Electrodialysis transport mechanism

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

In a hydrodynamically free system the flow of solution may be induced by the boundary conditions, as for example when a solution is fed forcibly into an electrodialysis (ED) cell. This type of flow is known as forced convection. The flow may also result from the action of the volume force entering the right-hand side of (1.6a). This is the so-called natural convection, either gravitational, if it results from the component defined by (1.6c), or electroconvection, if it results from the action of the electric force defined by (1.6d). In most practical situations the dimensionless Peclet number Pe, defined by (1.11b), is large. Accordingly, we distinguish between the bulk of the fluid where the solute transport is entirely dominated by convection, and the boundary diffusion layer, where the transport is electro-diffusion-dominated. Sometimes, as a crude qualitative model, the diffusion layer is replaced by a motionless unstirred layer (the Nemst film) with electrodiffusion assumed to be the only transport mechanism in it. The thickness of the unstirred layer is evaluated as the Peclet number-dependent thickness of the diffusion boundary layer. 

FIG. 22-48 Transport mechanisms for separation membranes a) Viscous flow, used in UF and MF. No separation achieved in RO, NF, ED, GAS, or PV (Z ) Knudsen flow used in some gas membranes. Pore diameter < mean free path, (c) Ultramicroporous membrane—precise pore diameter used in gas separation, d) Solution-diffusion used in gas, RO, PV Molecule dissolves in the membrane and diffuses through. Not shown Electrodialysis membranes and metallic membranes for hydrogen. 

M.E. Green and M. Yafuso, A study of the noise generated during ion transport across membranes, J. Phys. Chem., 1968, 72, 4072-4078 I. Rubinstein. Mechanism for an electrodiffusional instability in concentration polarization, J. Chem. Soc., Faraday Trans. 2. 1981, 77, 1595-1609 F. Maletzki, H.-W. Rosier and E. Staude, Ion transport across electrodialysis membranes in the overlimiting current density range Stationary voltage current characteristics and current noise power spectra under different conditions of free convection, J. Membr. Sci., 1992, 71, 105-115. 

Very large areas of membrane are used in an electrodialysis cell and hence they should be as cheap as possible. Thus while the perfluoropolymers have excellent properties, they are too expensive and the membranes used are copolymers of styrene and divinylbenzene. The cation-selective materials are activated by sulphonation while the anion-selective membranes are substituted with quaternary ammonium centres. To give the membranes the required mechanical and dimensional stability, the copolymerization reaction is initiated around a reinforcing mesh or within a porous polyethylene or thermoplastic sheet the first method is generally preferred since it gives membranes of lowest resistance. The ratio of styrene to divinylbenzene determines the extent of crosslinking and hence, after activation, the amount of water absorbed into the polymer. The water creates channels across the membrane through which the ions can diffuse and the quantity of water controls the maximum size of ion which can be transported. 

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