Liquid membranes transport mechanisms

A.M. Neplenbroek, D. Bargeman and C.A. Smolder, Nitrate removal using supported liquid membranes transport mechanism, J. Membr. Sci., 1992, 67, 107. 

Figure 7. Various liquid membrane transport mechanisms.

Figure 51 (a) Apparatus for liquid membrane transport under photoirradiation conditions, (b) Plausible mechanisms for Li" ion... 

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. 

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. 

Figure 2. Cells for a) Bulk Liquid Membrane and b) Emulsion Liquid Membrane Transport. (1) pH electrode, (2) mechanical stirrer, (3) organic (membrane) phase, (4) aqueous source phase, (5) aqueous receiving phase, (6) magnetic stirrers.

Figure 5.4.4. Various liquid membrane permeation mechanisms. (After Marr and Kopp (1982).) (a) Simple permeation of sp des A (b) simple permeation enhanced by reaction of A with an agent E in permeate (c) facilitated transport with a reversible complexing agent B in the membrane (d) facilitated transport in the presence of a reactive agent E in permeate (e) countertransport (f) cotransport.

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

Fig. 15.2 Basic mechanisms of liquid membrane extraction (a) type I facilitated transport (A + B AB) (b) type II facilitated transport (A + B B + C).

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. 

Fortunato, R. et al.. Supported liquid membranes using ionic liquids study of stability and transport mechanism, /. Membr. Sci., 242,197, 2004. 

Danesi, P.R. Horwitz, E.P. Rickert, P.G. Rate and mechanism of facilitated americium(III) transport through a supported liquid membrane containing a bifunctional organophosphorous mobile carrier, J. Phys. Chem. 87 (1983) 4708-4715. 

The multi-faceted functionality of a GDL includes reactant distribution, liquid water transport, electron transport, heat conduction and mechanical support to the membrane-electrode-assembly. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

Coupled transport was the first carrier facilitated process developed, originating in early biological experiments involving natural carriers contained in cell walls. As early as 1890, Pfeffer postulated that the transport in these membranes involved carriers. Perhaps the first coupled transport experiment was performed by Osterhout, who studied the transport of ammonia across algae cell walls [1], A biological explanation of the coupled transport mechanism in liquid membranes is shown in Figure 11.2 [2],... 

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... 

The discussion in Section 4.4.1.3 on transport mechanisms in SLM has manifestly demonstrated another facet of tuning analyte-selective extraction. For example, Figure 4.5 clearly demonstrates the selective extraction of a basic compound—all that is required here is a simple adjustment of the pH on either side of the membrane. Also, Figure 4.6 neatly illustrates the possibility of performing such selective extraction of anionic and cationic species in another transport mechanism that employs selective carriers. Thus, by fine-tuning the chemistry/composition of the sample, membrane liquid, and acceptor phases, analyte-selective extraction can be tailor-made. 

Paugam, M.F. and Buffle, J. (1998) Comparison of carrier-facilitated copper (II) ion transport mechanisms in a supported liquid membrane and in a plasticized cellulose triacetate membrane. Journal of Membrane Science, 147, 207. 

FIGURE 37 Mechanisms of carrier-facilitated immobilized liquid membrane extraction, also referred to as coupled transport. The species, R, refers to the carrier component responsible for complexation. 

In the fourth subtechnique, flow FFF (F/FFF), an external field, as such, is not used. Its place is taken by a slow transverse flow of the carrier liquid. In the usual case carrier permeates into the channel through the top wall (a layer of porous frit), moves slowly across the thin channel space, and seeps out of a membrane-frit bilayer constituting the bottom (accumulation) wall. This slow transverse flow is superimposed on the much faster down-channel flow. We emphasized in Section 7.4 that flow provides a transport mechanism much like that of an external field hence the substitution of transverse flow for a transverse (perpendicular) field is feasible. However this transverse flow—crossflow as we call it—is not by itself selective (see Section 7.4) different particle types are all transported toward the accumulation wall at the same rate. Nonetheless the thickness of the steady-state layer of particles formed at the accumulation wall is variable, determined by a combination of the crossflow transport which forms the layer and by diffusion which breaks it down. Since diffusion coefficients vary from species to species, exponential distributions of different thicknesses are formed, leading to normal FFF separation. 

In renewable-based energy systems PEM electrolysis seems to have an advantage over alkaline in that the thin membrane and ion transport mechanism can react to nearly instantaneously with the rapidly changing energy output of renewable sources, especially wind. Stacks involving the circulation of a liquid electrolyte have inherently more inertia in the transport of ions in solution than the PEM systems. 

Capillary condensation provides the possibility of blocking pores of a certain size with the liquid condensate simply by adjusting the vapor pressure. A permporometry lest usually begins at a relative pressure of 1, thus all pores filled and no unhindered gas transport. As the pressure is reduced, pores with a size corresponding to the vapor pressure applied become emptied and available for gas transport. The gas flow through the open mesopores is dominated by Knudsen diffusion as will be discussed in Section 4.3.2 under Transport Mechanisms of Porous Membranes. The flow rate of the noncondensable gas is measured as a function of the relative pressure of the vapor. Thus it is possible to express the membrane permeability as a function of the pore radius and construct the size distribution of the active pores. Although the adsorption procedure can be used instead of the above desorption procedure, the equilibrium of the adsorption process is not as easy to attain and therefore is not preferred. 

The mechanisms by which various components in a liquid or gaseous feed stream to the membrane system are transported through the membrane structure determine the sq>aiation properties of the membrane. These transport mechanisms are quite different in liquid and in gas or vapor phases. So are their effects on permeate flux (or permeability) and retention (or rejection) coefficient or separation factor in the case of gas separation. 

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