Carrier facilitated transport liquid membranes

FIGURE 37 Mechanisms of carrier-facilitated immobilized liquid membrane extraction, also referred to as coupled transport. The species, R, refers to the carrier component responsible for complexation. 

Immobilized Liquid Membranes. Facilitated transport liquid membranes for gas separations can be prepared In several configurations. The complexatlon agent solution can be held between two nonporous polymer films (2j1), Impregnated Into the pore structure of a micro-porous polymer film (25), or the carrier can be exchanged for the counterion In an Ion exchange membrane (it). 

The facilitated-transport liquid membranes for the production of oxygen-enriched air has been much improved using cobalt-Schiff s base complexes as an oxygen carrier [36]. Examples are [A, A -bis(salicylidene)-n-propyl-dipropylene-triaminejcobalt, Structure (10) and [A, A -bis(3-methoxysali-cylidene)tetramethylethylenediamine)]cobalt. In Structure (10) the five-coordinated deoxy-form is supplied by the cyclic ligand itself. Liquid membranes were prepared by immersing a microphorous membrane into a solution of the complex. The solution was strongly held within the pores by capillary action. A typical example of microporous membranes is Ultipor... 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

Fig. 9. Facilitated transport of oxygen by a carrier in a liquid membrane.

Because the carrier facilitated transport process employs a reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far larger than the selectivities achieved by other membrane processes. This one fact has maintained interest in facilitated transport for the past 30 years, but no commercial applications have developed. The principal problem is the physical instability of the liquid membrane and the chemical instability of the carrier agent. In recent years a number of potential solutions to this problem have been developed, which may yet make carrier facilitated transport a viable process. 

Figure 1.6 Schematic examples of carrier facilitated transport of gas and ions. The gas transport example shows the transport of oxygen across a membrane using hemoglobin as the carrier agent. The ion transport example shows the transport of copper ions across a membrane using a liquid ion-exchange reagent as the carrier agent...

Liquid membranes are the final membrane category. The selective barrier in these membranes is a liquid phase, usually containing a dissolved carrier that selectively reacts with a specific permeant to enhance its transport rate through the membrane. Liquid membranes are used almost exclusively in carrier facilitated transport processes, so preparation of these membranes is covered in that chapter (Chapter 11). 

Carrier facilitated transport membranes incorporate a reactive carrier in the membrane. The carrier reacts with and helps to transport one of the components of the feed across the membrane. Much of the work on carrier facilitated transport has employed liquid membranes containing a dissolved carrier agent held by capillary action in the pores of a microporous film. 

Carrier-facilitated transport is used successfully to extract various organic and inorganic substances from a feed mixture in liquid membranes. Liquid membranes are employed as bulk liquid membranes, emulsion liquid membranes, and supported liquid membranes. 

Membranes with carrier-facilitated transport for specific gas components have also been in focus for some years, but some of these membranes studied seem to be dead-end developments. Of special interest are however those reported for CO2 capture, either as liquid membranes or as fixed-site carriers (FSC) in polymers [14,15]. 

Huang CR, Wang KC, and Zhou DW. Mathematical modelling of carrier-facilitated transport in emulsion liquid membranes. In Bartsch RA, Way JD, eds. Chemical Separations with Liquid Membranes, Washington, DC American Chemical Society, ACS s3miposium series 642, 1996 115-122. 

Shukla, J.P. and Mishra, S.K., Carrier facilitated transport of plutonium(IV) through tributylphosphate/dodecane liquid membranes. Ind. J. Chem., Sect. A, 1995, 34 778-786. 

The rising need for new separation processes for the biotechnology industry and the increasing attention towards development of new industrial eruyme processes demonstrate a potential for the use of liquid membranes (LMs). This technique is particularly appropriate for multiple enzyme / cofactor systems since any number of enzymes as well as other molecules can be coencapsulated. This paper focuses on the application of LMs for enzyme encapsulation. The formulation and properties of LMs are first introduced for those unfamiliar with the technique. Special attention is paid to carrier-facilitated transport of amino acids in LMs, since this is a central feature involved in the operation of many LM encapsulated enzyme bioreactor systems. Current work in this laboratory with a tyrosinase/ ascorbate system for isolation of reactive intermediate oxidation products related to L-DOPA is discussed. A brief review of previous LM enzyme systems and reactor configurations is included for reference. 

Behr and Lehn (14) first demonstrated carrier-facilitated transport of amino acids through "liquid membranes" composed of a toluene layer floating on top of two isolated aqueous solutions. The carriers used were the quaternary ammonium salt Aliquat 336 (trioctylmethylammonium chloride) and the alkylated arylsulfonic acid dinonylnapthalenesulfonic acid. Amino acids were transported in the form of anions or cations, respectively, with the above carriers. The process is an ion-exchange process, as the carrier must exchange an ionized atom or molecule each time it forms a new ion pair (Figure 2). The net result of this type of carrier-facilitated transport process is that a solute ion is transported into the LM while an equal number of counterions are transported out of the LM. 

Figure 7. Schematic representation of the complete tyrosinase / ascorbate system in a Liquid Membrane showing diffusion of oxygen, carrier-facilitated transport of substrate and product through the LM, and the reactions occurring in the internal aqueous phase.

It has been shown that carrier-facilitated transport of amino acid ions plays a central role of development of LM enzyme systems. Better anion carriers and new cation carriers are needed to exploit enzymatic processes where amino acids (or derivatives) and their products must transport readily through the LM. Facile transport with little or no enzyme deactivation is required. Design of new carriers tailored specifically for LM processes will help pave the way for the industrial development of liquid membrane enzyme reactors. 

Lamb, J. D. Christensen, J. J. Izatt, S. R. Bedke, K. Astin, M. S. Izatt, R. M. "Effects of Salt Concentration and Anion on the Rate of Carrier-Facilitated Transport of Metal Cations through Bulk Liquid Membranes Containing Crown Ethers" J. Am. Chem. Soc., 1980 102 (10), p. 3399. 

The commonly accepted mechanism for the transport of a solute in LM is solution-diffusion. The solute species dissolve in the liquid membrane and diffuse across the membrane due to an imposed concentration gradient. Different solutes have different solubilities and diffusion coefficients in a LM. The efficiency and selectivity of transport across the LM may be markedly enhanced by the presence of a mobile complexation agent (carrier) in the liquid membrane. Carrier in the membrane phase reacts rapidly and reversibly with the desired solute to form a complex. This process is known as facilitated or carrier-mediated liquid membrane separation. In many cases of LM transport, the facilitated transport is combined with coupling counter- or cotransport of different ions through LM. The coupling effect supplies the energy for uphill transport of the solute. 

Mechanisms and Kinetics of Carrier-Facilitated Transport Through Liquid Membranes... 

Sastre, A., Madi, M., Cortina, J.L. and. Mira lies, N. (1998). ModeUing of mass transfer in facilitated supported liquid membrane transport of gold(III) using phospholene derivatives as carriers. J. Membr. Sci., 139, 57-65. 

In carrier facilitated gas transport through liquid immobilized membranes, the overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. At the high-pressure feed side, the gas molecule that has to be selectively transported complexes with the carrier molecule diffuses along with the mobile carrier through the liquid membrane phase and desorbs at the low-pressure permeate side of the membrane [2, 4]. 

F.J. Alguacil, H. Tayibi, Carrier-facilitated transport of Cd(II) from a high-salinity chloride medium across a supported liquid membrane containing Cyanex 923 in Solvesso 100, Desalination 180 (2005) 181-187. 

An actual commercially viable gas separation process based on carrier-facilitated transport in liquid membranes remains to be synthesized, although that goal has come close to attainment. 

J.D.Lamb, J.J.Christensen, S.R.Izatt, K.Bedke, M.S.Astin, R.M.Izatt, Effects of Salt Concentration and Anion on the Rate of Carrier Facilitated Transport of Metal Cations Through Bulk Liquid Membrane Containing Crown Ethers, J.Am.Chem.Soc., 102, 3399 (1980) H.Tsukube, Effects of Cation on Transport Efficiency and Selectivity of Amino Acid Derivative Anions, Bull.Chem.Soc.Jpn.,... 

Biswas, S., Pathak, P.N. Roy, S.B. (2012) Carrier facilitated transport of uranium across supported liquid membrane using dinonyl phenyl phosphoric acid and its mixture wifli neutral donors. Desalination, 290, 74-82. 

Joshi, J.M., Pathak, P.N., Pandey, A.K. Manchanda, VK. (2009) Study on synergistic carriers facilitated transport of uranium(VI) and europium(III) across supported liquid membrane from phosphoric acid media. Hydrometallurgy, 96 (1-2), 117-122. 

Kedari, C.S., Pandit, S.S., Misra, S.K. Ramanujam, A. (2001) Mass transfer mechanism of the carrier-facilitated transport of uranium VI across 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester immobilised liquid membranes. Hydrometallurgy, 62 (1), 47-56. 

Use of membrane-based separation processes represents a promising alternative to traditional processes, since they do not require high energy and chemical consumption, thus significantly improving the sustainable energy approach. In particular, innovative methods based on the carrier facilitated transport across a liquid membrane (LM) show great potential since they do not produce by-products of difficult disposal and they can be operated continuously. 

An artificial membrane set-up that mimics carrier-facilitated transport is the liquid membrane. It consists of two aqueous phases that are separated by a water-immiscible organic phase to which receptors (carriers) are added to facilitate selective transport. 

In the search for improved metal ion separation schemes, considerable attention has been focused upon the use of liquid membranes (7,2). In a liquid membrane system, a liquid or quasi-liquid phase separates two other liquid phases in which the membrane is immiscible. In the most common arrangement, a hydrophobic liquid phase, such as chloroform or toluene, separates two aqueous phases. If chemical species have some solubility in the membrane, they may pass from one aqueous phase through the membrane into the second aqueous phase by simple diffusion. More frequently, a carrier molecule which resides in the membrane provides carrier-facilitated transport of metal ions across the membrane. Compared with simple diffusion, the carrier-facilitated transport is usually more efficient and selective. 

Carrier-facilitated transport of actinides across bulk, supported, and emulsion liquid membranes, as well as plasticized membranes and recently developed emulsion-free liquid membranes, are reviewed. The discussion includes the effects of important experimental variables upon the solute flux for various types of liquid membranes. Applications of liquid membranes in the recovery and removal of radiotoxic actinides from the nitric acid wastes generated during reprocessing of spent fuel by the PUREX process and wastes produced by other radiochemical operations are surveyed. 

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