The feasibility of using Kelex 100, Cyanex 272, 301 and 302, and D2EHTPA as reagents for the supercritical fluid extraction of toxic heavy metals is demonstrated. High extraction efficiencies are observed for a range of heavy metals using Cyanex 301 and 302 and D2EHTPA ligands from a variety of matrices and from samples of high acidity. Extraction efficiencies increased with increasing pressure. Solubility of a range of complexes was determined and found to vary between 0.15 and 12 g L -1 . The extraction of toxic metals from environmental matrices was shown to be efficient for a range of metals using the Cyanex 302 ligand. Kelex 100 was found to be very selective for the extraction of Cu 2+ using supercritical fluid CO 2 . In many cases the extraction process did not quantitatively remove the metals, but the extraction profiles were demonstrated to be initially rapid followed by a very low level of metal extraction, indicating a limit of the extraction process to leachable toxic metals.

The solubility of a ferf-butyl-substituted dibenzobistriazolo-crown ether in methanol (5 mol %) modified CO2 has been determined to be in the order of 1 x 10"1 23 M at 60 °C and 200 atm. Using this proton-ionizable crown ether as an extractant in methanol (5%) modified CO2, quantitative extraction ofHg2 5" from sand and cellulose-based filter papers can be achieved at 60 °C and 200 atm if a small amount of water is present in the solid matrix. Other divalent metal ions including Cd2+, Co2+, Mn2+, Ni2+, Pb2+, and Zn2+ are virtually not extractable imder these conditions. Gold (Au3+) can be extracted by the macrocyclic ligand with a lower efficiency than Hg2+. Extraction of Hg2+ from aqueous samples can also be achieved using methanol-modified CO2 containing the macrocyclic ligand.Formation of extractable metal chelates with the ligand at the pH defined by the CO2-H2O system appears to be an important factor determining the extraction efficiencies of the metal ions by this in situ chelation-supercritical fluid extraction technique. Supercritical fluid extraction (SFE) of metal species from solid and liquid materials has been the subject of several recent studies.1-5 One approach for extracting metal ions in supercritical CO2 is to convert the charged metal species into neutral metal chelates by use of a suitable ligand dissolved in the fluid phase. This in situ chelation-SFE method has been shown to exhibit high efficiencies for extracting metal ions from solid and liquid materials when a fluorinated chelating agent such as lithium bis-(trifluoroethyl) dithiocarbamate (IiFDDC) or a fluorinated /3-diketone is used as an extractant in supercritical CO2.1,2,4 It is conceivable that selective extraction of metal ions in supercritical fluids may be achieved if the chelating agent possesses an ion recognition capability.Dithiocarbamate and /3-diketones are not selective chelating agents because they complex with a large number of metals and non-metals. These chelating agents are also difficult to regenerate because of their instability in acid solutions. Crown ethers are a class of selective ligands that form stable complexes with metal ions based primarily on the ionic radius-cavity size compatibility concept.6 Modification of crown structure by attaching negatively

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