Liquid membrane system carrier type

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... 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

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. 

Transport Across Emulsion Liquid Membranes. The transport of transition metal cations across emulsion liquid membrane experiments were conducted in the cell shown in Figure 2b. The liquid membrane consisted of the carrier and an emulsifier dissolved in kerosene. The liquid membrane system was a water-in-oil-in-water type of emulsion which was obtained by stirring the aqueous receiving phase solution with the organic phase to form an emulsion which was then mixed with the aqueous source phase solution. 

In our laboratories, research and development studies have been conducted on the separation of uranium and various lanthanides by common extractants (carriers) and of actinides by crown ether carriers using different types of liquid membrane systems. Also our studies have been directed toward determining optimal support systems for supported liquid membranes (SLM) which may offer improved flux... 

Light-driven membrane transport. Cations may be transported through liquid membranes using crown ethers. For example, a typical system is of the type water-phase(I)/organic-phase/water-phase(II). The metal ion is added to water-phase(I) and the crown ether to the organic phase (to yield the liquid membrane). The crown acts as carrier for metal ions from water-phase(I) across the liquid membrane phase into water-phase(II). There have now been a very large number of studies of this type reported and a fuller discussion of this topic is given in Chapter 9. 

An example of light-assisted transport of the first type involves (200) as the carrier in the liquid membrane. In this case, irradiation of the membrane alternatively with UV and visible light significantly increases the rate of K+ and Rb+ transport in the presence of picrate ion. This system also exhibits discrimination since the transport of K+ is favoured over Rb+ (Shinkai, Shigematsu, Sato Manabe, 1982). 

A membrane can be either a liquid or a solid. Its electrical properties arise when it allows transport of an ion of one charge but not that of another. Membranes are usually sufficiently thick that one can distinguish an inside region and two outer boundary regions which are in contact with electrolyte solutions. Two types of membranes are considered here (1) membranes of solid and glassy materials (2) liquid membranes with dissolved ion-exchanging ions or neutral ion carriers (ionophores). In fact all of these membranes are involved in ion exchange. It is important to understand how this process affects the potentials which develop in the system at both sides of the membrane. 

The application of mercury is widespread in agriculture, for example, as insecticide in seed treatment, and different types of industry [60]. A promising method for the removal and preconcentration of mercury from wastewater has been the application of liquid membranes containing calixarenes as carriers [61]. A three-phase system for the extraction of Hg from industrial wastewater has been reviewed by Ersoz [62]. Models and implication of theoretical conclusions are presented. 

Promising results are shown by recently developed integrated SLM-ELM [84, 85] systems. These techniques are known as supported liquid membrane with strip dispersion (SLMSD), pseudo-emulsion-based hollow fiber strip dispersion (PEHFSD), emulsion pertraction technology (EPP), and strip dispersion hybrid Hquid membrane (SDHLM). AH techniques are the same the organic phase (carrier, dissolved in diluent) and back extraction aqueous phase are emulsified before injection into the module and can be separated at the module outlet. The difference is only in the type of the SLM contactors hoUow fiber or flat sheet and in the Hquid membrane (carrier) composition. These techniques have been successfuUy demonstrated for the removal and recovery of metals from wastewaters. Nevertheless, the techniques stiU need to be tested in specific apphcations to evaluate the suitabUity of the technology for commercial use. 

At present, the most widely used type of micro-ISEs are the liquid membrane type. As has been the case with the previously mentioned clinical chemistry systems and catheter designs, the need for many of the glass membrane electrode systems has been obviated thanks to the increasing number of highly selective neutral carrier molecules that have become avail-... 

Despite the physical strength offered by a macroporous support, most immobilized liquid membrane (ILM) systems are not practical for industrial separations because they are not sufficiently stable. The two most important types of instability are solvent evaporation and loss of solvent and/or carrier from the support caused by a pressure differential across the membrane. These Instabilities can be completely eliminated by removing the solvent, l.e., replacing the liquid membrane with a polymeric membrane (PM). 

The versatility of ISEs was enhanced considerably by the introduction of membranes containing neutral ion carriers (ionophores). The first ISE of this type, with a membrane containing valinomycin and selective for potassium ions, was described by Stefanac and Simon in 1966. There are many liquid chemical systems that interact highly selectively with ions through, e.g., ion exchange, ion association, or solvent extraction. Practically useful ISEs based on these systems and on neutral ionophores have been obtained due to the gradual perfection of the technology of plasticized poly(vinyl chloride) (PVC) matrix membranes. 

Anion Type. Typically, discussion of liquid membranes focuses on the transport of cations with little mention of the anionic species involved. However, in order to maintain electroneutrality, many membrane carrier systems require that an anion be cotransported along with the cation. Because the anion must also enter and cross the organic phase, it is bound to influence transport efficiency. In fact, for transport by... 

Fig. 19. Schematic design of a flow injection analysis (FIA) system. A selection valve (top) allows a selection between sample stream and standard(s). The selected specimen is pumped through an injection loop. Repeatedly, the injection valve is switched for a short while so that the contents of the loop are transported by the carrier stream into the dispersion/reaction manifold. In this manifold, any type of chemical or physical reaction can be implemented (e.g. by addition of other chemicals, passing through an enzyme column, dilution by another injection, diffusion through a membrane, liquid-liquid extraction, etc. not shown). On its way through the manifold, the original plug undergoes axial dispersion which results in the typical shape of the finally detected signal peak...

There is another type of electrochemical flow microreactors that can be used for electrolyte-free electrolysis (Figure 9.5) [24]. In this system, two carbon fiber electrodes are separated by a spacer (porous PTFE membrane, pore size 3 pm, thickness 75 pm) at a distance of micrometer order. A substrate solution is introduced into the anodic chamber. The anodic solution flows through the spacer membrane into the cathodic chamber. The product solution leaves the system from the cathodic chamber. In this system, the electric current flow and the liquid flow are parallel. Using this electrochemical flow microreactor having a serial electrode configuration, the anodic methoxylation of p-methoxytoluene was accomplished effectively without intentionally added electrolyte. Protons generated by the anodic oxidation acted as carriers of the electricity. This process will be discussed in detail in the practical part of this chapter. The device could also be used for the anodic methoxylation of N-methoxycarbonyl pyrrolidine and acenaphthylene. 

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