Multistep pH-Peak-Focusing Countercurrent Chromatography with a Polyethylene Glycol-Na2SO4 Aqueous Two Phase System for Separation and Enrichment of Rare Earth Elements

Multistep pH-peak-focusing countercurrent chromatography was developed for separation and enrichment of rare earth metal ions using a polyethylene glycol-Na2SO4 aqueous two phase system (ATPS) and pH stepwise gradient elution. Metal ions in a sample solution are chromatographically extracted in a basic stationary phase (polymer-rich phase of the ATPS) containing a complexation ligand such as acetylacetone at the top of the countercurrent chromatography (CCC) column. After the sample solution is introduced, the mobile phases of which the pH values have been adjusted with buffer reagents are delivered into the column by stepwise gradient elution in order of decreasing pH. Each metal ion is concentrated at a pH border formed between the zones of different pH in the CCC column through extraction with a complexing agent into the stationary phase at the front side of the border (basic region) and back extraction into the mobile phase at the back side of the border (acidic region), moving toward the outlet of the column with the pH border. Mutual separations of La(III), Ce(III), Nd(III), Yb(III), and Sc(III) were achieved by the present method using five step pH gradient elution, and each rare earth metal ion was effectively enriched at each of the five pH borders. The mechanism for formation of pH profile of the column effluent and the potential of this technique for preparative scale separation are also discussed.