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Carrying Gold in Supercritical C02 Jeremy D Glennon, Stephen J Harris, Andrew rtalker, Conor C McSweeney and Mark O'Connell Analytical Division, Department o f Chemistry, University College Cork, Ireland, Email.j.glennon@ucc.ie Received: 23 March 1999 A series of fluorinated calixarene derivatives, with lower rim sulfur-donor atoms, were synthesized and studied for the sequestration of gold using supercritical fluid extraction. Following temperature and pressure optimization and using extracts collected in methyl iso-butyl ketone, analysis by flame atomic absorption spectrometry reveals that Au(III) can be efficiently complexed and extracted by a novel calix[4]arene thiourea reagent in supercritical CO2, INTRODUCTION TO SUPERCRITICAL FLUIDS Supercritical fluids, especially those based on inert substances, are considered as 'clean' solvents, free from the environmental concerns of disposal, handling and toxicity associated with organic solvents. A pure supercritical fluid is a substance above its critical temperature and pressure (1). Above its critical temperature, it does n o t condense or evaporate to form a liquid or a gas but is a fluid with properties changing continuously from gas-like to liquid- like as the pressure increases. Above the critical temperature Tc and pressure Pc, increasing the pressure increases the density and thus the solvating power of the fluid. It is this solvating power that makes superaitical fluids useful i n such processes as the decatieinstion o f cotiee a n d i n the extraction of many important industrial chemicals including medicinal compounds, natural oils and flavours and even, organicpollutants. The critical parameters for just three of the many important substances useful as supercritical fluids are given in Table 1. Supercritical fluids can replace liquid solvents in many processes such as extractions from solids, counter-current multistage separations, chromatographic separations and chemical reactions. Table 1 Critical Parameters for SelectedSupercriticalFluids Tc{°C) Pc{atm) CO2 31.3 72.9 N20 36.5 72.5 In reacting systems, supercritical fluids can make possible homogeneous reactions by dissolving reactants, influence reaction equilibria and rates, and open up the way for new syntheses. In materials science, solid materials can be processed into very uniform fine particles and thin film production, material coating, impregnation and dyeing have recently been demonstrated (2). One compound, CO2, has so far been the most widely used for supercritical fluid extraction (SFE) because of its convenient critical temperature, cheapness, non-explosive character and non-toxicity. While generally applicable to the extraction of non-polar organic compounds, the addition of small amounts of modifiers such as the lower alcohols, extends the use of supercritical CO2 (SF-C02) to many polar compounds. Highly valuable extracts from medicinal plants have been obtained using SF-C02 for extraction and it is the solvent of choice in many applications including 52 COO' Cold Bulletin 1999,32(2) H 2 O 374.1 217.7 Figure 1 Phase diagram for carbon dioxide ~ Pc 73 P:: Supercritical Fluid 31 Tc T/oCPDF Image | Carrying Gold in Supercritical C02
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