Perfluorocarboxylic acid counter ion enhanced extraction of aqueous alkali metal ions with supercritical carbon dioxide

Abstract

Supercritical carbon dioxide (scCO₂) extraction of alkali metal ions from aqueous solutions (K⁺, Na⁺∶1.7 × 10^–4 mol dm^–3) with crown ethers [18-crown-6 (18C6), cis-dicyclohexyl-18-crown-6 (DC18C6), 15-crown-5 (15C5)] as ligands was investigated at 40 °C and 8–18 MPa. Ligands were chosen that form 1∶1 (cation–ligand) complexes with the alkali metal ions. The recovery for potassium ion was 1–2% with 18C6, and was insensitive to pressure. In the presence of an additive, pentadecafluorooctanoic acid (HPFOA), the recovery for potassium ion could be increased to 43–54%. This is because the HPFOA has CO₂-philic fluoroalkyl chains and also easily generates counter ions for the metal–ligand complex. Similar behavior was also observed when DC18C6 ligand was used, although the recovery relative to 18C6 was lower. The recovery for sodium ion was 5–15% with 18C6 and HPFOA and 20–40% when 15C5 or DC18C6 ligands with HPFOA were used. For these cases, the increased recoveries for sodium ion can be attributed to the smaller cavity sizes of the 15C5 and the hydrophobicity of DC18C6 ligands, in addition to the HPFOA counter ion effect. The extraction mechanism is considered to involve as cation–ligand complex formation, HPFOA dissociation and HPFOA anion interaction with the complex as counter ion. Based on the mechanism, a chemical equilibrium model was developed which confirmed that the extracted complexes (alkali metal ion–ligand–counter ion) for all systems were monomeric (1∶1∶1). These results show that alkali metals can be extracted with scCO₂ using 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.