Factors Influencing the Efficiencies of Multistage Separations of Lanthanides

Abstract

Centrifugal partition chromatography (CPC) a multistage liquid-liquid countercurrent distribution technique has been demonstrated to be valuable for the complete separation of adjacent tervalent lanthanide metal ions. Using bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) and 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP) as the extractants in the heptane-H2O and toluene-H2O phase pairs respectively, the base line separations of light and heavy lanthanides at 25 °C were achieved. It was also shown that a mixture of light and heavy lanthanides could be separated in a single run by the use of a pH gradient in the aqueous mobile phase. These separation studies clearly indicated that the main factor limiting the CPC separation efficiencies was the slow kinetics of back-extraction of the lanthanide complexes. The mechanism of the complex dissociation reactions could be discerned from the dependency of the width of the CPC bands on the extractant concentration and pH. These mechanism were independently verified by stopped-flow kinetic studies in micelles formed by the neutral surfactant Triton X-100. Further a linear correlation was found between CETP (channel equivalent of a theoretical plate) determined from the CPC band widths and the half-lives of the dissociation reactions determined by stopped-flow. The importance of CPC not only as a tool for the separation of tervalent lanthanides but also for gaining fundamental understanding of their complex formation and dissociation kinetics was established in this study.