Liquid-liquid extraction of metal ions with a cyclic ligand calixarene carboxyl derivative

The extraction behavior of alkaline, alkaline-earth, and transition metals in liquid-liquid extraction was systematically investigated with a cyclic host compound calixarene carboxyl derivative. Using a novel host ligand, an extraction equilibrium experiment was carried out and the complexation mechanism between the metal ions and the cyclic ligand was investigated. Calixarene carboxyl derivatives showed high extractability for all of the metal ions compared to that of the monomer analog. The extraction behavior of alkaline and alkaline-earth metals is closely related to the cavity size of the cyclic ligands; thus, the tetramer selectively extracted sodium and calcium ions, while the hexamer did so for cesium and barium ions. On the other hand, the extractability of transition metals could not be explained by a size effect, and increased in the order: monomer<tetramer<hexamer. Among transition metals, the higher the stability constant between a metal and the carboxyl group, the more preferential extraction was occurred. These results indicate that the cyclic structure is effective for the extraction of alkaline and alkaline-earth ions, rather than transition-metal ions. All of the metal ions were confirmed to be extracted with the cyclic ligands by forming a 1:1 complex in a toluene phase.