Metal Ion Separations by Supported Liquid Membranes - American

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Ind. Eng. Chem. Res. 1999, 38, 2182-2202

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 throughout 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 wastewater treatment (Prasad and Sirkar 1988, 1990; Chiarizia 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 aqueous 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 transported across the membrane against their own concentration gradient as a consequence of an existing concentration 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 encouraged 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 researchers observed that the carrier could assist in the trans* Author to whom correspondence may be addressed. Phone: (525) 6223792. E-mail: [email protected].

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 cotransport. When the extractant exhibits acidic properties, coupled countertransport takes place and the extraction reaction proceeds via

M+ + HX(membrane) S MX(membrane) + H+

(1)

However, when basic or neutral extractants are used, coupled cotransport takes place according to

h S EMX(membrane) M+ + X- + E

(2)

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 supported liquid membranes (SLM). In the case of nonSLMs 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), respectively. 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 another 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 selectively through the membrane by interacting with the specific carrier contained in the organic phase. On the

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Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2183

opposite side of the membrane the reaction between the solute and the carrier is reversed due to the different prevailing conditions of the strip aqueous phase. In a specific case, the prevailing conditions favor the formation of a stronger complex between a counterion present in the stripping solution and the metal species. The solute passes into the stripping solution and the extractant stays in the SLM to repeat the cycle. Experimental designs usually consist of a two-compartment cell. However, cells composed of three compartments have also been used where two supports of equal (Zuo et al., 1996; Wo´dzki and Sionkowski, 1995) or different nature (Kislik and Eyal, 1996a,b) separate the organic and aqueous phases. In these two cases, since the supports are not impregnated, the middle compartment contains the carrier-diluent stirred or circulated solution. While Zuo et al. (1996) demonstrate the suitability of their cell to overcome problems due to the evaporation of the diluent and Wo´dzki and Sionkowski (1995) report, as the main advantage of using two ion-exchange membranes (multimembrane hybrid system, MHS), the possibility of avoiding some problems caused by the instability of organic liquid membranes in contact with aqueous solutions and an enhancement of the overall transport process, Kislik and Eyal (1996a,b) suggest several economical and operational advantages such as low carrier losses, long membrane lifetime, different driving forces, and compact equipment for their threecompartment cell identified as “hybrid liquid membrane” (HLM). However, the authors have observed that when back-extraction kinetics is a limiting factor for metal mass transfer, HLM and HFCLM (hollow fiber contained liquid membrane) (Guha et al., 1994 and Yang et al., 1996) present similar performances. Nakamura and Akiba (1989) used a combination of two SLM systems to study the transport of europium, which consisted of two supports of equal nature but impregnated with different carriers separating aqueous solutions of different composition. This system exhibited good stability but poor extraction diffusion rates. Finally, for this same element, Danesi (1984-85) has proposed a system consisting of a series of composite SLMs, interposed between compartments containing identical aqueous solutions. Composite SLMs have proved to be useful to perform multistage separations and concentrations when single-stage processes present limitations due to the very similar chemical behavior of the metal species. In the case of HF-SLMs, several modes of operation are described in the literature. In one mode, the carrier is absorbed in the microporous walls of polymeric supports shaped as tiny hollow tubes (i.d. 0.5-1.0 mm). Usually the feed solution is circulated through the lumen and the strip solution on the shell side of the hollow fiber. In such a case, extraction and backextraction of the metal species take place simultaneously. In a second mode, aqueous and organic solutions flow continuosly in the same way as described above, with both phases coming into contact through the pores of the fiber wall, and a differential pressure is applied in one of the phases to avoid phase entrainment. Here, only one separation operation is realized, either extraction or back-extraction. To carry out these two separation operations simultaneously contained liquid membranes, CLM (Sengupta et al., 1988; Guha et al., 1994), can be used. In the case of CLMs, the extracting organic solvent is contained in the interstices

of two sets of microporous hollow fibers in a permeator. In this HFCLM, the aqueous feed flows in the lumen of a fiber of one of the sets. The aqueous strip solution passes through the lumen of the other set. Each aqueous-organic interface is immobilized at the respective fiber by applying the correct phase-pressure conditions. For hydrophobic fibers, the aqueous feed and strip solutions flow at higher pressures than the organic liquid phase. Recently, Yang et al. (1996) introduced a three-set module which allows simultaneous separations of cations and anions using two different carriers and two different stripping solutions. Finally, a different type of liquid membrane has been developed by Teramoto et al. (1989) named “flowing liquid membrane” which was made into a spiral-type module. In this type of SLM, the microporous membranes and mesh spacers are spirally wound around acrylic resin pipes through which the feed solution, the strip solution, and the organic membrane solution are supplied to the module. The main advantages reported for the recovery of chromium(VI) and zinc(II) are high stability (about 100 h of operation) and high performance. However, to commercially use the flowing liquid membrane, further technical improvements of the module were suggested by the authors to prevent leakage of the organic solution. In general, the literature concerning membranes is extensive. Books edited by Araki and Tsukube (1990), Baker et al. (1991), Mulder (1991), Osa and Atwood (1991), Osada and Nakagawa (1992), Ho and Sirkar (1992), and Bartsch and Way (1996) provide extensive theoretical and practical treatments on membranebased separation processes. Several summarizing papers provide a substantial amount of information concerning basic principals and technological applications of liquid membranes (Marr and Kopp, 1982; Danesi, 1984-85; Noble et al., 1988, 1989; Boyadzhiev, 1990; Baker and Blume 1990; Kopunec and Mahn, 1994; Visser et al., 1994). Modeling of SLM Transport Mechanisms The development of theoretical models which account for the experimental results is fundamental for a complete understanding of SLM transport mechanisms. Modeling allows one not only to explain but also to predict the behavior of a system under different experimental conditions when a set of relevant parameters are identified (concentration of the different chemical species, forward and reverse rate constants, thickness of the membrane, diffusion coefficients of the species or mass transfer coefficients, etc.). Furthermore, mathematical modeling is essential to accurate scale-up SLM systems. Such models have been proposed and applied in SLMs, both in FS and HF configurations. Depending on the degree of complexity of the model, the mathematics used can dim the direct physical meaning of the role that chemical, hydrodynamic, and geometric parameters play, making it difficult to clearly analyze the influence of changing these relevant parameters. This is the reason ad initium simplified models were preferred over the more complex ones. However, with the availability of high-speed computers and the development of fast algorithms and computer programs, numerical simulations can be more readily performed, making use of more sophisticated models commonly used nowadays. Although it is not the objective of this paper to provide an exhaustive analysis of the many

2184 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999

models which have been postulated to study carriermediated transport through SLMs, a brief panorama of the models most frequently used by researchers in the field will be given in order to serve as a guide for future developments. Models for a variety of conditions such as interfacial reactions, mass transfer resistances and different geometries and structures for FS-SLMs, ELM, and HFSLM are given in Noble and Way (1987) and in Inoue (1990) in which diffusion, passive transport, and active transport are reviewed. Schultz (1977) has also previously discussed mathematical models for various systems which were globally nonreactive assuming equal diffusion coefficients for all forms of the transported species and reaction rates much faster than diffusion rates in the organic phase. A nonequilibrium approach considering the problem of transport in the aqueous liquid film boundary layers has also been addressed (Schultz et al., 1974; Goddard et al., 1974; Goddard, 1977). FS-SLMs. In SLMs, the transport mechanism can be described as a series of elementary steps which constitute the overall reaction scheme: (1) The metal ion diffuses from the bulk phase to the membrane interface across the aqueous diffusional layer. (2) The carrier reacts with the solute at the feed interface. (3) The complexed carrier diffuses across the membrane. (4) The solute is released by the carrier at the strip interface. (5) The released metal ion diffuses from the strip interface to the bulk phase across the aqueous diffusional layer. (6) The carrier returns across the membrane. Furthermore, for consideration of the co- or countertransport of the chemical species which serves as the driving force of the transport process, additional steps can be included in the reaction scheme: (7) diffusion of the co- or counterspecies across the aqueous diffusional layers; (8) chemical reaction of the co- or counterspecies with the carrier. In addition to steps 1-8, a change from the common facilitated transport mechanism to a “jumping” membrane transport was theoretically predicted by Cussler et al. (1989) and experimentally applied by Kalachev et al. (1992). Mainly, two approaches have been used to calculate the solute flux: the simplified and the integral. In the former, only some of the steps of the reaction scheme are considered, while, in the latter, all of them are taken into account. Table 1 summarizes some representative models which have been proposed and briefly describes the methods, assumptions, and restrictions involved. It is noticeable that almost all models deal with the application of Fick’s first law. This law can be conveniently used for the description of stationary transfer processes. Nonsteady processes are better described by Fick’s second law. Because the steady states of the diffusion layers change as the equilibria are approached, the transfer conditions also vary making the steadystate assumption not completly valid. Double film theory models can be extended to nonstationary processes by taking into account the mass transfer coefficient timedependent functions. The assumption of steady state is the reason usually initial fluxes and permeabilities are measured in experiments, even when the SLM lifetime is evaluated. The mathematical and time-consuming complications introduced in the more sophisticated models are only justified when the predictions of the model, over a broad range of experimental conditions,

agree with the experimental data in such a form that a simplified model is not able to predict. In this form, Danesi’s model has been successfully applied to an extensive number of systems cited in the literature because the experimental conditions usually employed (extremely low distribution coefficients in the membrane strip interface, sufficiently low membrane phase polarity, and a concentration of the metal-containing species much lower than that of the carrier and the ion which serves as driving force) allow satisfactory application of the equations. However, Youn et al. (1995) have shown the deviations observed between model predictions and experimental values when the proton transfer resistance and carrier concentration variation are not considered in the cobalt-HEH(EHP) system. Moreover, it is well-known (Tarasov and Yagodin, 1988; Freiser, 1988) that different types of metal extraction mechanisms can take place depending on the pH of the aqueous phase, the distribution coefficients, acid dissociation constant, extractant concentration, etc., for the same chemical system. In this form, when only one ratedetermining step is considered in a model, a variation of the experimental conditions in a broad scope may restrict its correct application. At the same time, the constant value of the diffusion coefficient is valid only in very diluted solutions of the species and the thickness of the diffusion layer depends both on the hydrodynamics and the value of this coefficient. The assumption usually made concerning the employment of the “bulk” equilibrium extraction constant at the interfaces should be carefully analyzed; the reaction conditions change significantly within the interface region in the course of the extraction and with the variation in the hydrodynamic situation. In other words, the rate of extraction (stripping) depends on the local reaction equilibrium constant at the aqueous phase-boundary layer but not on the global equilibrium constant (Tarasov and Yagodin, 1988). Aggregation of the extractant, polymerization of the metal-containing organic species, adduct formation, and participation of several equilibria in the overall extraction reaction are commonly found phenomena in liquidliquid extraction systems. As SLMs resemble liquidliquid extraction, these phenomena are expected to be present in membrane systems. However, the complexity and poor knowledge of the chemistry involved make it difficult to determine how these effects should be considered. Chiarizia et al. (1983) found that the influence of the feed metal concentration and carrier dimerization on membrane permeability was entirely explainable in terms of a variable monomer concentration of the carrier molecule. Chemical reactions taking place simultaneously between the monomeric and dimeric species of the carrier and the metal ion (Kanungo and Mohapatra, 1995) and/or the diffusion of both carrier species in the membrane phase (Juang and Lee, 1996b) have been reported. In these models the dimerization constant is incorporated as a new equation to be satisfied in the resolution of the coupled equations during the numerical treatment. Polymerization of Eu(III)-D2EHPA complexes has been observed in SLMs when the transport rate diminished due to the use of a thick support material or the presence of acetate anions at high Eu(III) concentration (Hung and Lee, 1990). Concerning the number of migrating species usually only one is taken into account. The contribution of the other species is neglected by diffusion considerations

2-4, 6, 8

2-4

1-3

2-4, 6

1-8

1-8

2-4, 6, 8

Baker et al. (1977)

Danesi et al. (1981), Komasawa et al. (1983)

Ibań˜ez et al. (1989)

Kiani et al. (1984), Prasad et al. (1986), Prasad and Sirkar (1987a,b), Zha et al. (1995c)

Plucinski and Nitsch (1988), Youn et al. (1995), Daiminger et al. (1996)

Hernańdez-Cruz et al. (1998)

reacn scheme steps

Cussler (1971)

authors Analytical expressions for the fluxes and the maximum concentration difference were proposed. Equations show the roles that diffusion, facilitated diffusion, and pumping play.

advantages

A study of the concentration change in the receiving phase allows the determination of the equilibrium constant and the maximum initial flux through the membrane. Analytical solutions for the permeant concentration in the receiving phase as a function of time are derived under different conditions. The approach is similar to the treatment of liquid-liquid extraction in conventional solvent extractors.

Boundary layer resistances in the aqueous phases are not considered. Diffusion coefficients are assumed constants and equal for all the species, partition coefficients are assumed equal at both interfaces, and uncoupled fluxes are considered. Boundary layer resistances in the aqueous phases are not considered; local equilibrium near the membrane is assumed. The simplifications incorporated in the model do not allow explicit analysis of the influence of the driving force and calculation of the maximum solute concentration difference that the system can achieve. The model does not apply when additional resistances are present. Boundary layer resistances in the aqueous phases are not considered. Assumptions of equal diffusivity of the chemical species, steady-state concentrations, and local equilibrium near the membrane are made.

restrictions

The aqueous films and the membrane pore Mass transfer in the films is stationary and diffusion resistances are combined as a oneof molecular nature. The concentrations dimensional series of diffusion resistances in have a linear profile; it is considered that which the solute transport is described by equilibrium is instantaneously established simple film-type mass transfer coefficients at the interfaces. (film theory). Application of Fick’s first diffusion law in The solute flux can be estimated over a broad Assumptions of steady-state and linear combination with the governing rate laws for range of experimental conditions. If all the concentration gradients throughout the the chemical reaction taking place between characteristics parameters are known, membrane and the aqueous boundary the solute and the carrier. The resulting it leads to correct predictions. layers. equations are numerically solved simultaneously in combination with the mass balance equations. Fick’s second law is applied for the diffusion The model accounts for non-steady-state Diffusion through the membrane phase is of the carrier species in the membrane, situations and predicts the concentration considered to be rate controlling; i.e., combined with the extraction equilibrium profiles of the chemical species within the diffusion of the ion through the aqueous constant at the two interfaces and the aqueous membrane as a function of time, as well as boundary layers is neglected. complex equilibrium constants. Equations are the concentration of the solute species in numerically solved. the source and receiving phase.

Assumption of steady-state concentration of the carrier complex at both the source and receiving interfaces, combined with the mass balances of the chemical species and the equilibrium constant. Fick’s first diffusion law for the metal-carrier complex in the membrane is assumed.

Application of Fick’s first law for the metalSimple model. In its interval of validation, carrier complex in the membrane phase with the model correctly accounts for the the assumption of local equilibrium at the two influence of pH, maximum attainable interfaces. Steady-state flux. concentration factor, and coupling effects. Incorporation of the kinetic mechanism of Simple model; an analytical solution metal extraction with Fick’s first law for the is obtained. diffusion of the ion through the feed aqueous boundary layer and the metal-carrier complex in the membrane phase.

Calculation of the steady-state flux of two solutes diffusing simultaneously across a carrier-containing membrane separating two well-stirred solutions applying the continuity equations for steady-state diffusion of the solute, the carrier complex, and the carrier.

method of evaluation

Table 1. Reported Model Mechanisms of FS-SLM Transport



provided the diffusion of large molecules is slower in comparison with the small ones (Cianetti and Danesi, 1983; Chiarizia, 1991a). Multiple metal-organic species diffusion models have also been developed. Fernańdez et al. (1986) fitted the theoretical permeability equations and the experimental values to propose a two-species scheme that accounts for Zn(II) transport with D2EHPA. More recently, analytical expressions for the transport of three species in the membrane phase have been established (Kanungo and Mohapatra, 1995a). Competitive reaction schemes in which two or more metal ions react with a given extractant have been treated with the numerical methods indicated above (Juang, 1993a; Juang and Liang, 1993b) or by considering only the diffusion in the membrane phase as the principal resistance. In the latter case, this approach can be used in the reported systems because transport limited by the kinetics is essentially never observed when using crown ethers as carriers (Fyles, 1991). The model equations, similar to the Michaelis-Menten type equations, permit handling of competitive equilibria if the concentration of the interfering ions and the equilibrium constants are known (Izatt et al., 1989). At this point it it must be emphasized that supplement independent data obtained with other physicochemical techniques are required before drawing conclusions about SLM transport predictions of a model which considers aggregation of the extractant, polymerization of the metal-containing species, adduct formation, and participation of several equilibria in the overall permeation rate. Unnecessary complexity in a model could be introduced if all the possible reactions are not previously properly analyzed. HF-SLMs. Two main alternate approaches have been employed in the description of HF-SLM transport. One considers the continuity mass conservation equations and the associate boundary conditions, while the other applies the film theory. In the former, the simultaneous presence of a two-dimensional mass transport processes occurring inside of the fiber lumen and a one-dimensional mass transport process occurring inside the fiber wall, coupled to chemical reactions occurring at the aqueous interfaces, require the use of complicated mathematical algorithms (Danesi, 1984). Assuming the steady state for the fluid film initially containing the solute, Kim and Stroeve theoretically and numerically analyzed the mass transfer of a solute in both the axial and transverse directions of the channel. They describe the velocities and concentration profiles along the HF module for the cases of reactive hollow fibers (Kim and Stroeve, 1988), countertransport (Kim and Stroeve, 1989), and cotransport (Kim and Stroeve, 1990). Alonso et al. (1994) satisfactorily applied this type of analysis for modeling the extraction process of Cr(VI) with Aliquat 336. For systems working in non-steady-state conditions, the description of the change of the solute concentration with time is also needed. The macroscopic mass balances of the permeating solute in the fluid phases through the HF modules were solved simultaneously with the mass balances in the stirred tanks for the homogenization of the aqueous and organic solutions during the extraction and back-extraction process of Cr(VI) in a two hollow fiber configuration (Ortiz et al., 1996a,b). Danesi (1984) described a simplified model with the main assumption that the radial and the axial solute concentration gradients were linear. A parameter, the

permeation coefficient, is considered to be constant throughout the entire process. The approximate and simple analytical equations of the model derived for the limiting cases of low and high metal concentration in the feed solution were applied in the once-through and the recycling modes. The equations contain only a few, simple, and often independently measurable parameters and have the advantage that they can be compared with those for FS-SLMs. Shiau and Chen (1993) also developed a model considering steps 1-5 in the scheme for Cu(II) transport with D2EHPA. The analysis was made by coupling Fick’s first law-type flux equation with the rate expressions for the forward and reverse chemical reactions and the macroscopic mass balance of the system. A rigorous model and a simple model with variable permeation coefficients were proposed. Both the once-through mode and the recycling mode were analyzed. The results when compared with Danesi’s model proved to be a better approach. Coelhoso et al. (1997) evaluated the mass-transfer coefficient during the extraction and stripping of lactate in hollow fiber contactors considering a constant distribution coefficient or alternatively the equilibrium constant of the reaction. They concluded that using the equilibrium relationship is a better approach because the variation of the distribution coefficient with solute concentration is implicitly assumed. Applying a diffusion model with interfacial reaction, and taking into account the laminar velocity distributions of the aqueous and organic solutions which flow along the hollow fiber, Yoshizuka et al. (1986) studied the kinetics and mechanism of metal extraction in a hollow fiber extractor. Kinetic data were fitted parametrically to determine the rate constants. They suggested that this method could be used to acquire kinetic rate constants where diffusion and reaction resistances are competing factors. The model was applied to Cu(II) transport with LIX 34 (Yoshizuka et al., 1986), Zn(II) with PC-88A (Sato et al., 1989b), and Mo(II) with PC-88A (Sato et al., 1989a). The second approach, i.e., application of the film theory, has been used extensively to evaluate performance and to design HF-contactors for carrying out dispersion-free solvent extraction (Prasad and Sirkar, 1988, 1990, and references therein). Extensive theoretical treatment of different membrane types and forms has been published for membrane-based solvent extraction processes (Prasad and Sirkar, 1992) and HF-CLMs (Majumdar and Sirkar, 1992). A comparison of both methods of description of HFSLM modules based on analyzing the influence of the internal mass transfer on the design of such systems has been reported (Urtiaga and Irabien, 1993). The authors indicate that when the value of the Peclet (Pe) number > 40, a correlation which includes a modified local Sherwood number can be used for the design of the HF module. When Pe < 40, the linking of the mass transfer in the inner fluid and across the immobilized liquid membrane does not allow this expression to be used and the design must be accomplished according to the continuity mass conservation equation and the related boundary conditions. Recently, Rogers and Long (1997) incorporated film theory, published mass transfer correlations, ramdom sequential addition, Voronoi tessellations, and facilitation factors to model HF-membrane contactors. The improvements made concern (a) the randomness of the HF-membrane module, i.e., nonordered packing, and (b)


combination of the film theory and facilitation factors in a simple way as has been done for FS-SLMs (Niiya and Noble, 1985; Leiber et al., 1985; Noble et al., 1986; Noble and Way, 1987; Smith and Quinn, 1979; Chaara and Noble, 1989; Jemaa and Noble, 1992; Dindi et al., 1992; Way and Noble, 1992). Scaling up HF-SLMs requires correlation equations between the relevant dimensionless parameters of the system. Some reported liquid-liquid solvent extraction correlations have been applied in HF-SLMs and compared with reverse osmosis and gas-liquid correlations in these type of modules by Sekino (1995) as a form of verifying the appropriateness of all of them. The author considers that the effect of the packing density of the hollow fibers has strong influence in comparing these correlations. Crowder and Cussler (1997) have pointed out how the nonuniformity of hollow fibers can make the average mass transfer coefficient in the module deviate from the one expected from the average properties of a single fiber. New mass transfer correlations have to consider the influence of these irregularities. So far, all the models presented in FS-SLMs and HFSLMs assume that the reaction between the target species and the carrier takes place in the membrane or its surface (the “small carrousel” of Mogutov and Kocherginsky (1993)). In the case where the carrier is hydrophilic enough to leave the membrane, and the reactions thereby take place mainly in the aqueous phases, for example, the thin near-membrane water layers (“big carrousel”), macrokinetic effects are expected. Equations for this situation without stirring (Mogutov and Kocherginsky, 1993) and with stirring (Mogutov and Kocherginsky, 1994) have been proposed. It is also shown how the equations for the so-called “big carrousel” may be simplified to the well-known transport equations where the stagnant aqueous layers, chemical reactions at the membrane surface, and diffusion through the membrane resistances are the fluxcontrolling processes. These results have been demonstrated quantitatively in experiments with quinones (Kocherginsky et al., 1991). To overcome single-stage limitations on separations the use of membrane cascades has been suggested. Design and modeling of such processes has been done by Prasad and Sirkar (1992) and McCandless (1997). Hollow fiber membrane modules with various fiber passes were analyzed making an analogy between the theory of mass transfer in fiber-and-shell mass exchangers and that of heat transfer in shell-and-tube heat exchangers by Yeh and Hsu (1998). Modeling and simulation of an integrated membrane process for the recovery of Cr(VI) with Aliquat 336 in HF modules has also appeared in the membrane literature (Alonso and Pantelides, 1996b). Mass transport may be strongly affected by coupling effects. These can involve coupling between flows of one character and forces of another as well as coupling of the solute flows. These effects can alter the efficiency of the separation. Irreversible thermodynamics deals with the relationships among the fluxes, forces, and their interdependency. A review applied to membrane transport is presented by Baranowski (1991). Simon et al. (1996) evaluated theoretically the degree of coupling in membrane separations. An experimental application can be found in Yang et al. (1992). The shift in the counter- or co-ion concentration is the most usual form

of performing uphill separations. However, this is not the only alternative. Thermally enhanced facilitated transport presents potential opportunities as has been pointed out by Rockman et al. (1995). Equations describing this type of process can be found in their article. Membrane Efficiency Several advantages have been described for the use of SLMs, namely the following: high selectivities, high concentration and/or separation factors in single stage operations, relative high fluxes, possibility of using expensive extractants, and low capital and operating costs. Additionally, hollow fiber configurations present several other advantages (Prasad and Sirkar, 1987a,b; 1990; Alonso et al., 1996a): very high and well-defined interfacial area per unit extractor, ability to handle particles, independent variation of phase flow rates without flooding, no need for differences in phase densities, and the possibility for scale-up operations. In contrast, low permeation rates and the degradation of the membranes are identified as the main dissadvantages of SLMs. However, the degree of importance of these factors depends greatly on the type of configuration employed. With the aim of improving the performance of SLMs, most of the research work published in the last 10 years deals mainly with the study of three parameters: permeation, stability, and selectivity (Table 2). In a broad sense, these three parameters can be used to characterize membrane efficiency. Permeability. This measures the quantity of a solute transported through a specific area of membrane surface in a given unit of time and is related to the permeation flux J which is defined as J ) PC ) -V(dC/A) dt, where P ) permeability coefficient, C ) solute concentration in the feed solution at time t, V ) volume of feed solution, A ) membrane area, and ) membrane porosity. In relation to the support, fundamental aspects, such as membrane structures, membrane fabrication (material selection), membrane characterization, and characteristics of commercially available membranes, are thoroughly discussed by Kulkarni and co-workers (1992), Tsujita (1992), and Feng and Huang (1997). In SLMs, traditionally hydrophobic or hydrophilic porous membranes have been used. The effect of porous support composition on the performance of SLMs on the transport of copper and neodymium using D2EHPA as carrier was studied by Takigawa (1992) by comparing commercial hydrophobic and hydrophilic supports and a synthesized microporous polybenzimidazole support. Improved performance using the latter support was found for the removal of these metal ions; metal ion transport reached completion in significantly less time than with the other supports investigated. Prasad and Sirkar (1992) analyzed the behavior of hydrophobic, hydrophilic, and composite (hydrophobic-hydrophilic) membranes on the basis of an appropriate applied pressure difference between the aqueous and the organic phases in order to wet the membrane by the phase of interest. Conclusions about the usefulness of a hydrophobic or hydrophilic membrane from a mass transfer point of view depended on if the solute preferred the organic or the aqueous phase. Overall mass transfer coefficients and individual coefficient relations for these three different membrane types and for the flat and hollow fiber forms are proposed. More recently, Zha et al. (1995c) evaluated the performance of hydro-

2188 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Table 2. Review of Extraction Systems Using SLM (1989-1997)a Flat-Sheet SLM ions

feed

strip

carrier/diluent

refs

Organophosphorus Reagents Am(VI) Ca(II) Co(II) Co(II) Co(II) Co(II), Ni(II) Co(II), Ni(II) Co(II), Ni(II) Co(II), Ni(II) Co(II), Ni(II), Cu(II) Cu(II) Eu(III) Eu(III)

0.05 M Na2SO4, pH 3.5-6.0 SO42SO420.5 M (Na,H)SO4, pH 3.0-5.50 1 M (Li,H)NO3 0.5 M Na2SO4, pH 3-5 acetate buffer, pH 5.0 1.9 M NaAcO + 0.1 M AcOH, pH 6.0 0.1 M (Li,H)NO3 pH 1.9-3.3

Eu(III) Eu(III) La(III) lanthanides, Th

5 M (H,Na)ClO4 0.1 M HNO3 0.3 M (Na,H)Cl, pH 1.1-2.1 0.1 M (K,H)NO3, pH 1.1-4.8

lanthanides Mn(II) Nd(III), Pr(III) Ni(II) rare earths rare earths

pH 1.5-5.0 0.009 M Na2SO4 + acetate, pH 2-4.5 0.1 M (Na,H)NO3 0.1 M acetate buffer 0.1 M NaNO3 + 0.0013 M DTPA 0.005-0.5 M HNO3, 0.995-0.5 M NaNO3 (Na,H)NO3 variable, (Na,H)Cl variable

rare earths

1 M HClO4, 1-3 M H3PO4 Cl-

HDEHP/n-decane HDEHP/n-dodecane D2EHPA/CCl4 D2EHPA/kerosene HEH(EHP)/kerosene HEH(EHP)/kerosene) D2EHPA/kerosene HEH(EHP)/kerosene HDEHP/kerosene D2EHPA/kerosene HDEHP/dodecane DTMPPA/kerosene DTMPPA/kerosene

Mikheeva, 1994 Nair, 1991 Chaudry, 1990a Mohapatra, 1992b Youn, 1995 Youn, 1997b Juang, 1993a Juang, 1993b Huang, 1991 Marchese, 1993 Takigawa, 1992 Juang, 1996b Nakamura, 1994a

CMP/kerosene DIDPA/kerosene HDEHPA/kerosene DOPA/kerosene, DPPA/kerosene DTMPPA/kerosene D2EHPA/kerosene DEHPA/kerosene D2EHPA/n-dodecane PC-88A/kerosene DEHPA/n-dodecane

Nakamura, 1989/90 Nakamura, 1989 Kataoka, 1995 Hrdlicka, 1996

HDEHP,TBP/n-dodecane

Kopunec, 1991

D2EHPA/kerosene D2EHPA/CCl4 D2EHPA/kerosene D2EHPA/kerosene

Chaudry, 1994a Chaudry, 1992c Juang, 1996a Juang, 1994a

D2EHPA/kerosene CYANEX272/n-dodecane EHPA/kerosene CYANEX 272/kerosene CYANEX 272 /dodecane

Juang, 1994b Chiarizia, 1990 Akiba, 1997 Kanungo, 1995 Cox, 1990 Szpakowska, 1997

2 wt % HCl

AcorgaP-50/n-octane, mesitylene LIX 984N/kerosene AcorgaP-50/n-octane, mesitylene LIX 84/kerosene LIX 84/kerosene LIX 84/kerosene

Amines 0.1-4 M HNO3

TOA/kerosene

Fu, 1997

primary, secondary, tertiary and quaternary amines/heptane

Barnes, 1993

Cd2+: Alamine 336/o-xylene, kerosene CrO42-: Aliquat 336/kerosene Alamine 336/kerosene TOA/n-decane ADOGEN 364/kerosene

Molinari, 1989

0.001-0.1 M HNO3 0.1-4 M HCl 1 M H2SO4 H2SO4 3 N H2SO4 0.5 M (Na,H) 1 M (Li,H)NO3 0.9 N H2SO4 H2SO4, pH 2.3 2 M H2SO4 0.2 M HNO3 0.01, 0.03, 0.1 M HNO3 0.1 M HNO3 5 M HNO3 0.5-2.0 M H2SO4 0.015-0.1 M HNO3 0.01-0.4 M HNO3 9 M H2SO4 0.1 M HNO3 0.1 m HNO3 H2O, pH 1-4 1 M HNO3 0.1 M EDTA, 1 M HCl, 1 M HNO3, 1 M (Na,H)Cl 0.5-4 M HNO3 0.1-2.5 M NH4F (Na,H)SO4 (Na,H)SO4, pH 0.22-1.10 pH 0.58-0.94 HEDPA 1 M H2SO4 4.5 M H2SO4 0.1 M HCl

Sr(II) Ti(IV) V(IV) V(IV)

0.005-0.1 M citric acid, pH 4.7 1-3 M H2SO4 0.5 M (Na,H)SO4, pH 1.30-3.66 0.5 M (Na,H)SO4, pH 0.96-3.04

V(IV) U(VI) Y(III), Fe(III) Zn(II) Zn(II)

0.5 M (Na,H)SO4, pH 1.28-2.52 H2SO4, pH 2.0 (groundwater) 0.01 M H2SO4 0.2 M (Na,H)SO4, pH 2.0-5.0 acetate, pH 4.7

Cu(II)

sulfate, pH ) 2.63

Cu(II) Cu(II)

sulfate, pH ) 2.5 sulfate, pH ) 2.63

2 M H2SO4 0.5-2.0 M H2SO4

Cu(II) Cu(II) Cu(II), Cd(II), Cd(II)

1.47 M NH3, pH ) 11 sulfate, pH 1-5 nonbuffered, acetate buffered

1.5 M H2SO4

Au(III), Ir(IV), Pd(II), Pt(II), Ru Au(I)

0.1-5 M HCl

Cd(II), Cr(VI)

Cd2+: 1 M NaCl

0.1 M KH2PO4/Na2HPO4, pH ) 7.0, 0.1 M Na2CO3/NaHCO3, pH )10.0

Hydroxyoximes 1-2 M H2SO4

0.1 M NaOH (tertiary, secondary, and primary amines), 1 M Tu + 0.25 M H2SO4 (quaternary) Cd2+: 0.5-0.25 (AcONa + NH4Cl)

CrO42-: aqueous

CrO42-: 4 M LiCl

Co(II), Ni(II), Cu(II) Co(II), Fe(III) In(III)

7 M HCl 6 M HCl 2.2-4 M HCl

distilled water 1 M NH4OH, pH 11.5 2.2-4 M AcONa

Ir(IV) Mo(VI) Mo(VI) Mo(VI), W(VI) Pd(II)

0.15 M HCl 0.001-0.1 M HCl, pH 1-3 0.001-0.1 M HCl 0.1 M HCl + 0.1 M tartaric acid 0.5 M HCl

4 M HNO3, 1 M HClO4 0.01-1 M NaOH 0.01-1 M NaOH 1 M NaOH 0.1-4 M HNO3, 0.1-3 M HClO4

TOA/kerosene TOA/xylene TOA/xylene TOMAC/kerosene TOA/kerosene

Nakamura, 1994b Mohapatra, 1992a Moreno, 1993 Zha, 1995a Kojima, 1994 Kopunec, 1993

Yang, 1997 Szpakowska, 1996 Zha, 1995b Yi, 1992 Kralj, 1996

Marchese, 1995 Kalachev, 1992 Rodrı´guez de San Miguel, 1996 Fu, 1995b Chaudry, 1992a Chaudry, 1990b Mahmoud, 1996 Fu, 1995d

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2189 Table 2 Continued Flat-Sheet SLM ions

feed

strip

Tc(VII), Cr(VI)

Amines 0.1-9 M HCl HClO4, dimethylamine, NaSCN, Tu + HCl, HNO3, NaClO4, NH3 0.001-10 M HCl, HNO3, H2SO4 + 0.1 M NaOH (all) 0.01% K2Cr2O7 Tc: aqueous nitrate NaOH

V(V)

acetate buffer, pH 4-7

ammonia buffer, pH 8-13

W(VI)

0.005-1 M HF

0.5-3 M NaOH

Pt(IV) Tc(VII)

Cu(II) Ga(III) Pt(IV), Pd(II) rare earths

alkali and alkaline earth Eu Hf(IV), Zr(IV) rare earths rare earths

4.5 M chloride + 5 M NH3, pH 9 NaOH-NaCl buffer, synthetic Bayer liquor 0.5-4.0 M H2SO4 1 M HNO3, 0.8-3 M HCl 1 M HAc/Ac buffer, pH 4.6

0.5 M (H,Na)Cl2CHCO2, pH 3 0.05 M H2SO4

0.01-1 M perchlorate

Solvating Reagents deionized water

4 M NaNO3 + 1 M HNO3 2-12 M HNO3 0.14-M NH4SCN, pH 1-6 2 M NaNO3, pH 2.0

rare earths

2-6 M NaNO3 + HNO3, pH 0.6-5.3

Tc(VII)

1-8 M HCl + 0.01% K2Cr2O7

Th(IV) U(VI)

1-9 M HCl 0.1-5.5 M HNO3 + 0.1-1.9 M NaNO3 U(VI) 3 M HNO3 U(VI) 2 M HNO3 V(V), Zn(II), Be(III) V(V): 0.01-1 M H2SO4 Zn(II): 1-3 M HCl + 3 M CaCl2 Be(III): 5 M NH4SCN + 0.5 M HCl Zr(IV) 2-12 M HNO3

Ba(II), Ca(II), K, Na, Li, Ba, Ca, Sr: NO3Cd(II), Hg(II), K(I), Li(I), Ni(II), Hg, Cd: SCNSr(II), Zn(II) Cd, Zn: BrCd: ClCs(I) demineralized water, 1 M HNO3, 5.8 M LiNO3 + 1 M HNO3, synthetic nuclear fuel concentrate soln Cs(I), Sr(II) 4 M NaNO3+ + 1 M HNO3 Cu(II) Cu(II)

β-Diketones 2.1 M H2SO4 1.5 M HCl

0.01 M MES-LiOH buffer, pH 6.0 0.01 M LiClO4 + 0.01 M MES-LiOH buffer, pH 6.0

Hg(II)

0.001-0.03 M NaNO2, pH 1-10

Na(I), K(I), Rb(I)

aq nitrate media

Sr(II)

synthetic concentrate

carrier/diluent TOA/kerosene

Fu, 1995a

TOA/xylene

Chaudry, 1996

Primene JM-T, Amberlite LA-2, TLA/n-dodecane Aliquat 336/cumene, dodecane TOA/xylene

Chiarizia, 199a

LIX54-100/kerosene Kelex100 + Versatic 10/kerosene LIX26/kerosene HBTA, HTTA, HFTA/mixtures ONPOE; ONPPE, PNPHE + TBEP

TPP/o-nitrophenyl octyl ether 0.25 M sodium citrate CMPO/1,2-NPOE distilled, deionized water TBP/xylene 0.01-1 M HCl TOPO, TBP/kerosene 0.05 M EDTA + 1 M NaNO3, TOPO/n-dodecane pH 4.2 0-6 M NaNO3 + 0.05 M TBP/n-dodecane complexone (EDTA, DCTA, HEDTA, DTPA, NTA), pH variable 3 M NaCl, 1 M NaOH, TBP/kerosene 0.212-3.5 M LiCl 0.943 M Na2CO3 TBP/benzene 0.01 M HNO3 TBP/kerosene 0.01 M HNO3 1 M (NH4)CO3 V(V): 0.2 M Na2CO3 Zn(II): distilled water

refs

Palet, 1995 Chaudry, 1991 O’Hara, 1989 Zha, 1995c Fu, 1995c Sugiura, 1989a

Saito, 1993 Hill, 1996 Chaudry, 1992b Chitra, 1997 Manh, 1992 Kopunec, 1992

Chaudry, 1993 Rasul, 1995 Chaudry, 1995

TBP/kerosene TBP/dodecane TBP/kerosene

Chaudry, 1994b Shukla, 1991 Chaudry, 1989

distilled water

TBP/xylene

Chaudry, 1989/1990

Crown Ethers distilled, deionized H2O

R2DC18C6/phenylhexane

Izatt, 1989

Be(III): 1 M NaOH

H 2O

DB21C7, B21C7, tBuB21C7, Dozol, 1995 nDecB21C7/n-hexylbenzene

demineralized water, 0.25 M dtBuDC18C6, 10-B21C7, sodium citrate Cp4,Cp5,Cp6,Cp7,Cp8/ 1,2-NPOE 5E-4 M CDTA, pH 6.0 22DD + fatty acid/toluene + phenylhexane (1:1) NaNO3, Na4P2O4 in 0.01 M 22DD + HDEP/toluene, LiClO4 + MES, LiOH, toluene + kerosene HNO3, pH 6 0.001-0.009 M Na2S2O3, 18C6, DC18C6, DB18C6, pH 1-10 DA18C6, 15C5, DA15C5/CHCl3 distilled, deionized water DC18C6 + 2-NPOE + T2BEP + CTA + MeCl demineralized water DC18C6/various

Hill, 1996 Parthasarathy, 1994 Parthasarathy, 1991 Parham, 1994 Schow, 1996 Dozol, 1993b

2190 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Table 2 Continued Flat-Sheet SLM ions Au(III) Cd(II), Ni(II), Pb(II) Cd(II) Cr(III) Cu(II) Ga(III) Ga(III)

feed

strip

0.5-3.0 M HCl Cd2+: I-, NO3-, Br-, ClNi2+: ClO4-, NO3-, Br-, Cl-, SO42Pb2+: SCN-, NO3-, Br-, Cl0-0.8 M LiCl Cr3+: sulfate, pH ) 4.2-4.5

carrier/diluent

Others Tu, Na2S2O3/1% HCl water water Cr3+: 1 N H2SO4 1 M H2SO4 pH 0.5 pH 0.5

Ga(III) In(III)

acetate, pH 6-7 various pH values (0.5-6.0) KCl + HCl, pH 1.5; KHPh + HCl, pH 2.5-4.0; KHPH + NaOH, pH 4.5-6.0 0.05-4.0 M total sulfate chloride buffer solutions (pH 1-4)

Hg(II), Ag(I)

HNO3, HClO4, pH various

Hg(II)

aq solutions of metal (Hg, Cu, Zn, Cd, Ag) salt(s) (Cl, NO3-) 0.01-3 M HCl 2.25 × 10-3 M HCl 0.1 M LiCl 0-0.1 M LiCl

Tu + 1 M H2SO4, Ts in deionized water deionized water

Pd(II) Se(IV) Zn(II) Zn(II)

1 M H2SO4 pH 0.5

2 M aqueous NH3 0.24 M H2O2 in 1-3 M HCl deionized water purified water

refs

DTH, NTH/CHCl3 tetradentate oxoiminato Schiff bases/CHCl3

Zuo, 1996 Upadhyay, 1994

bathocuproine/dibenzyl ether Cr3+: DNNSA/o-xylene, kerosene, n-heptane, kerosene-o-xylene tetradentate hydroxamate/CHBr3 ODPHA/kerosene ODPHA/kerosene

Saito, 1991 Molinari, 1989

2-BDA/n-dodecane N-nitroso-N-p-octadecylphenylhydroxyalamine ammonium salt/kerosene BTPD/kerosene, dodecane

Teramoto, 1993 Okushita, 1990

LPB/NPOE

Ikeda, 1989

DETE/toluene Na(DDTC)/kerosene bathocuproine/dibenzyl ether bathocuproine/dibenzyl ether

Yoshizuka, 1995 Noguerol, 1997 Saito, 1992 Saito, 1990

Bromberg, 1992b Okushita, 1996 Okushita, 1995

Bromberg, 1993

Hollow Fibers ions Ag(I) Cd(II)

Cu(II), Zn(II) Mo(VI) rare earths U(VI)

operation mode extraction/back-extraction; one HF impregnated module extraction module, polymeric HF; back-extraction module, ceramic multichannel extraction/back-extraction; two HF modules extraction extraction/back-extraction

Zn(II)

extraction/back-extraction; one HF impregnated module extraction/back-extraction

Cu(II), Cr(VI),

simultaneous

feed

strip

Organophosphorus Reagents 0.1 M HNO3 + 0.1 M NH3 + [NO3-]T ) 0.5 M 0.5 M NO330% P2O5 from 85% H3PO4

4 M HCl

0.1 M acetate buffer HCl solution and/or HCl HNO3, pH 0-4.5 sodium acetate-HNO3, HNO3 NaNO3-HNO3 H2SO4, pH 2.0 0.1 M HEDPA 0.1 M acetate buffer solution and/or HCl

Zn(II), Hg(II) extraction/back-extraction three sets of HF module Cr(VI) simultaneous extraction/ back-extraction; two HF modules Cr(VI) simultaneous extraction/ back-extraction; two HF modules Cr(VI) extraction Cr(VI) extraction Cr(VI), Hg(II) simultaneous extraction/ Cr: 0.1 M H2SO4, back-extraction; contained pH 2.5 liquid membrane, Hg: 0.1 M HCl two sets HF Cr(VI) V(V)

extraction extraction/back-extraction; one HF impregnated module

Cu(II)

extraction/back-extraction; one ceramic HF impregnated module

acetic acid/acetate media

refs

Cyanex471X/cumene

Galindo, 1989

Cyanex 302/kerosene

Alonso, 1997

PC-88A/n-heptane

Sato, 1990

PC-88A/n-heptane PC-88A/n-heptane

Sato, 1989a Kubota, 1995

Cyanex 272/n-dodecane Chiarizia, 1990a

HCl

PC-88A/n-heptane

set 1 (Cu): 2 M H2SO4 set 2 (Cr): 0.1 M NaOH 1 M NaCl

TOA + LIX 84/kerosene Yang, 1996

Aliquat 336/kerosene

Ortiz, 1996a

1 M NaCl

Aliquat 336/kerosene

Ortiz, 1996b

Aliquat 336/kerosene Aliquat 336/kerosene TOA/xylene

Alonso, 1996a Alonso, 1994 Guha, 1994

Amines pH 3.4-4.3

carrier/diluent

Cr: 0.5 M NaOH Hg: 1 M NaOH +4M NaCl

TOA/xylene ammonia/ammonium Aliquat 336/kerosene nitrate media

Hydroxyoximes sulfate media, 1.5 M H2SO4 pH 3.0

LIX 84

Sato, 1989b

Yun, 1993 Rosell, 1997

Yi, 1995

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2191 Table 2 Continued Hollow Fibers SLM ions

operation mode

feed

strip

carrier/diluent

refs

Hydroxyoximes Cu(II)

extraction

pH 1.5-6.0

Cu(II) Cu(II) Cu(II)

extraction/back-extraction extraction simultaneous extraction/ back-extraction; contained liquid membrane, two sets HF

pH > 2.0 Cu: pH 2.95-4.51 sulfate media

Ca(II), Sr(II), Ba(II), Na(I), K(I), Cs(I) Cu(II), Cd(II), Pb(II)

extraction/back-extraction; one HF impregnated module extraction/back-extraction; one HF impregnated module K(I), Na(I), simultaneous extraction/ Ba(II), Ca(II), back-extraction; Sr(II) two HF modules Sr(II) extraction/back-extraction; one HF impregnated module

E-HNBPO LIX 84/n-decane Cu: LIX 84/n-heptane Cu: LIX 84/n-heptane

Matsumoto, 1990 de Haan, 1989 Yun, 1993 Guha, 1994

Crown Ethers 18C6 + 0.001 M 0.5 M HCl picric acid

DCB

Mackova, 1994

nitrate media + MES, pH 6.0

CDTA, pH 6.0

22DD/phenylhexane-toluene (1 + 1 v/v)

Parthasarathy, 1997

nitrate aqueous media

water

DC18C6/2-octanone, hexane, Lamb, 1990 toluene, octane, 1-octanol, octanal, 4-methyl-2-pentanone 0.02 M HS/DCB, DCB, DCB Mackova, 1996

H2SO4 Cu: 2 M H2SO4

18C6 + 0.01 M HCl, 0-4 M HCl, 18C6 + 4 M LiNO3, H2O, H2O 18C6 + 4M HNO3

a Acorga P-50 ) 94% 5-nonylsalicylaldoxime. ADOGEN 364 ) 50% mixture of C and C aliphatic substituted tertiary amines. Alamine 8 10 336 ) tri(C8-C10)amine. Aliquat 336 ) tricaprylmethylammonium chloride. Amberlite LA-2 ) mixture of branched secondary amines with 24C atoms. B21C7 ) decylbenzo-21-crown-7. Bathocuproine ) 4,7-diphenyl-2,9-dimethyl-1,10-phenanthroline. 2-BDA ) 2-bromodecanoic acid. BTPD ) bis(bis(2-ethylhexyloxy)thiophosphoryl) disulfide. 15C5 ) 15-crown-5. 18C6 ) 18-crown-6. CDTA ) cyclohexanediaminetetraacetic acid. CDTA ) trans-cyclohexanediaminetetraacetic acid. CMP ) dihexyl-N,N-diethylcarbamoylmethylphosphonate. CMPO ) octylphenyl-N,N′-diisobutylcarbamoylmethylenephosphine oxide. Cp4 ) 1,3-dioctyloxy-2,4-crown-6-calix[4]arene. Cp5 ) ((1,3bis(2-nitrophenyl)octyl)oxy)-2,4-crown-6-calix[4]arene. Cp6 ) bis-crown-6-calix[4]arene. Cp7 ) bis(1,2-benzo)crown-6-calix[4]arene. Cp8 ) bis(1,2-naphtol)crown-6-calix[4]arene. CTA ) cellulose triacetate. CYANEX 272 ) bis(2,4,4-trimethylpentyl)phosphinic acid. Cyanex 302 ) bis(2,4,4-trimethylpentyl)thiophosphinic acid. Cyanex 471X ) triisobutylphosphine sulfide. 22DD ) 1,10-didecyldiaza-18-crown-6. D2EHPA ) bis(2-ethylhexyl)phosphoric acid. DA15C5 ) diaza-15-crown-5. DA18C6 ) diaza-18-crown-6. DB18C6 ) dibenzo-18-crown-6. DB21C7 ) dibenzo-21-crown-7. DBC ) 1,2-dichlorobenzene. DC18C6 ) dicyclohexano-18-crown-6. DETE ) 3,3-diethylthietane. DIDPA ) diisodecylphosphoric acid. DNNSA ) dinonylnaphthalenesulfonic acid. DOPA ) di-n-octylphosphoric acid. DPPA ) di-n-pentylphosphoric acid. dtBuDC18C6 ) di-tert-butylcyclohexyl-18-crown-6. DTH ) dodecylthiourea. DTMPPA ) bis(2,4,4-trimethylpentyl)phosphinic acid. DTPA ) diethylenetriaminepentaacetic acid. E-HNBPO ) E-2-hydroxy-5-nonylbenzophenone oxime. EHPA ) 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. HBTA ) benzoyltrifluoroacetone. HDEHP ) bis(2-ethylhexyl)hydrogen phosphate, bis(2-ethylhexyl)phosphoric acid. HDEP ) bis(2-ethylhexyl)phosphoric acid. HEDPA ) 1-hydroxyethane-1,1-diphosphonic acid. HEH(EHP) ) (2-ethylhexyl)phosphonic acid mono-2-ethylhexyl ester. HFTA ) furoyltrifluoroacetone. HS ) dinonylnaphthalenesulfonic acid. HTTA ) thenoyltrifluoroacetone. Kelex100 ) 96% 7-(4-ethyl-1-methyloctyl)-8-quinolinol. KHPh ) potassium hydrogen phthalate. LIX 26 ) alkylated derivative of 8-quinolinol. LIX 54-100 ) phenyl-alkyl-β-diketone. LIX 84 ) 2-hydroxy-5-nonylacetophenone. LIX 984N ) mixture of 5-nonylsalicylaldoxime and 2-hydroxy-5-nonyl-acetophenone oxime. LPB ) polybutadiene. MeCl ) methylene chloride. MES ) N-morpholinoethanesulfonic acid. Na(DDTC) ) sodium diethyldithiocarbamate. nDecB21C7 ) n-decylbenzo-21-crown-7. 1,2-NPOE ) 1,2-nitrophenyl octyl ether. 2-NPOE ) 2-nitrophenyl octyl ether. NPOE ) o-nitrophenyl octyl ether. NTH ) nonylthiourea. ODPHA ) N-octadecanoylN-phenylhydroxylamine. ONPOE ) o-nitrophenyl n-octyl ether. ONPPE ) o-nitrophenyl phenyl ether. PC-88A ) (2-ethylhexyl)phosphonic acid mono-2-ethylhexyl ester. PNPHE ) p-nitrophenyl n-heptyl ether. Primene JM-T ) long-chain primary alkylamine (t-C18H37NH2 to t-C22H45NH2). R2DC18C6 ) bis(1-hydroxyheptylcyclohexano)-18-crown-6. T2BEP ) tris(2-butoxyethyl) phosphate. TBEP ) tris(2-nbutoxyethyl) phosphate. TBP ) tributyl phosphate. tBuB21C7 ) tert-butylbenzo-21-crown-7. TLA ) trilaurylamine. TOA ) tri-n-octylamine. TOPO ) trioctylphosphine oxide. TPP ) tripentyl phosphate. Ts ) thiosulfate. Tu ) thiourea.

phobic and hydrophilic membranes in the recovery of gallium from alkaline solutions. From mass transfer resistance relations, it can be deduced that in the most common situation, when mf > 1 and ms , 1 (where mf and ms are the solute distribution coefficient in feed and strip solution, respectively), the use of hydrophobic membranes as the support in membrane extraction processes gives rise to lower mass transfer resistance. In contrast, when one deals with the stripping operation, the resistance for the solute to transfer through a hydrophilic membrane is less than in a hydrophobic membrane. They also observed when comparing three hydrophobic membranes that the one with the smaller pore size presents the slowest mass transfer rate through the membrane. Due to the instability of hydrophobic membranes in strong alkaline solutions, hydrophilic membranes had to be used in their experiments in both extraction and back-extraction operations. To overcome chemical environmental difficulties, as well

as other problems exhibited by polymeric supports, Yi and Tavlarides (1992, 1995) have suggested the use of ceramic membranes for copper separation from aqueous solutions, which also offer the possibility of achieving high permeabilities. In this same direction, Alonso et al. (1997) have reported a combination of polypropylene hollow fiber and multichannel tubular ceramic modules for the extraction and back-extraction operations, respectively, in the concentration-separation process of cadmium from phosphoric acid using Cyanex 302 as the extractant. In this work the kinetic modeling of the extraction system is also reported. Finally, in the novel HLM systems, Wo´dzki and Sionkowski (1995) and Kislik and Eyal (1996a,b) used porous membranes with ion-exchange or hydrophobic-hydrophilic properties to entrap the carrier solution. SLMs operate on the basis of two physicochemical processes: extraction and back-extraction. Subsequently, to efficiently use this method, i.e., to achieve


experimental conditions to specifically and quantitatively extract or strip the metal species under consideration, the chemistry of such processes must be profoundly understood. It is well-known that the theory of SLM extraction involves theory of solution chemistry, chemical thermodynamics of solutions, transport phenomena, and chemical kinetics. Furthermore, the correct selection of the carrier compound and, particularly, the design of a selective carrier requires basic knowledge of metal-carrier bonding. Zolotov and co-workers (1986) have surveyed these latter aspects for inorganic liquidliquid extraction methods and emphazised the importance of the incorporation of coordination chemistry in the theory of extraction. Under this same theory another fundamental consideration is the definition of the extraction system. Sekine and Hasegawa (1990) have classified solvent extraction systems on the basis of the chemical properties of the extractant as acidic, basic, and solvating. Other authors (Zolotov et al., 1986; Barnes and Marshall, 1995) classify the extractants as neutral, cation-exchange, and anion-exchange reagents. Once the distribution equilibrium is defined, it is possible to study the factors affecting it from the standpoint of chemical equilibria (i.e. ionic strength, acid-base, oxidation-reduction, and complexation reactions in aqueous phases, association reactions in the organic phase, formation of uncharged molecular species in both phases, etc.). Liquid-liquid extraction processes which involve changes in stereochemistry, ligand exchange, and rates of extraction have been summarized by Hudson (1982). When working with SLMs, it is a common practice to optimize the liquid-liquid extraction system before studying its performance using a membrane barrier. Plots of % E vs pH find application in the selection of the feed and strip aqueous pH in which to perform membrane studies. Baker and Blume (1990) and Kojima et al. (1994) have studied the influence of complexing reagents in the feed phase introduced as a buffer and as masking reagent, respectively. In the first case, concentration polarization effects were found to depend on the type of buffer used while in the second case a selective permeation of the metal of interest is achieved. As stated in the analysis of the transport mechanisms, hydrodynamical factors, such as the flow rates of the feed and strip solutions, are known to influence the diffusion rates through the SLMs, in such a form that optimization of these factors is a frequent practice. As expected, permeation increases as temperature rises due to the dependence of the complexation reaction rates and diffusion resistance on temperature (Molinari et al., 1989; Chaudry et al., 1991; Saito, 1991; Rasul et al., 1995). With the aim of avoiding the formation of a third phase, the effect of a long chain alcohol used as modifier on the extraction equilibria and permeability has been studied by Nakamura and Akiba (1989, 1989-90). Although the authors observed that the presence of the modifier improved the performance of the membrane system, the concentration of the modifier had to be optimized to avoid a decrease in the distribution ratio of the metal species extracted. Szymanowski et al. (1997) in the study of interfacial activity of bis(2ethylhexyl)phosphoric acid in liquid-liquid extraction systems observed a strong competition between this extractant and long chain alcohols for adsorption at the interface which may result in a reduction in the rate of metal extraction.

The influence of the viscosity of the solvent has been analyzed using the Stokes-Einstein equation and the Hiss and Cussler (1973) approach. In the case for some aromatic solvents modified with 1-decanol, a good correlation was found between the metal permeability and the ratio of the dynamic viscosity and the square root of the molecular weight of the membrane solvent (Dozol et al. 1993b). Visser et al. (1995) found that the diffusion constants of the carrier complexes increase linearly with the reciprocal of the solvent viscosity as predicted by the Stokes-Einstein equation when using calix[4]arenes as carriers. Dreher and Stevens (1998) obtained similar conclusions finding that the copper ion flux was proportional to the viscosity to the power -0.84 with Newtonian and non-Newtonian fluids using Acorga P5100 as carrier. However, data taken over a wider viscosity range are needed in order to clarify which approximation is more accurate. Selectivity. Once the permeation factors for a given cation are optimized, appropriate variations of the experimental conditions could be employed in order to achieve high selectivities during competitive metal transport. When thermodynamic selectivity is the key factor in the separation, preliminary studies on liquidliquid extraction and back-extraction are usually employed to find out such conditions. The efficiency of the mutual separation of several cations in a hollow fiber extractor, based on the differences of extraction rates, was encountered to be nearly equal to that in the process based on the difference of liquid-liquid extraction equilibria using PC88A as carrier (Kubota et al., 1995). In this paper, the authors reported a linear relationship between the logarithm of the interfacial forward reaction rate constant and the equilibrium constant in two kinds of solvents for rare earth metal transport. A comparison between the separations factors in liquid-liquid extraction and transport experiments for some lanthanoids with DTMPPA can be found in Nakamura et al. (1994b). By choice of an appropriate pH value, Ga(III) is selectively separated from a solution containing 50-300-fold more Al(III) with a separation factor above 300 (Okushita and Shimidzu, 1995), while In(III) is separated and concentrated from 10- to 100-fold excess of Zn(II) with a separation factor between 200 and 250 (Okushita and Shimidzu, 1990). The flux ratio Hf(IV)/Zr(IV) in a solution containing 2.4% of Hf with respect to Zr using a TBP xylene-based SLM was reported to be > 125 under optimal conditions (Chaudry and Ahmed, 1992b). This ratio was observed to be dependent on the nitric acid concentration in the feed solution, TBP concentration, and even the temperature. Pt(IV) and Pd(II) were completely taken up from HCl, HNO3, and H2SO4 feed solutions through a LIX 26/kerosene/1-octanol SLM, but due to the bad strip of Pd(II), only Pt(IV) was released and separated into the product solution in H2SO4 and HNO3 systems (Fu et al., 1995c). The separation factor of Pd(II) and Ir(III) was found to be dependent on the feed HCl concentration according to their equilibrium distribution coefficients in a trioctylamine-based SLM (Fu et al., 1997). Recently, Hayashita (1996) has observed that selectivity of the heavy metal ions Pb(II) and Cd(II) depends on the membrane area, the metal ion concentration in the source phase, the carrier concentration in the membrane phase, and the complex formation in the receiving phase.


Figure 1. Variation of the temporal derivative of % of extraction as a function of time for In(III) and Fe(III) separation at different linear flows of the aqueous feed solution in an HF-SLM. Flow rate of organic phase: 63 cm/min.

Figure 2. Relative variation of feed metal concentration as a function of time. The open symbols represent unicomponent transport experiments while solid symbols represent multicomponent systems. Feed: [M]0 ) 0.0001 M, 3 M HCl. Membrane: 0.15 M ADOGEN 364/kerosene. Strip: 3 M CH3COONa.

Data concerning the influence of hydrodynamical factors in selectivity are scarcely found in the literature. However, it is expected that these factors will be correlated with the kinetic behavior of the system and specifically to the ratio between the diffusion time to the reaction time, as can be accounted by the Damko¨ler number. SLM transport is a kinetic process, and it is no wonder that kinetic selectivity can also be achieved. Such type of separating behavior has been reported during Y(III)/Fe(III) (Akiba et al., 1997) and U(VI)/Fe(III) (Chiarizia et al., 1990b) transport. The influence of the flow velocities on the separation of In(III) from Fe(III) in a hollow fiber membrane-based solvent extraction process is shown in Figure 1. As can be seen, indium extraction is dependent on the flow velocities used; the slow reaction rate of Fe(III) with CYANEX 272 makes this system independent of the flow rate, allowing the indium separation. Finally, it is important to note that the fluxes which can be measured in a single-ion solution and multicomponent mixtures are not always equal. The fluxes of the solutes cannot be independent due to multicomponent diffusion effects (Cussler, 1976). Interaction effects are expected to occur according to the solvent extraction experience. The phenomenon known as the “crowding effect” explains the differences observed for iron flux in the presence and absence of copper during LIX64N transport experiments and is also important in the separation of uranium from vanadium (Baker and Blume, 1990). The importance of measuring the selectivity of a membrane system in conditions where all the permeating species are present in solution has been remarked by Behr et al. (1985). Figure 2 illustrates this by comparing the variation in solute concentration as a function of time in single-ion and multicomponent solutions. Stability. One of the breakthroughs needed for applying liquid membrane processes in industrial applications is related to the stability control of the SLM systems. Although it seems that this could be a tailormade specific quality for a given system, some experimental evidences which can serve as a guide for SLM design are outlined in this section. Further discussion

in this topic can be found in the work of Kemperman et al. (1996). Different approaches can be found in the literature in order to determine SLM lifetime or stability: flux decline measurement over extended periods of time (Yang and Fane, 1997; Zha et al., 1995a; Chiarizia, 1991b), frequent measurement of the permeability coefficient variation of fresh solutions (Ngo Manh and Kopunec, 1992; Danesi et al., 1987; Shinbo et al., 1993, Chiarizia, 1991b), the membrane-liquid (ML) loss weighing method (Yang and Fane, 1997; Zha et al., 1995a), monitoring the amount of carrier in the membrane (Zha et al., 1995a; Neplenbroek et al., 1992b), following the decrease of the solute’s initial permeability coefficient in fresh solutions with the number of runs (Hill et al., 1996), measuring the countertransport factor (Neplenbroek et al., 1992a,b), measuring the aqueous transport through the membrane (Dozol et al., 1993a), following the constance of the selectivity between two solutes (Deblay et al., 1991; Shinbo et al., 1993), analyzing the deviations from linearity in kinetics analysis (Szpakowska and Nagy, 1997), following the progressive wetting of SLMs by aqueous solutions (Danesi et al., 1987; Takeuchi et al., 1987; Takeuchi and Nakano, 1989), detecting the amount of a nontransferring colored component in the strip solution (Dreher and Stevens, 1998), or using noninvasive techniques such as impedance spectroscopy (Zha et al., 1994; Moreno and Valiente, 1997). Some of these methods are worked out in complementary form, so that the stability is interpreted on the basis of a comparison among the results. On the other hand, sometimes only one method is used. Several physical and physicochemical properties of the system have been correlated with SLM stability: pore diameter, support thickness, dielectric constant of the solvent, viscosity, interfacial tension, contact angle, bubble and drop points, water solubility, and osmotic pressure gradient. In relation to the support, Chiarizia (1991b) has shown that very thin membranes give rise to shorter membrane lives probably because of the very small inventory of organic phase absorbed in their pores, which enhances the effect of the loss of organic phase due to aqueous solubility, and because of the small


length of the pores, which enhances the occurrence of water bridges between feed and strip solutions through pores devoid of organic phase. The high inventory of organic phase could explain the fact that several authors have reported that the Accurel support provides the most stable SLM when comparing different supports (Duffey et al., 1978; Parkinson et al., 1983; Tromp et al., 1988). It was observed that the SLM lifetime depends on the type of carrier-diluent mixture used for the liquid membrane (Danesi et al., 1987). This factor determines losses and hydrophilization of the membrane liquid. According to these authors, when the wettability of the support pores by water increases, it is expected that less stable SLMs are formed. Also, it has been reported that instability effects depend strongly on the type of solvent (Takeuchi, 1987) and the molecular structure of the carrier used (Neplenbroek et al., 1992a,b). The membrane liquid should have an appreciable lower surface tension than the critical surface tension of the support and should have high drop point values. The membranes can also be damaged by partition of the carrier between the organic diluent and the aqueous solution (Stolwijk et al., 1989). According to Hill et al. (1996), the higher the initial permeability coefficient and partition constant, the longer the stability of the SLM. Dreher and Stevens (1998) observed that lifetime increases with membrane liquid viscosity to the power of 0.64. However Dozol et al. (1993) obtained stable SLM with diluents having both low and high viscosities. Hydrophilization of the SLM could also be due to some other factors. Nakano et al. (1987) explained the Ni(II)/ HDEHP/n-heptane system flux decline due to water liberation from the Ni complex in the membrane during its diffusion. Colinart et al. (1984) took this factor into account in analyzing water transport in emulsified liquid membranes. Thien and Hatton (1988) have reported reverse micelle formation employing emulsion liquid membranes. Paatero and Sjo¨blom (1990) have recommended to avoid conditions that favor the formation of reversed micelles and microemulsion in order to increase SLM stability. It is also expected that the formation of liquid crystalline phases inside the membrane will dramatically slow the transport properties. Membrane impregnation is said to affect water transport through the empty pores and, therefore, reproducibility (Szpakowska and Nagy, 1997). Yang and Fane (1997) pointed out that the preparation method had no influence on the initial fluxes but affected stability. In “dry surface membranes” no ML loss was observed in contrast with “wet surface membranes” in which significant ML loss was measured. The intrinsic nature of an SLM consisting not only of the ML but also the aqueous solutions the membrane liquid is in contact with attract the attention toward not to underestimating the physicochemical nature of these aqueous phases. The interfacial behavior of the carrier in an SLM plays a major role in determining the membrane stability. To maximize the lifetime of an SLM, it is essential to use organic liquid phases exhibiting a high organic-water interfacial tension. Molinari et al. (1989) explained the fast degradation of an o-xylene-SLM in comparison with a kerosene-SLM based on the fact that aromatic solvents are reported in the literature to have lower interfacial tensions than aliphatic hydrocarbons. Neplenbroek et al. (1992a,b) have shown that there are large differences in the

stability of SLMs depending on the composition of the membranes and somewhat on the composition of the aqueous phases. The removal of the carrier from the SLM was shown to depend on the salt concentration in the aqueous phases and the nature of the salt used. Szpakowska and Nagy (1997) observed that membrane lifetime decreases with increasing phase acceptor acidity. Zha and Fell (1995b) observed that when the feed pH value was close to that of the strip, serious leakage and direct channeling between the feed and strip occurred. Some authors indicate that SLMs appear to be unstable under high osmotic pressure gradients. Fabiani et al. (1987) observed volume and salt flow under an applied osmotic gradient and no hydrostatic pressure difference. On the contrary, other researchers (Neplenbroek et al., 1992a,b; Szpakowska and Nagy, 1997) showed that instability effects are not caused by an osmotic pressure difference. The osmotic pressure gradient and transport of water are viewed as a consequence of SLM instability and not as a cause. A possible explanation for this contradiction has been reported by Deblay et al. (1991). These authors observed lifetime independency of the osmotic pressure gradient across the SLM when water concentration in the organic phase was sufficiently low and the interfacial tension high enough. In the case that these conditions are not fulfilled, the organic phase can be progressively removed from the micropores of the support by a flux of water transported from the lower to the higher ionic strength solution, and the lifetime of the SLM decreased with an increase in the osmotic pressure gradient. Danesi et al. (1987) reported that most of the SLMs they studied were capable of withstanding large differences in externally applied pressures as long as the organic phase wetted the support and observed that the presence of a large pressure gradient is far more effective in rendering the SLM unstable when the diluent for the carrier is a strong surfactant exhibiting a good solubility for water. Pressure differences larger than the drop point of the SLM drops off stability (Deblay et al., 1991). From liquid-liquid extraction experience it is known that adsorption of certain substances at the interfaces may result in the formation of interfacial films that change the properties of the interface and its neighborhood (Tarasov and Yagodin; 1988). These films have strong effects on the extraction kinetics, and their irregular formation, which is due to an uneven access of the interface under hydrodynamic flow conditions, may cause the interface to become unstable according to the Marangoni-Gibbs effect. Spontaneous surface convection (SSC), the phenomenum that correlates mass transfer and hydrodynamics, is mainly initiated in the presence of a critical gradient of interfacial tension. Hydrodynamic instabilities (Kelvin-Helmholtz, Rayleigh-Taylor, Bernard, Tollmien-Schichting instabilities, and capillary forces) tend to occur at the interface between different liquids (Zha and Fell, 1995b). According to Dreher and Stevens (1998), due to the fact that in an SLM the aqueous fluids are always under forced flow across the membrane by stirring or pumping, it is expected that capillary ripples and the Kelvin-Helmholtz instability will have an important contribution over stability in comparison with any other type of hydrodynamic instabilities. An estimation of the minimum velocity difference between the feed and strip solutions to stabilize the membrane interface is reported


by these authors. Hydrodynamic instabilities lead to ML loss by emulsion formation, probably a dominant cause for the instability of SLMs. Adding electrolyte to the aqueous solutions reduced the ML loss and improved stability by preventing or reducing the emulsion formation (Neplenbroek et al., 1992b; Zha et al., 1995a). Instability effects are said to depend strongly on the flow velocity of the aqueous phases (Takeuchi et al., 1987; Neplenbroek et al., 1992a). The lower the stirring speed, the longer the lifetime of SLMs (Takeuchi et al., 1987; Neplenbroek et al., 1992a; Szpakowska and Nagy, 1997; Dreher and Stevens, 1998). This effect is due to an increase in the shear force across the membrane as the stirring speed is increased. The addition of surface active substances to the system, on one hand, favors the appearance of SSC because of the decrease in interfacial tension but, on the other hand, prevents it by decreasing the interfacial mobility, particularly when interfacial films are formed. Thus, depending on what predominates, surface active substances can engender or suppress SSC (Tarasov and Yagodin; 1988). The addition of polymers to the liquid membrane as a means to increase stability has been reported (Dreher and Stevens, 1998). Because elastic fluids exhibit greater resistance to motion than purely viscous ones, longer SLM lifetime is expected with elastic fluids. However, no conclusion could be drawn about this by the authors due to the low shear rates encountered at the membrane surface which hinders the elastic characteristics of the studied fluids. According to Zha and Fell (1995b), four principal SLM instability mechanisms have been reported, based on osmotic pressure, progressive wetting, pore-blockage, and shear-induced phenomena. However, according to Kemperman and co-workers (1998), only two mechanisms for SLM instability seem to be of major importance and can explain the loss of membrane solvent and carrier in different ratios: the solubility of the LM components in the adjacent feed or strip solutions and emulsification due to lateral shear forces. Whatever the mechanism of SLM degradation is, it has been observed that SLM failure occurs progressively. A restricted breakthrough of the membrane can occur while the larger part of the membrane is still intact and keeps on functioning (Chiarizia, 1991b; Neplenbroek et al. 1992b). Despite the ML loss increasing with time, the rate at which this phenomenon occurs is not constant; ML loss was reported to be fast in the initial stage and then it waned (Zha and Fell, 1995b). The complex nature of membrane stability has been pointed out with amine systems. The stability of Primene JM-T membranes was found to be strongly dependent on the concentration of the carrier, reaching a maximum value. This behavior was explained by a combined mechanism in which the interfacial tension, membrane inventory, water penetration into the pores, emulsion formation, and amine solubility took part in the instabilization process (Chiarizia, 1991b). In this reference it was suggested that the formation of solid or gelatinous precipitates in the pores of the membrane, when the saturation limit of the carrier was reached in the liquid membranes phase, had the effect of slowing down the diffusion of permeating species but enhanced the stability of the membrane, by preventing the formation of an emulsion with the aqueous phase and by acting as a barrier against bridging in semidevoid pores.

Long-term experiments have shown complex phenomena difficult to predict. A possible interaction between the support and the carrier solution was found by Chiarizia et al. (1990a) from long-term experiments (1.5 years) which led, in the long run, to a change in support properties that, in turn, affected the permeability. In these experiments, an increase in permeability was observed with the support reimpregnation. Neplenbroek et al. (1992a), in their long-term experiments with amine carriers, observed that air bubble adhesion to the hydrophobic material can be a general problem for the application of SLM-separation processes. The authors indicated that the magnitude of this effect depended on the hydrodynamic conditions. Among the several avenues proposed for improvement of SLM stability are the following: (1) Synthesis of selective carriers which possess very low solubility in water (Danesi et al., 1987). Improvement of the compatibility of the carrier with the other membrane components should increase membrane stability and flux rates. Danesi et al. (1987) have shown that for HDEHP, a carrier with very limited aqueous solubility ([HDEHP]octane/[HDEHP]water ) 4 × 104), the permeability coefficient decreases in about 20% during 1 month of observation, while, for the more watersoluble HDBP carrier ([HDBP]octane/[HDBP]water ) 0.5), the value of this coefficient rapidly declines after only 5 days of operation. Bartsch and Way (1996) have collected contemporary techniques and methods for the design, synthesis, and evaluation of new carrier species. (2) Modification of the aqueous phases in order to achieve high organic/water interfacial tension. Chiarizia (1991b) has found that the high value of the interfacial tension for a TLA membrane in n-dodecane accounts for its stability over 26 days of continuous operation without measuring any significant decline of the membrane performance. (3) Maintenance of an interface-immobilizing pressure difference across the porous membrane in a direction and of a magnitude effective to oppose the tendence of leakage (Kiani et al., 1984; Sikar 1991). Majumdar and Sirkar (1992) report that the copper flux remained stable over a 1 month-long period when using a 10% v/v solution of LIX 84 in n-heptane in an HFCLM. (4) Various degrees of binding the extractant to the polymeric membranes (Baniel et al., 1990; Schow et al., 1996). The membrane type, polymer inclusion membrane (PIM) or polymeric plasticizer membrane (PM) (Sugiura et al., 1987, 1989b), is claimed to combine the virtue of rapid transport with high selectivity and ease of set up and operation, exhibiting at the same time excellent durability. Because this type of membrane is independent upon organic solvents due to the fact that the carrier molecules are trapped within the cellulose triacetate matrix, they do not suffer from loss of organic solvents nor as much leaching of carrier into the aqueous phases. Peterson and Lamb (1996) have run a PIM for more than 100 days with little variation in cation flux and no signs of structural weakening. (5) Minimizing emulsion formation by reducing surface shear, gelling the immobilized liquid, or application of a protective coating (Neplenbroek et al., 1992c, 1990; Wijers et al., 1996; Kemperman et al., 1998; Bromberg et al., 1992a,b). Few reports of the stability of gelled SLM (GSLM) in metal ion separations have been found in the literature. Bromberg et al. (1992a) have reused several times a GSLM without observing leakage even


under strong acidic conditions. Bromberg et al. (1992b) also found that these membranes were very stable over a 30-day testing period. However, Neplenbroek et al. (1992c) in their studies concerning nitrate anions report problems with reproducibility of the results and the impediment for applying the gel at the lumen side of an HF. Interfacial polymerization was proposed as a solution by Kemperman et al. (1998). The authors demonstrated that while an uncoated SLM shows no flux after 1 day of enhanced degradation, the best coated membranes with this technique show no flux decrease after 350 h of operation. (6) Utilization of alternative configuration systems such as self-impregnating (Danesi and Rickert, 1986) or continuous regenerating (Nakano et al., 1987) modules, contained liquid membranes (Sengupta et al., 1988; Guha et al., 1994), hybrid liquid membranes (Kislik and Eyal, 1996a,b), spiral type flowing liquid membrane (Teramoto et al., 1987), or periodic dispersion of the organic membrane solution into the aqueous receiving phase (Youn et al., 1997a). A comparison between all these type of configurations and stabilizing methods is required in order to decide which of them is the best option in a particular system. It is expected that besides the system’s performance, economical factors play an important role in determining the final choice. Data Acquisition and Evaluation Optimization in data acquisition is required in order to easily and promptly obtain information concerning membrane efficiency and to compare different systems. The incorporation of an FIA manifold was suggested as an option for rapid optimization of SML systems by Barnes and Van Staden (1992). This type of configuration is claimed to overcome the time-consuming process of data acquisition when employing the classical SLM configuration and to allow minimal consumption of reagents, reproducibility, and simplicity of the experimental setup. Optimization in data evaluation is a more difficult task to accomplish, particularly because of the many types of extractants, solvents, metals, ionic media and experimental configurations employed in transport experiments (Table 2). The use of a system that allows intercomparison of the performance of different SLM systems might be the solution to overcome such problem. The necessity of using recommended systems for testing purposes in the development of new extraction equipment has been suggested by the Europen Federation of Chemical Engineering Working Party on Distillation, Absorption and Extraction (Bart et al., 1994). In the context of ion exchange reactive extraction, the Zn/ D2EHPA system is favored due to the fact that most of the requirements for a reactive liquid-liquid test system can be fulfilled. Reactive extraction using SLMs requires the proper knowledge of the mechanism (site of chemical reaction, limiting mass transfer steps), kinetic data (rate laws, mass transfer coefficients), and thermodynamic data (stoichiometry and equilibrium constants of the reaction(s)) for the process. However, in the extraction system Zn/D2EHPA a certain disagreement has been encountered in the literature with respect the ratecontrolling extraction steps, rate constants, and reaction orders, as well as the nature of the species present in the extraction process (Bart et al., 1994). A review of

some models for the equilibrium extraction and reaction kinetics proposed by several authors and an attempt to analyze them was reported in this work. Some of the discrepancies observed by the authors may be related to the use of different experimental techniques. As indicated by Freiser (1988), “simple” well-known extraction chelating systems can show complex reaction mechanisms difficult to elucidate when only the Lewis cell type or single drop methods are employed. In this paper, Freiser reported that the introduction of a microporous phase separator in extraction kinetic studies resulted in clear evidence of reaction pathways occurring at the interface and in the bulk aqueous phase during LIX 65N-copper and KELEX 100-nickel extractions. Likewise, Juang and Lo (1994b), and Yoshizuka et al. (1986, 1995) performed kinetic-membranebased studies. The former using a diaphragm-type liquid membrane permeation cell for the D2EHPA-vanadium(IV) system and the latter a hollow fiber membrane extractor for copper extraction and stripping with LIX 34 and palladium(II) extraction with DETE, respectively. In the field of the Zn/D2EHPA system, some studies in FS-SLM (Fernańdez et al., 1986; Huang and Juang, 1987), HF-SLM (Daiminger et al., 1996), and spiral-type (Teramoto et al., 1989) configurations can be found in the literature. The authors agree that when low values of the distribution coefficient are employed (high metal: extractant ratio), the permeation is limited by membrane diffusion, while for high values of the distribution coefficient (low metal:extractant ratio) the mass transfer resistance is situated in the aqueous extraction phase and/or the membrane. Although encouraging results are obtained by these authors, due to the fact that the aim of their studies was not oriented toward the deduction of kinetic laws, no conclusion can be derived concerning the use of membrane-based techniques as a better alternative in kinetic analysis for the recommended system. To change the balance of the controlling rate process with D2EHPA (e.g. from diffusion to kinetic regime), studies with other metals, principally copper, cobalt, and nickel, have been suggested (Bart et al., 1994). Huang and Juang (1988) predicted different permeabilities for these cations in FS-SLM and calculated the relative values of the resistances presented in the system by assuming a three-step simultaneous controlling scheme. Finally, it is important to mention that the proper modeling of the equilibrium and extraction kinetics in industrial applications requires a considerable effort in obtaining equilibrium and kinetic data in industrial solvents, consideration of nonidealities of the organic phases, evaluation of degradation effects, accumulation of impurities (when the chemicals are not purified), and unification of the concentration ranges, ionic strengths, and diluents used in order to allow simple comparisons among the different works and to readily apply the obtained knowledge for practical purposes. Future Trends in SLMs Until today, one of the major challenges of SLM systems has been the commercialization of metal extraction processes at a large scale. Attempting to optimize a process means looking more deeply into its physicochemical and technical elements. SLM scaling up will fail unless a complete understanding of the efficiency parameters is done and reported in such a way


that a concise and global insight of the separation characteristics of a given system can be easily drawn. A major trend in the future for SLM development is concerned, in this way, with establishing a convenient data representation form. The type of data representation required must allow intercomparison between the different systems and cover, as far as possible, the permeability, selectivity, and stability properties of the system. In this direction, the selection and recommended use of a reference system is also an interesting alternative. Furthermore, the possibility of development of membrane systems that perform clean, specific, and lowenergy separations of metal species calls for the development of more commercially available selective extractants. More information concerning the effects of impurities present in commercially available extractants is needed to conclude about the efficiency of a given system. In the future, new support materials will be requiered in order to overcome environmental difficulties and mechanical instabilities in modular configurations. Extractant and support will probably constitute a unique element, considering that the support will be designed to contain a specific carrier within. The main concern will continue to be the search for long-term stability. The experimental conditions in which an SLM metal separation is to be applied determines the complexity of the phenomena that a given theoretical model should account for. In such a case that a broad change range of the experimental variables is expected, models dealing with all the aspects concerning the chemical, kinetical, and hydrodynamical factors will be favored in the future. As a consequence a great quantity of goodquality relevant parameters data will be required. At the same time, the implementation of new approaches directed to the scrutiny of the interfacial phenomena and the issues within the membrane phase by modern physicochemical techniques will give important information for the comprehensive understanding and the correct application of the theory in these systems. The elucidation of the nature and number of the transported species, the evaluation of the influence of interfacial phenomena in SLM transport, and the influence of impurities in membrane efficiency have to get research priority. SLM transport optimization must fulfill a complete set of conditions to get a balance between different factors. The role that mass transfer has in membrane stability (the reader should compare Szpakowska and Nagy (1997), Dreher and Stevens (1998), Zha and Fell (1995b), Molinari et al. (1989), and Deblay et al. (1991)), the relation between support thickness and magnitude of the initial flux (Szpakowska and Nagy 1997; Zha et al. 1995a), and the influence of the viscosity in stability (Dreher and Stevens, 1998; Dozol et al., 1993) are some topics which have to be clarified to evaluate SLM performance. Since relatively few studies concerning the separation of two or more competitive solutes are encountered in the actual literature, more work is required in this field in order to approach more practical separation problems. Conclusions Despite the enormous amount of information found in the literature, a critical evaluation of SLM metal transport is difficult due to the diversity of operational conditions employed by researchers in the field. Factors

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Received for review June 1, 1998 Revised manuscript received December 10, 1998 Accepted December 17, 1998 IE980374P

Metal Ion Separations by Supported Liquid Membranes - American

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