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Review date: July 24, 2003
PNNL-SA-27883

Extraction of Metal Ions from Liquid and Solid Materials by Supercritical Carbon Dioxide

KE Laintz, CM Wai, CR Yonker, and RD Smith. Anal. Chem. 64:2875-2878 (1992).

Abstract: Supercritical fluid extraction (SFE) of organic compounds has been the subject of many studies in recent years. A suitable solvent for this purpose is carbon dioxide due to its moderate critical constants (Tc = 31.1 °C, Pc = 72.8 atm), inertness, and easy availability in purified form. Little information is available in the literature regarding SFE of metal ions. Direct extraction of metal ions by supercritical carbon dioxide is known to be highly inefficient because of the charge neutralization requirement and the weak solute-solvent interactions. However, if metal ions are bound to organic ligands, their solubility in supercritical carbon dioxide may be significantly increased. A widely used chelating agent for trace metal extraction known in the literature is diethyl dithiocarbamate (DDC) which forms stable complexes with over 40 metals and nonmetals. Recently, Laintz et al. have shown that metal-DDC complexes have limited solubilities in supercritical carbon dioxide. If fluorine is substituted for hydrogen in the ligand, as in the case of bis(trifluoroethyl) dithiocarbamate (FDDC), the resulting metal-FDDC complexes exhibited significantly high solubilities in supercritical carbon dioxide. The solubilities of a number of metal-FDDC complexes in supercritical carbon dioxide are in the order of 10^-3M at 50°C and 100 atm which is about 2-3 orders of magnitude greater than the nonfluorinated analogues. The solubility of metal-FDDC in supercritical CO2 appears adequate for analytical applications. However, the feasibility of applying FDDC for metal extraction using supercritical CO2 as solvent has not been demonstrated. This paper describes an experimental approach for the extraction of Cu²⁺ from an aqueous solution and from a silica surface using a supercritical carbon dioxide fluid containing LiFDDC as an extractant.