Facilitated cotransport of Ag(I), Cu(II), and Zn(II) ions by DB18C6 and DA18C6 crown ethers as carriers: Interface behavior on ion transport

The interface behavior in the facilitated cotransport of Ag(I), Cu(II), and Zn(II) ions through supported liquid membranes (SLMs) made of a flat polypropylene membrane sheet containing dibenzo-18-crown-6 (DB18C6) or 4,13-diaza-18-crown-6 (DA18C6) as carriers was studied. The liquid-liquid extraction tests showed a maximum distribution coefficient when the carrier concentration was greater than 0.01 mmol/L. In transport experiments the transmembrane flux increased with increasing carrier concentrations that reached a limiting value over 5 mmol/L. The calculation of the diffusion coefficients in membranes showed a lower diffusivity of DA18C6-metal complexes than did DB18C6-metal complexes for all 3 ions: (Ag+) > (Cu2+) > (Zn2+). The diffusivity was significantly higher using a Celgard 2500 support than with Celgard 2400 or 2402. The immobilized interfaces were soaked for various times. The membranes with the highest capability to retain ions were immersed for 30 days. Phase fluxes of almost zero, at less than 10(-2) mol/L concentration, were obtained. In the transient state of the transport through the SLM, different molar flow rates at the feed-membrane and membrane-strip inter-faces were observed. The selectivity of the interfaces containing DB18C6 in the separation binary mixtures of ions showed the following separation factors: SFAg-Zn=2.9; SFAg-Cu=2.5; SFCu-Zn=1.2.