Extractive separation of metal ions with inorganic ligands such as halide ion into supercritical carbon dioxide

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A supercritical fluid extraction (SFE) by a supercritical fluid instead of a conventional organic solvent has recently been focused on the advanced method to remove the metal ions from aqueous solutions and solid materials. Among the supercritical fluids used in SFE, a carbon dioxide has been widely used because of its desirable properties, i.e. relatively low critical temperature and pressure, non-toxicity, non-flammability and high purity, although its solvating power is lower than those of organic solvents, such as chloroform.Extraction of most metal ions into supercritical fluid carbon dioxide (SF-CO_2), which is highly nonpolar as a solvent, from liquid and solid samples is difficult due to the requirement of charge neutralization of metal ions. The formation, however, of an uncharged complex enabled the extraction of metal ions into SF-CO_2, as well as a conventional solvent extraction. To select a suitable chelating extractants, therefore, is essential in the SF-CO_2 extraction.This work performed fundamental studies for the extraction of gallium(III) with 2-methyl-8-quinolinol (HMQ) and 2-methyl-5-butyloxymethyl-8-quinolinol (HMO_4Q) into supercritical fluid carbon dioxide from a weakly acidic solution. The distribution constants of HMO_4Q between aqueous and SF-CO_2 phases were determined at 45℃, 8.6-20.4 MPa and I = 0.1 M (H, Na)NO_3. At 45℃ and 15.3 MPa, gallium(III) was hardly extracted with HMQ into SF-CO_2, but was quantitatively extracted with HMO_4Q in the pH range of 2.20-2.84. The extraction constant, K_<ex-co2> ( [Ga(OH)(MO_4Q)_2]_<SF-CO_2>[H^+]^3[Ga^<3+>]^<-1> [HMO_4Q])^<-2>_<SF-CO2>), of gallium(III) with HMO_4Q was determined to be 10^<-2.6> at 45℃, 15.7 MPa and I = 0.1 M(H, Na)NO_3, which was 63 times larger than that in heptane at 45℃ and 0.10 MPa. It was also found that the addition of 3 ,5-dichlorophenol as a synergist enhanced the extractability of gallium(III) with HMO_4Q into SF-CO_2.