Rapid Separation of Tl+and Pb2+from Various Binary Cation Mixtures Using Dicyclohexano-18-crown-6 Incorporated into Emulsion Membranes

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Abstract
Relative transport rates of metal cation nitrates (Na+, K+, Rb+, Cs+, Ag+, Tl+, Ca2+, Sr2+, Ba2+, and Pb2+) in a water-toluene-water emulsion membrane system were measured. The toluene component contained the surfactant Span 80 and the crown ether dicyclohexano-18-crown-6. The aqueous receiving phase contained Li4P2O7. When each metal cation was individually present in the aqueous source phase, metal extraction was complete within 10 min with the order of extraction being Tl+ > Cs+ > Ag+ > Rb+ > K+ ≥m Na+ and Pb+ > Ca2+ > Sr2+ > Ba2+ for uni-and bivalent cations, respectively. Significant extraction was found for all cations except Na+, K+, and Ba2+. Some metal ions were concentrated nearly 10-fold in a 10-min period. Relative transport rates were determined when binary cation mixtures of either Tl+ or Pb2+ were present at equal concentrations with each of the remaining metal ions in the source phase. Tl+, when present with either Na+, Cs+, or Rb+, was selectively extracted from the source phase. Complete and nearly exclusive extraction of Pb2+ was observed in the presence of all cations including Tl+. The enrichment ratios of Pb2+ in the binary mixtures were approximately 10 while those of the second cation were less than 0.5 except for Sr2+ which was 0.86. Corresponding separation factors for Pb2+ ranged from 1000 to > 6000.