Interfacial effect of coexisting salt cations on extraction of erbium using gas bubble-supported organic liquid membrane

The conventional understanding about the specific ion effect from the coexisting electrolyte salts on the extraction and separation of target metal ions during liquid-liquid solvent extraction cannot explain some abnormal phenomena when more than one kind of background ions coexisting in the aqueous feed solutions to be separated. In this work, the specific ion effect of coexisting ions, Mg 2+ and Al 3+ as the example here, on extraction of Er 3+ ion was investigated using gas bubble-supported organic liquid membrane. A very interesting phenomenon was noticed that, the difference in Er 3+ extraction percentage was only 11.7 % in the Er-Mg solution system with addition of Al 3+ ions or not, while it could reach 18.0 % in that of Er-Al with addition of Mg 2+ ions or not. When Mg 2+ and Al 3+ coexisted simultaneously in the feed solutions, their specific ion effect on improving the extraction percentage of Er 3+ was not equal to the sum of that when only single Mg 2+ or Al 3+ exists, respectively. Furthermore, a thinner organic liquid membrane supported by gas bubbles was in favor of enhanced interfacial enrichment of Er 3+ and its diffusion mass transfer near the interface. During gas bubble-supported organic liquid membrane extraction, Mg 2+ ions exhibited more obvious specific ion effect due to a weaker interaction with P507 molecules. However, a stronger hydration ability of Al 3+ than Mg 2+ ions induces a stronger competition of Al 3+ with P507 molecules. Therefore, the coexistence of Mg 2+ and Al 3+ ions could play a synergistic role in enhancing enrichment of Er 3+ on the basis of the mutual interference from the synergistic specific ion effect of coexisting Al 3+ and Mg 2+ ions. The research helps thoroughly understand the regulation mechanism on the synergistic specific ion effect for selective extraction of specified ions from the complex solutions containing multiple coexisting background ions.