Solvent effect on distribution ratio of Pd(II) in supercritical carbon dioxide extraction and solvent extraction using 2-methyl-8-quinolinol

The distribution ratio (DM) of Pd(II) by the extraction with 2-methyl-8-quinolinol (HMQ) was determined using the supercritical carbon dioxide medium (SF-CO2) and organic solvent media such as perfluoro-methylcyclohexane, heptane, cyclohexane, carbon tetrachloride and benzene. From experimental results of the slopes of log DM versus pH plot and log DM versus HMQ concentration plot, the extracted species both in the SF-CO2 extraction (SFE) and the solvent extraction (SE) were determined to be Pd(MQ)2. The distribution constant of HMQ (KD,HMQ) in the SFE and SE systems were determined from the dependence of the distribution ratio of HMQ (DHMQ) on the pH. A linear relationship was observed between log KD,HMQ and the solubility parameter (δ) of the extraction medium based on the regular solution theory in both the SFE using SF-CO2 at the pressure of 8.5–40 MPa and the SE systems. The difference in the slope of the log KD,HMQ versus δ plot between the SFE and the SE systems is attributable to the extent of the specific interaction of the solute HMQ with the solvent molecules, i.e. CO2 molecules and the organic solvent molecules. The DM versus δ plot obtained under a given extraction condition using SF-CO2 (11–40 MPa) and organic solvents showed clear linearity. The DM obtained using SF-CO2 at relatively low pressure range from 8.5 to 11 MPa was independent of the pressure and the δ of SF-CO2, which coincides with the experimental fact that the solubility of Pd(MQ)2 in the SF-CO2 at 8.5–11 MPa was practically constant.