Bulk liquid membrane systems

Selectivity parameters, needed for the BOHLM or BAHLM module design and their determination techniques, are analyzed. Selectivity can be controlled by adjusting the concentration, volume, and flow rate of the LM phase. Such control of the selectivity is one of the advantages of the bulk liquid membrane systems in comparison with other liquid membranes configurations and Donnan dialysis techniques. The idea of dynamic selectivity and determination techniques are presented and discussed. 

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) ... 

We have made use of the cation-selective properties of macrocycles to design liquid membrane [6], supported liquid membrane [6], and macrocycle-bonded silica gel [7] systems for specific cation separations. The selective transport of Ag [8, 9], Pb " [8], K" [10] and Li [11] was observed in a bulk liquid membrane system using various macrocyclic ligands. In the supported liquid membrane (thin sheet and hollow fiber) systems, selective and predicted separations of Na /K , Cd " / Hg ", and others have been achieved [12, 13],... 

Figure 1 shows several types of mass transfer or diffusion cells, which are of the simplest design for performing bulk liquid membrane (BLM) processes. Each of the devices is divided into two parts a common part containing the membrane liquid, M and a second part in which the donor solution F and acceptor solution R are separated by a solid impermeable barrier. The liquid, M contacts with the two other liquids and affects the transfer between them. All three liquids are stirred with an appropriate intensity avoiding mixing of the donor and acceptor solutions. For a liquid-ion exchange in a BLM system. Fig. 2 shows the transfer mechanism of cephalosporin anions, P , from donor (F) to acceptor (R) solution... 

In order to develop the liquid membrane techniques, i.e., emulsion Hquid membrane (ELM), supported liquid membrane (SLM), non-dispersive extraction in hollow fiber membrane (HFM), etc., for practical processes, it is necessary to generate data on equilibrium and kinetics of reactive extraction. Furthermore, a prior demonstration of the phenomena of facilitated transport in a simple liquid membrane system, the so-called bulk liquid membrane (BLM), is thought to be effective. Since discovery by Li [28], the liquid membrane technique has been extensively studied for the separation of metal ion, amino acid, and carboxyHc acid, etc., from dilute aqueous solutions [29]. 

Much effort has been expended in attempting to use membranes for separations. Reverse osmosis membranes are used worldwide for water purification. These membranes are based on size selectivity depending on the pores used. They do not have the ability to selectively separate target species other than by size. Incorporation of carrier molecules into liquid membrane systems of various types has resulted in achievement of highly selective separations on a laboratory scale. Reviews of the extensive literature on the use of liquid membrane systems for carrier-mediated ion separations have been published [15-20]. A variety of liquid membranes has been studied including bulk (BLM), emulsion (ELM), thin sheet supported (TSSLM), hollow fiber supported (HFSLM), and two module hollow fiber supported (TMHFSLM) types. Of these liquid membranes, only the ELM and TMHFSLM types are likely to be commercialized. Inadequacies of the remaining... 

The mechanism of the competitive pertraction system (CPS) is presented schematically in Fig. 5.4 together with the compartmental model necessary for constructing the reaction-diffusion network. The simple flat-layered bulk liquid membrane of the thickness En and interface area S separates the two reservoirs (f, feed and s, stripping) containing transported divalent cations A2+ and B2+ (most frequently Zn2+ and Cu2+ or Ca2+ and Mg2+) and/or antiported univalent cations H+. At any time of pertraction t, their concentrations are [A]f, [B]f, and [H]f and [A]s, [Bj, and [H]s, for the feed and stripping solution, respectively. The hydrophobic liquid membrane contains a carrier of total concentration [C]. Its main property is the ability to react reversibly with cations at respective reaction zone and to diffuse throughout the liquid membrane phase. 

MHS with pervaporation of water from LM (MHS-PV) is presented in Fig. 5.9. Contrary to the simple MHS with an agitated bulk liquid membrane, separated from the feed and strip solutions by flat hydrophobic or hydrophilic or ion-exchange membranes, the MHS-PV system exploits a Hquid membrane continuously flowing between the two flat cation-exchange and two pervaporation membranes. To couple the separation and pervaporation processes, the LM is simultaneously pumped through the MHS and PV modules. The pervaporation membranes are placed on stainless steel porous supports. Aqueous feed and strip solutions are intensively agitated. 

Sahoo GC, Ghosh AC, Dutta NN, Mathur RK. Facilitated transport of 7-aminocephalosporanic acid in a bulk liquid membrane. J Membr Sci 1996 112 147-154. Ersoz M, Vura US, Okdan A, Pehkvan E, Yildiz S. Transport studies of amino acids through a liquid membrane system containing carboxylated poly (styrene) carrier. J Membr Sci 1995 104 263-269. 

There are many problems associated with water-immiscible, organic hybrid bulk liquid membrane (BOHLM) systems (see Chapter 5) such as ... 

Figure 6.11 Schematic representation of experimental arrangement for a multimembrane hybrid system feed solution (1), strip solution (2), tube-in-tube diaphragm cell (3), glass vessel (4), bulk liquid membrane (5), magnetic stirrer (6), feed and strip inlet (7), feed and strip outlet (8), peristaltic pump (9), feed (10) and strip (11) ion-exchange membranes. From Ref. [90] with permission. 

The selectivity displayed for F-relative to other halide anions encouraged Sessler and coworkers to investigate whether sapphyrin would act as a fluoride anion carrier in a model three-phase H2O-CH2CI2-H2O bulk liquid membrane transport system.When two aqueous solutions with different fluoride anions concentrations, were separated by a dichloromethane solution, slow transfer of fluoride anions from the more concentrated... 

LM systems include bulk liquid membrane (BLM), emulsion liquid membrane (ELM) and supported hquid membrane (SLM). 

Other techniques have been developed to improve upon SX among them of particular interest are liquid membranes with the main t)q)es of these membranes being bulk liquid membranes (BLMs), emulsion hquid membranes (ELMs) and supported liquid membranes (SLMs) (Kolev, 2005) (Fig. 10.1). While these all have advantages compared to SX systems, they have not yet achieved wide eommercial acceptance. The following paragraphs present a brief deseription of the principles utilized by BLMs, ELMs and SLMs. For more information about liquid membranes please refer also to Chapters 7 and 8 of this volume. 

Bulk Liquid Membranes. Figure 4 shows four different cells which have been utilized in BLM transport experiments (11-13). The upper two are U-tube cells (12,13) and the lower two are so-called "tube-within-a-shell cells (12,13). The apparatus for conducting bulk liquid membrane transport experiments has the advantage of simplicity. However due to the thickness of the membrane, the amount of species transported is very low. Therefore, bulk liquid membrane transport systems are utilized in studies of transport mechanisms and assessing the influence of carrier structure upon transport efficiency and selectivity, but have no potential for practical application. 

Figure 3. Comparison of Bulk Liquid Membrane (BLM), Supported Liquid Membrane (SLM), and Emulsion Liquid Membrane (ELM) Systems. (A is the source (feed) phase, B is the liquid membrane, and C is the receiving phase.)...

The transport of cobalt(II), copper(II), nickel(II), and zinc(II) from aqueous sulfate solutions by novel di(p-alkylphenyl)phosphoric acid carriers in bulk and emulsion liquid membrane transport processes is reported by Walkowiak and Gega in Chapter 13. To probe the mechanism of the liquid membrane transport processes, interfacial tension measurements are conducted. A multistage emulsion liquid membrane system for separation of the transition metal cation mixtures is developed. 

In Chapter 14, Smith utilizes boronic acid carriers for the separation of hydrophilic sugars from aqueous solutions in bulk and supported liquid membrane systems. Boronic acids also function as carriers for the transport of sugars through lipid bilayers. 

Transport Through a Bulk Liquid Membrane. All theoretical models concerning carrier-assisted transport through SLMs are based on the theoretical work for carrier-assisted transport through BLM systems reported by Reusch and Cussler (5). They described the transport of different alkali salt mediated by dibenzo-18-crown-6 through a BLM. 

Figure 3. Synergistic Effect of Mixed Solvents on Na Transport Through a Bulk Liquid Membrane. The source phase is 1 M NaCl. The receiving phase is distilled water and dibenzo-18-crown-6 is the carrier. Three mixed solvent systems were tested ( ) chloroform(l)-nitrobenzene(2) ( ) dichloroethane( 1 )-nitrobenzene(2) and (O) chloroform( 1 )-dichloroethane(2). (Reproduced with permission from ref. 44. Copyright 1992 American Chemical Society.)...

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