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supercritical fluid chromatography. In addition, it has several advantages as an enzymatic reaction medium over conventional organic solvents, which include gas- like diffusivities and liquid-like densities. There is also at present, considerable interest in the fascinating properties of subcritical and supercritical water as solvent and reaction media. METAL COMPLEXES IN SUPERCRITICAL C02 Surprisingly, the first demonstration of the solvating power of supercritical fluids involved metal species ie cobalt and iron chlorides in supercritical ethanol in 1879. However, since then the emphasis has been on the extraction of organic compounds, with recently a revival in interest in metal extraction. As in organic solvents, free metal ions are insoluble in supercritical CO2 because of the charge neutralization requirement and the weak solute-solvent interactions. However, when metal ions are bound to well chosen organic complexing reagents or ligands, they can become quite soluble in supercritical CO2. Such reagents, depending on chemical structure, can complex metal ions in a polydentate or macrocyclic manner. Metal complexes have been separated and extracted using SF-C02, Wenclawiak et a1 have described the chromatographic separation of cobalt and chromium acetylacetone complexes using CO2 modified with methanol (3). In other work, a mixture of ferrocenes was shown to dissolve in pure supercritical C O 2 (4). A fluorinated diketone, namely 2,2,-dimethyl- 6,6,7, 7,8,8,8-heptafluoro-3, 5-octanedione has also been employed to remove lanthanides and actinides from solid materials (5). More recently, Wai has demonstrated that Cu (II) and Hg (II) can be extracted from solid matrices by supercritical CO2 containing lithium bisftrifluoroethyl] dithiocarbamate (LiFDDC). The choice of the fluorinated ligand is based on the fact that the solubilities of the fluorinated metal complexes are several orders of magnitude higher than the non-fluorinated analogues (6). OXIDATION STATES AND COMPLEXATION OF GOLD Precious metals such as gold and silver, in their normal oxidation states (gold in I and III and silver in I), tend to form their most stable complexes with ligands containing 'soft' donor atoms (7). Thus complexing reagents with N- and especially S-donor atoms form strong complexes with the precious metals. In particular, Au(I) forms its most stable compounds when bonded to 'soft' ligands, containing sulfur or phosphorus and is stabilized with respect to disproportionation by such complexation (8) . As medicinal compounds, goldll) thiolates (AuSR)n have been studied in relation to chemotherapy for rheumatoid arthritis. Au (III) is invariably complexed in all solutions, usually as anionic species such as [AuC14r and can be solvent-extracted by ion pairing with an oxonium ion from dilute HCl solutions into ethyl acetate and diethyl ether with a very high partition coefficient (9). Gold metal is chemically unreactive and is not attacked by oxygen or sulfur, but reacts readily with halogens or with solutions containing or generating chlorine such as aqua regia; it also dissolves in cyanide solutions in the presence of air or hydrogen peroxide to form [Au(CN)2r. The reduction of solutions of AIC14- by various reducing agents may, under suitable conditions, give highly coloured solutions containing colloidal gold. The complexation of gold for the purpose of extraction from ores has mainly been carried out using cyanide as reagent (10). The most important method for extracting gold is by treating finely crushed ore with sodium cyanide in the presence of oxygen to give a sodium gold cyanide complex, which is typically adsorbed onto activated carbon, before being re-extracted and reduced to the metal (11). Major problems with the cyanidation process are the relatively slow dissolution kinetics and the health and environmental problems associated with it. Generally, alternative gold complexing reagents while less toxic, are less robust, excessively consumed and less economically viable. Complexation has also been shown to playa role in controlling the oxidative power of Fe(III), used in acidic solution as an oxidant for Au(0) (12). In a patent specification Kenna suggested that complexation of the ferric ions could be utilized in reducing their oxidative power to a level where oxidation of gold still proceeded at an acceptable rate, while the oxidation of thiourea was greatly reduced (13). In a separate patent, Nagaraj found that when alkyl hydroxamic acids or their salts were used alone or in conjunction with traditional sulfide collectors on sulfide ores, the kinetics of flotation and overall recovery of the precious metals had increased significantly (14). Precious metals usually exist together with a large (fji;} ColdBulletin1999,32(2) 53PDF Image | Carrying Gold in Supercritical C02
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