Liquid Membranes Carrier Mediated Transport

With membranes of this class, transport is not determined in any way by the membrane (or membrane material) but by a very specific carrier-molecule. The carrier containing liquid is located inside the pores of porous membrane. The permselectivity towards a component depends mainly on the specificity of the carrier molecule. 

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In SLM extraction, the transport mechanism is influenced primarily by the chemical characteristics of the analytes to be extracted and the organic liquid in the membrane into which the analytes will interact and diffuse. Analyte solubility in the membrane and its partition coefficient will have the main impact on separation and enrichment. Analyte transport in SLM extraction can be substantially categorized into two major types one is diffusive transport (or simple permeation) and the other covers facilitated transport (or carrier-mediated transport).73... 

Shukla JP and Misra SK. Carrier-mediated transport of uranyl ions across tributyl phosphate—dodecane liquid membranes. J Mem Sci, 1991 64(1-2) 93-102. 

Akiba et al. investigated the carrier-mediated transport of uranium (VI) by a liquid membrane of 7-dodecenyl-8-quinolinol (Kelex 100) [60]. The apparent rate constant of uranium transport was not affected significantly with the increase in the carrier concentration and the pH of the feed solution. They had also reported that uranium can be effectively recovered from seawater through the liquid membranes without any preliminary treatment. Compounds analogous to Kelex 100, with long-chain aliphatic linkages, have been used for this study [61]. 

Kopunec, R. and Thang, N.M., Carrier-mediated transport of rare earth elements through liquid membranes. IE. Transport of Sc, Y, Ce, Eu, Gd, Tm, Yb through supported liquid membrane containing DEHPA. J. Radioanal Nucl Chem., 1993, 170 51-66. 

Schultz, J. S. "Carrier-Mediated Transport in Liquid-Liquid Membrane Systems" In Recent Developments in Separation Science, vol.III, Li, N. N., Ed. CRC Press Inc., West Palm Beach, CA, 1977. 

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. 

The effect of carrier, contained in the terms Ff. and Fbex lowers the resistance of tlie liquid membrane [42, 65]. Basic features ot carrier-mediated transport (1) the flux is proportional to the carrier concentration ... 

Chrisstoffels LAJ, de Jong F, Reinhoudt DN, Mechanistic studies of carrier-mediated transport through supported liquid membranes. In Bartsch RA, Way JD, Eds., Chemical Separations with Liquid Membranes, ACS Symposium Series 642. American Chemical Society, Washington, DC, 1996 Chapter 3 18-56. 

J P Behr, M Kirch, J M Lehn, Carrier-mediated transport through bulk liquid membranes Dependence of transport rates and selectivity on carrier properties in a diffusion-limited process. J. Am. Chem. Soc. 1985 107, 241. 

Brinchi L, Gerniani R, Mancini MV, Savelli G, Spreti N. Carrier-mediated transport of toxic heavy metal ions in bulk liquid membranes. Eur J Org Chem 2004 1330-1335. 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

Singh, S.K., Misra, S.K., Sudersanan, M., Dakshinamoorthy, A., Munshi, S.K., Dey, P.K., Carrier-mediated transport of uranium from phosphoric acid medium across TOPO/n-dodecane-supported liquid membrane, Hydrometallurgy 87, 190, 2007. 

Kopunec, R. and Manh, T.N. 1994. Carrier-mediated transport of actinides and rare earth elements through liquid and plasticized membranes. J. Radioanal. Nucl. Chem. 183 181-204. 

The vast majority of cation transport model studies have been conducted using crown ethers as ionophores and bulk liquid membranes as bilayer models. Much information has been obtained about carrier-mediated transport [1,7] but the mechanism for transport of molecules through biological membranes is predominantly of the channel type [8]. The synthesis of a cation-conducting channel is a daunting task. We felt that a suitable compound would require at least three basic features. First, it must be capable of insertion into a lipid bilayer. Second, it must span the membrane. Third it must have some residue integral to it that would... 

Scheme 1. Carrier-mediated transport of cations through liquid membranes. M+ cation, C carrier, C -M complex.

Three different types of passive carrier-mediated transport mechanism may be distinguished similar to transport in liquid membranes (see chapter VI) i) fiicilitated diffusion ii) co-transport and iii) counter-transport. 

Far higher selectivities can be obtained by adding a carrier molecule to the liquid (membrane) which has a high affinity for one of the solutes in phase 1. The carrier accelerates the transport of this specific component. This type of transport is called carrier mediated transport or facilitated transport. The mechanism of facilitated transport can be demonstrated by the simple experiment dqpicted schematically in figure VI > 32. 

Mechanistic Studies of Carrier-Mediated Transport Through Supported Liquid Membranes... 

Figure 9.4. Schematic representation of carrier-mediated metal-ion transport through a liquid membrane (A = anion).

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

Kirch and Lehn have studied selective alkali metal transport through a liquid membrane using [2.2.2], [3.2.2], [3.3.3], and [2.2.C8] (146, 150). Various cryptated alkali metal picrates were transported from an in to an out aqueous phase through a bulk liquid chloroform membrane. While carrier cation pairs which form very stable complexes display efficient extraction of the salt into the organic phase, the relative rates of cation transport were not proportional to extraction efficiency and complex stability (in contrast to antibiotic-mediated transport across a bulk liquid membrane). Thus it is [2.2.Ca] which functions as a specific potassium ion carrier, while [2.2.2] is a specific potassium ion receptor (Table VI). 

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