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Ion Exchange Membranes
Preparation, Characterization, Modification and Application
By Toshikatsu Sata
The Royal Society of ChemistryCopyright © 2004 The Royal Society of Chemistry
All rights reserved.
The separation of substances by membranes is essential in industry and human life. Of the various separation membranes, the ion exchange membrane is one of the most advanced and is widely used in various industrial fields: electrodialysis, diffusion dialysis, separator and solid polymer electrolyte in electrolysis, separator and solid polymer electrolyte of various batteries, sensing materials, medical use, a part of analytical chemistry, etc.
The ion exchange membrane originated from two different sources: the finding of ion exchange phenomena in soil and in explaining biological phenomena in cell membranes. The ion exchange phenomenon was found by the English agriculturist, H.P. Thompson and the chemist J.T. Way in 1850, who noted the adsorption of ammonium sulfate on soil as follows,
Ca–Soil + (NH4)2SO4 = 2NH4–Soil + CaSO4
Their finding led to the synthesis of inorganic ion exchangers (aluminosilicate compounds)., Subsequently, the English chemists, B.A. Adams and E.L. Holmes synthesized organic cation and anion exchange resins by the condensation reaction of phenolic compounds having ionic groups and formaldehyde. D'Alelio developed polymerization type ion exchange resins and laid the basis of today's ion exchange resins.
Studies on ion permeable membranes, collodion type membranes, were carried out by L. Michaelis. He recognized the effect of charge in the membrane on ion permeation through the membrane. In 1939, K.H. Meyer, J.F. Sievers and T. Teorell proposed a theory of membrane potential. The modified collodion membrane was the first charged artificial membrane. In 1949, Sollner published a paper concerning bi-ionic potential, which is a measure of permselectivity between ions with the same charge through the membrane. They and many other researchers established the basis of studies on electrochemical properties of the ion exchange membrane.
M.R.J. Wyllie, W. Juda and M.R.C. McRae reported the synthesis of cation and anion exchange membranes around 1950. The former membrane is prepared by using a powdery ion exchange resin and binding polymer and the latter is composed of ionic condensation type polymers. After these works, studies on the ion exchange membranes, synthetic methods, theoretical explanation of membrane phenomena and trials for industrial applications became very active. Though the main purpose of the studies on ion exchange membranes in the USA and European countries was the electrodialytic desalination of saline water, in Japan the aim was to produce sodium chloride by electrodialytic concentration of seawater. An ion exchange membrane for seawater concentration has to meet severe requirements compared with desalination of saline water: high permselectivity at high concentration of the solution (seawater), high electrical conductivity and permselectivity for specific ions due to selective concentration of sodium chloride from seawater. In 1972, the production of edible salt in Japan was completely converted to the electrodialytic concentration of seawater and evaporation of the concentrated solution, instead of a salt field method.
It was proposed in 1950 that the ion exchange membrane could act as a separator in the electrolysis of sodium chloride solution to produce chlorine gas, hydrogen gas and caustic soda. However, there was no membrane for such a purpose. After the development of perfluorocarbon carboxylic acid membranes in around 1974 in Japan, electrolysis in Japan was totally converted from the mercury method to the process using the ion exchange membrane by 1986, to avoid mercury pollution. The process is the most energy-saving among other processes, the mercury method and diaphragm method, and produces highly pure caustic soda without pollution. The process is now replacing other processes worldwide. This technology is based on the development of perfluorocarbon ion exchange membranes, Nafion®, by E.I. du Pont de Nomours & Co. (Inc), which also diversifies new and practical applications of the ion exchange membrane.
The ion exchange membrane, a non-porous, membranous polymer having ionic groups, is a typical functional polymer. The characteristics of the membrane are: (1) ion conductivity, (2) hydrophilicity and (3) the existence of fixed carrier (ion exchange groups). According to these characteristics, various applications have arisen. Table 1.1 shows example applications.
The ion exchange membrane is ion conductive, namely, ions can permeate through the membrane together with water molecules. There are many driving forces to transport ions and solvents through the membrane. The electrochemical potential has been mostly used: electrodialysis, e.g., concentration and desalination of electrolyte solutions, separation between electrolyte and non-electrolyte, acid and base production using bipolar ion exchange membranes, electrodialytic ion exchange across the membrane, electro-deionization which is a desalination method based on a combination of ion exchange resins and the ion exchange membrane, electrodialytic double decomposition, etc. A typical application of the membrane to electrolysis is the chlor-alkali process. The ion exchange membrane is also used in electrochemical organic synthesis as a separator when anolyte and catholyte should not mix. Oxidizing agents, O3, H2O, etc. are synthesized using a composite of the membrane and electrodes.
Many separations based on differences of chemical potential have also been proposed and practiced. Diffusion dialysis simply uses differences in chemical potential as the driving force and the process is industrially used in the recovery of acid and alkali from waste. Donnan dialysis enables recovery of trace amounts of precious metals by proton transport through the cation exchange membrane as a driving force, to soften hard water in the presence of the concentration difference of sodium chloride solution, etc. Neutralization dialysis is based on the neutralization energy between acid and alkali across the membrane, which is large, being used to separate salt from non-electrolyte, etc. Many studies on up-hill transport have been made using the ion exchange membrane. Because the concentration of the electrolyte in the membrane phase is higher than that of the solution, the electrolyte solution with higher concentration permeates through the mosaic ion exchange membrane in the presence of hydraulic pressure, which enables the desalination of salt solutions (piezo-dialysis). Because a small amount of the concentrated solution permeates through the membrane, and a large amount of desalted solution remains in the upper stream, piezodialysis is, in principle, efficient in desalinating saline water compared with reverse osmosis. When a temperature difference exists across the ion exchange membrane, salt and solvent (water) permeate through the membrane according to the ionic species and the particular membrane (thermo-dialysis).
The ion exchange membrane has been examined as a component of batteries and is now used in batteries as a separator and solid polymer electrolyte. In alkali batteries it is used as a separator instead of a porous separator. Batteries having a large capacity have been actively studied for load leveling of electricity: the zinc-chlorine battery, zinc-bromine battery, redox flow battery such as Fe–Cr system, vanadium system, etc. These batteries, especially redox flow batteries, require the cation or anion exchange membrane to separate anolyte and catholyte. Conversely, when the ion exchange membrane separates a concentrated solution from the dilute one, a membrane potential arises, which results in a concentration cell. Recently, hydrogen–oxygen and methanol–oxygen fuel cells have been actively studied as new energy generation methods, which require a proton conductive membrane or a cation exchange membrane. It is important, however, that tubular-type cation exchange membranes are examined to prepare an actuator such as a catheter for medical use, in which the stimulus is an electrochemical potential.
Because ion exchange membranes have ionic groups, which are hydrated, these membranes are hydrophilic. Utilization of ion exchange membrane in a perva-poration process, which dehydrates water-miscible organic solvents by evaporation across the membrane (liquid phase/vapour phase), is an example application of these characteristics. Though membranes for pervaporation processes do not require ion exchange groups to exist in the membrane, the ion exchange membrane shows excellent performance in this process. These characteristics are also effective in the dehumidification of various gases. Also, because the membrane adsorbs water molecules according to humidity, the membrane also shows the properties of a humidity sensor (hygrometer).
The charged groups of the membrane act as a fixed carrier for various ionic materials and provide new applications of the membrane. An ion exchange membrane ion-exchanged with amines selectively permeates acidic gases, e.g., carbon dioxide, by carrier transport and a membrane with silver ions selectively permeates olefin gases compared with paraffin gases. The membrane can also be used to prepare modified electrodes and to immobilize enzymes due to the ion exchange groups (fixed carrier). Membrane applications are spreading in analytical chemistry. The characteristics of ion conductivity, hydrophilicity and a fixed carrier provide various applications for sensing materials.
Finally, the author is thankful for studies on ion exchange membranes because he has learned physical chemistry, organic synthesis, polymer chemistry, polymer modification, electrochemisty, etc. while preparing the ion exchange membrane, evaluating its properties exactly and seeking its application, because studies on ion exchange membranes are typically interdisciplinary.CHAPTER 2
Theory of Membrane Phenomena in Ion Exchange Membranes
Because ion exchange groups, anionic or/and cationic charged groups, exist in non-porous membranes, various interesting phenomena are observed in the ion exchange membrane: permselectivity of cations or anions through the membrane, generation of membrane potential and bi-ionic potential, ion conductivity, diffusion of electrolyte and non-electrolyte through the membrane, water transport such as osmotic water and electro-osmotic water, salt rejection in the presence of hydraulic pressure, etc. Studies on these membrane phenomena mainly originate from attempts to explain phenomena in biological membranes.
There are three basic concepts that explain membrane phenomena: the Nernst–Planck flux equation, the theory of absolute reaction rate processes, and the principle of irreversible thermodynamics. Explanations based on the theory of absolute reaction rate processes provide similar equations to those of the Nernst–Planck flux equation. The Nernst–Planck flux equation is based on the hypothesis that cations and anions independently migrate in the solution and membrane matrix. However, interaction among different ions and solvent is considered in irreversible thermodynamics. Consequently, an explanation of membrane phenomena based on irreversible thermodynamics is thought to be more reasonable. Non-equilibrium thermodynamics in membrane systems is covered in excellent books and reviews, to which the reader is referred. The present book aims to explain not theory but practical aspects, such as preparation, modification and application, of ion exchange membranes. In this chapter, a theoretical explanation of only the basic properties of ion exchange membranes is given.
2.2 Flux Equation
The Nernst–Planck flux equation has been widely applied to explain transport phenomena in ion exchange membranes and solution systems. When ion i diffuses through the ion exchange membrane (x axis is perpendicular to membrane surface), the flux, Ji (mol cm-2 s-1), is expressed by a product of the gradient of the chemical potential, –(dµi/dx), and the concentration of i and a constant,
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.1)
where Di is the diffusion coefficient of i, R the gas constant, T the absolute temperature, ai activity of i (ai = Ciγi: Ci, the concentration of i;γi, activity coefficient of i).
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.2)
When there is an electrical potential gradient, including diffusion potential, the flux of i, Ji(e), is proportional to the gradient of the electrical potential, (dΨ/dx), the concentration, Ci, and valence, zi, of ion i and its electrochemical mobility ui,
Ji(e) = -uiziCi dΨ/dx (2.3)
The Nernst–Einstein equation is
ui = Di F/RT (2.4)
where F is the Faraday constant, thus,
Ji(e) = –Di F/RT ziCi dΨ/dx (2.5)
Therefore, the total flux of ion i is
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.6)
This is known as the Nernst–Planck flux equation and is applicable to ideal systems.
Ion exchange membranes have a large number of hydrated counter-ions in the membrane phase. The counter-ions impart more momentum to the solvent than co-ions do and solvent transfer takes place to the respective electrode chamber (due to osmosis and electro-osmosis). If the membrane has an anionic charge (cation exchange membrane), solvent transfer takes place from anolyte to catholyte. The velocity of this solvent transfer is proportional to the strength of the electrical potential gradient and is inversely proportional to the resistance to flow of the membrane matrix. If the flow velocity of the solvent is v and [bar.C]i is the concentration of i (counter-ion) in the membrane, the flux of liquid in the membrane (convection), Ji(conv), is expressed as follows,
Ji(conv) = [bar.C]ivi (2.7)
Thus, the total flux of i ion through the membrane is expressed as
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.8)
This equation can apply to all ions under the condition of electro-neutrality.
Based on electro-neutrality, we deduce the following equation for the ion exchange membrane, where X is the fixed ion concentration of the membrane, and ω is the sign of the fixed charge (-1 for negatively charged membranes, +1 for positively charged membranes),
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.9)
At steady state, the concentrations are independent of time, and therefore,
dJi/dx = 0
As the ionic species move in the presence of an electric potential gradient they give electric current I, which is related to flux, Ji,
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2:10)
The specific conductivity, [bar.κ], and transport number of i in the membrane, [bar.t]i, may be written as
[bar.κ] = -[I/(dΨ/dx)] (2.11)
[bar.t]i = FJi/I (2.12)
2.3 Permselectivity of Ions Through the Ion Exchange Membrane
Although cations can ion exchange into a cation exchange membrane as counter-ions, anions basically cannot enter into the cation exchange membrane. Similarly, cations are also rejected by the cationic charge (anion exchange groups) of the anion exchange membrane. This selective uptake of counter-ions and rejection of co-ions by the charged membrane is explained a Donnan membrane equilibrium.,
When there is a distribution of electrical potential, Ψ, in the system, the electrochemical potential of cation A is expressed by ηA, that of anion X as ηx, and that of solvent (water) by ηw
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.13)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.14)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.15)
where µ0A, µ0X and µ0W are the chemical potential of A, X and water under standard conditions (1 atm, in pure water), respectively, R is the gas constant, T the absolute temperature, αA, αXand αW the activity of the respective components, P, pressure, P0, pressure under standard conditions, VA, VX and VW, partial molar volume of cation A, anion X and water, zA and zx are the valence of cation A and anion X, and F the Faraday constant. Eqs. (2.13), (2.14) and (2.15) are basic equations for exchange equilibrium between the membrane phase and solution phase.
Excerpted from Ion Exchange Membranes by Toshikatsu Sata. Copyright © 2004 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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