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

Abstract

The extraction of alkali metal ions from aqueous solution (K⁺, Na⁺ : 1.7 × 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.