Partition coefficients of cobalt chelates and chelating agents between aqueous solutions and supercritical carbon dioxide

Distribution (or partition) coefficients (Dx) of several chelating agents and cobalt-chelates between aqueous phases and SC CO2 were determined. β-Diketones and dithiocarbamates fluorinated or not were the investigated chelating agents for cobalt. Different solute concentrations and different aqueous phase pH values were experienced for fixed operating conditions at 270 bar pressure and 50 °C temperature. The results obtained were compared with similar measurements performed with hexane as surrogate of SC CO2. HFAAH distribution ratio is weaker than AAH when SC CO2 is the extracting solvent. Distribution coefficients of cobalt-chelates, CoAA2 and CoHFAA2, are roughly equal when the solvent is SC CO2. With hexane, CoAA2 exhibits a higher distribution coefficient than CoHFAA2. However, distribution ratios for cobalt β-diketonates are very weak (10−2–10−3) because of their tendency to form hydrated chelates in aqueous medium. On the opposite, cobalt dithiocarbamates (CoDDC3 and CoFDDC3) are very stable under acidic conditions, which explains their distribution ratios much higher than 1. Hence, chelating agents NaDDC and LiFDDC seem to be more suitable than β-diketonates to extract divalent cobalt from aqueous medium, in spite of their instability in an acidic medium formed by H2O in contact with SC CO2.