Facilitated transport of metal ions through composite and polymer inclusion membranes

The application of activated composite membranes (ACMs) and polymer inclusion membranes (PIMs) for the transport of Pb(II), Cd(II), and Zn(II) in a continuous extraction–reextraction system using organophosphorous acids as carriers was examined. The ion carriers, DEHPA, Cyanex 301, and Cyanex 302, were incorporated into PIMs composed of cellulose triacetate as a support and o-nitrophenyl pentyl ether as a plasticizer. ACMs were pepared by interfacial polymerization and PIMs by physical immobilization of carrier into plasticized cellulose triacetate. The polyamide top layer of the ACM immobilizes organophosphorous acids by trapping them in the polymer net during the interfacial polymerization process. Transport of Pb(II), Cd(II), and Zn(II) ions through ACM and PIM membranes was determined. Highly selective Pb(II) separation was achieved in the ACM system with Cyanex 301; the transport rate of process, however, was higher for the PIM system. It was found that ACM has much higher stability than PIM. A membrane-based process for Zn(II) or Pb(II) recovery from hydrometallurgical aqueous effluents has been proposed.

The following conclusions can be derived from the results of the present study: The flux values for PIMs were considerably higher compared with activated composite membranes. On the other hand, the metal transport was comparable between PIMs and ACMs, taking into account the different membrane parameters in each case. The investigations show that the mechanism of transport across PIMs is a facilitated counter transport. At higher ion carrier concentration, however, a fixed size mechanism was observed, i.e., the same as for ACMs. High long-term stability of PIMs and ACMs was established, thus allowing the use of this process in hydrometallurgical operations. The transport through PIMs with Cyanex 301 and Cyanex 302 was faster as in the case of ACMs; the selectivity, however, was higher for ACMs. The increase of transport rate as well as higher selectivity were observed for ACMs formed from the PIM with D2EHPA. The possibility of using metal ion transport through ACMs for technological solution treatment was also shown. Finally, the stability of PIMs and ACMs was checked by the study of transport cycles. With D2EHPA, the initial flux was very stable, while with D2EHPA it oscillated remarkably. Nevertheless, fluxes achieved an almost stable value, indicating that these membranes are more stable and durable than analogous supported liquid membranes.