Spectrophotometric study on solubility of UO2(β-diketonato)2dmso complexes (β-diketonate = acetylacetonate, trifluoroacetylacetonate, hexafluoroacetylacetonate; dmso = dimethyl sulfoxide) in supercritical carbon dioxide
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,
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2019, Green Sustainable Process for Chemical and Environmental Engineering and Science: Supercritical Carbon Dioxide as Green SolventSpectrophotometric studies on solubility of lanthanoid(III) complexes with fluorinated β-diketonates in supercritical carbon dioxide
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2013, Journal of Organometallic ChemistryCitation Excerpt :A resultant of the LA–LB interaction and the C–H⋯O hydrogen bond is the increased polarization of one of the CO bonds of CO2 that subsequently enhances the solubility of a number of polar solutes in SCCO2 [96]. Recent 1H, 13C and 19F NMR studies conducted on β-diketones and UO2(β-diketonato)2dmso [dmso = dimethyl sulfoxide] complexes found that their solvations in SCCO2 are attributed to a balance of van der Waals, LA–LB and the C–H⋯O interactions [95,100]. Further reading on the polar attributes of CO2 and their contribution to solvation can be obtained from Refs. [94–96].
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2011, Fluid Phase EquilibriaCitation Excerpt :In the following, we summarize some points that are important for experimental methods for phase equilbrium measurement. Almost 800 articles with experimental data on high pressure phase equilibria were found [163–818]. More than 2400 systems have been investigated, from pure-components, binary systems up to complex mixtures with many components.
Solubility of β-diketonate complexes for cobalt(III) and chromium(III) in supercritical carbon dioxide
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Present address: Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.