Transport in ion-exchange membranes

Reverse osmosis can be used for the separation of ions om an aqueous solution. Neutral membranes are mainly used for such processes and the transport of ions is determined by their solubility and diffusivity in the membrane (as expressed by the solute permeability coefficient, see eq V 162). The driving force for ion transport is the concentration difference, but if charged membranes or ion-exchange membranes are used instead of neutral membranes ion transport is also affected by the presence of the fixed charge. Teoreil [45] and Meyer and Sievers [46] have used a fixed charge theory to describe ionic transport through these type of systems. This theory is based on two principles the Nemst-Planck equation and Dorman equilibrium. 

If an ion-exchange membrane in contact with an ionic solution is considered, then ions with the same charge as the fixed ions in the membrane ate excluded and caimotpass through the membrane. This effect is known as Donnan exclusion and can be described by equilibrium tbetnuxiynamics which allow the chemical potential of the ionic component in the two phases present to be Calculated when an ionic solution is in equilibrium with an ionic membrane. Thus, in the ionic solution itself  

Quantities with the subscript m refer to the membrane phase. At equilibrium the electrochemicai potentials in toth phases are equal, thus 

If the reference states for both phases are also assumed to be equal (n j. = j ), the 

This equation enables some simple calculations to be undertaken. For a given monovalent ionic solute at a concentration difference of 10, the equilibrium potential difference established at the interface is E on = [(8.314 298)/ (96500)] In (1/10) = - 59 mV. 

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LeBlanc OH, Ward WJ, Matson SL, and Kimura SG. Facilitated transport in ion-exchange membranes, J. Membr. Sci. 1980 6 339-343. 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

Way et al. ( ) applied this analytical model to predict facilitation factors for 00 facilitated transport In Ion exchange membranes. As shown In Figure 8, there was good agreement between experimental and predicted facilitation factors. 

Tanaka, Y. (2006) Irreversible thermodynamics and overall mass transport in ion-exchange membrane electrodialysis. Journal of Membrane Science 281, 517-531. 

Moya AA, Homo J. Application of the network simulation method to ionic transport in ion-exchange membranes including diffuse double-layer effects. J Phys Chem B 1999 103(49) 10791-9. 

Meares, P. (Ed.), Membrane Separation Processes, Elsevier, Amsterdam, 1976. Meares, P., J. F. Thain, and D. G. Dawson, Transport across ion-exchange membranes The frictional model of transport, in Membranes—A Series of Advances (Ed. G. Eisenman), Vol. 1, p. 55, M. Dekker, New York, 1972. Schlogl, R., see page 415. 

Graft copolymers of ethyleneimine on polyepichlorohydrine or polystyrene chelated with heavy metal ions like Hg2 , Cu2 and Cd2 + may have possibility in ion-exchange membranes for the transport of ions 31). 

The Nemst-Planck flux equation has been widely applied to explain transport phenomena in ion exchange membranes and solution systems. When ion i diffuses... 

R. Yamane, T. Sata, Y. Mizutani and Y. Onoue, Concentration polarization phenomena in ion-exchange membrane electrodialysis. II. The effect of the condition of the diffusion-boundary layer on the limiting current density and on the relative transport numbers of ions, Bull. Chem. Soc. Jpn., 1969, 42, 2741. 

Way ( ) applied the competitive transport model of Nllya and Noble ( ) to the prediction of facilitation factors for competitive transport of COj and HjS In Ion exchange membranes containing organic amine carriers. The results of the numerical simulations are shown in Table 2. The agreement Is very good for CO, and qualitative for H,S. 

Way and Noble (22) reported facilitated transport of HjS in ion exchange membranes containing organic diamine cations at ambient conditions. The lEMs were highly selective for H S over CH and had... 

Ethylene diamine HzN(CH2)2NH2, was chosen as the carrier to study the facilitated transport of HzS In Ion exchange membranes for several reasons. It can be singly protonated to produce a carrier which can then be exchanged Into an Ion exchange membrane to form the facilitated transport membrane. The mechanisms for the reactions of EDA with acid gases have been studied and some kinetic data exist as described below. 

P. Meares, Transport in ion exchange mem branes in Synthetic Membranes Science Engineering and Applications (Eds. P. M Bungay, H. K. Lonsdale, M. N. Pinho) NATO ASI Series, D. Reidel Publishing Dordrecht, Holland, 1986. 

E. Riande, Transport Phenomena in Ion-Exchange Membranes, In H. Jean (ed.). Physical Electrolytes, Academic Press, NewYork (1972), p. 401. 

Permselective membranes exclude specific molecules from transport due to their size or other properties. With dialysis membranes, the molecular size is aitical. An important application is the exclusion of protein molecules which may contaminate the electrode surface. Lipophilic membranes block the transport of polar molecules, and in ion-exchanger membranes, only ions with a defined charge sign are mobile. Best known are Nafion membranes (Du Pont), which act as cation exchangers in wet state. Anions cannot penetrate the membrane. 

Hie loss of selectivity as a result of membrane dehydration described above is similar to results reported previously for Ag(I)-facilitated transport of alkenes in perfluorosulfonic acid membranes.(13) For cation transport in ion-exchange materials, it is known that loss of membrane hydration results in a dramatic decrease in conductivity suggesting that the cations become much less mobile in dry membranes. Ftesumably, a similar phenomenon is responsible for the diminished benzene fluxes observed for dehydrated PVA-AgX membranes. Even though a Ag(I)-benzene complex can still form, hydration is necessary for the complex to be mobile and thereby provide effective facilitation of benzene. 

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