Optical Absorption, Stability and Structure of NpO 2 + Complexes with Dicarboxylic Acids
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Optical Absorption, Stability and Structure of NpO2+ Complexes with Dicarboxylic Acids Guoxin Tian and Linfeng Rao Glenn T. Seaborg Center, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ABSTRACT Complexation of NpO2+ with oxalic acid (OX), 2,2'-oxydiacetic acid (ODA), 2,2'iminodiacetic acid (IDA) and 2,2'-thiodiacetic acid (TDA), has been studied using spectrophotometry in 1 M NaClO4. Both the position and the intensity of the absorption band of NpO2+ at 980 nm are affected by the formation of NpO2+/dicarboxylate complexes, providing useful information on the complexation strength, the coordination mode and the structure of the complexes. INTRODUCTION The dioxoactinide(V) and (VI), or actinyl, cations of U, Np, Pu, and Am are of importance because of their unique properties and behavior in the reprocessing of nuclear spent fuel and in the migration of actinide elements. As hard Lewis acids like the tri- and tetravalent f-element ions, the actinyl cations tend to form primarily ionic bonds and prefer oxygen donor ligands to softer ligands containing nitrogen and sulfur donor atoms [1]. The bonding strength usually depends on the charge density on the cations due to the electrostatic nature of the bonds. The pentavalent actinyl cations An(V), with the lowest charge density of the actinide ions, have very low tendency to form strong complexes with ligands, even with hard bases such as OH-, and F-. Consequently, the pentavalent actinyl cations are very difficult to separate from high-level nuclear wastes by using traditional extractants, and they are highly mobile in the subsurface, raising concern about the migration of these radionuclides in the environment [2]. To better understand the chemistry of actinides related to separations and environmental migration, studies of the complexation of pentavalent actinyl cations with various ligands are necessary. NpO2+ is the best representative of pentavalent actinyl cations for such study, because it is the most stable An(V) cation and it has characteristic optical properties very useful for spectrophotometric studies. The absorption bands of NpO2+ at 980 and 1024 nm follow Beer-Lambert behavior, and are sensitive to the coordination environment of NpO2+ [3-5]. A recent study from this group has shown that the intensities of these bands are related to the symmetry of the NpO2+ species because they originate from f-f transitions [6]. It is of interest to investigate if the correlation between the absorption spectra and the symmetry also exists for NpO2+ complexes with dicarboxylate ligands such as oxalate (OX), oxydiacetate (ODA), iminodiacetate (IDA) and thiodiacetate TDA). By reviewing the data on the complexation of NpO2+ with dicarboxylates in the literature [7, 8, 9,10], it has been found that the majority of previous studies put emphasis on the calculation of stability constants, but insufficient discussions were given on the coordination modes and the symmetry in the NpO2+ complexes. In this study, we have re-visited the features of t
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