Supercritical fluid extraction of alkali metal ions using crown ethers with perfluorocarboxylic acid from aqueous solution

The extraction of alkali metal ions from aqueous solution (K⁺, Na⁺ : 1.7 x 10^-4 mol dm^-3) with supercritical CO₂ (scCO₂) and crown ethers [18-crown-6 (18C6), 15-crown-5 (15C5)] as ligand was investigated at 40°C and 8.0-18.0 MPa. The recovery for potassium ion was 1-2% with 18C6 and was independent of pressure. Remarkably, the recovery increased to 40-60% in the presence of the additive pentadecafluoro-n-octanoic acid (PFOA), which is probably because PFOA has CO₂-philic fluoroalkyl chains, and easily generates counter ions for the metal-ligand complex. In the case of sodium, the recovery was found to be low even with PFOA, which may be related to the cavity size of 18C6. The 15-crown-5 ligand, having the smaller cavity size increased sodium recovery up to 20-40%. These results show that alkali metals can be extracted with scCO₂ and crown ether ligands and further that the recovery of the metals can be greatly increased by adding a substance that can supply a CO₂-philic counter ion.