Leaching supercritical

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... 

Poly(7V-vinyl-2-pyrrolidone)-Rh complex was used to catalyze the carbonylation of MeOH to MeOAc and AcOH in supercritical C02 at rates approximately 50% of those in liquid solution, but with minimal catalyst leaching.72... 

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

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). 

TACOM TBA TCLP THC TNB TNBA TNT TOC TRBP TW-SCWO Tank-Automotive and Armaments Command tributylamine toxicity characteristic leaching procedure total hydrocarbons trinitrobenzene trinitrobenzoic acid trinitrotoluene (an energetic material) total organic carbon thermal reduction batch processor transpiring-wall supercritical water oxidation... 

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... 

T0746 STC Remediation, Inc., Solidification/Stabilization Technology T0756 Supercritical Water Oxidation—General T0773 Terra Vac, Heap Leaching... 

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. 

Fly ash increases the density, decreases the permeability, and increases the leaching resistance of Ordinary Portland Cement (OPC). It is a truism that The leach resistance of solidified cement-waste systems can be improved by any process which accelerates curing, limits porosity, or chemically bonds fission product or actinide elements. (Jantzen et al., 1984). Supercritical C02 treatment of a modified Portland cement is expected to further increase the density over the untreated material, so that a reduced porosity and improved leachability should result. In addition, the high silica content of fly ash, with its well-known sorbent properties toward actinides and certain other radionuclides, enhances the immobilization characteristics. 

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]. 

Ultrasound-assisted leaching versus supercritical fluid leaching... 

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. 

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 ... 

Leachant removal after depressurization in supercritical-C02 leaching allows the leached species to be dissolved in a fairly low volume of appropriate solvent. 

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].)... 

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