Liquid membrane system application

In general, high selectivities can be obtained in liquid membrane systems. However, one disadvantage of this technique is that the enantiomer ratio in the permeate decreases rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed would be an approach to tackle this problem or, alternatively, using a system containing the two opposite selectors, so that the feed stream remains virtually racemic [21]. Another potential drawback of supported enantioselective liquid membranes is the application on an industrial scale. Often a complex multistage process is required in order to achieve the desired purity of the product. This leads to a relatively complicated flow scheme and expensive process equipment for large-scale separations. 

The study of the ion transfer through artificial liquid membrane systems is important for the elucidation of the ion transfer through biological membranes. In this respect, the Interface between two inmiscible electrolyte solutions (ITIES) constitutes a biomimetic medium suitable for studying several fundamental processes, ranging from biocatalysis to cellular respiration of photosynthesis, and many others [18-22], The first studies of liquid/liquid interfaces (L/L) under the application of an external potential were carried out by Gavach et al. [23], laying the basis for the current electrochemical treatments of ITIES. 

In some disciplines, certain multiple emulsions have been termed liquid membrane systems, as the liquid film which separates the other liquid phases acts as a thin semi-permeable film through which solute must diffuse moving from one phase to another. There are, therefore many potential practical applications of multiple emulsions. 

This emulsification technique aims to maximize membrane surface area. The internal phase of the droplets can be placed inside the pores of polymeric films, or inside hollow fibers. The membrane phase can contain surfactants or no surfactants for easy coalescence. This aspect of the liquid membrane system will be discussed in more detail in the application section. 

There have been several modified systems since the invention of liquid membranes, including a facilitated transport mechanism. One of them is to disperse the receiving solution in an organic membrane phase on one side of a porous hollow fiber.Two plants to treat contaminated groundwater were built and operated based on this revised liquid membrane system. More discussion about this application is given below in the application section. 

With the advancement of membrane technologies and engineering in the integration of MF, UF, NF, and RO membrane systems, and possibly a liquid membrane system, wastewater reclamation can provide high quality water for underground water replenishment and direct household nonpotable use, and source water for ultra-pure water applications. Membrane technology provides several advantages over conventional treatment processes ... 

Liquid membrane systems were first introduced in 1968 (34), and since then they have been evaluated for various chemical and biochemical applications (35). Some of the applications include the selective extraction of hydrocarbons (36), the recovery of rare earths from process streams (37), the extraction of organic contaminants like phenol from water streams (38), and amino acid recovery (39). 

The BAHLM differs from all bulk liquid membrane systems by application of polyelectrolyte aqueous solutions as carriers and charged (ion-exchange) membranes (lEM) as barriers. As can be seen in Fig. 6.2, the physicochemical aspects of the BAHLM processes are complicated. Transport of solutes or their complexes consist of the following steps (see Fig. 6.2A) ... 

A recent study with biotechnology applications relates to amino acid extraction. Schugerl and co-workers (71 ) used a quaternary ammonium carrier in an emulsion liquid membrane system for enzyme catalyzed preparation of L-amino acids. Frankenfield et al. (72) discuss a wide variety of biomedical ELM applications including enzyme encapsulation, blood oxygenation, and treatment of chronic uremia. 

Multiple emulsions usually refer to series of complex two-phase systems that result from dispersing an emul sion into its dispersed phase. Such systems are often referred to as water-in-oil-in-water (W/OAV) or oil-in-water-in-oil (O/W/0) emulsions, depending on the type of internal, intermediate, and continuous phase. Multiple emulsions were early recognized as promising systems for many industrial applications, such as in the process of immobilization of proteins in the inner aqu eous phase (37) and as liquid membrane systems in extraction processes (38). W/O/W emulsions have been discussed in a number of technical applications, e.g., as prolonged drug-delivery systems (39-44), in the context of controlled-release formulations (45), and in pharmaceutical, cosmetic, and food (46) applications. 

S. Mukhopadhyay, Dispersion and emulsion based liquid membrane systems in hollow fiber contactor, in S.A. Ansari, A.K. Pandey, P.K. Mohapatra, A. Goswami (eds.), Proceedings of Theme Meeting on Membrane Separation for Fuel Cycle Applications, BARC, Mumbai, India, September 16-18, 2013, p. 11. 

Polymeric substances have been used in analytical electrochemical applications to impart ion selectivity to electrodes (Buck, 1974, 1976, 1978). They have been used either to form solid electrodes [see also (SN) ,Nowak et al., 1977 Voulgaropoulos et al., 1978] or to form liquid membrane systems and neutral carrier systems exhibiting ion selectivity. 

In Chapter 6, characteristic features of emulsion liquid membrane systems are examined by Yurtov and Koroleva. The effects of surfactant and carrier concentrations and external and internal phase compositions upon the properties of the extracting emulsions are discussed. Several mathematical models for the rheological curves are considered, and regions of applicability for the models are evaluated. An influence of nanodispersion formation on mass transfer through the interface and on the properties of extracting emulsions for cholesterol is demonstrated. 

New approaches for development of specific carriers for use in liquid membrane are described (i) computer-aided design of cation-specific carriers and (ii) functionalization of rare earth complexes as anion carriers. A new series of Li(I) and Ag(I) ion-specific carriers are successfully designed using MM2, MNDO and density functional calculations. Computer chemistry provides a rational basis for design and characterization of cation-specific carriers of armed crown ether-and podand-types. Lipophilic lanthanide tris(p-diketonates) are shown to be a new class of membrane carriers. They form 1 1 complexes with anionic guests and mediate transport of amino acid derivatives. Since these complexes exhibit different anion transport properties from those of crown ethers, further applications of rare earth complexes offer promising possibilities in the development of specific anion carriers for liquid membrane systems. 

Spherical liquid membranes consist, in simplest terms, of an emulsion suspended in a liquid that does not destroy the emulsion. In a typical application, small droplets of aqueous solution are encapsulated in a thin-film oil this emulsion is then suspended in another aqueous solution. Alternatively, small droplets of oil can be emulsified with water and the emulsion suspended in oil. In the first case, the oil phase is the liquid membrane in the second case, the water is the Uquid membrane. A typical droplet might be about 100 p,m in diameter. These spherical liquid membrane systems have many potential medical applications in the emergency treatment of drug overdoses and for oxygenating the blood system. Spherical liquid membranes may be applied in resource recovery and water purification, as encapsulated cells as well as liquid membrane encapsulated enzymes [331). 

Gerritsma, D. A., Robertson, A., McNulty, J. Capretta, A. (2004). Heck reactions of aryl halides in phosphonium salt ionic liquids library screening and applications. Tetrahedron Letters, 45,41, 7629-7631, ISSN 0040-4039 Giiell, R., Antico, E., Salvado, V. Fontas, C. (2008). Efficient hollow fiber supported liquid membrane system for the removal and preconcentration of Cr(VI) at trace levels. Separation and Purification Technology, 62,2,389-393, ISSN 1383-5866 Guibal, E., Vincent, T. Jouannin, C. (2009). Immobilization of extractants in biopolymer capsules for the synthesis of new resins a focus on the encapsulation of tetraalkyl phosphonium ionic liquids. Journal of Materials Chemistry, 19, 45, 8515-8527, ISSN 0959-9428... 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

As described above, the application of classical liquid- liquid extractions often results in extreme flow ratios. To avoid this, a completely symmetrical system has been developed at Akzo Nobel in the early 1990s [64, 65]. In this system, a supported liquid-membrane separates two miscible chiral liquids containing opposite chiral selectors (Fig. 5-13). When the two liquids flow countercurrently, any desired degree of separation can be achieved. As a result of the system being symmetrical, the racemic mixture to be separated must be added in the middle. Due to the fact that enantioselectivity usually is more pronounced in a nonaqueous environment, organic liquids are used as the chiral liquids and the membrane liquid is aqueous. In this case the chiral selector molecules are lipophilic in order to avoid transport across the liquid membrane. 

Two-phase liquid systems or liquid membranes have found applications in various separation technologies. Liquid membranes have an advantage over traditional... 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

Other applications of supported liquid membranes have been related to metal speciation. For example, recently a system for chromium speciation has been developed based on the selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two SLM units connected in series. Aliquat 336 and DEHPA were used respectively as carriers for the two species and graphite furnace atomic absorption spectrometry used for final metal determination. With this process, it was possible to determine chromium in its different oxidation states [103]. 

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