Chapter 6 Ion-exchange and electrodialysis ALISTAIR S. GRANDISON, Department of Food Science and Technology, The University of Reading, Reading RG6 6AP, UK Ion-exchange and electrodialysis are distinct methods of separation, but can conveniently be treated together, as the basic criterion for separation in both cases is the molecular electrostatic charge. While ion-exchange involves retention of ionised solutes on a solid support material, electrodialysis permits the separation of ions using selective ion exchange membranes 6.1 ION-EXCHANGE Ion-exchange methods can potentially be used for separations of many types of molecules such as metal ions, proteins, amino acids or sugars. The technology is utilised in many sensitive analytical chromatography procedures, frequently on a very small scale. On the other hand industrial-scale production operations, such as demineralisation or protein recovery, are possible. This chapter will consider only the larger-scale applications which have current or potential use for production in the food and biotechnology industries 6.1.1 Theory, materials and equipment a brief summary of the theory of ion-exchange will be given here. More detailed ac- counts can be found elsewhere(e. g. Vermeulen et al., 1984: Walton, 1983; Helfferich, Solute/ion-exchanger interactions Ion-exchange could be defined as the selective removal of a single, or group of, charged pecies from one liquid phase followed by transfer to a second liquid phase by means of a solid ion-exchange material. In practice this involves the process of adsorption - the transfer of specific solute(s) from a heterogeneous feed solution on to the solid ion exchanger. The mechanism of adsorption is electrostatic, involving opposite charges on the solute(s) and the ion- exchanger. The feed solution is washed off, and this is followed
Chapter 6 Ion-exchange and electrodialysis ALISTAIR S. GRANDISON, Department of Food Science and Technology, The University of Reading, Reading RG6 6AP, UK Ion-exchange and electrodialysis are distinct methods of separation, but can conveniently be treated together, as the basic criterion for separation in both cases is the molecular electrostatic charge. While ion-exchange involves retention of ionised solutes on a solid support material, electrodialysis permits the separation of ions using selective ionexchange membranes. 6.1 ION-EXCHANGE Ion-exchange methods can potentially be used for separations of many types of molecules such as metal ions, proteins, amino acids or sugars. The technology is utilised in many sensitive analytical chromatography procedures, frequently on a very small scale. On the other hand industrial-scale production operations, such as demineralisation or protein recovery, are possible. This chapter will consider only the larger-scale applications which have current or potential use for production in the food and biotechnology industries. 6.1.1 Theory, materials and equipment A brief summary of the theory of ion-exchange will be given here. More detailed accounts can be found elsewhere (e.g. Vermeulen et al., 1984; Walton, 1983; Helfferich, 1962). Solutelion-exchanger interactions Ion-exchange could be defined as the selective removal of a single, or group of, charged species from one liquid phase followed by transfer to a second liquid phase by means of a solid ion-exchange material. In practice this involves the process of adsorption - the transfer of specific solute(s) from a heterogeneous feed solution on to the solid ionexchanger. The mechanism of adsorption is electrostatic, involving opposite charges on the solute(s) and the ion-exchanger. The feed solution is washed off, and this is followed
156 A.S. Grandison by desorption, in which the separated species are recovered back into solution in a much purified form The ion-exchange solids bear fixed ions which are covalently attached to a solid matrix. There are two basic types of ion-exchanger (1) Cation exchangers(sometimes called 'anionic exchangers)which bear fixed nega tive charges and are therefore able to retain cations, and (2) Anion exchangers(sometimes called 'cationic exchangers) which bear fixed posi tive charges Ion-exchangers can be used to retain simple ionised species, but may also be used in the separation of polyelectrolytes which possess both positive and negative charges (i.e amphoteric molecules such as proteins) as long as the overall charge on the polyelectrolyte is opposite to the fixed charges on the ion-exchanger. This overall charge depends on the isoelectric point of the polyelectrolyte and the ph of the solution. At pH values lower than the isoelectric point the net overall charge will be positive and vice versa. In some circumstances it is even possible for ion-exchangers to retain macro- nolecules of like charge, presumably if a portion of the molecule carries a sufficient opposite charge(Peterson, 1970). The main interaction is via electrostatic forces, and in the case of polyelectrolytes the affinity is governed by the number of electrostatic bonds etween the solute molecule and the ion-exchanger. However, particularly with large molecules such as proteins, multiple interactions may occur involving steric effects. Size and geometric properties, and the degree of hydration of the ions may affect these interactions, and hence the selectivity of the ion-exchanger for different ions. Charge density may be more important than overall charge in determining the relative selectivity Figure 6. 1 is a schematic diagram showing a generalised anion exchanger -1e bearing fixed positive charges. To maintain electrical neutrality these fixed ions must be balanced by an equal number of mobile ions of the opposite charge (i.e. anions) which re held by electrostatic forces. These mobile ions can move in and out of the porous molecular framework of the solid matrix and may be exchanged stoichiometrically with other dissolved of the same charge, and are termed counterions. Ion-exchange systems can be considered to consist of two aqueous liquid phases-one confined within Matrix Fig. 6. 1 Schematic diagram of a generalised anion exchang
156 A. S. Grandison by desorption, in which the separated species are recovered back into solution in a much purified form. The ion-exchange solids bear fixed ions which are covalently attached to a solid matrix. There are two basic types of ion-exchanger: (1) (2) Cation exchangers (sometimes called ‘anionic exchangers’) which bear fixed negative charges and are therefore able to retain cations, and Anion exchangers (sometimes called ‘cationic exchangers’) which bear fixed positive charges. Ion-exchangers can be used to retain simple ionised species, but may also be used in the separation of polyelectrolytes which possess both positive and negative charges (i.e. amphoteric molecules such as proteins) as long as the overall charge on the polyelectrolyte is opposite to the fixed charges on the ion-exchanger. This overall charge depends on the isoelectric point of the polyelectrolyte and the pH of the solution. At pH values lower than the isoelectric point the net overall charge will be positive and vice versa. In some circumstances it is even possible for ion-exchangers to retain macromolecules of like charge, presumably if a portion of the molecule carries a sufficient opposite charge (Peterson, 1970). The main interaction is via electrostatic forces, and in the case of polyelectrolytes the affinity is governed by the number of electrostatic bonds between the solute molecule and the ion-exchanger. However, particularly with large molecules such as proteins, multiple interactions may occur involving steric effects. Size and geometric properties, and the degree of hydration of the ions may affect these interactions, and hence the selectivity of the ion-exchanger for different ions. Charge density may be more important than overall charge in determining the relative selectivity. Figure 6.1 is a schematic diagram showing a generalised anion exchanger - i.e. bearing fixed positive charges. To maintain electrical neutrality these fixed ions must be balanced by an equal number of mobile ions of the opposite charge (Le. anions) which are held by electrostatic forces. These mobile ions can move in and out of the porous molecular framework of the solid matrix and may be exchanged stoichiometrically with other dissolved ions of the same charge, and are termed counterions. Ion-exchange systems can be considered to consist of two aqueous liquid phases - one confined within Counter-ions Imbibed solvent Fig. 6.1. Schematic diagram of a generalised anion exchanger
Ion-exchange and electrodialysis 157 the structure of the solid matrix in equilibrium with an outside phase. The interface between the two phases acts as a semipermeable membrane which allows the passage of any mobile ionic species depending on the Donnan equilibrium. This states that the chemical potential of a salt must be the same inside and outside the ion-exchanger-eg in the simplest case where the only mobile ions present are Na and CI, then at equilibrium, [Na'cI ]Inside phase=[Na][Cl outside phase Thus a certain proportion of co-ions(mobile ions having the same sign- Nat in this example-as the fixed ions) will be present even in the internal phase. Therefore, if an anion exchanger(as in Fig. 6. 1)is in equilibrium with a solution of NaCl, the internal phase contains some Na ions, although the concentration is less than in the external ase because the internal concentration of Cl ions is much larger When an ion-exchanger is contacted with an ionised solution, equilibration betweer the two phases rapidly occurs. Water moves into or out of the internal phase so that equivalent basis. If two or more species of counterion are present in the solution aar? osmotic balance is achieved Counterions also move in and out between the phases on an ill be distributed between the phases according to the proportions of the different present and the relative selectivity of the ion-exchanger for the different ions differential distribution of different counterions which forms the basis of separation by ion-exchange. The relative selectivity for different ionised species results from a range of factors. The overall charge on the ion and the molecular or ionic mass are the primary determining factors, but selectivity is also related to degree of hydration, steric effects and environmental factors such as pH or salt content In the adsorption stage, a negatively charged solute molecule(e.g. a protein P)is attracted to a charged site on the ion-exchanger(r)displacing a counterion(x) R+X-+P→R+P+X In the desorption stage, the anion is displaced from the ion-exchanger by a competing salt ion(S), and hence is eluted RtP+S-→Rts-+P lon-exchangers may be further classified in terms of how their charges vary, with in pH, into weak and strong exchangers. The terms strong or weak do not refer strength of binding of the ions to the exchanger, or the mechanical strength of the matrix but to the ph range over which the materials are effective. Strong ion-exchangers are nised over a wide range, and have a constant capacity within the range, whereas weak exchangers are only ionised over a limited ph range(e. g. weak cation exchangers may lose their charge below pH 6 and weak anion exchangers above pH 9). Thus exchangers may be preferable to strong ones in some situations, for example desorption may be achieved by a relatively small change in pH of the buffer in the region of the pka of the exchange group. Regeneration of weak ion-exchange groups is easier than with strong groups, and therefore has a lower requirement of costly chemicals
Ion-exchange and electrodialysis 157 the structure of the solid matrix in equilibrium with an outside phase. The interface between the two phases acts as a semipermeable membrane which allows the passage of any mobile ionic species depending on the Donnan equilibrium. This states that the chemical potential of a salt must be the same inside and outside the ion-exchanger - e.g. in the simplest case where the only mobile ions present are Na' and C1-, then at equilibrium, [Na'] [Cl-lInside phase = iNa'l [C1-lOutside phase Thus a certain proportion of co-ions (mobile ions having the same sign - Na' in this example - as the fixed ions) will be present even in the internal phase. Therefore, if an anion exchanger (as in Fig. 6.1) is in equilibrium with a solution of NaC1, the internal phase contains some Na' ions, although the concentration is less than in the external phase because the internal concentration of C1- ions is much larger. When an ion-exchanger is contacted with an ionised solution, equilibration between the two phases rapidly occurs. Water moves into or out of the internal phase so that osmotic balance is achieved. Counterions also move in and out between the phases on an equivalent basis. If two or more species of counterion are present in the solution, they will be distributed between the phases according to the proportions of the different ions present and the relative selectivity of the ion-exchanger for the different ions. It is this differential distribution of different counterions which forms the basis of separation by ion-exchange. The relative selectivity for different ionised species results from a range of factors. The overall charge on the ion and the molecular or ionic mass are the primary determining factors, but selectivity is also related to degree of hydration, steric effects and environmental factors such as pH or salt content. In the adsorption stage, a negatively charged solute molecule (e.g. a protein P-) is attracted to a charged site on the ion-exchanger (R') displacing a counterion (X-): R+X- + P- -+ R'P- + XIn the desorption stage, the anion is displaced from the ion-exchanger by a competing salt ion (S), and hence is eluted: R'P- + S- -+ R'S- + PIon-exchangers may be further classified in terms of how their charges vary, with changes in pH, into weak and strong exchangers. The terms strong or weak do not refer to the strength of binding of the ions to the exchanger, or the mechanical strength of the matrix, but to the pH range over which the materials are effective. Strong ion-exchangers are ionised over a wide range, and have a constant capacity within the range, whereas weak exchangers are only ionised over a limited pH range (e.g. weak cation exchangers may lose their charge below pH 6 and weak anion exchangers above pH 9). Thus weak exchangers may be preferable to strong ones in some situations, for example where desorption may be achieved by a relatively small change in pH of the buffer in the region of the pKa of the exchange group. Regeneration of weak ion-exchange groups is easier than with strong groups, and therefore has a lower requirement of costly chemicals
158 A.S. Gr lon-exchange group Some common examples of cation exchangers are 3(Ht)2(medium -pKa 2-3) Base function is almost invariably present as amines or imines. These are introduced into the matries by chloromethylation, followed by reaction with the appropriate amine produce weakly to strongly basic ion-exchangers. Some common examples are O-CH2--CH2-NHT-(CH2-CH3)(diethylaminoethyl- DEAE l2-NH](amino ethyl -AE) O-CH2-CH2-NT(C2H5)2-CH2--CH(OH)-CH3(quaternary amino ethyl- QAE -CH2-NT(CH3)3(quaternary amine -Q) Q and QAE are strong anion exchangers while DEAE and aE are weak lon-exchange materials All ion-exchangers basically consist of a solid insoluble matrix to which are attached the active, charged groups on which ion-exchange occurs, Various terms are used to describe this material including resin, adsorbent, medium, or just ion-exchanger. There is no general agreement on which is correct, and the usage is sometimes confusing -e. g. the term 'resin is sometimes used as a general term for ion-exchangers, or sometimes spe cifically for synthetic organic materials, while a resin is strictly a naturally occurring organic compound(Kanekanian and Lewis, 1986) The solid support must have an open molecular framework which allows the mobile ions to move freely in and out, and must be completely insoluble throughout the process Most commercial ion-exchangers are based on an organic polymer network, although inorganic materials may be used. The support material does not directly determine the ionic distribution between the two phases, but it is a major factor in determining the physical and chemical stability of the ion-exchanger. Hence this will determine factors such as the capacity, the flow rate through a column, the diffusion rate of counterions into and out of the matrix, the degree of swelling and the durability of the material. The materials tend to be of two main types- xerogels or aerogels. Xerogels are insoluble synthetic polymers containing a cross-linking agent. Their structure and porosity depends on the solvent and degree of solvation and they are compressible to some degree Xerogels make up the majority of commercially available ion-exchangers including polyacrylamides, polystyrene and dextrans. The pore size of these materials can be ontrolled by the manufacturing conditions, especially the degree of cross-linking Aerogels have a much more fixed rigid structure (e. g. porous silica) and are therefore incompressible, which has obvious advantages for production scale
158 A. S. Grandison Ion-exchange groups Some common examples of cation exchangers are -SO;H+ (strong - pK, 1-2) --POi-(H+), (medium - pK, 2-3) -COOH (weak - pK, 3.5-8) Base function is almost invariably present as amines or imines. These are introduced into the matries by chloromethylation, followed by reaction with the appropriate amine to produce weakly to strongly basic ion-exchangers. Some common examples are -O-CH2 -CH2 -NH+-(CH2 -CH3), (diethylaminoethyl - DEAE) -0 - CH2 - CH2 - NH; (amino ethyl - AE) - 0 - CH, - CH2 -N+(C2H5), -CH2 - CH(0H)-CH3 (quaternary amino ethyl - QAE) -CH2-N+(CH3), (quaternary amine - Q) Q and QAE are strong anion exchangers while DEAE and AE are weak. Ion-exchange materials All ion-exchangers basically consist of a solid insoluble matrix to which are attached the active, charged groups on which ion-exchange occurs. Various terms are used to describe this material including resin, adsorbent, medium, or just ion-exchanger. There is no general agreement on which is correct, and the usage is sometimes confusing - e.g. the term ‘resin’ is sometimes used as a general term for ion-exchangers, or sometimes specifically for synthetic organic materials, while a resin is strictly a naturally occurring organic compound (Kanekanian and Lewis, 1986). The solid support must have an open molecular framework which allows the mobile ions to move freely in and out, and must be completely insoluble throughout the process. Most commercial ion-exchangers are based on an organic polymer network, although inorganic materials may be used. The support material does not directly determine the ionic distribution between the two phases, but it is a major factor in determining the physical and chemical stability of the ion-exchanger. Hence this will determine factors such as the capacity, the flow rate through a column, the diffusion rate of counterions into and out of the matrix, the degree of swelling and the durability of the material. The materials tend to be of two main types - xerogels or aerogels. Xerogels are insoluble synthetic polymers containing a cross-linking agent. Their structure and porosity depends on the solvent and degree of solvation and they are compressible to some degree. Xerogels make up the majority of commercially available ion-exchangers including polyacrylamides, polystyrene and dextrans. The pore size of these materials can be controlled by the manufacturing conditions, especially the degree of cross-linking. Aerogels have a much more fixed rigid structure (e.g. porous silica) and are therefore incompressible, which has obvious advantages for production scale
Ion-exchange and electrodialysis 159 As the adsorption is a surface effect, the available surface area is a key parameter. For industrial processing the maximum surface area to volume should be used to minimise plant size and product dilution. It is possible for a l ml bed of ion-exchanger to have a total surface area >100 m2. The ion-exchange material is normally deployed in packed beds, and involves a compromise between large particles(to minimise pressure drop)and small particles to maximise mass transfer rates. Porous particles are employed to increase surface area/volume. However, the surface must also be accessible to the solute molecules, and hence materials with an enormous surface area due to the presence of minute pores may be of very limited use, because much of this surface is inaccessible even to small solute molecules. Manufacturers of ion-exchange materials generally quote the exclusion limit of products with respect to molecular size. Particularly in the case of biopolymers, the shape of the pores and the three-dimensional structure of the solute may be a further consideration Capacity The capacity of an ion-exchanger is defined as the number of equivalents of exchange ble ions per kilogram of exchanger but is frequently expressed in meq/g(usually in the ry form), and can be determined by titration of the charged groups with strong acid or ase.This property depends on the nature of the fixed ions as well as the available surface area. Most commercially available materials have capacities in the range 1-10 uivalents/kg of dry material Blinding and fouling The operational life of an ion-exchanger, or at least the time between major clean-up campaigns, is limited by blinding or fouling. This is non-specific adsorption onto the matrix surface, or within the pores, which effectively reduces the capacity, and certainly affects the choice of ion-exchanger for a particular separation. The susceptibility of an on-exchanger to blinding or fouling with a particular feedstock may exclude its use fo that function despite having otherwise excellent binding capacity and specificity for the molecules in question. For example, the presence of significant lipid levels in a feedstock may exclude the use of some exchangers for protein separations Elution The choice of method of elution depends on the specific separation required. In some cases the process is used to remove impurities from a feedstock, while the required compound(s) remains unadsorbed. No specific elution method is required in such cases, although it is necessary to regenerate the ion-exchanger with strong acid or alkali. In other cases the material of interest is adsorbed by the ion-exchanger while impurities are washed out of the bed. This is followed by elution and recovery of the desired solute(s) In the latter case the method of elution is much more critical- for example, care must be taken to avoid denaturation of adsorbed protei Elution of the adsorbed solute is effected by changing the ph or the ionic strength of the buffer, followed by washing away the desorbed solute with a flow of buffer Increasing the ionic strength of the buffer increases the competition for the charged sites on the ion-exchanger. Small buffer ions with a high charge density will displace
Ion-exchange and electrodialysis 159 As the adsorption is a surface effect, the available surface area is a key parameter. For industrial processing the maximum surface area to volume should be used to minimise plant size and product dilution. It is possible for a 1 ml bed of ion-exchanger to have a total surface area >IO0 m2. The ion-exchange material is normally deployed in packed beds, and involves a compromise between large particles (to minimise pressure drop) and small particles to maximise mass transfer rates. Porous particles are employed to increase surface area/volume. However, the surface must also be accessible to the solute molecules, and hence materials with an enormous surface area due to the presence of minute pores may be of very limited use, because much of this surface is inaccessible even to small solute molecules. Manufacturers of ion-exchange materials generally quote the exclusion limit of products with respect to molecular size. Particularly in the case of biopolymers, the shape of the pores and the three-dimensional structure of the solute may be a further consideration. Capacity The capacity of an ion-exchanger is defined as the number of equivalents of exchangeable ions per kilogram of exchanger but is frequently expressed in meq/g (usually in the dry form), and can be determined by titration of the charged groups with strong acid or base. This property depends on the nature of the fixed ions as well as the available surface area. Most commercially available materials have capacities in the range 1-10 equivalents/kg of dry material. Blinding and fouling The operational life of an ion-exchanger, or at least the time between major clean-up campaigns, is limited by blinding or fouling. This is non-specific adsorption onto the matrix surface, or within the pores, which effectively reduces the capacity, and certainly affects the choice of ion-exchanger for a particular separation. The susceptibility of an ion-exchanger to blinding or fouling with a particular feedstock may exclude its use for that function despite having otherwise excellent binding capacity and specificity for the molecules in question. For example, the presence of significant lipid levels in a feedstock may exclude the use of some exchangers for protein separations. Elution The choice of method of elution depends on the specific separation required. In some cases the process is used to remove impurities from a feedstock, while the required compound(s) remains unadsorbed. No specific elution method is required in such cases, although it is necessary to regenerate the ion-exchanger with strong acid or alkali. In other cases the material of interest is adsorbed by the ion-exchanger while impurities are washed out of the bed. This is followed by elution and recovery of the desired solute(s). In the latter case the method of elution is much more critical - for example, care must be taken to avoid denaturation of adsorbed proteins. Elution of the adsorbed solute is effected by changing the pH or the ionic strength of the buffer, followed by washing away the desorbed solute with a flow of buffer. Increasing the ionic strength of the buffer increases the competition for the charged sites on the ion-exchanger. Small buffer ions with a high charge density will displace
polyelectrolytes which can subsequently be eluted, Altering the buffer pH so that the harge on an adsorbed polyelectrolyte is neutralised or made the same as the charges on the ion- exchanger will result in desorption Ion-exchange columns Fixed bed operations consisting of one, or two columns connected in series(depe on the type of ions which are to be adsorbed), are used in most ion-exch paration Liquids should penetrate the bed in plug flow, in either downward or direction The major problems with columns arise from clogging of flow and the formation of channels within the bed. Problems may also arise from swelling of organic matrices when the pH changes Mixed bed systems These may be used to avoid prolonged exposure of the solutions to both high and low ph environments, as is frequently encountered when using anion and cation exchange columns in series(e. g. during demineralisation of sugar cane juice to prevent hydrolysis of sucrose as described below ) Cation and anion exchangers are intimately mixed during he adsorption phase so that the feed solution remains at high or low pH only for the time required to pass from one particle to the next. Regeneration is possible on the basis that the two exchange materials have different specific gravities, and thus separate into two layers on backwashing. By the use of a regenerant distributor, strong acids and alkalis may be used to regenerate the resins independently. After rinsing, the ion-exchangers are remixed using compressed air Stirred tanks The flow and swelling problems encountered with fixed beds are obviated by the use of stirred tanks; however, these systems are less efficient and expose the ion-exchangers to mechanical damage as there is a need for mechanical agitation. The system involves mixing the feed solution with the ion-exchanger and stirring until equilibration has been achieved(typically 30-90 min in the case of proteins- Kanekanian and Lewis, 1986) After draining and washing the ion-exchanger, the eluant solution is then contacted with the bed for a similar equilibration time before draining and further processing 6.1.2 Applications of ion-exchange in the food and biotechnology industries One method of classifying the applications of ion-exchange could be by industries or ommodities. The main areas of the food industry where the process is currently used or applications occur outside these to render this classification unsatisfactory. Ion-exchange is widely employed in the recovery, separation and purification of biochemicals, monoclonal antibodies and enzymes Another way of categorising the applications is by the type of separations attained, for (1) removal of minor components, e.g. deashing or decolorising (3)isolating valuable compounds very of protg of purified enzymes (2) enrichment of fractions, e.g. red om whey or bloo
160 A. S. Grandison polyelectrolytes which can subsequently be eluted. Altering the buffer pH so that the charge on an adsorbed polyelectrolyte is neutralised or made the same as the charges on the ion-exchanger will result in desorption. Ion-exchange columns Fixed bed operations consisting of one, or two columns connected in series (depending on the type of ions which are to be adsorbed), are used in most ion-exchange separations. Liquids should penetrate the bed in plug flow, in either downward or upward direction. The major problems with columns arise from clogging of flow and the formation of channels within the bed. Problems may also arise from swelling of organic matrices when the pH changes. Mixed bed systems These may be used to avoid prolonged exposure of the solutions to both high and low pH environments, as is frequently encountered when using anion and cation exchange columns in series (e.g. during demineralisation of sugar cane juice to prevent hydrolysis of sucrose as described below). Cation and anion exchangers are intimately mixed during the adsorption phase so that the feed solution remains at high or low pH only for the time required to pass from one particle to the next. Regeneration is possible on the basis that the two exchange materials have different specific gravities, and thus separate into two layers on backwashing. By the use of a regenerant distributor, strong acids and alkalis may be used to regenerate the resins independently. After rinsing, the ion-exchangers are remixed using compressed air. Stirred tanks The flow and swelling problems encountered with fixed beds are obviated by the use of stirred tanks; however, these systems are less efficient and expose the ion-exchangers to mechanical damage as there is a need for mechanical agitation. The system involves mixing the feed solution with the ion-exchanger and stirring until equilibration has been achieved (typically 30-90 min in the case of proteins - Kanekanian and Lewis, 1986). After draining and washing the ion-exchanger, the eluant solution is then contacted with the bed for a similar equilibration time before draining and further processing. 6.1.2 Applications of ion-exchange in the food and biotechnology industries One method of classifying the applications of ion-exchange could be by industries or commodities. The main areas of the food industry where the process is currently used or is being developed are sugar, dairy and water purification, although sufficient applications occur outside these to render this classification unsatisfactory. Ion-exchange is widely employed in the recovery, separation and purification of biochemicals, monoclonal antibodies and enzymes. Another way of categorising the applications is by the type of separations attained, for example: (1) (2) (3) removal of minor components, e.g. deashing or decolorising; enrichment of fractions, e.g. recovery of proteins from whey or blood; isolating valuable compounds, e.g. production of purified enzymes
Ion-exchange and electrodialysis 161 Alternatively the chemical nature of the adsorbed ions could be used as a basis for classification. Any ionisable component of a foodstuff can potentially be adsorbed on to an ion-exchanger and thus separated The following is an attempt to classify applications in food and biotechnoloy sis of the function of the process Softening Softening of water and other liquids involves the exchange of calcium and magnesium ions for sodium ions attached to a cation exchange resin, e. g R-(Na)2+ Ca(HCO3)2-R-Ca-T+ 2NaHCO The sodium form of the cation exchanger is produced by regenerating with NaCl solution. Apart from the production of softened water for boiler feeds and cleaning of food and processing equipment, softening may be employed to remove calcium from sucrose solutions prior to evaporation(which reduces scaling of heat exchanger surfaces in sugar manufacture), and from wine(which improves stability)(Cristal, 1983) Demineralisation Demineralisation using ion exchange is an established process for water treatment, but over the last 20 years it has been applied to other food streams. Typically the process employs a strong acid cation exchanger followed by a weak or strong base anion exchanger. The cations are exchanged with H ions, e. g 2R"H++CaSO 4>(R")2Ca2++H2SO4 R-H++Na+ Na++HcL and the acids thus produced are fixed with an anion exchanger, e. g R+OH-+HtCl-→RCl+H2O Demineralised cheese whey is desirable for use mainly in infant formulations, but also in many other products such as ice cream, bakery products, confectionery, animal feeds etc The major ions removed from whey are Na*, K, Ca, Mg+, CI", HPO4, citrate and lactate. lon-exchange demineralisation of cheese whey generally employs a strong cation exchanger followed by a weak anion exchanger(Houldsworth, 1976). This can produce more than 90% reduction in salt content, which is necessary for infant formulae. Lower levels of demineralisation, obtained using a by-pass system, may be adequate for other applications. Due to the high salt content of whey, the system must be regenerated after the treatment of 10-15 bed volumes of whey. This is achieved, following rinsing, by the treatment of cation and anion exchangers separately with strong acids and alkalis respectively. Typically a cycle is about 6 h, of which 4 h are required for regeneration, therefore two or three parallel systems may be necessary. The use of recurrent regeneration reduces the consumption of regeneration chemicals Jonsson(1984)described the SMr(Swedish Dairies Association) process for whey demineralisation, in which the whey first enters a weak anion column in which the whey anions are exchanged for HCO3 ions. Following this a weak cation column exchanges the
Ion-exchange and electrodialysis 16 1 Alternatively the chemical nature of the adsorbed ions could be used as a basis for classification. Any ionisable component of a foodstuff can potentially be adsorbed on to an ion-exchanger and thus separated. The following is an attempt to classify applications in food and biotechnology on the basis of the function of the process. Softening Softening of water and other liquids involves the exchange of calcium and magnesium ions for sodium ions attached to a cation exchange resin, e.g. R-(Na+)2 + Ca(HC03)2 -+ R-Ca2+ + 2NaHC03 The sodium form of the cation exchanger is produced by regenerating with NaCl solution, Apart from the production of softened water for boiler feeds and cleaning of food and processing equipment, softening may be employed to remove calcium from sucrose solutions prior to evaporation (which reduces scaling of heat exchanger surfaces in sugar manufacture), and from wine (which improves stability) (Cristal, 1983). Deminerulisution Demineralisation using ion exchange is an established process for watsr treatment, but over the last 20 years it has been applied to other food streams. Typically the process employs a strong acid cation exchanger followed by a weak or strong base anion exchanger. The cations are exchanged with H+ ions, e.g. 2R-Hf + CaS04 -+ (R-)2Ca2+ + H2S04 R-H' + Na'C1- + R-Na' + H+Cland the acids thus produced are fixed with an anion exchanger, e.g. R'OH- + H'Cl- -+ R'CI- + H20 Demineralised cheese whey is desirable for use mainly in infant formulations, but also in many other products such as ice cream, bakery products, confectionery, animal feeds etc. The major ions removed from whey are Na', K+, Ca2+, Mg2+, C1-, HPO,, citrate and lactate. Ion-exchange demineralisation of cheese whey generally employs a strong cation exchanger followed by a weak anion exchanger (Houldsworth, 1976). This can produce more than 90% reduction in salt content, which is necessary for infant formulae. Lower levels of demineralisation, obtained using a by-pass system, may be adequate for other applications. Due to the high salt content of whey, the system must be regenerated after the treatment of 10-15 bed volumes of whey. This is achieved, following rinsing, by the treatment of cation and anion exchangers separately with strong acids and alkalis respectively. Typically a cycle is about 6 h, of which 4 h are required for regeneration, therefore two or three parallel systems may be necessary. The use of countercurrent regeneration reduces the consumption of regeneration chemicals. Jonsson (1984) described the SMR (Swedish Dairies Association) process for whey demineralisation, in which the whey first enters a weak anion column in which the whey anions are exchanged for HCOT ions. Following this a weak cation column exchanges the
162 A S. Grandison whey cations for NH4. The whey salts are thus exchanged for ammonium bicarbonate which decomposes to NH3, CO2 and water during subsequent evaporation, the NH3 and CO2 being recovered. Jonsson and Arph(1987)compared conventional ion-exchange demineralisation of cheese whey to the Smr process and concluded that the requirement for regeneration chemicals and production of waste chemicals are much reduced in the SMR process Demineralisation by ion-exchange resins is used at various stages during the manufac ture of sugar from either beet or cane, as well as for sugar solutions produced by hydroly sis of starch. In the production of sugar from beet, the beet juice is purified by liming and carbonatation and then may be demineralised by ion-exchange(McGinnis, 1971). The carbonated juice is then evaporated to a thick juice prior to sugar crystallisation Demineralisation may, alternatively, be carried out on the thick juice which has the advantage that the quantities handled are much smaller, but is limited by the fact that diffusion rates are low at high sugar concentrations, To produce high-quality sugar the juice should have a purity of about 95%. Rousseau(1984)described the new demineralisation/ demi' process which utilises a mixed bed of weak cationic and weak anionic resins in a batchwise process to treat the thick juice(dry matter 70%0). This gives rise to a very pure juice with minimum dilution, with the bonus of a decolorisation at no extra cost. A further application in beet sugar production is the Quentin process by which the sugar level of molasses can be decreased. This is achieved by exchanging potassium and sodium ions of the juice prior to the final crystallisation, for magnesium using a strongly acidic cation exchanger. Magnesium is less molassigenic than alkali Ions Ash removal or complete demineralisation of cane sugar liquors has been described by Chen(1985).The process is carried out on liquors that have already been clarified and decolorised. so the ash load is at a minimum The use of a mixed bed of weak cation and strong anion exchangers in the hydrogen and hydroxide forms, respectively, reduces the prolonged exposure of the sugar to strongly acid or alkali conditions which would be necessary if two separate columns were used. Destruction of sucrose is thus minimised The cation and anion resins are sometimes used in their own right for dealkalisation or deacidification, respectively, Weak cation exchangers may be used to reduce the alkalinity of water used in the manufacture of soft drinks( Carney, 1988)and beer( Cristal 1983), while anion exchangers can be used for deacidification of fruit and vegetable juices(Lue and Chiang, 1989; Dechow et aL., 1985). In addition to deacidification, anion exchangers may also be used to remove bitter flavour compounds(such as naringin or imonin)from citrus juices (Johnson and Chandler, 1985). Anion or cation exchange resins are used in some countries to control the pH or titratable acidity of wine(rankin 1986: Bonorden et al., 1986)although this process is not permitted by other tradition wine producing countries. Acidification of milk to pH 2. 2, using ion-exchange during casein manufacture by the Bridel process, has also been described(Pierre and Douin 1984) Ion-exchange processes can be used to remove specific metals or anions from drinking ater and food fluids, which has potential application for detoxification or radioactive decontamination. For example, procedures have been described for the removal of lead Brajter and Slonawska, 1986), barium and radium(Snoeyink et al., 1987), aluminium
162 A. S. Grandison whey cations for NH;. The whey salts are thus exchanged for ammonium bicarbonate which decomposes to NH3, C02 and water during subsequent evaporation, the NH, and C02 being recovered. Jonsson and Arph (1 987) compared conventional ion-exchange demineralisation of cheese whey to the SMR process and concluded that the requirement for regeneration chemicals and production of waste chemicals are much reduced in the SMR process. Demineralisation by ion-exchange resins is used at various stages during the manufacture of sugar from either beet or cane, as well as for sugar solutions produced by hydrolysis of starch. In the production of sugar from beet, the beet juice is purified by liming and carbonatation and then may be demineralised by ion-exchange (McGinnis, 197 1). The carbonated juice is then evaporated to a thick juice prior to sugar crystallisation. Demineralisation may, alternatively, be carried out on the thick juice which has the advantage that the quantities handled are much smaller, but is limited by the fact that diffusion rates are low at high sugar concentrations. To produce high-quality sugar the juice should have a purity of about 95%. Rousseau (1984) described the ‘new demineralisation/demi’ process which utilises a mixed bed of weak cationic and weak anionic resins in a batchwise process to treat the thick juice (dry matter 70%). This gives rise to a very pure thick juice with minimum dilution, with the bonus of a decolorisation at no extra cost. A further application in beet sugar production is the Quentin process by which the sugar level of molasses can be decreased. This is achieved by exchanging potassium and sodium ions of the juice prior to the final crystallisation, for magnesium using a strongly acidic cation exchanger. Magnesium is less molassigenic than alkaline ions. Ash removal or complete demineralisation of cane sugar liquors has been described by Chen (1985). The process is carried out on liquors that have already been clarified and decolorised, so the ash load is at a minimum. The use of a mixed bed of weak cation and strong anion exchangers in the hydrogen and hydroxide forms, respectively, reduces the prolonged exposure of the sugar to strongly acid or alkali conditions which would be necessary if two separate columns were used. Destruction of sucrose is thus minimised. The cation and anion resins are sometimes used in their own right for dealkalisation or deacidification, respectively. Weak cation exchangers may be used to reduce the alkalinity of water used in the manufacture of soft drinks (Carney, 1988) and beer (Cristal, 1983), while anion exchangers can be used for deacidification of fruit and vegetable juices (Lue and Chiang, 1989; Dechow et al., 1985). In addition to deacidification, anion exchangers may also be used to remove bitter flavour compounds (such as naringin or limonin) from citrus juices (Johnson and Chandler, 1985). Anion or cation exchange resins are used in some countries to control the pH or titratable acidity of wine (Rankine, 1986; Bonorden et al., 1986) although this process is not permitted by other traditional wine producing countries. Acidification of milk to pH 2.2, using ion-exchange during casein manufacture by the Bride1 process, has also been described (Pierre and Douin, 1984). Ion-exchange processes can be used to remove specific metals or anions from drinking water and food fluids, which has potential application for detoxification or radioactive decontamination. For example, procedures have been described for the removal of lead (Brajter and Slonawska, 1986), barium and radium (Snoeyink et al., 1987), aluminium
Ion-exchange and electrodialysis 163 (Pesavento et aL., 1989), uranium(Sorg, 1988)and nitrates(Lauch and Guter, 1986)from drinking water. Removal of a variety of radionuclides from milk has been demonstrated Radiostrontium and radiocaesium can be removed using a strongly acidic cation ex changer(Tait et al., 1989; Koga et al, 1968), while I 3I can be adsorbed on to a variety of anion exchangers(Barth et al., 1970). The production of low sodium milk, with potential dietetic application, has been demonstrated (Nakazawa and Hosono, 1989) Decolorisation however, other cases where colour removal is required without demineralisation. re are Demineralisation processes may have the added benefit of colour removal. There are Sugar liquors from either cane or beet contain colourants such as caramels. melanoidins, melanins or polyphenols combined with iron. Many of these are formed ring the earlier refining stages, and it is necessary to remove them in the production of a marketable white sugar. The use of ion-exchangers just before the crystallisation stage results in a significant improvement in product quality. It is necessary to use material with an open, porous structure to allow the large colourant molecules access to the adsorption sites. Chen(1985)described the use of strongly basic resins operated in the chloride cycle for decolorisation during cane sugar refining. These are sometimes the only decolorising systems used, but in other cases complement the use of carbon adsorbents. Bohm and Schafer (1969)described the decolorisation of beet sugar juice on an industrial scale using ion-exchange resins a new approach to the use of ion-exchange for decolorisation of sugar solutions is the application of powdered resin technology. Finely powdered resins(0.00.5-0. 2 mm diam- eter) have a very high capacity for sugar colourants due to the ready availability of dsorption sites. The use of such materials on a disposable basis eliminates the need for and the accompanying disposal problems of, chemical regenerants, as well as removing he problem of sugar dilution which occurs during column operation. However, the advantages must be weighed against the added expense of discarding expensive resins after a single use( Chen, 1985). Colour reduction of fermentation products such as wine has also been described. Brown et al.(1988)used a strongly basic anion exchanger to remove colouring matter, followed by a strong cation exchanger to restore the pH. It is claimed that colour reduction can be achieved without substantially deleteriously affect ing the other wine qualities Protein purification Ion-exchange can be used successfully in many protein purification processes in the food and pharmaceutical industries. High purity protein isolates can be produced in a single step from dilute solutions containing other contaminating materials. The process compares favourably with competing techniques in terms of cost and efficiency. The amphoteric nature of protein molecules permits the use of either anion or cation exchang ers, depending on the pH of the environment. Elution takes place by either altering the oH or increasing the ionic strength. The eluate can be a single bulk, or a series of fractions produced by stepwise or linear gradients, although fractionation may be too complex for large-scale industrial production. Separation of a single protein may take place on the basis that it has a higher affinity to the charged sites on the ion-exchanger
Ion-exchange and electrodialysis 163 (Pesavento et al., 1989), uranium (Sorg, 1988) and nitrates (Lauch and Guter, 1986) from drinking water. Removal of a variety of radionuclides from milk has been demonstrated. Radiostrontium and radiocaesium can be removed using a strongly acidic cation exchanger (Tait et al., 1989; Koga et al., 1968), while I*31 can be adsorbed on to a variety of anion exchangers (Barth et al., 1970). The production of low sodium milk, with potential dietetic application, has been demonstrated (Nakazawa and Hosono, 1989). Decolorisation Demineralisation processes may have the added benefit of colour removal. There are, however, other cases where colour removal is required without demineralisation. Sugar liquors from either cane or beet contain colourants such as caramels, melanoidins, melanins or polyphenols combined with iron. Many of these are formed during the earlier refining stages, and it is necessary to remove them in the production of a marketable white sugar. The use of ion-exchangers just before the crystallisation stage results in a significant improvement in product quality. It is necessary to use materials with an open, porous structure to allow the large colourant molecules access to the adsorption sites. Chen (1985) described the use of strongly basic resins operated in the chloride cycle for decolorisation during cane sugar refining. These are sometimes the only decolorising systems used, but in other cases complement the use of carbon adsorbents. Bohm and Schafer (1969) described the decolorisation of beet sugar juice on an industrial scale using ion-exchange resins. A new approach to the use of ion-exchange for decolorisation of sugar solutions is the application of powdered resin technology. Finely powdered resins (0.005-0.2 mm diameter) have a very high capacity for sugar colourants due to the ready availability of adsorption sites. The use of such materials on a disposable basis eliminates the need for, and the accompanying disposal problems of, chemical regenerants, as well as removing the problem of sugar dilution which occurs during column operation. However, the advantages must be weighed against the added expense of discarding expensive resins after a single use (Chen, 1985). Colour reduction of fermentation products such as wine has also been described. Brown et al. (1988) used a strongly basic anion exchanger to remove colouring matter, followed by a strong cation exchanger to restore the pH. It is claimed that colour reduction can be achieved without substantially deleteriously affecting the other wine qualities. Protein purification Ion-exchange can be used successfully in many protein purification processes in the food and pharmaceutical industries. High purity protein isolates can be produced in a single step from dilute solutions containing other contaminating materials. The process compares favourably with competing techniques in terms of cost and efficiency. The amphoteric nature of protein molecules permits the use of either anion or cation exchangers, depending on the pH of the environment. Elution takes place by either altering the pH or increasing the ionic strength. The eluate can be a single bulk, or a series of fractions produced by stepwise or linear gradients, although fractionation may be too complex for large-scale industrial production. Separation of a single protein may take place on the basis that it has a higher affinity to the charged sites on the ion-exchanger
164 A.S. grandison compared to other contaminating species, including other proteins present in the feed. In such cases, if excess quantities of the feed are used, the protein of interest can be adsorbed exclusively, despite initial adsorption of all the proteins in the feed (Kanekanian and Lewis, 1986). Alternatively it may be possible to purify a protein on the basis that it has a much lower affinity for the ion-exchanger than other proteins present in the feed, nd thus the other proteins are removed, leaving the desired protein in solution ionic strength and temperature must be avoided to prevent denaturation of the protein? One limitation of the process for protein treatment is that extreme conditions of An area of great potential is the recovery of proteins from whey. It is estimated (van hoogstraten, 1987)that about 110 million tonnes of whey are produced each year as a by-product of the manufacture of cheese and related products such as casein. Typically whey contains 0.6-0.8% protein, which is highly nutritious and also displays excellent hysical properties, yet the vast majority of this is wasted or under-utilised. The Vistec protein recovery process employs carboxymethyl cellulosic anion-exchange materials to produce high purity functional protein from cheese whey (Jones, 1976; Palmer, 1977) The system uses a stirred tank reactor into which the whey is introduced at low pH. Following rinsing of non-adsorbed material, the protein fraction is eluted at high pH, and further purified by ultrafiltration so that the final protein content is approximately 97% (on a dry matter basis). The product is commercially exploited by the Bio-isolates com- pany(Fig. 6.2). Ayers and Petersen(1985)have described a similar process based on sulphopropyl cellulosic materials which is also used for commercial recovery of cheese are rigid and do not swell or contract when the pH or ionic strength oras ntage that they whey protein Silica-based ion-exchangers such as Spherosil have the advan Fig. 6. 2. lon-exchange recovery of food proteins(with permission of Bio-isolates plc)
164 A. S. Grandison compared to other contaminating species, including other proteins present in the feed. In such cases, if excess quantities of the feed are used, the protein of interest can be adsorbed exclusively, despite initial adsorption of all the proteins in the feed (Kanekanian and Lewis, 1986). Alternatively it may be possible to purify a protein on the basis that it has a much lower affinity for the ion-exchanger than other proteins present in the feed, and thus the other proteins are removed, leaving the desired protein in solution. One limitation of the process for protein treatment is that extreme conditions of pH, ionic strength and temperature must be avoided to prevent denaturation of the protein. An area of great potential is the recovery of proteins from whey. It is estimated (van Hoogstraten, 1987) that about 110 million tonnes of whey are produced each year as a by-product of the manufacture of cheese and related products such as casein. Typically whey contains 0.648% protein, which is highly nutritious and also displays excellent physical properties, yet the vast majority of this is wasted or under-utilised. The Vistec protein recovery process employs carboxymethyl cellulosic anion-exchange materials to produce high purity functional protein from cheese whey (Jones, 1976; Palmer, 1977). The system uses a stirred tank reactor into which the whey is introduced at low pH. Following rinsing of non-adsorbed material, the protein fraction is eluted at high pH, and further purified by ultrafiltration so that the final protein content is approximately 97% (on a dry matter basis). The product is commercially exploited by the Bio-isolates company (Fig. 6.2). Ayers and Petersen (1985) have described a similar process based on sulphopropyl cellulosic materials which is also used for commercial recovery of cheese whey protein. Silica-based ion-exchangers such as Spherosil have the advantage that they are rigid and do not swell or contract when the pH or ionic strength of the environment Fig. 6.2. Ion-exchange recovery of food proteins (with permission of Bio-isolates plc)
