Supercritical fluid leaching

Ultrasound-assisted leaching versus supercritical fluid leaching... 

USAL allows leaching of a wide variety of compounds, whatever their polarity, as it can be used with any solvent. On the other hand, supercritical fluid leaching uses almost exclusively CO2 as leachant (with or without a co-leachant as a modifier), which restricts its scope to non-polar or low-polar analytes. 

USAL is usually more expeditious than supercritical fluid leaching. 

On the other hand, USAL falls short of supercritical fluid leaching in the following respects ... 

Supercritical fluid extraction can be performed in a static system with the attainment of a steady-state equilibrium or in a continuous leaching mode (dynamic mode) for which equilibrium is unlikely to be obtained (257,260). In most instances the dynamic approach has been preferred, although the selection of the method probably depends just as much on the properties of the matrix as those of the analyte. The potential for saturation of a component with limited solubility in a static solvent pool may hinder complete recovery of the analyte. In a dynamic system, the analyte is continuously exposed to a fresh stream of solvent, increasing the rate of extraction from the matrix. In a static systea... 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Another solution to the problem of ionic liquid loss to the organic phase is to extract the product from the ionic liquid using a supercritical fluid (See Chapter 8, Section 8.2.2.3). It has been demonstrated that this can be done continuously for a variety of reactions including the hydroformylation of long chain alkenes [20], and that neither the ionic liquid nor the catalyst are leached to significant extents. The only problem here is the high pressures involved (see section 9.8). 

In mercury speciation studies, pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SEE) are employed [33]. In particular, methyl-mercury is extracted by the Westoo method [33,34], which consists in a leaching process with hydrochloric acid, the extraction of the metal chloride into benzene or toluene, the addition of ammonium hydroxide that converts the metal species to hydroxide and the saturation with sodium sulfate. Most of the HPLC methods reported in literature for the determination of organomercury compounds (mainly monomethyhnercury, monoethyhnercury, and monophenylmercury) are based on reversed... 

Meguro,Y. Iso, S. Yoshida,Z. Tomioka, O. Enokida,Y. Yamamoto,I.Decontamination of uranium oxides from solid wastes by supercritical C02 fluid leaching method using HN03-TBP complex as a reactant, J. Supercrit. Fluids 31 (2004) 141-147. 

Numerous extraction methods and techniques have been developed and reported, especially if one takes into account the variety of modifications. The most common and simple general classification of these methods is similar to that introduced in chromatography and based on the kind of phase to which the analyte is transferred. One can distinguish the extractions as liquid, solid, gas, and supercritical fluid phase extractions. More precise description specifies the two phases between which the analyte is distributed (e.g., liquid-liquid or solid-liquid [leaching] extractions). The latter methods are all called solvent extraction. 

Supercritical fluids, most commonly carbon(IV) oxide, occasionally modified by a small addition of a polar solvent (methanol, acetonitrile, or water). Supercritical fluid extraction (SEE) uses water as the most popular additive, because increasing the temperature from 50 to 400 °C at a pressure exceeding the critical level makes it possible to achieve transition of extractant from the subcritical to the supercritical state and leaching of the compoimds in the order of polar to moderately polar [86]. 

The equipment needed is much simpler, so the overall cost of leaching is much lower. This can be of interest to routine laboratories with a limited budget, unable to afford a supercritical fluid extractor. 

Figure 4.13. Scheme of a supercritical fluid leacher assisted by ultrasound (A). Supercritical leaching efficiency of oil from almond for a 3-4 mm particulate size with and without ultrasound assistance (B). (Reproduced with permission of Elsevier, Ref [178].)... 

In many situations, ultrasound-assisted leaching is an expeditious, inexpensive, efficient alternative to conventional extraction techniques and, in some cases, even to supercritical fluid and microwave-assisted extraction. A number of applications to both organic and inorganic analytes in a wide variety of samples exist. Most are conducted by hand. As with automatic extractions, applications involving continuous systems are still very scant and hence one possible target for future research. 

The term solvent extraction can apply to the removal of pollutants from soil with a solvent. However, this is properly called leaching and will be covered separately toward the end of this chapter along with supercritical fluid extraction (SEE). Initially,... 

It must be also noted that supported ionic liquid phase (SILP) catalysis can also be successfully combined with supercritical fluids. Cole-Hamilton et al. [127] have reported recently high activity (rates up to 800 h ), stable performances (>40 h) and minimum rhodium leaching (0.5 ppm) in the hydroformylation of 1-octene using a system that involves flowing the substrate, reacting gases and products dissolved in... 

R. A. Leach and J. M. Harris, Thermal Lens Absorption Measurements by Flow Injection into Supercritical Fluid Solvents. Anal. Chem., 56 (1984) 2801. 

SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. 

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