Extraction of Rare Earth Metals Using Liquid Surfactant Membranes Prepared by a Synthesized Surfactant

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Abstract
Three surfactants, l-glutamic acid dioleyl ester ribitol (nonionic, 2C 18Δ9 GE), l-glutamic acid dioleyl ester quaternary ammonium chloride (cationic, 2C 18Δ9 GEC 2 QA), and dioleyl dimethyl quaternary ammonium chloride (cationic, 2C 18Δ9 QA) were synthesized for potential use in liquid membrane operations. These surfactants have strongly hydrophobic, twin oleyl chains as the hydrophobic moiety. Using the synthesized surfactants, extraction of rare earth metals was carried out by liquid surfactant membranes in a stirred tank. The extraction behavior of 12 kinds of rare earth metals was systematically studied with 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (commercial name: PC-88A) as a carrier. Different surfactants having an identical hydrophobic moiety can have significantly different behaviors in rare earth extractions by liquid surfactant membranes, where extraction efficiency appears to be governed by the nature of the interfacial microenvironment between oil and water. An interfacial reaction model which takes into account the adsorption of a surfactant at the interface has been proposed to evaluate the permeation rate of rare earth metals by liquid surfactant membranes. It was found that a cationic surfactant strongly enhances the extraction rate of rare earth metals compared with the conventional surfactant, Span 80. The cationic surfactant 2C 18Δ9 GEC 2 QA appears to be one of the best surfactants currently available for rare earth extraction by liquid surfactant membranes.