Numerous studies have been performed to develop efficient separation and concentration processes of metals with a solvent-extraction technique, which is well known to be one of the promising techniques to separate and concentrate metals on an industrial scale. The extractant is known to have a crucial effect on the separation and extraction efficiency in the solvent-extraction process. Therefore, it is very important to develop an efficient extractant for a target metal ion. Thus, the choice of the extractant often determines the success of the metal-separation process.Recently, a cyclic ligand calixarene derivative has been noted to be a novel host compound of metal ions.1-3 Calixarene has plural phenolic hydroxyl groups, and various derivatives are readily synthesized by introducing a suitable functional group. To date, some investigators have synthesized attractive calixarene derivatives as a novel cyclic receptor for a target material: it had a remarkable selectivity to the target material based on the cavity size and the rigid molecular structure. However, apart from a large number of studies concerning the synthesis of novel calixarene compounds, its application to metal extraction is still limited. Izatt et al. 4 first reported on the extraction of alkaline metal ions with a calixarene; then, novel calixarene ligands have been synthesized for the extraction of several metal ions.
5-9In a previous paper we reported on the extraction behavior of rare-earth metals with a calixarene derivative functionalized by carboxyl groups. 10 In the present study, we investigated the extraction behavior of twelve kinds of metal ions, such as alkaline (Li, Na, K, Cs), alkaline-earth (Mg, Ca, Sr, Ba) and transition-metal ions (Cu, Zn, Co, Ni) with two cyclic compound calixarenes (o-carboxylate derivatives of p-toctylcalix[n]arene (n=4,6)) along with the monomer analog (n=1). The relationship between the cyclic structure of the calixarene and the extraction behavior of the metal ions was discussed. In particular, we focused on the ring-size effect of the cyclic ligands and the stability constants between the functional carboxyl group and the metal ions.
ExperimentalReagents Figure 1 shows the structures and abbreviations of the extractants used in the present study. 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 extractabilit...
