Spectrophotometric study on solubility of UO2(β-diketonato)2dmso complexes (β-diketonate = acetylacetonate, trifluoroacetylacetonate, hexafluoroacetylacetonate; dmso = dimethyl sulfoxide) in supercritical carbon dioxide

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Abstract

We have investigated the molar extinction coefficients (ɛ) and solubility of UO2(β-diketonato)2dmso (β-diketonate = acetylacetonate (acac), trifluoroacetylacetonate (tfacac), and hexafluoroacetylacetonate (hfacac); dmso = dimethyl sulfoxide) in supercritical CO2 (scCO2) at 40 °C in the pressure range of 10–25 MPa (density; 0.6 < ρ (g cm−3) < 0.9) by using UV–vis spectrophotometer equipped with high-pressure cell. The ɛ values (M−1 cm−1, M = mol dm−3) for UO2(acac)2dmso, UO2(tfacac)2dmso, and UO2(hfacac)2dmso in scCO2 were estimated as 1.06 × 102 at 355 nm, 9.59 × 101 at 440.5 nm, and 7.95 at 486 nm, respectively, and were similar to that in C6H14. We also found out that the solubility of UO2(acac)2dmso, UO2(tfacac)2dmso, and UO2(hfacac)2dmso increases with increasing pressure and in order of UO2(acac)2dmso < UO2(tfacac)2dmso < UO2(hfacac)2dmso, i.e., 2.67 × 10−3 < 1.37 × 10−2 < 3.73 × 10−1 M at 40 °C and 25 MPa. This is consistent with the order of an increase in the number of –CF3 group in the coordinated β-diketonates. Furthermore, the equation proposed by Chrastil was found to hold for the solubility of UO2(β-diketonato)2dmso, that is, the logarithmic plot of solubility versus densities of scCO2 showed the linear relationship. These phenomena suggest that the solubility of UO2(β-diketonato)2dmso is related with CO2 solvation and the strength of interactions of CO2 with the coordinated β-diketonates. In order to examine such interactions, we measured the dependence of 13C chemical shifts of CO2 in scCO2 containing β-diketones on the density of scCO2. As a result, it was proposed that the solubility of UO2(β-diketonato)2dmso is mainly controlled by the van der Waals interactions between scCO2 and particular sites (–CH3 and –CF3 groups) of ligands in UO2(β-diketonato)2dmso.

Introduction

Supercritical carbon dioxide (scCO2) is of interest as an alternative to organic solvents for the extraction of metal ions from solid and liquid wastes, because use of scCO2 should make it possible to reduce the amount of wastes and to simplify the extraction processes [1], [2]. However, the direct extraction of metal ions to scCO2 phase is known to be difficult, because of very poor solubility of metal ions in scCO2. Previous studies have demonstrated that non-charged metal complexes with chelating ligands lead to an increase in solubility of metal ions in scCO2 [3], [4], [5]. The β-diketones being one of the most important chelating ligands have been applied to the extraction of uranyl and lanthanoid ions from the radioactive wastes generated from the reprocessing processes of spent nuclear fuels [2], [3], [4], [6], [7]. The extractability can be enhanced by adding oxygen donor reagents such as tributyl phosphate (TBP) to scCO2 containing β-diketones, because of synergistic effect [3], [8]. To clarify the extraction mechanism and to optimize the extraction conditions, it is necessary to examine the solubility of metal–chelate complexes in scCO2 systematically. In spite of existence of the solubility data for many metal–chelate complexes in scCO2 [9], [10], [11], [12], [13], limited information is available concerning solubility of uranyl complexes in scCO2. Solubility of uranyl complexes should be helpful in designing supercritical extraction processes for decontamination of nuclear wastes.

Hence, we have examined the solubility of uranyl β-diketonato complexes, UO2(β-diketonato)2dmso (β-diketonate = acetylacetonate (acac), trifluoroacetylacetonate (tfacac), and hexafluoroacetylacetonate (hfacac); dmso = dimethyl sulfoxide), in scCO2 by using UV–vis spectrophotometer equipped with high-pressure cell. Furthermore, we discuss their solubility from the viewpoint of the interactions between scCO2 and ligands on the basis of data for 13C NMR chemical shifts of CO2 in scCO2 containing β-diketones.

Section snippets

Materials

The UO2(acac)2dmso, UO2(tfacac)2dmso, and UO2(hfacac)2dmso complexes (abbreviated as UO2(ACAC), UO2(TFA), and UO2(HFA), respectively) were synthesized using the procedure reported in the previous paper and recrystallized from ethyl acetate [14]. Acetylacetone (Hacac), trifluoroacetylacetone (Htfacac), hexafluoroacetylacetone (Hhfacac), and thenoyltrifluoroacetylacetone (Htta) (Wako Pure Chemical Ind. Ltd., 99%) were purified by distillation. Dimethyl sulfoxide (dmso) (Wako, 99%) was stored over

Molar extinction coefficients

The UV–vis absorption spectra of scCO2 samples containing various known amounts of UO2(β-diketonato)2dmso were measured at 40 °C in the pressure range of 10–25 MPa (0.6 < ρ < 0.9). The absorbances of scCO2 samples were constant within ±2% regardless of pressure. Plots of absorbances versus concentrations of UO2(β-diketonato)2dmso in scCO2 were found to be linear over the range of concentrations (UO2(ACAC) 1.1 × 10−5 to 1.3 × 10−4 M, UO2(TFA) 1.3 × 10−4 to 1.0 × 10−3 M, and UO2(HFA) 1.0 × 10−4 to 2.0 × 10−3 M). For

Conclusions

The ɛ values and solubility of UO2(β-diketonato)2dmso in scCO2 were examined at 40 °C in the pressure range of 10–25 MPa by using UV–vis spectrophotometer equipped with high-pressure cell. It was found that the ɛ values of UO2(β-diketonato)2dmso in scCO2 are similar to that in C6H14, and that the solubility of UO2(β-diketonato)2dmso increases with increasing pressure and the number of –CF3 group in the coordinated β-diketonates. The solubility of UO2(HFA), UO2(TFA), and UO2(ACAC) were 2.67 × 10−3,

References (28)

  • M.D. Samsonov et al.

    Dissolution of uranium dioxide in supercritical fluid carbon dioxide

    Chem. Commun.

    (2001)
  • Y. Lin et al.

    Supercritical fluid extraction of lanthanides and actinides from solid materials with a fluorinated β-diketone

    Anal. Chem.

    (1993)
  • Y. Lin et al.

    Supercritical fluids extraction of thorium and uranium ions from solid and liquid materials with fluorinated β-diketones andtributyl phosphate

    Environ. Sci. Technol.

    (1994)
  • A.F. Lagalante et al.

    Solubilities of copper(II) and chromium(III) β-diketonates in supercritical carbon dioxide

    Inorg. Chem.

    (1995)
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    Present address: Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

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