A direct measurement of the pH of water in contact with supercritical CO2 was made by observing the spectra of a pH indicator with a UV-vis spectrophotometer. The pH was analyzed under pressures of 70-200 atm and temperatures of 25-70 °C. The measured pH varied from 2.80 to 2.95, and relative standard deviations of <1.5% were achieved. The effects of pH on the efficiency of supercritical fluid extraction of metals and ionizable organic species in water-containing systems are discussed.Supercritical fluid extraction (SFE) has become an attractive alternative to conventional solvent extraction for the recovery of organic compounds and metal chelates from solid and aqueous samples.1•2 Carbon dioxide is the gas of choice for SFE because of its moderate critical constants, inertness, low cost, and availability in pure form. In many environmental applications, water is often present in an SFE system, either as a part of the original sample or added deliberately. The presence of water has been shown to facilitate the extraction of metal ions from solid materials using the in situ chelation technique in supercritical C02.2 3 The role of water in the SFE of organics and metals is not well understood. Water in contact with carbon dioxide becomes acidic due to the formation and dissociation of carbonic acid: C02 + H20 -H2C03 -»H+ 4-HC03D etermining the pH of water in contact with supercritical C02 is essential to understanding SFE of both metal ions and ionizable organic species. For SFE of metal ions, complexing agents are used to neutralize the metal charge and to transport the metals into the CO2 phase. The degree of dissociation of the complexing agents is dependent on the pH of the solution. If the complexing agents are not ionized at the solution pH, complexation and

Experiments have been performed to evaluate the feasibility of extracting solid uranyl nitrate with organophosphorus ligands using supercritical carbon dioxide as a solvent. Ligands investigated include tri-n-butylphophate (TBP), tributyl phosphine oxide (TBPO), trioctyl phosphine oxide (TOPO) and triphenyl phosphine oxide (TPPO). Of these ligands, TBP demonstrated the most favorable ligand behavior, forming stable complexes with uranyl nitrate, and showing excellent transport capabilities. TOPO and TBPO showed reasonable extraction efficiencies for uranyl nitrate but the resulting complexes were difficult to transport, presumably due to solubility limitations. TPPO showed poor extraction efficiency and poor transport capabilities. Further investigation of TBP-C0 2 phase behavior did not indicate secondary liquid phase formation. Kinetics of extraction were rapid, with quantitative extraction being achieved in 30 -40 minutes. Solubility of U0 2 (N0 3 ) 2 · xH 2 0 · yTBP was estimated at 1.3 X 10~3-2.0X 10" 3 M under the conditions 60°-120°C and 250 atm C0 2 .

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Uranyl ions in nitric acid solutions can be effectively extracted by supercritical CO 2 containing tributyl phosphate (TBP) or organophosphine oxides. The form of the extracted uranyl nitrate-TBP complex and the kinetics of the supercritical extraction are similar to those reported for the conventional solvent extraction of uranyl nitrate with TBP. On-line back-extraction of uranium in supercritical CO 2 with an aqueous solution has also been demonstrated. The results suggest that supercritical CO 2 could be as effective as the organic solvents used in the PUREX process. Supercritical CO 2 containing organophosphinic acids such as Cyanex 301 and Cyanex 302 has been shown to extract heavy lanthanides selectively from the light lanthanides in aqueous solutions. This in situ chelation-SFE technique is also capable of removing leachable uranium from solid samples such as mine tailings as indicated by the EPA Toxicity Characteristics Leaching Procedure.There has been considerable interest in the past two decades to utilize supercritical fluids as solvents for chemical separations (i). The reasons for developing supercritical fluid extraction (SFE) technologies are mostly due to the environmental regulations and waste disposal costs for conventional solvents. Supercritical fluids have both gas-like and liquid-like properties. The solvation power of a supercritical fluid depends on pressure and temperature; thus, one can achieve the optimum conditions for a particular separation process by manipulating the temperature and pressure of the fluid phase. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from solid matrices. Carbon dioxide is the most widely used gas for SFE because of its moderate critical constants (T c = 31.3 °C, P c = 72.9 atm), nontoxic nature, and availability in pure form. In SFE processes, solutes dissolved in supercritical carbon dioxide are separated by reducing 390 In Metal-Ion Separation and Preconcentration; Bond, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999. 391the pressure of the fluid phase causing precipitation of the solutes. The fluid phase is usually expanded into a collection vessel to remove the solutes and the gas is recycled for repeated use. Typical examples of large-scale industrial applications of the SFE technology using supercritical C0 2 include the preparation of decaffeinated coffee and hop extracts (1).Until recently, little information was available in the literature regarding SFE of metal species. Direct extraction of metal ions is highly inefficient because of the charge neutralization requirement and the weak solute-solvent interactions. However, when metal ions are chelated with organic ligands, they may become quite soluble in supercritical C0 2 (2). This in situ chelation-SFE technique appears to have a wide range of applications including the treatment of metal contaminated or radioactive waste materials and mineral processing.Background