Extraction of Lanthanoids by Liquid Surfactant Membranes

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Abstract
Separation and concentration of lanthanoids such as La3+, Nd3+, Sm3+, Eu3+, Gd3+, Dy3+, and Yb3+ were carried out using liquid surfactant membranes containing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as a carrier. Under the condition of sufficiently high distribution ratio, more than 98% of these metal ions was extracted within 5 min even if the volume ratio of W/O emulsion to the external aqueous phase was as low as 1/32. The ratio of the concentration of metal ions in the internal aqueous phase to that in the external feed phase reached about 50,000 within 20 min. In the conventional solvent extraction of Yb3+, the aggregates of metal-carrier complexes, which were insoluble in the organic membrane phase, were formed at high loading ratio. In the extraction of Yb3+ by liquid surfactant membranes, however, formation of such aggregates was suppressed because both extraction and stripping occurred simultaneously on both sides of the membranes. The rate of interfacial reaction between lanthanoids and the carrier was remarkably reduced by the presence of the emulsifier, and the forward reaction rate was represented by rf =kf [Ln3+][(HR)2]3/[H+]3 where [(HR)2] is the concentration of the dimer of the carrier. The rate of the extraction by liquid surfactant membranes was satisfactorily simulated by a proposed permeation model, i.e., a “multilayer shell model.”