Separations using LEMs

Separations using LEMs can be effected in two different ways (5). The first of these (called TYPE I transport) is a simple diffusion process in which the solute partitions into the membrane phase from the exterior phase, diffuses across the membrane to the dispersed interior phase droplets, and partitions into the interior phase. A reaction takes place in the internal phase which converts the solute into a species which is incapable of partitioning back into the membrane phase. TYPE I transport is limited to uncharged solutes only, since only uncharged solutes will be able to favorably partition into the membrane phase. 

Current methods of detection for CAs in biological fluids (urine, plasma, and serum) involve chromatographic separation coupled to either electrochemical 119,120) or optical 121) techniques. However, most optical methods rely purely on the native fluorescence of CAs (lex 280 nm, lem 310 nm) 122), which have small Stokes shifts and suffer from signal losses due to reabsorption others involve pre- or postcoliunn derivatization with various fluorophores, such as naphthalene-2,3-dicarboxaldehyde 123), 1,2-diphenylethylenediamine 124,125), or fluorescamine 126). These methods all require significant time for separation using expensive instrumentation and thus are not feasible for rapid CA detection. 

Foam can also be broken with a rotating perforated basket [Lem-lich, Principles of Foam Fractionation, in Periy (ed.), Progre.ss in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1]. If the foamate is aqueous (as it usually is), the operation can be improved by discharging onto Teflon instead of glass [Haas and Johnson, Am. In.st. Chem. Fng. J., II, 319 (1965)]. A turbine can be used to break foam [Ng, Mueller, and Walden, Can. J. Chem. Fng., 55, 439 (1977)]. Foam which is not overly stable has been broken by running foamate onto it [Brunner and Stephan, Ind. Fng. Chem., 57(5), 40 (1965)]. Foam can also be broken by sound or ultrasound, a rotating disk, and other means [Ohkawa, Sakagama, Sakai, Futai, and Takahara,y. Ferment. Technol, 56,428, 532 (1978)]. 

Electrodialysis is a well-proven technology with a multitude of systems operating worldwide. In Europe and Japan, electrodialysis dominates as a desalting process with total plant capacity exceeding that of reverse osmosis and distillation [3]. Electrodialysis with monopolar membranes is applied to different food systems, to demineralization of whey [5-8], organic acids [9], and sugar [10,11], separation of amino acids [12] and blood treatments [13], wine stabilization [14—16], fruit juice deacidification [17-19], and separation of proteins [20-22]. These applications use the sole property of dilution-concentration of monopolar lEMs in a stack of as many as 300 in an electrodialysis cell. 

Two kinds of lEMs are used in electrodialysis (ED) homopolar membranes bearing charges of same sign and BPMs bearing positive and negative charges located separately on each side of a same plane. 

Applications of liquid emulsion membranes (LEMs) to biomedical and biochemical systems are reviewed and other potential applications identified. The LEM-mediated downstream processing of small, zwitterionic biochemicals (e.g. amino acids) is examined using chloride ion counter-transport to separate and concentrate the amino acid phenylalanine from stimulated fermentation broth. The effect of agitation rate and osmotic swelling of membranes on separation is shown to be significant. 

Since they were first developed in 1967 (1 ), liquid emulsion membranes (LEMs) have been used in a variety of separations (see reviews by Frankenfeld and Li (2), Marr and Kopp (3), and Way e t al. (4)). Conceptually, LEMs are quite simple. They consist of an... 

In spite of the potential problems of swelling and breakage, LEMs have been tested on the pilot plant scale with good results (2,9,11, 12), and will soon be used for the commercial separation of zinc from Viscose wastewater from rayon and cellophane processing (13). 

In terms of the amount of literature developed, biochemical separations have been largely ignored by those in the field of LEM-mediated separations. One application that has enjoyed some experimental scrutiny is that of the use of LEMs in drug delivery and overdose prevention systems. They have been used to separate or release several different types of drugs including acetylsalicyclic acid (18), phenobarbital (19), and several barbiturates (20,21). 

LEM systems have also been shown to be successful in separating commodity-type biochemicals such as propionic acid (10) and acetic acid (10,22) and have been used for the preparation of L-amino acids from racemic D,L mixtures by means of enzymatic hydrolysis of amino acid esters (23). In addition to biochemical separations, the work of Mohan and Li showed that enzymes could be encapsulated in liquid emulsion membranes with no deleterious effect on enzyme action (24). Later work by these authors indicated that encapsulated live cells could remain viable and function in the LEM interior phase for period as long as five days (25). 

These various uses of liquid emulsion membranes show the versatility of LEM-mediated separations and point to possible applications of liquid emulsion membranes in the biochemicals field. 

The versatility of LEMs is clear. From the encapsulation of living cells to the removal of toxic or inhibiting substances, and in their use as a downstream process, liquid emulsion membranes remain a powerful and, as of yet, virtually untapped resource for biochemical engineers. The ability of LEMs to separate and concentrate amino acids demonstrated here gives strength to this observation, and it is anticipated that these systems will enjoy increasing attention in the years to come. 

The use of two types of liquid membranes is described in [302] liquid emulsion membranes (LEMs), and supported liquid membranes (SLMs), where isoparaffin or kerosene and their mixtures were used as organic phases. A surfactant of the type of Span 80 served as emulsifier. LEMs are used, for example, for selective separation of L-phenylalanine from a racemic mixture of L-leucine biosynthesis as well as conversion of penicillins to 6-APA (6-aminopenicillanic acid). SLMs have a higher stability. A number of their commercial applications have been studied, e.g. in separation of penicillin from fermentation broth, as well as in the recovery of citric acid, lactic acid and some aminoacids. Compared with other separation methods (ultrafiltration, ultracentrifugation and ion exchange), LEMs and SLMs are advantageous in the separation of stereospecific isomers in racemic mixtures. 

An alternative method for the preparation of facilitated transport membranes is the subject of the first paper in this section. Way and Noble (113) report a study of H,S facilitated transport in reactive ion exchange membranes. The use of a perfluorosulfonic acid lEM as a support for organic amine counterions avoids problems of solvent and carrier loss often encountered with ILMs. High carrier loadings of greater than 8 M in the lEMs were attained which helped to account for the high facilitation factors of 26.4 which are observed at low partial pressures. An analytical model predicted facilitation factors in excellent agreement with the experimental data. Separation factors for HjS over CH., of 792 to 1200 are reported. Implications of the mathematical model for industrial applications are also discussed. 

Unlike many routine clinical chemistry tests, clinical analyses of lEM almost always involve a multiple metabolite analysis. The results form the basis of a metabolic profile in which both individual concentrations of metabolites and their relationship to each other can be viewed either in tabular form or in a graphical display. Perhaps the most comprehensive and historically significant test in lEM studies is gas chromatography/mass spectrometry (GC/MS) of a derivatized extract of urine. Figure 2 is a chromatogram from an infant with propionic acidemia, an organic acid disorder of leucine metabolism. Hundreds of volatile compounds of carbohydrate, amino acid, fatty acid, and nucleic acid metabolism are separated in 40 min using capillary GC. Addition... 

Two separate field tests were conducted at a copper mine in Arizona. The objectives of these tests were to evaluate the technical and economical feasibility of selectively recovering copper from a variety of dilute mine solutions using the LEM technique. The first field test of the LEM mobile system was conducted in September of 1993, and the second in May-June of 1994. 

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