Metal complexation/supercritical fluid extraction

Separation Techniques. Current methods for separating fatty acids are by solvent crystaUi2ation or by the hydrophili2ation process. Other methods that have been used in the past, or perhaps could be used in the future, are panning and pressing, solvent extraction, supercritical fluid extraction, the use of metal salts in assisting in separation, separations using urea complexes, and adsorption/desorption. 

The iridium catalyst was found to be sufficiently soluble for catalysis when in the form of the substrate complex, but precipitated quantitatively once all substrate had been consumed. Supercritical fluid extraction at that stage yielded the solvent- and metal-free product in crystalline form leaving the active and selective catalyst behind for... 

Wai CM, Wang S (1997) Supercritical fluid extraction metals as complexes. J Chromatogr A 785 369-383... 

Metal-complexation/SFE using carbon dioxide has been successfully demonstrated for removal of lanthanides, actinides and various other fission products from solids and liquids (8-18), Direct dissolution of recalcitrant uranium oxides using nitric acid and metal-complexing agents in supercritical fluid carbon dioxide has also been reported (79-25). In this paper we explored supercritical fluid extraction of sorbed plutonium and americium from soil using common organophosphorus and beta-diketone complexants. We also qualitatively characterize actinide sorption to various soil fractions via use of sequential chemical extraction techniques. 

Interestingly, the phase separation in this case results from a change in the composition of the reaction mixture rather than temperature or pressure. The iridium catalyst was found to be sufficiently soluble for catalysis in the form of the substrate complex, but it was precipitated quantitatively once all substrate has been consumed. Supercritical fluid extraction at that stage yielded the solvent- and metal-free product in crystalline form, leaving the active and selective catalyst behind for further use. Under batch operation, noticeable deactivation occurred after the fourth cycle, but this might be avoided by working in a closed system. Recently, ionic liquids have been found to stabilize the iridium catalyst for better recycling in a two-phase approach [30]. 

Although modifiers are added to supercritical fluids to increase their polarity, they can also impart decreased polarity, aromaticity, chirality and the ability to further complex organometallic compounds. Just as carbon dioxide is the most popular substance for use as a supercritical fluid, it is also that to which modifiers are most frequently added. This is so because modifiers are seen as the means for enabling the use of CO, in situations where it may not be the best solvent. For example, methanol is added to supercritical CO, to increase its polarity, aliphatic hydrocarbons to decrease it, toluene to impart aromaticity, [/ ]-2-butanol to add chirality and tributyl phosphate to enhance the solvation of metal complexes. The amount of modifier to be added depends on the properties of the extractant and those of the analyte and matrix usually, it ranges from a few... 

At the liquid-SC CO2 inter ce, a constant, and sufficient high concentration of extractant Cyanex 302 is assumed. Concentration effects of generated metal-complexes are assumed to be negligible, due to diffusion coefficient of solutes in supercritical fluids of about 10" m /s (17), which is approximately 2 orders of magnitude fester than in the aqueous phase. A continuous flow of solvent during extraction even reduces surfece effects, due to both continuous supply of exbactant and continuous removal of metal-extractant complex. Thus, mass transfer is a limiting factor at the liquid-SC CO2 interface, as studied previously by Tai et al. (18). Furdier research is required to study a possible impact on the overall extraction of SFE from humid MSWI fly ash. 

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