Solubility of Organophosphorus Metal Extractants in Supercritical Carbon Dioxide

The solubility (S) of several liquid organophosphorus compounds, tributyl phosphate (TBP), diisodecylphosphoric acid (DIDPA), di-(2-ethylhexyl)phosphoric acid (DEHPA), dihexyl-(N,N-diethylcarbamoyl)methylphosphonate (CMP), and octyl(phenyl)(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO), in supercritical CO2 was determined over temperature and pressure ranges of 30−90 °C and 7.5−25 MPa. The solubility of the compound increases with increasing density (ρ) of CO2 fluid, showing a linear relationship: ln S = p ln ρ + q (p and q are constants). The slope p measured at 60 °C is 21.8, 11.4, 13.1, 10.8, or 7.5, and the constant q, given solubility at ρ = 1 g/mL, is 26.3, 0.4, 2.7, 5.1, or −0.1 respectively for TBP, DIDPA, DEHPA, CMP, or CMPO. A homogeneous mixture of TBP or CMP with CO2 is readily obtained even at relatively low pressure, where the density of CO2 is relatively low. It is found that all the compounds examined are soluble enough in CO2 to prepare an organophosphorus compound−CO2 mixture which can be used in supercritical CO2 fluid extraction of metal ions.