Metal Ion Separations by Supported Liquid Membranes Josefina de Gyves* and Eduardo Rodrı ´guez de San Miguel Departamento de Quı ´mica Analı ´tica, DEPg, Facultad de Quı ´mica, UNAM, Ciudad Universitaria, 04510 Me ´ xico, D.F. Me ´ xico Carrier-mediated transport through supported liquid membranes is currently recognized as a potentially valuable technology for selective separation and concentration of toxic and valuable metal ions. In this paper, a review of the fundamental aspects concerning metal ion transport and the influencing factors are surveyed in terms of data modeling, membrane efficiency (permeability, selectivity, stability), and data acquisition and evaluation. An account of the information reviewed demonstrates the need for critical reflection on system performances in order to accomplish scaling up operations. On the same basis, an attempt to outline some future trends in the field is presented. Overview Membranes for the separation and concentration of metal ions have received considerable attention through- out the past three decades due to characteristics such as ease of operation, energy and selectivity advantages, and low cost operation factors. From a practical point of view, separation membranes find applications in the industrial (Lee et al., 1978; Danesi, 1984-1985; Dwozak and Naser, 1987; Guerriero et al., 1988; Kopunec and Manh, 1994), biomedical (Uragami,1992), and analytical fields (Jonsso ¨n and Mathiasson, 1992; Taylor et al., 1992; Parthasarathy et al., 1997) as well as in waste- water treatment (Prasad and Sirkar 1988, 1990; Chiari- zia et al., 1990a,b; Yun et al., 1993 and Ortiz et al., 1996a,b). Nowadays, they constitute basic materials that stimulate scientific research and technological developments. Consequently, continual efforts are being made to improve the performance of these membranes. In general, a membrane may be regarded as a semipermeable barrier. When placed between two aque- ous phases, chemical species can move through the membrane from a region of high solute concentration into a region of low solute concentration by means of a purely diffusional process. However, it has long been observed (Cussler, 1971) that species can also be trans- ported across the membrane against their own concen- tration gradient as a consequence of an existing con- centration gradient of a second species present in the system (coupled transport). Furthermore, the transport process may take place in the presence of an extractant or carrier contained within the membrane (facilitated transport). Studies of facilitated transport originated in biochemistry using natural carriers contained in cell walls. The development of microporous polymer films together with the high fluxes, high selectivities, and specific concentrations achieved in biomembranes en- couraged investigation into artificial membranes and carriers. Over 25 years ago, Bloch (1970) first proposed the use of extraction reagents dissolved in an organic solution and immobilized on microporous inert supports for removing metal ions from a mixture. An interesting historical account of coupled transport is presented by Baker and Blume (1990). Subsequently, other research- ers observed that the carrier could assist in the trans- port process (coupled facilitated transport) by reacting competitively with the two species which were being transported across the membrane (Baker et al., 1977, Babcock et al., 1980a,b). Depending on the nature of the extraction reagent, facilitated coupled transport can be of two types (Danesi, 1984-85): counter- and cotrans- port. When the extractant exhibits acidic properties, coupled countertransport takes place and the extraction reaction proceeds via However, when basic or neutral extractants are used, coupled cotransport takes place according to where pH and counterion concentration are used as driving forces, respectively. Liquid membranes, depending if they contain only liquid phases or if a polymeric support is involved in addition to the liquid phases, may be divided into two categories: nonsupported liquid membranes and sup- ported liquid membranes (SLM). In the case of non- SLMs the most common types are emulsion liquid membranes (ELM) and bulk liquid membranes (BLM). A more detailed description of the nature, performance, and applications of ELMs and BLMs may be found in Noble et al. (1989) and Bartsch et al. (1996), respec- tively. For SLMs, common configurations available commercially include: flat sheets (FS) and hollow fibers (HF). Also, many types of membrane modules are produced. A typical SLM consists of a polymeric (organic or inorganic) support impregnated or in contact with an extractant or carrier dissolved in an organic solvent and two aqueous solutions. The organic phase is immiscible with the aqueous media and sometimes contains an- other component which is called the modifier. A modifier is added in order to favor the extraction of a selected species in a synergetic fashion or to avoid microemulsion or third phase formations. In the case of FS-SLMs, the support is generally a laminar-form inert porous material. The solute, initially present in the aqueous feed solution, permeates selec- tively through the membrane by interacting with the specific carrier contained in the organic phase. On the * Author to whom correspondence may be addressed. Phone: (525) 6223792. E-mail: degyves@servidor.unam.mx. M + + HX(membrane) S MX(membrane) + H + (1) M + + X - + E h S EMX(membrane) (2) 2182 Ind. Eng. Chem. Res. 1999, 38, 2182-2202 10.1021/ie980374p CCC: $18.00 © 1999 American Chemical Society Published on Web 05/01/1999