《食品和生物分离过程》（英文版） Chapter 6 Ion-exchange and electrodialysis.pdf

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