vibration of the linear PuO+2 ion. After six days had elapsed evidence for the disproportionation of PuO+2 ion as a function of time was obtained by Raman spectroscopy: a decrease of the 748 cm−1 band and the appearance of bands at 833 and 817 cm−1 was observed. Raman spectroscopy is seen to be a powerful tool for studying the aqueous chemistry of actinyl ions. Due to the high scattering factor observed and the simplicity of the spectra, Raman spectroscopy should be the selected technique for further studies on aqueous Pu(V) chemistry.