Ion exchange refers to the interaction of ionic species in aqueous solutions with adsorbent solid materials. It is distinguished from conventional Adsorption by the nature and morphology of the adsorbent material which in most cases is either a dynamic polymer matrix or an inorganic structure containing exchangeable functional groups. All modern ion exchange resins are polymeric structures, generally based either on styrene or an acrylic matrix. Polystyrene sulphonic acid cation resins are cross-linked copolymers of styrene (vinylbenzene), with divinylbenzene (DVB-containing pendant sulphonic acid groups capable of exchanging cations in solution over the entire pH range. Acrylic cation resins are made by copolymerizing acrylic or methacrylic acid with DVB. These resins have a high capacity and will exchange cations in the alkaline pH range 7−14. Polystyrene onion exchange resins are cross-linked copolymers of polystyrene-DVB, with pendant functional groups comprising primary, secondary and tertiary amines. Strong base resins operate over the entire pH range, whereas weak base resins exchange anions in the pH range 0−7. Polystyrene copolymers possess a gel type structure, swell and contract in aqueous solutions and are physically robust in most process operations. Macroporous or macroreticular resins have been developed which have a porous, sintered structure possessing free water channels within the matrix of about 1,000 Ådiameter. Large molecules can travel freely within these pores and therefore, these materials are less prone to organic fouling. Inorganic materials are either naturally-occurring or synthetic materials. Typical examples are natural and synthetic zeolites; insoluble salts, e.g., hydrous zirconium phosphate; heteropolyacids, e.g., ammonium phosphomolybdate; natural clays, e.g., montmorillonite.
The important properties of ion exchange materials are given below:
In order to preserve electroneutrality, ion exchange is stoichiometric and the capacity is independent of the nature of the counterion;
Ion exchange is nearly always a reversible process;
Ion exchange is a rate-controlled process, usually governed by diffusion in the solid phase or in the surrounding liquid film.
Simple binary cation exchange equilibrium can be expressed by the law of mass action. Assume that the ion exchanger is initially in the B form and the solution contains ions A.
Overbars denote the ionic species in the resin phase. The selectivity coefficient, (Kc) , can be defined in terms of the concentrations in the solution and resin phase.
It is usual to express the selectivity coefficient in terms of equivalent or mole fractions of the ionic species.
A typical equilibrium plot of univalent-univalent exchange, i.e., zA = zB = 1 is shown in Figure 1. In the industrially-important case of divalent-univalent exchange, i.e., zA = 2, zB = 1, the selectivity coefficient becomes:
Figure 1. Equilibrium isotherm for uni-univalent exchange. From Rousseau R.W. (1987) Handbook of Separation Process Technology, John Wiley & Sons Inc, with permission.
Figure 2. Equilibrium isotherm for di-univalent exchange. From Rousseau R.W. (1987) Handbook of Separation Process Technology, John Wiley & Sons Inc, with permission.
The equilibrium curve is given in Figure 2. In this case, selectivity is strongly dependent on the total ionic concentration of the solution phase.
Ion exchange is a rate-controlled process and the kinetics depends on the following five steps:
Diffusion of the counterions through the bulk solution to the surface of the ion exchanger;
Diffusion of the counterions within the solid phase;
Chemical reaction between the counterions and the ion exchanger;
Diffusion of the displaced ions out of the ion exchanger;
Diffusion of the displaced ions from the exchanger surface into the bulk solution.
The kinetics of ion exchange are governed by either a diffusion or mass action mechanism, depending on which is the slowest step. Diffusion of ions in the external solution is termed liquid film control, but is hydrodynamically ill-defined. The diffusion and transport of ions within the ion exchanger is termed particle diffusion control. Chemical reaction (step 3) is uncommon but can be rate-controlling in certain specialized cases.
Homogeneous diffusional mass transfer processes can be described by Fick's Law:
Ji is the flux if the diffusing species i of concentration Ci and D is the diffusion coefficient. Fundamental treatment of the laws of ion exchange is given by Helfferich (1962). The concentration profiles within a spherical ion exchange bead are depicted in Figure 3.
Fractional conversion or attainment of equilibrium can be calculated under infinite solution volume conditions by the solution of Pick's Law for spherical geometry:
The mathematical treatment is quite different if the exchanger is treated as a solid phase and the mechanism is assumed to be a heterogeneous chemical reaction. The conceptual models to describe this case study are similar to those developed for noncatalytic fluid-solid reactions. Shell progressive, shrinking core or ash-layer models have been applied to ion exchange, especially if there is a complexing reaction. Figure 4 gives a schematic representation of a partially-reacted ion exchange bead.
Figure 3. Radial concentration profiles at different times for ideal particle diffusion and film diffusion control. The right hand sides of the diagrams show the profiles of species A (initially in the resin) and the left sides, those of species B (initially in solution). From Helfferich, F. (1962) Ion Exchange, McGraw Hill Co., with permission.
Figure 4. Schematic diagram of a partially-reacted resin bead. From Rousseau, R.W. (1987) Handbook of Separation Process Technology, John Wiley & Sons Inc., with permission.
The following relationships have been obtained for each kinetic mechanism:
Liquid film diffusion control
Ash-layer diffusion control
Chemical reaction control
Conventional ion exchange reactions can often be explained by homogeneous diffusional kinetics. For example, simple ion exchange reactions encountered in water treatment, such as Na+-H+ or C1−-OH− with polyelectrolyte gels and macroreticular resins, can be fitted by diffusional theory. However, more complex ion exchange reactions involving complex ions or chelating ion exchange materials are not satisfied by diffusional relationships. For example, the sorption and desorption of copper by an iminodiacetic acid exchanger is better fitted by chemical reaction rate models. Also, sorption into inorganic ion exchangers is better explained by chemical reaction rate models. Slater (1991) has brought together most of the available ion exchange theory and discussed the interpretation and prediction of mass transfer data.
Industrial applications of ion exchange are extremely widespread and range from purification of low-cost commodities, such as water, to the purification and treatment of high-cost pharmaceutical products as well as precious metals such as gold and platinum, Dorfner (1991) has reviewed the industrial applications of ion exchange and comprehensive references are to be found in his textbook.
The largest single application, measured in terms of ion exchange resin usage, is water treatment, i.e., softening, demineralization for high pressure boilers and dealkalization. (See also Water Preparation.) Enormous advances in ion exchange technology have occurred because of the relentless requirement for pure and ultrapure water. Other major industrial applications are the processing and decolorization of sugar solutions and the recovery of uranium from relatively low-grade mineral ore leach solutions. Ion exchange is also used in the fields of medicine, pharmaceuticals, chemicals processing, catalysis and laboratory analysis.
The removal of salts and other ionic impurities from water is based primarily on the exchange of cations (Na+, K+, Ca2+, Mg2+, etc.) with the hydrogen form of a cation exchanger and the exchange of anions (C−, HCO , CO , SO, etc.) with the hydroxide form of an anion exchanger. Usually, softening of water is achieved with cation exchangers and demineralization, with a mixed bed of cation/anion exchange resin. Natural groundwaters contain appreciable amounts of long-chain aliphatic acids, e.g., humic and fulvic acid and these tend to foul conventional anion exchange resins. Macroreticular polymeric anion exchange resins overcome this problem and offer relatively easy regeneration. Ultrahigh purity water for the nuclear power, semiconductor and pharmaceutical industries is produced using mixed bed ion exchange processes. Ion exchange is now widely used in effluent treatment and pollution control. Bolto and Pawlowski (1987) have reviewed these procedures in detail. The process strategy depends entirely on the waste to be treated, concentration of pollutants and flow rates. The treatment of mine drainage water, removal of ammonia, nitrates and pesticides from groundwater and the treatment of nuclear waste solutions are examples of typical applications.
The principal applications of ion exchange in the purification and treatment of sugar solutions, juices and syrups are:
Softening and demineralization of sugar juices to remove scale-forming elements prior to evaporation;
Decolorization with anion exchange resins;
Catalytic inversion of sucrose to fructose and glucose;
Ion exchange is predominantly used in the sugar beet industry since the syrup contains significant amounts of calcium and magnesium, and these can be exchanged with conventional cation exchangers in sodium form. Deionization of sugar syrups using both cation and anion exchange resins also reduces molasses and thus, sugar yield is increased. The inversion of sucrose can be catalyzed by cation exchange resins in the hydrogen form.
Some important uses of ion exchange are: processing of pharmaceuticals, use in artificial organs and analytical applications in medicine. Antibiotics, such as streptomycin, can be separated from fermentation broths with cation exchange resins under neutral pH conditions. Weak acid carboxylic ion exchange resins are usually employed in sodium form and regenerated with a mineral acid. Similarly, vitamin B12 can be recovered from microbial fermentation broths using weak acid ion exchangers. In medicine, ion exchange resins have also found use as preparative media and for various clinical applications.
Ion exchange has found widespread use in the recovery of metals from mineral leach solutions and from secondary waste solutions. The most important application is the recovery of uranium from low-level ore bodies in the mineral deposits of Australia, Canada, South Africa and the USA. Ion exchange has also been used for the recovery of thorium, rare earth elements, transition metals, transuranic elements, precious metals such as gold, silver and platinum, and base metals such as chromium.
Uranium is recovered from mineral leach solutions as an anionic sulfate or carbonate complex, depending on the nature of the ore body. Uranyl sulfate anions are sorbed preferentially at pH values in the range 0.5−1.5 on conventional strong base anion exchange resins and can be eluted using sulfuric acid (usually heated to improve regeneration). The uranium uptake is extremely favorable and for this reason, high enrichment of uranium is achieved in the eluant. Uranium is normally precipitated from the eluant with alkali. The process was traditionally carried out in a cascade of fixed bed columns operating on a "merry-go-round" principle, but in recent years sophisticated continuous countercurrent ion exchange contactors have been developed and installed on mines in South Africa.
Gold can be recovered from low-concentration side-streams with activated carbon. Countercurrent adsorption has also been applied to this process, although the regeneration step is more difficult and requires high temperature elation and thermal reactivation of the carbon. Recently, much work has been devoted to the development of novel ion exchange resins capable of recovering gold from cyanide liquors and amenable to chemical regeneration using conventional reagents at ambient temperatures.
A typical mixed-bed ion exchange column is shown in Figure 5. The inlet and bottom collector systems are major design features. Most processes use two, three or four columns operated in sequence to maintain a continuous flow of treated product solution. There have been great advances in the development of continuous countercurrent ion exchange columns based on multistage fluidized bed contactors. These have been used on several uranium recovery plants in South Africa and a schematic installation is shown in Figure 6. Column diameters of up to 5 m are commonplace. The advantages of continuous countercurrent ion exchange are realized for processes having very favorable equilibrium or treating high concentration solutions for the recovery of high value solutes, e.g., uranium and precious metals such as gold and platinum.
Figure 5. Schematic arrangement of a typical mixed bed ion exchange column. From Arden, T.V. (1968) Water Purification by Ion Exchange, Butterworth Heinemann, with permission.
a stoichiometric coefficient
c concentration in solution phase
concentration in the particle phase
cA0 concentration in bulk solution phase (Figure 4)
cAs concentration at liquid/particle interface (Figure 4)
D diffusion coefficient in solution phase
diffusion coefficient in particle phase
De effective diffusion coefficient
J mass flux of ions from solution to particle
(Kc) selectivity coefficient
kmA mass transfer coefficient of species A in the liquid film
ks rate constant based on surface area
r0 particle radius
X extent of resin particle conversion
x equivalent or mole fraction of ionic species in solution phase
y equivalent or mole fraction of ionic species in particle phase
z valency of ionic species
A ionic species A
B ionic species B
i ionic species i
Bolto, B. A. and Pawlowski, L. (1987) Wastewater Treatment by Ion Exchange. E and F N Spon. London.
Dorfner. K. (1991) Ion Exchangers. Walter de Grauter. Berlin and New York.
Helfferich, F. (1962) Ion Exchange. McGraw Hill. New York.
Slater, M. I. (1991) The Principles of Ion Exchange Technology. Butterworth-Heinemann. Oxford.