Effect of Receiving Phase Anion on Macrocycle-Mediated Cation Transport Rates and Selectivities in Water—Toluene—Water Emulsion Membranes

Abstract

Relative transport rates of metal nitrates (Na⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, T1⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺, and Pb²⁺) were measured alone and in combination with either Pb²⁺, Ag⁺, or T1⁺ in a water-toluene-water emulsion membrane system. The toluene phase contained the surfactant Span 80 and the crown ether dicyclohexano-18-crown-6 (DC18C6). The aqueous receiving phase contained the lithium salt of one of the following anions: pyrophosphate, thiosulfate, hydroxide, chloride, formate, nitrate. In the case of the metal combinations, chloride and formate ions were not studied. Unless significant complexation occurred both between the transported cation and the anion in the receiving phase and between the cation and DC18C6 in the membrane phase, there was little or no transport of the cation from the source phase to the receiving phase. Selective removal of Pb²⁺ and of Ag⁺ from binary mixtures of these cations with each of the cations listed was demonstrated using the emulsion membrane.