Download - Unit 9 Ion Exchange Chromatography

5

Ion Exchange Chromatography UNIT 9 ION EXCHANGE

CHROMATOGRAPHY Structure 9.1 Introduction

Objectives 9.2 Basic Features of Ion Exchange Mechanism 9.3 Classification of Ion Exchangers

Natural Ion Exchangers Synthetic Ion Exchangers Liquid Ion Exchangers

9.4 Synthesis of Ion Exchange Resins Cation Exchangers Anion Exchangers Amphoteric Exchangers

9.5 Trade Names and Nomenclature 9.6 Resin Properties

Moisture Content Particle Size Cross Linkages Capacity Distribution Ratio Equivalency of Exchange Resin Selectivity

9.7 Operating Methods Batch Operation Column Operation Moving Bed Operation

9.8 Ion Exchange in Mixed Aqueous - Organic Media 9.9 Specific Cation Exchangers 9.10 Synthetic Inorganic Ion Exchangers

Different Types and Their Characteristics Special Properties and Applications

9.11 Applications Separation of Metal Ions and Anions Separation of Organics Separation of Ionized from Nonionized Separation of Actinide Elements Miscellaneous Applications

9.12 Summary 9.13 Terminal Questions 9.14 Answers

9.1 INTRODUCTION Amongst various separation techniques, ion exchange is the most popular name because of its use for water softening. It is also unique in terms of its versatility and historical developments. Besides the well-known use of ion exchangers in water treatment, they find use in industry, nuclear fuel processing, hydrometallurgy, agriculture and biology. The treatment of water by solid adsorbents is as old as civilization. There are records available that in the time of Aristotle, sand filters were used for purification of sea water. Moses used a tree branch for making bitter water sweet. But the credit of recognizing the ion exchange phenomenon goes to two agricultural chemists-Thompson and Way. They observed the exchange of ammonium ions with calcium ions in soils. The realization of the fact that certain clay minerals were responsible for the exchange, led to the attempts to use such materials for water softening. It also prompted scientists to synthesize materials with similar properties.

6

Chromatographic Methods-III

The first synthetic ion exchanger was prepared in 1903 by two German chemists-Harm and Rumpler. Another German, Gans, worked on several pioneering applications of permutits. But the permutits could not stand in the market because of their poor reproducibility and chemical stability.

A real breakthrough in the subject came in 1935 when two English chemists, Adams, and Holmes, observed that crushed phonograph record exhibited ion exchange properties. This observation led to the synthesis of several organic ion exchangers which had better properties. It was illustrated that stable and high capacity cation exchangers could be prepared as sulphonic acid resins and polyamine type resins exhibited anion exchange properties. The area of ion exchange blossomed at a very fast rate. The versatility of ion exchange resins was readily recognized. Many attempts have been made to modify and improve the existing materials. It is possible to tailor make ion exchange resins for specific applications.

Ion exchange is firmly established as a unit operation. All over the world, numerous plants are in operation accomplishing the tasks that range from the recovery of metals from industrial wastes to the separation of rare earths and from catalysis of organic reactions to the decontamination of cooling water of nuclear reactors. In the laboratory, ion exchangers prove themselves as useful materials for accomplishing analytical separations. The ion exchange membranes find quite a good use in physiological chemistry and biophysics. Ion exchange separation played a major role in the identification of trans-uranium elements by Glen T. Seaborg. The identify of each element of 5f series was established beyond any doubt by the sequence of their appearance a analogous to the appearance of the corresponding 4f elements

The above applications clearly indicate that a variety of ion exchangers are available and these materials can be used for different applications. In view of this, it is important to understand the basic ion exchange mechanism, and a broad classification of ion exchangers. Ion exchange resins are synthesized by following different chemical routes. An idea about it can be had by illustrating the synthesis of some well- known ion exchange resins. The practical utility of an ion exchanger depends upon its properties, both chemical and physical. Another point which is important in this context is as to how the material is being operated. The discussion on ion exchangers will not be complete if we do not talk about some special type of ion exchangers viz. chelating resins and synthetic inorganic ion exchangers. Finally, a discussion on various types of applications will be taken up. It may be noted that some of these uses may not be directly based on separations.

Objectives After studying this Unit, you should be able to

discuss basic ion exchange mechanism,

classify different types of ion exchangers,

describe the synthesis of ion exchange resins,

explain the properties which characterize an ion exchanger,

describe the operating methods for ion exchangers,

explain the behaviour of specific cation exchangers,

present a complete picture about the different types of synthetic inorganic ion exchangers and their advantages alongwith applications, and

discuss different types of applications of ion exchangers.

7

Ion Exchange Chromatography 9.2 BASIC FEATURES OF ION EXCHANGE

MECHANISM The term ion exchange generally means exchange of ions of like sign between a solution and a solid highly insoluble in it. The solid known as ion exchanger carries exchangeable cations and anions. When the exchanger is in contact with an electrolyte, these ions can be exchanged for a stoichiometrically equivalent amount of other ions of same sign. Carriers of exchangeable cations are known as cation exchangers and carriers of exchangeable anions as anion exchangers. Certain materials are capable of both cation and anion exchange. These are known as amphoteric exchangers. A typical cation exchange reaction is shown below:

2 NaX + CaCl2(aq) CaX2 + 2 NaCl(aq) Similarly, typical anion exchange reaction is as follows:

2 XCl + Na2SO4(aq) X2SO4 + 2 NaCl(aq) where, X represents a structural unit of the ion exchanger.

In the first process, a solution containing dissolved CaCl2, say something like hard water, is treated with a solid exchanger, NaX, containing exchangeable Na+ ions. The exchanger removes the Ca2+ ions from the solution and replaces them with Na+. Thus, a cation exchanger in Na+ form is converted to Ca2+ form.

Ion exchange, with very few exceptions, is a reversible process. In water softening, a cation exchanger has lost its Na+ ions and can be regenerated with a solution of a sodium salt such as NaCl. Ion exchange resembles adsorption in that, in both cases, a dissolved species is taken up by a solid. The characteristic difference between the two is that the ion exchange in contrast to sorption, is a stoichiometric process. Every ion removed from the solution is replaced by an equivalent amount of another ionic species of the same sign. However, in the case of sorption a solute, an electrolyte or non-electrolyte, may be taken up without any species being replaced.

Ion exchangers owe their characteristics to a particular feature of their structure. They are built of a framework which is held together by chemical bonds or lattice energy. The framework carries a positive or negative surplus charge which is compensated by ions of opposite charge, called counter ions. The counter ions are free to move within the framework and be replaced by other ions of same sign. The framework of cation exchanger may be regarded as a macromolecule or a crystalline polyanion, that of an anion exchanger as a polycation.

From the above discussion, it emerges out that a useful ion exchanger must have the following requisites: i) It should have negligible solubility in the medium to be used. ii) It must contain sufficient number of accessible ion exchange groups and it must

be chemically stable. iii) It should be sufficiently hydrophilic to permit diffusion of ions through the

structure at a finite and usable rate. iv) The swollen exchanger must be denser than water.

SAQ 1 What is the basic difference between adsorption and ion exchange?

...

8


SAQ 2 A sodium phosphate solution is passed through a column of an anion exchanger in the chloride form. The PO43 ions are taken up by the ion exchanger. Write down the ion exchange equilibria.

...

...

...

...

9.3 CLASSIFICATION OF ION EXCHANGERS Many different natural and synthetic products show ion exchange properties. These exchangers can be either cation or anion exchangers. Therefore, a simple broad classification can be as i) Natural ii) Synthetic However, within these two categories the material can be i) Organic ii) Inorganic For the purposes of simple presentation, we will select the first classification i.e., natural and synthetic.

9.3.1 Natural Ion Exchangers Most of the natural ion exchange materials are crystalline aluminosilicates with cation exchange properties. The typical representative of this group of materials are zeolites which include among others, the minerals like analcite Na[SiAlO6]2. H2O, chabazite (CaNa)[SiAlO6]2.6H2O and naturalite Na2[Si2Al2O10].2H2O. All these minerals have a relatively open three dimensional framework with channels and interconnecting cavities in the aluminosilicate lattice. The zeolite lattice consists of SiO4 and AlO4 tetrahedra. These have their oxygen atoms in common. Because aluminium is trivalent, the lattice carries a negative charge. The charge is balanced by alkali and alkaline earth cations which do not occupy fixed positions and are free to move in the lattice framework. These ions behave as counter ions and can exchange with other counter ions.

There are other aluminosilicates with loose layer structure having cation exchange properties. These materials carry their counterions in between the layer of the lattice. The typical mineral of this type is montmorillonite with the approximate composition Al2[Si4O10(OH)2].nH2O. Such minerals swell in one direction increasing the interlayer distance.

It may be important to mention here that certain aluminosilicates can also behave as anion exchangers. In montmorillonite, kaolinite and feldspar of sodalite and camerinite groups the exchange of OH for Cl, 24SO and

34PO has been observed. There are

some problems with the use of zeolites as ion exchangers because of some of their properties. The zeolites are soft minerals and thus, are not very abrasive resistant. They have poor mechanical strength. Their frameworks are more rigid hence less open. They swell very little and the counter ions in their pores do not move very freely. Above all, they suffer partial decomposition by acids and alkalis.

9

Ion Exchange Chromatography

Another lesser known variety of natural ion exchangers is some types of coals. They contain carboxylic and possibly other weak acid groups. They, thus, can be used as cation exchangers. Most of these materials swell excessively and are decomposed by alkali. They are, therefore, stabilized before use. Soft and hard coals are stabilized by metal ion solutions. Most lignites and bituminous coals and anthracites can be converted into strong cation exchangers by sulphonation with fuming strong sulphuric acid. These coals have very limited applications.

9.3.2 Synthetic Ion Exchangers Virtually the field of ion exchange has been dominated by organic ion exchange resins. An almost unlimited variety of resins with different compositions and degrees of cross linking can be prepared. The resins consist of an elastic three-dimensional network of hydrocarbons which carry fixed ionic groups. The charge of the group is balanced by mobile counter ions. As a matter of fact, these resins are cross-linked polyelectrolytes. In a cation exchanger, the matrix carries ionic groups like

SO3, COO, PO33 and in an anion exchanger, it carries groups such as

NH3+, >NH2 , > N+

An ion exchange resin particle is one single macromolecule. The chemical, thermal and mechanical stability and the ion exchange behaviour of the resin depend chiefly on the structure and the degree of cross-linking of the matrix and on the nature and the number of fixed ionic groups. The degree of cross-linking determines the mesh width of the matrix which in turn affects the swelling of the resin and the mobilities of the counter ions. This finally affects the rate of ion exchange and other processes and the electrical conductivity. It should be clear that ion exchange resins do not have unlimited chemical and thermal stability. The common causes of resin degradation are chemical and thermal deterioration. A majority of commercial ion exchange resins are stable in all common solvents except in the presence of strong oxidizing and reducing agents. They can generally withstand temperatures slightly higher than 100C.

As pointed out earlier that the ion exchange behaviour of the resin is mainly determined by the fixed ionic groups. The number of groups determines the ion exchange capacity. The chemical nature of groups to a great extent affects the ion exchange equilibria. One of the important factors is the acid and base strength of the group. This can be illustrated by taking a few examples. The groups COO are ionized only at high pH and at low pH, they combine with H+ forming the undissociated COOH. Thus, they no longer act as fixed charges. On the other hand, strong acid groups like 3SO remain ionized even at low pH. Similarly, weak base group NH3

+

lose a proton, forming an uncharged NH2 when pH is high and strong base groups such as N(CH3)3+ remain ionized even at high pH. Thus, the operative capacity of weak acid and weak base exchanges is more pH dependent.

In this unit, we will mainly focus on the properties of organic resins and these will be discussed in more detail in section 9.5. Inspite of the fact that different types of resins have a variety of applications, there are some pronounced limitations of these types of exchangers. They are not very stable at high temperatures and cannot withstand high dose of ionizing radiations and highly oxidizing media. From 1950s onwards, interest in the management of nuclear waste grew at a very fact pace. This led to resurgence of interest in inorganic ion exchange and a complete subject of synthetic inorganic exchangers became prominently important. A variety of amorphous and crystalline inorganic ion exchangers have been synthesized. The list of these materials is large. Many of these exchangers show specificity for particular ions and they are used to separate them. No doubt the area of synthetic inorganic ion exchangers initially developed for nuclear waste management purposes but with the time, it has attracted

10


the interest of different types of research groups. A detailed discussion on synthetic inorganic ion exchangers will be taken up towards the end of this unit.

9.3.3 Liquid Ion Exchangers You can recollect that in Unit 2, sub-Sec. 2.3.4, it was pointed out that high molecular weight amines and quaternary ammonium salts behave as liquid anion exchangers. They extract the anions and anionic metal complexes. With a similar analogy, some authors classify alkylphosphoric acids, sulphonic acids and carboxylic acids as liquid cation exchangers (Unit 3, sub-Sec. 3.2.4). It was also pointed out at the same time that this analogy should not be extended too far. Besides other complications, the operation of transfer of solute in solvent extraction and ion exchange chromatography is different. However, one situation remains to be considered when these extractants mainly high molecular weight amines are loaded on inert supports and the supports are used in columns for separations. This is classified under the head of extraction chromatography. For this, you may refer to Unit 4, sub-Sec. 4.2.3 where a brief mention has been made about extraction chromatography. A variety of metal ion separations are achieved using this technique. In this context, there may be some justification for having liquid ion exchangers as a distinct class of ion exchangers. However, this unit does not discuss them in detail. An idea about liquid ion exchangers has already been given in Unit 2 and 3.

SAQ 3 What are the two distinct classes of aluminosilicates based on their structure?

...

...

...

...

SAQ 4 Under what conditions the organic resinous ion exchangers deteriorate fast?

...

...

...

...

SAQ 5 Is there any justification of including liquid ion exchangers as a distinct category of ion exchangers?

...

...

...

9.4 SYNTHESIS OF ION EXCHANGE RESINS It has been made clear earlier that we will mainly focus on synthesis and properties of organic resins. If we take synthesis, there are too many types of resins and different chemical routes are followed to prepare them. Therefore, it may be difficult to cite

11


here even the few important ones. Hence, to highlight the synthetic chemistry of ion exchange resins, some discussion will be taken up on general terms and that will be accompanied by a few examples of synthesis. One point which is very clear about synthesis of ion exchange resin is that it must yield a three dimensional cross-linked matrix of hydrocarbon chains carrying fixed ionic groups. This can be achieved in the following ways: i) Monomeric organic electrolytes can be polymerized in such a way that a cross

linked network is formed. ii) The matrix can be built from nonionic monomers and the fixed ionic groups

are then introduced into the completed network. iii) The fixed ionic groups are introduced while the polymerization is still in

progress. While synthesizing resinous exchanger, it should be kept in mind that it should be sufficiently cross-linked to have negligible solubility. The cross linking should be such that it should be able to swell. Polymers which are too highly cross-linked cannot swell. The mobility of counter ions in such resins is so low that ion exchange is difficult to take place. The method of synthesis should be such that the degree of crosslinking can be controlled. Most of the ion exchange resins are made by either condensation polymerization or addition polymerization. Now the addition polymerization processes have more or less replaced the condensation processes.

9.4.1 Cation Exchangers A broad variety of cation exchangers with fixed ionic groups of different character and different acid strength are commercially available. The most common of these are strong-acid resins with ( 3SO ) and weak acid resins with carboxylic acid groups (COO). Even if we consider these two types of resins, the resins of various strength can be made since dissociation constants are affected by the nature and configuration of the units to which the groups are attached. The arylsulphonic acids are stronger than alkylsulphonic acids. Many ion exchangers contain two or more different types of ionic groups and they are known as bifunctional or polyfunctional.

a) Condensation polymers The earliest known cation exchange resin was a condensation product of phenol and formaldehyde. The list became broader and more extensive. Other monovalent or polyvalent phenols like resorcinol and naphthol instead of phenol and other aldehydes instead of formaldehyde can be used. Phenolic group can act as a fixed ionic group but the resins have a very low acid strength. Groups with higher acid strength can be introduced by various methods. The easiest course is sulphonation of phenol prior to polymerization.

b) Addition polymers The area of synthesis of ion exchange resins is now dominated by addition copolymers prepared from vinyl monomers. They are more chemically and thermally stable than the condensation polymers. Moreover, in addition polymerization, the degree of cross-linking and particle size are easy to control. A well known cation exchange resin is obtained by the copolymerization of

12


styrene and a small proportion of divinylbenzene followed by sulphonation by treatment with concentrated sulphuric acid or chlorosulphonic acid.

The role of divinylbenzene is as a crosslinking agent. Pure divinylbenzene is not easily available. The commercial product consists of different divinylbenzene isomers (around 50%) and ethylenestyrene (around 50%). Therefore, ethylenestyrene is also introduced in the matrix. The degree of crosslinking can be adjusted by varying the divinylbenzene content.

9.4.2 Anion Exchangers The earliest anion exchangers synthesized were with weak base amino groups

Subsequently, resins with strong-base quaternary ammonium groups were prepared

It was followed by synthesis of resins with strong-base quaternary phosphonium groups and tertiary sulphonium groups.

13


Like cation exchangers, the earlier known anion exchangers were condensation polymers and they are replaced by addition polymers.

a) Condensation polymers The earliest known anion exchange resins were prepared from aromatic amines like m- phenylenediamine by condensation with formaldehyde.

The aldehyde reacts with amino groups. In the process, the secondary and tertiary amino groups are formed. Thus, the resins are polyfunctional. Aliphatic polyamines which are not as weakly basic can also be condensed with aldehydes.

b) Addition polymers Like cation exchangers, a commonly used anion exchange resin is prepared by copolymerization of styrene and divinylbenzene followed by chloromethylation (introduction CH2Cl grouping) say, in the para position and interaction with a base such as trimethylamine. The polymers containing quaternary ammonium groups are strong bases and those with amino or substituted amino groups show weakly basic properties.

9.4.3 Amphoteric Exchangers The ion exchangers which contain both acidic and basic groups are known as amphoteric exchangers. A number of exchangers of this type has been synthesized but only a few have found application.

A well known resin containing both strong base and acid groups is prepared by copolymerization of styrene, vinylchloride and a cross-linking agent followed by quaternization and sulphonation of the product.

14


Among the amphoteric resins, the most important are the ones known as snake- cage polyelectrolytes. They are conventional cation or anion exchangers within which polycation or polyanions, respectively have been formed by polymerization. A typical example is that a snake- cage polyelectrolyte can be prepared by converting a strong base anion exchanger to acrylate form and then acrylate anion is polymerized in the resin. The linear chains of the poly-counter ions are so intricately interwined with the crosslinked matrix that they cannot be displaced by other counter ions. The situation is something like a snake trapped in a cage. One significant difference these snake cage polyelectrolytes show from other amphoteric exchangers is that the poly-counter ions are not attached to the matrix. Therefore, the charges of poly-counter ions of the matrix have more freedom to move. As a result, it is not necessary for the resin to have mobile counter ions (counter ions to the poly-counter ions) to remain electrically neutral provided the charges of fixed ionic groups and poly-counter ions are balanced. These exchangers are excellent reversible sorbents for electrolytes. This will be discussed later when the applications of ion exchangers are being cited.

At the end of this section on the synthesis of ion exchange resins, it may be important to point out that the chemical structures of the polymers shown are hypothetical. It is difficult to establish the resin structure exactly. Furthermore, the structures of the polymers do not represent repeating identical units since the sequence of the monomeric component is essentially random.

SAQ 6 What are the advantages of addition polymeric resins over their condensation counterparts?

...

...

SAQ 7 What is the role of divinylbenzene in the synthesis of styrene-divinylbenzene polymeric resin?

...

...

9.5 TRADE NAMES AND NOMENCLATURE A number of manufacturers of ion exchange resins sell their products with different trade names. Some of these are given in Table 9.1.

15


Table 9.1: Some Commercially Available Ion Exchange Resins

Manufacturer

Trade name

Dow Chemical Co., USA Dowex

Rohm & Hass Co., USA Amberlite

Permutit Co., UK Zeo- Karb/ De Acidite

Chemical Process Co., USA Duolite

Bayer-Farben, Germany Lewalit

Wolfen-Farben, Germany Wolfatit Sicso, India Seralite

Nomenclature The trade names of resins are generally so named that the basic structure is readily apparent. Taking the example of Dowex resin, it will include i) Type i.e. Dowex 50, 50 W( cation exchangers); Dowex 1, 2, 4, 21K (anion

exchangers) ii) X- Number or percent divinylbenzene like X8 iii) Mesh size i.e. 20- 50 ( based on US Standard screen) iv) Ionic form i.e. Na The label will carry something like

Type % DVB Mesh size Ionic form

50 X8 20- 50 Na

9.6 RESIN PROPERTIES As a matter of fact, the resin is a very complex material and there are several properties which are to be known and clearly understood before putting it to any particular application. Some of the important properties are i) Moisture content ii) Particle size iii) Crosslinkage iv) Capacity v) Distribution coefficient vi) Equivalency of exchange vii) Resin selectivity Let us now study them in detail.

9.6.1 Moisture Content The moisture content of the resins is determined in the usual manner by heating it at 110 115C overnight to constant weight. However, several precautionary steps are necessary in this exercise. For example, some resins are thermally unstable in the hydrogen and hydroxyl form and therefore, these should be converted to a stable form before oven drying. Samples which decompose at these temperatures are occasionally dried at room temperature over P2O5 for longer periods of time.

16


9.6.2 Particle Size The importance of particle size for proper column performance in an ion exchange unit is quite obvious. Rate of exchange, pressure drop and back wash expansions are all dependent on particle size. The resin beads or particles may be formed with diameters ranging from 1mm to less than 0.04 mm. For most of the ion exchange operations, an effective size of 0.4 0.6 mm diameter is preferred. This corresponds to particle size distribution falling between the 20- and 50-mesh screens. The ion exchange reactions are mostly conducted in the aqueous media in which the particles have fully hydrated diameter. This is the value that is to be taken into consideration. The size of the water swollen resin will depend on the type of functional group and the amount of cross linking of the polymer.

The size of the particle is one of the parameters affecting the rate of ion exchange reaction. Besides this, the other parameters affecting rate are size and charge of the ion involved, degree of cross linking and the temperature. As a matter of fact, decreasing the size of the particle materially decreases the time required for the equilibrium to be attained with the contacting solution. Since the time required to achieve the equilibration is decreased the efficiency of a given volume of resin increases. In other words, the volume of the resin required to perform a specific operation decreases. The physical aspects of operation are also considerably altered by the change in the particle size. With the decreasing particle size, the friction loss or pressure drop of a liquid flowing through the column increases. This means that for a given flow rate, with decreasing particle size, the pressure drop in a column increases.

An ion exchange column is usually backwashed at the end of an operating cycle to remove the foreign material and reclassify the particles. The back washing step expand the bed to different extents depending upon the specific gravity of the resin. The finer the mesh size and the lower the density, the greater will be the bed expansion.

Generally, the smaller resin particles (~ 50 mesh) are physically more stable. This is important when the resin is mechanically moved or it goes through large volume changes.

9.6.3 Cross Linkages The second variation which can be introduced into the copolymer bead is that of cross linkage. As mentioned earlier, the cross linkage in a styrene- divinylbenzene polymer refers to the fraction of divinylbenzene content. Thus, a resin of 8% crosslinkage is made with beads containing 8% divinylbenzene and 2% styrene and other monovinyl monomers.

The cross linkage affects the resin in two ways. As the amount of cross linkage increases, the dry weight capacity decreases. This decreased capacity results from the greater difficulty of substituting active groups on the copolymers probably due to steric factors. However, as compared to this, the change in water content is more pronounced. Thus, as the cross linkage increases, the resin has a swollen volume for essentially the same number of sites and the wet volume capacity increases.

There are other properties which are affected by the degree of cross linkage. With the decrease in the cross linkage, the resin swells more and thus, the diffusion of ions within the resin becomes faster. This, in turn, gives faster equilibrium rate particularly, for large ions. On the other hand, if the cross linkage is increased, the diffusion paths may become small enough for the entrance of large ions. This offers a possibility of separation of ions based on ionic sizes. A typical example is the separation of sulphate from high molecular weight sulphonic acid by using highly cross linked anion

17


exchange resin. In the same light, we can say that if the cross linking is decreased, the permeable selectivity difference is also decreased.

Cross linkage affects the physical properties also. Highly crosslinked resin is brittle. On the other hand, low cross linked resins are highly swollen; therefore, soft and easily deformed.

9.6.4 Capacity If we consider an ion exchanger, it can be taken as a reservoir of exchangeable ions. In the ion exchange operation, it is the counter ions which are put to use. The counter ions content of a given amount of material is equal to the fixed charges which must be balanced by the counter ions and thus, is essentially constant. This amounts to the fact that it is independent of particle size and shape and of the nature of counter ions.

Ion exchangers are characterized in a quantitative manner by their capacity. In the common usage, it is defined as the number of ion equivalents in a specified amount of the material. But this simple definition is not sufficient and will have to be qualified. The definition becomes acceptable when the conditions are given. Capacity and related data are primarily used for two purposes, for characterizing ion exchange materials and for use in numerical calculations of ion exchange operations. In the second case, it is more practical to use other definitions or quantitatives which reflect the effect of operating conditions. The different types of capacity are given as under.

The total capacity of an ion exchange resin is the number of ionic (or potentially ionic) sites per unit weight or volume of resin. The dry weight total capacity is usually expressed in milliequivalents per gram of anhydrous resin. Scientifically, it is usually expressed as meq/ g dry H+ or Cl form. The wet volume capacity is the number of sites per unit volume of the water swollen resin. The performance of an ion exchange resin is generally based on volume and the wet volume total capacity is the theoretical or maximum capacity which the resin can show in any aqueous ion exchange application. It may be expressed in milliequivalents per milliliter.

The net number of sites which are utilized in a given volume of resin in a given cycle in known as the operating capacity of the resin in that particular cycle. It may be expressed in the same terms as total capacity or as a percent of total capacity.

There is another term which is known as useful capacity which is the capacity when equilibrium is not attained. It depends on experimental conditions viz. ion exchange rates etc. There is another capacity which is known as breakthrough (dynamic) capacity which is utilized in column operation. It depends on operating conditions. There is also a capacity known as sorption capacity which is the amount of solute taken up by sorption rather than ion exchange per specified amount of the exchanger.

9.6.5 Distribution Ratio It should be remembered that we should not speak of a resin to pick up a certain ion without noting that there is another ion in the resin phase. It is actually the tendency of an ion exchanger to pick up A+ at the expense of B+. This tendency of the exchanger to take up A+ will be different if the resin contains other ions C+ instead of B+. Thus, we can prepare a resin containing a certain counter ion and then compare a series of other ions containing this counter ion as a reference. For the ions in this series, we may simply mention distribution ratios. Distribution ratio simply expresses the partitioning of ion between the solution and the resin phases.

solutiontheinionsametheof.Concsinretheinionanof.Conc

=D

18


The conventional units are

solutionofLitre/Amountsinredryofkg/Amount

=D

The amount term, in milligram, moles or whatever may be is proper since the units cancel in calculating the D ratio.

The D values are generally determined by batch method. A known amount of resin is brought in contact with a known amount of metal ion in solution until equilibrium is attained. Because isotherms are non-linear, the D values are taken to be limiting slopes at very low values (Fig. 9.1). The best solution for this is to determine D values at low concentrations by taking labeled solutions using radioisotopes. The D value is determined by simply counting the solution before and after equilibrium with the resin.

Fig. 9.1: A typical curve of loading of an ion exchanger

Sometimes, the distribution ratio is expressed with different values, say

solutionofL/AmountvolumewetofL/Amount

=D

The conversion factor of D to Dv is the bed density, , where is in kg of dry resin per L of resin bed.

For any ion exchange, the importance is its use for the separation that means selectivity. For selectivity, the Dvalues should be different for the ions to be separated. It should be kept in mind that the Dvalues is conditional. It depends upon the nature of resin and the composition of the solution in contact with it. Composition will include pH, ionic strength, type and molarity of acid and the presence of water miscible organic solvents and other ions.

19


Distribution Coefficient There is a term synonymous to distribution ratio which is known as distribution coefficient (Kd). This is also used to express the distribution of the ion between the solution and the ion exchange resin. It is more or less the same as distribution ratio. This weight distribution coefficient of ion is given by

solutiontheofmL1in.Concsinretheofg1in.Conc

=dK

It is only the difference in terminology and it is determined in the same manner as distribution ratio. It is expressed as per gram of the dry resin. It is conditional and dependent on the nature of resin and the conditions prevailing in the solution.

9.6.6 Equivalency of Exchange It is well known that in the process, an equivalency of ion exchange is established. It amounts to the fact that as many ion equivalents of one charge must enter the resin phase as leave it during a reaction process. But there are a number of things which occur in an ion exchange to make it appear otherwise. The simplest case is that of an acid or base neutralization in which the effluent contains only water. There is another example where precipitates may form and be filtered out on the resin. Then too, many substances are physically adsorbed or occluded in the resin at least temporarily; e.g. organic acids and amines. Even so, material balances on an equivalent basis are usually fairly easy to obtain for an ion exchange processes.

9.6.7 Resin Selectivity The strong cation exchanger like Dowex 50 is comparable in acid strength with hydrochloric acid and will form stable salt like bonds with any cation. Similarly, a strong anion exchanger like Dowex 1 is comparable to sodium hydroxide and will form stable bonds with any anions. The only ions which cannot be held strongly by one or the other of these resins are complex ions or organic ions which due to their size or configuration are hindered from entering the interior of resin particle.

The above statement does not mean that all bonds between the strong resin and the different ions are of equal strength. The ion exchange resins will have preference for the particular types of ions they will like to hold if given the choice. It is this preference which is defined as the selectivity of the resin. In the resin systems, the typical physical chemistry equilibrium constant is not strictly applicable. It is substituted by a selectivity coefficient. For a resin containing B ion placed in a solution of ion A and allowed to come to equilibrium, the selectivity coefficient (K) AB for monovalent exchange is given as follows.

( ) ( )( ) ( ) solutionin A of Conc. resin in the B of Conc.

solutionin B of Conc. resin in theA of Conc.)( BA

=K

It can also be written as A + Br Ar + B (9.1)

r

rBA ]B][A[

]B[]A[)( =K (9.2)

Here, r in the subscript represents the resin phase. This definition ignores the activity coefficient of the ions in the two phases. There is no fully satisfactory method for determining the activity coefficient of ions in the resin phase and are thus omitted. The activity coefficient of ions in solutions can be obtained from the literature and can be applied in the above expressions for accurate results when working with other than

20


dilute solutions. In the case of concentrated solutions when the activity coefficient is significantly altered, the selectivity coefficients values should be applied with caution.

It should be kept in mind that selectivity is dependent upon many factors. It varies with temperature and pressure. The effect of pressure has not been investigated due to the nature of the ion exchange technique. However, there are several factors which are of more concern and these are discussed below:

i) Type of functional group Beyond the primary question of whether the resin is a cation or anion exchanger, the effect of functional groups upon the selectivity of the resin is largely a matter of acid and base strength. The difference between the weak and strong exchange resin is rather sharp. But there are shades of strength in both the categories. These differences are largely reflected in the position of hydrogen or hydroxyl ion occupies in the series. To elaborate this point, a typical example can be cited. In Dowex-1 (a strong anion exchanger) all the three groups are methyl groups.

Dowex-2 differs only in that one of the methyl groups is replaced by an ethanol group. This substitution of methyl group changes the selectivity to give a resin which can be converted to the free base form much more efficiently.

ii) Valence and nature of exchanging ions a) At low aqueous( less than 0.1 N) concentrations and ordinary

temperatures the extent of exchange increases with the increasing valency of the exchanging ion, i.e., Na+ < Ca2+< Al3+< Th4+ This means that divalent ions are more tightly held by the resin than monovalent ions and trivalent ions more tightly than divalent ions.

b) Under similar conditions and constant valence, for univalent ions the extent of exchange decreases with the size of hydrated cation. Thus,

Li+< H+ < Na+

21


of ferric ion in concentrated solution of chloride ions. In this case, ferric ion has a tendency to exist as 4FeCl (a complex ion) which is strongly held by a quaternary ammonium anion exchange resin. The iron can be removed from the resin by rinsing with water or dilute acid since the complex breaks down when chloride ions are not present in high concentration. Such examples are numerous in the literature and are successfully exploited for various metal ion separations.

iv) Ionic forms of resin It should be borne in mind that the selectivity of an ion exchange resin changes, usually decreases, as the resin is converted to that particular ionic form. In most cases, this effect is slight. The selectivity coefficient holds for one specific composition of the resin. The ionic form is usually expressed as the mole fraction of the resin which in the A form at equilibrium and is designated as XA. In general, the greater is the value of (K) AB , the greater is its change from XA.

v) Total solution ionic strength Mono-monovalent exchanges are usually little affected by the change in the total ionic strength. However, it becomes important if the exchanges are taking place in different valence say mono-divalent exchange.

Let us first consider mono-monovalent exchange NaCl + RSO3H HCl + RSO3Na ... (9.3)

(K ++NaH ) = r

r

]H][Na[]H[]Na[

++

++

... (9.4)

By substituting the values as follows,

X rNa+ = Equivalent fraction of Na+ in resin = (Na+)r/ C r (9.5)

X +Na = Equivalent fraction of Na+ in solution = (Na+) / C r (9.6)

where C r = total capacity or normality of resin (equiv./ L) and C = total normality of solution.

Eq. 9.4 takes the following form

r

r

X

X

+

+

Na

Na

1= K

+

+NaH

+

+

Na

Na

1 X

X ( 9.7)

This equation gives the equivalent fraction of Na+ in the resin as a function of the solution with which the resin is in equilibrium. It may be noticed that the terms C r and C do not figure in the Eq. 9.7.

Now consider the exchange of monovalent ion with divalent ion. CaCl2 + 2 RSO3Na 2 NaCl + (RSO3)2Ca ... (9.8)

K++

+CaNa = 2

r

2r

]Na][Ca[]Na[]Ca[

+++

+++

(9.9)

When Eq. 9.9 is expressed in terms of the equivalent fraction of Ca++ in the resin as a function of the solution, it becomes

22


2rCa

r

Ca

)1( ++++

X

X= K

++

+CaNa 2

Ca

Car

)1( ++++

XX

CC

(9.10)

In Eq. 9.10, the apparent selectivity coefficient is the term K++

+CaNa )( C

C r.

C r is the total capacity of the resin per unit volume and, therefore, is fixed for particular resin, the selectivity of the divalent ion in this exchange is inversely related to the total concentration of the solution. It can be concluded that the more dilute the solution, the more selective the resin becomes for the divalent ions. The normality of the resin phase (C r) will depend upon the swollen volume of the resin and thus, is a function of cross linkage. Similarly, the apparent selectivity coefficient has this same form for exchanges between divalent and trivalent ions. However, in the case of exchange between

monovalent and trivalent ion, the expression takes the form (K) ++++AB (2)

CC r

.

This means that the selectivity of the resin for the trivalent ions is inversely related to the square of total solution concentration.

It can be concluded that this polyvalent ion effect makes efficient water softening possible. The divalent cations ( Ca++ and Mg2+) are easily picked up from hard water ( a very dilute solution) and yet are easily displaced by Na+ ions of a relatively concentrated (10 15%) salt solution. For the same reason, rehardening of water i.e., replacement of Ca++ / Mg++ ions by Na+ is difficult by ion exchange. This polyvalent ion effect holds good for both cation and anion exchange.

SAQ 8 What are the main factors on which the swelling of the resin bead will depend when immersed in water?

...

SAQ 9 What is the main advantage if the degree of cross linkage of the resin is decreased?

...

SAQ 10 Name the important variables of the solution on which D/Kd value of an ion for a particular resin will depend?

...

SAQ 11 State whether the following statements with regard to the extent of exchange are TRUE/ FALSE. i) The extent of exchange for Fe3+ is more than for Ce4+. ii) Under similar conditions, the extent of exchange for Na+ is more than for Li+. iii) For divalent cations the uptake of ion is only determined by the ionic size of the

cation.

23

Ion Exchange Chromatography 9.7 OPERATING METHODS

So far you have learnt about different properties of the resins and their effect on the resin behaviour particularly with regard to their utility for separations. In the context of utility of resins, a question arises as to how are the resins operated or brought in contact with the solution. There are two main techniques for this. They are batch and column methods. The column method may be further sub-divided as to whether the resin bed is fixed or moving and whether the feed solution and regenerant solution flow past the resin in the same relative direction or in opposite direction.

9.7.1 Batch Operation The batch method consists of immersing the resin in the solution in a container and allowing the equilibrium to be established. The extent of exchange of ion is limited by the selectivity of the resin under equilibrium conditions. Therefore, unless the selectivity is quite favourable, only a relatively small part of the total capacity of the resin can be utilized. It is convenient to use a resin batchwise but it is generally impractical to regenerate the resin for reuse batch-wise.

9.7.2 Column Operation Column operation can be visualized as a large number of batch operations in series. The extent to which the exchange takes place in each one of these small batch operations is limited by the appropriate selectivity coefficient, the overall effect may be much more favourable. The successive batch operation in a simulated column may be considered to plates in a distillation column. In a majority of units, ion exchange containers are used and they are taken as vertical columns filled with ion exchanger (Fig. 9.2). The resin is supported on a bed of graded gravel or some other filter base and the feed and regenerant solution passed through the column (down-flow operation) or up through the resin (up-flow operation).

Fig. 9.2: Ion exchange column with fraction collector

Generally, fixed bed units are used with down flow operation. It gives a maximum of resin solution contact and a minimum of mechanical problems. A fixed bed ion exchange column may be operated with counter-current flows. In such a system, the feed is put through the column down flow and the regenerant is put through up-flow and vice-versa. A large number of column arrangements have been designed in which multiple column are piped together to give semi-continuous operations, maximum resin utilization, regenerant recovery or some other improvements.

24


9.7.3 Moving Bed Operation A somewhat different type of column operation is that encountered in a moving bed or continuous counter-current system. In such a system, the resin as well as the solution, is made to flow through the system. A typical unit consists of two stages in which the resin is contacted counter-currently with the exhausting stream and the regenerant stream. The chemistry of such an operation is similar to fixed bed operation.

The advantages of moving bed operation are those with continuous operation say a constant supply of a product of uniform quality and reduced cost of space, capital and labour. There are some design problems due to the movement of resin. Maintaining a counter- current flow of resin and solution depends upon the densities of two phases.

9.8 ION EXCHANGE IN MIXED AQUEOUS-ORGANIC MEDIA

Up to this point, you may be carrying an impression that ion exchange takes only when the aqueous solution is brought in contact with the solid exchanger. There must be ions in both the solution and the solid. The ions must be free to move and the exchange takes place. With water as a solvent and with solids which qualify as an ion exchangers, these conditions are usually met. Water, because of its high dielectric constant, is an excellent solvent for most inorganics and quite a number of organic acids, bases and salts. This is what justifies the choice of water as a phase for ion exchange. Water, however, is by no means the only solvent which allows ion exchange to take place. There are other solvents with high dielectric constant ( ) in which electrolytes can dissolve and dissociate and in which most of the ion exchangers are stable. These are ethylene glycol ( = 41), methanol ( =32), ethanol ( = 26) and acetone ( = 27). The last three solvents, in particular, have importance in ion exchange. They can be used with or without addition of water.

This particular section is devoted to ion exchange in mixed aqueous organic media. It has been observed that some irregular trends in the distribution of metal ions between mixed aqueous- organic media and the ion exchanger are exhibited when the percentage of the organic content is varied. The organic solvents used are water miscible oxygenated compounds like tetrahydrofuran, methanol, ethanol and acetone. The behaviour of the metal ions with varying organic content has been usefully exploited for achieving some difficult separations. Korkisch pioneered this technique and named it as combined ion exchange solvent extraction and coined the abbreviation CIESE for it. In order to explain the behaviour, in chromatography using these solution two processes are operative: ion exchange and liquid-liquid extraction. The mechanism can be explained on the following lines.

i) The addition of organic content reduces the dielectric constant of the solution promoting thereby ion pair formation between the ionic species and fixed ions of the ion exchanger. Consequently, with increasing organic content, the uptake of ionic species on the ion exchanger may increase.

ii) Sometimes, the addition of organic content of the aqueous solution causes a decrease in the distribution coefficient of the ion. This is probably due to the attachment of the anionic complex to the protonated organic solvent making thereby the anionic complex less available for the solid anion exchanger. It may be worthwhile to explain the formation of this ion-pair between the protonated organic solvent and the anionic metal complex on the lines similar to that discussed in sub-Sec. 3.2.4 of Unit 3. This mechanism in this case can be illustrated as follows:

25


(Organic solvent) + H+ ( Protonated organic solvent) (Protonated organic solvent) + ( Anionic complex of the metal)

complexnassociatioIon)} the metalcomplex of( Anionic solvent) ed organic{(Protonat

It may be important to point out that the force of attraction between the two ionic components of the ion association complex increases with the increasing organic content i.e., decreasing dielectric constant.

iii) In some rare cases, the organic solvent may form complexes with the metal ions bringing a change in their distribution coefficient values.

The ion exchange and solvent extraction may operate simultaneously in a particular system and compete with each other resulting in to some irregular trends. Sometimes, the irregularity of these trends is difficult to explain. Nevertheless, it does not reduce the potentiality of the technique for separation purposes. In order to highlight the utility of the technique, the distribution coefficient of Au (III) and Hg(II) for Amberlite IR 400 (anion exchanger) at 0.6 M HCl with changing percentage of tetrahydrofuran (THF) are given below in Table 9.2.

Table 9.2: I Values Data THF

Concentration

0 20 40 60 80 90

Au (III)

5232 4308 241 2.6 2.0 0.1

Hg (II)

>104 370 368 282 201 200

In Table 9.2, there is a decreasing trend in the distribution coefficients of both the metal ions with the increasing percentage of THF content. Without any THF at 0.6 M HCl, both Au(III) and Hg(II) are strongly adsorbed on Amberlite IR-400 and it is difficult to separate them. The best condition for separation is achieved at 0.6 M HCl with 90% THF.

SAQ 12 From the data given in Table 9.2, comment on the mechanism operating leading to decrease in the values of distribution coefficients of the two metal ions.

...

...

9.9 SPECIFIC CATION EXCHANGERS In the beginning of this Unit, it was mentioned that there have been so much developments in the field of ion exchange that you can more or less have ion exchangers tailor made for a specific job. Attempts in this direction led to what are known as specific cation exchangers. This means these ion exchangers show unusual selectivity towards a cation or a group of cations. If we look at the common general purpose cation exchangers,s they prefer certain counter ions. There is some sort of specificity. But here we mean the ion exchanger to be exclusively specific. The basic idea to synthesize such resins come from the fact that a reagent which either precipitates a cation or forms a strong complex may be introduced in the matrix of the

26


resin. The first attempt in this direction was made by Skogseid who synthesized a resin containing group with a configuration similar to that of dipicrylamine.

Dipicrylamine Dipicylamine is a known specific precipitating agent for K+ ions. The resin is synthesized from polystyrene by nitration, reduction, condensation with picrylchloride and again nitration.

Many compounds which form chelates with metal ions have been incorporated into resins while polycondensation with phenols and aldehydes. To cite an example, there is one with anthranilic acid. It is selective for zinc and other transition metal ions.

Other compounds which have been used are o- aminophenol, anthranilic acid- diacetic acid, m-phenylenediglycine.

There are chelating resins containing groupings similar to those of the more conventional chelating compounds, e.g., EDTA (ethylenediamine tetracetic acid) but attached to a cross linked matrix for gross insolubility. These compounds tightly bond certain metal species which tend to form highly stable structures. Dowex A- 1, chelating resin, contains iminodiacetate groups attached to a cross linked polystyrene matrix.

The resin has a very greater affinity for chelate forming di- and trivalent cations than for cations like Na+ or K+. This resin is particularly useful where one wishes to overcome the competing effect of high concentration of one or ions. Thus, it will effectively remove traces of heavy metal ions such as Fe3+, Cu2+ and Zn2+ from concentrated solutions of alkali and alkaline earth cations can be removed. Metals can be eluted from the resins with mineral acids. Selectivity among transition metal ions can be attained by adjustment of pH.

There is one unattractive feature which is common to all specific ion exchangers. As a matter of fact, the desired selectivity for certain ions is attained by introducing certain

27


groups for which the counter ion has the affinity. As a result of this, the mobility of counter ions is greatly reduced. Thus, the gain in selectivity is at the cost of rate of ion exchange. There is another problem which arises due to extreme specificity. It is difficult to replace the preferred counter ion except when it is replaced by H+ ions. This will mean that it may be difficult to regenerate the resin. In light of the above, one should choose a resin keeping in mind a compromise between selectivity, rate of ion exchange and ease of regeneration.

SAQ 13 What are the two main limitations of chelating resins?

...

...

9.10 SYNTHETIC INORGANIC ION EXCHANGERS In sub-Sec. 9.3.2, it was pointed out that the resinous exchangers cannot withstand high temperatures and radiation dose. Moreover, they get degenerated in highly oxidizing media. Towards the end of 1950s, there was a great deal of activity in nuclear fuel technology, particularly, reprocessing of nuclear fuel and the need of ion exchangers which can withstand high temperatures and radiation dose was felt. This revitalized the interest in inorganic ion exchangers and the chemists started synthesizing inorganic ion exchangers. In a span of about three decades, a huge variety of inorganic ion exchangers were synthesized and put to use for different separations and other applications. Most of these materials are amorphous in nature, almost gel like materials which after drying, are ground to the desired mesh size. Only a few compounds have been synthesized in a well-defined crystalline structure. Let us now study about different types and their characteristics.

9.10.1 Different Types and Their Characteristics The different synthetic inorganic ion exchangers can be broadly classified under the following categories. i) Hydrous oxides of polyvalent metals.

ii) Insoluble acidic salts of polyvalent metals.

iii) Salts of heteropolyacids.

iv) Insoluble ferrocyanides.

v) Synthetic aluminosilicates.

vi) Miscellaneous inorganic ion exchangers e.g., synthetic apatites, end sulphides.

i) Hydrous oxides of polyvalent metals The hydrous oxides are of particular interest because most of them can function both as cation and anion exchangers above and below a certain pH value. These substances are mostly amphoteric in nature and their behaviour mainly depends upon the basicity of the central atom and the strength of the M O and O H bonds. The hydrous oxides of tetravalent elements e.g., Zr (IV), Ti (IV), Mn(IV), Sn(IV) and Ce(IV) are the most studied compounds of this class. Mixed hydrous oxides of di- and tetravalent metals, tri- and tetra-valent have also been investigated. The hydrous oxides of quinquevalent and sexivalent metals generally show cation exchange properties and are stable towards most of

28


the commonly used reagents. The hydrous oxides are useful for column operations and can be easily regenerated for use. Apart from enabling routine separations of various cations and anions, hydrous oxides have been used for the purification and isolation of transuranium elements from highly radioactive fission products. Titanium oxides columns have been used for the recovery of uranium and plutonium from the spent nuclear fuels.

ii) Insoluble acidic salts of polyvalent metals The acidic salts of multivalent metals form one of the most extensively studies class of compounds. A wide range of compounds of this type has been described as ion exchangers. They include phosphates, arsenates, antimonates, vanadates, molybdates, tungstates, tellurates etc. mostly of trivalent and tetravalent metals like Al(III), Cr(III), Fe(III), Ti(IV), Zr(IV), Sn(IV), Ce(IV) and Th(IV). These salts mostly act as cation exchangers and their exchange properties prominently arise from the presence of readily exchangeable hydrogen ions associated with the anionic group. A bewildering array of these acidic salts are known, mostly in the form of gels. Mixed acidic salts like zirconium arsenophosphate, tin (IV) arsenophosphate etc. have also been explored as ion exchangers.

Gels of these acidic salts have a potential of use for separation of heavy metal cations by column chromatography. They have been used for paper chromatography by impregnating the papers with them. The gels do not have a definite composition and are not very stable towards the hydrolysis of the acidic group. Because of uncertainity about the exact composition and structure of gels, it is very difficult to understand the exact mechanism of ion exchange reaction. A real breakthrough in these exchangers came when some of them were prepared in definite crystalline forms. Phosphates and arsenates of Zr(IV), Ti(IV), Sn(IV), Th(IV) and Ce(IV) have been obtained in crystalline forms. One of the most studied compounds of this series is zirconium phosphate which has been obtained with different degree of crystallinity and in different crystalline forms.

iii) Salts of heteropoly acids The parent acids of the compounds are 12-heteropoly acids of the general formula Hm XY12 O40. nH2O (m = 3, 4, 5) where X may be phosphorus, arsenic, silicon, germanium and boron and Y different elements such as molybdenum, tungsten and vanadium. Amongst the exchangers of this category, work has been mainly reported on 12- molybdophosphates. Ammonium molybdophosphate and ammonium tungstophosphate are the two widely investigated compounds for their physicochemical behaviour and practical applications. Salts of heteropoly acids act mainly as cation exchangers. They have been mainly used for concentration and purification of Cs137from fission products. The behaviour of heteropoly acid salts with quaternary organic cations like pyridinium, picolinium, collidinium has been studied as ion exchangers.

iv) Insoluble ferrocyanides The ion exchange properties of a large number of insoluble ferrocyanides of various metals e.g., Ag(I), Zn(II), Cd(II), Cu(II), Ni(II), Co(II), Pb(II), Mn(II), Fe(III), Ti(IV), Zr(IV), V(V), Mo(VI), W(VI), U(VI), have been studied. The ferrocyanides act as cation exchangers with a high affinity for heavy alkali metal ions, specially for Cs+. The ion exchange mechanism in ferrocyanide is rather complicated and not yet clear. In order to improve the mechanical properties of ferrocyanides for use in column operations, the exchanger has been prepared by precipitation on solid inert supports e.g., bentonite, silica gel, etc., freezing the

29


gel or bonding the precipitate particles to insoluble polymers such as polyvinyl acetate.

v) Synthetic aluminosilicates These compounds represent a great family of inorganic ion exchangers and depending upon their structure they may be divided into the following three main groups:

Amorphous

Two dimensional layered aluminosilicates, and

Aluminosilicates with rigid three dimensional structures (zeolites) Among these groups, synthetic zeolites have attracted increasing attention because of their molecular and ion-sieving properties. They have been successfully employed in gas adsorption and catalysis.

vi) Miscellaneous inorganic ion exchangers Exchangers like apatites and sulphides are included in this class. The anionic and cationic components of the apatite structure, M10 (XO4)Y, are exchangeable, where M = Ca, Sr, Ba, Cd and Pb; X= P, As, V, Cr, Mn, Si, Ge and Y= F2, Br2 (OH)2, O and CO 23 . The structure, physicochemical properties, thermal stability and ion exchange properties of these materials have been reported. The ion exchange properties of a wide range of sulphides e.g., Ag2S, FeS, CuS, ZnS, PbS, CdS, NiS, As2S3 and Sb2S3 have been studied. The sulphides are selective towards cations forming insoluble sulphides. The metal of the sulphide is displaced by the appropriate ion in solution.

9.10.2 Special Properties and Applications Here, it may be important to point out that earlier a complete section has been devoted to the properties of ion exchange resins and the next section will be presenting a detailed overview of applications of ion exchange resins. In such a situation, this sub-section for the properties and applications of synthetic inorganic ion exchanger may at first sight appear out of place. But in this section, we are going to discuss about some unusual characteristics of some inorganic ion exchangers and the applications based upon them. One important point that has to be kept in mind is that it is difficult to highlight all the unusual features shown by different classes of inorganic exchangers and the applications based upon them. Therefore, only some important characteristics and applications are being discussed below.

i) Radiation and thermal stability Generally, it is taken as more or less granted that the synthetic inorganic ion exchangers are more resistant to radiation damage than their organic counterparts. However, the studies have shown that it is not justified to make a generalization about them. Each ion exchanger needs to be tested for its radiation stability. A large number of them have been used for processing fission products. The ferrocyanides and salts of heteropoly acids are used for the recovery of Cs137. The radiation stability combined with separation capability yields a system which is known as isotope generator. A radionuclide generator is a system/ device which makes the repeated recovery of a short-lived isotope in a pure form from a relatively long- lived parent isotope. A typical example is a Ba131- Cs131 generator on hydrous zirconium oxide. Short lived isotopes find use in nuclear medicine for diagnosis and therapy.

30


By now a fairly good amount of data is available that the thermal stability of inorganic ion exchangers particularly of those amorphous in nature has been over emphasized. A number of amorphous materials start losing their ion exchange capacity on heating. However, some of them are quite resistant to heat. Hydrous Ta2O5 is quite heat resistant upto 300C and is used for decontaminating nuclear reactor cooling waters. A promising use of these exchangers could be in fuel cells at high temperatures or for concentrating the nuclear waste. Thermally stable ion exchanger in the transition metal forms are used as high temperature catalysts. Layered zirconium phosphate as such or in other metal form has been used for catalyzing different organic reactions.

ii) Unusual selectivity Many of these inorganic ion exchangers show high selectivity for particular ions and therefore, the separation of these ions can be more conveniently carried out than on typical organic resins. Such examples are numerous in literature. Zirconium antimonate exchanger has been used for the separation of Rb+ and Cs+. Cerric antimonate is more or less specific for Hg2+ and is used for the separation of Hg2+ from Cd2+/ Pb2+. Zirconium phosphate like ferrocyanides and heteropoly acid salts shows unusual selectivity for Cs+ and is used for the removal of Cs+ from the nuclear reprocessing solution. Because of high capacity of zirconium phosphate for NH +4 , it is used in artificial kidney machine. The selectivity features are not only confined to column separations but they have been extended to paper chromatography in which the papers are impregnated with the inorganic ion exchangers.

The unusual selectivity and stability of synthetic inorganic ion exchangers make them suitable for use in ion selective electrodes. They have been investigated for use as materials for membranes.

Apart from the uses mentioned above, the studies on inorganic ion exchangers throw light on problems such as sorption of ions by precipitates, electrophoretic behaviour of suspensions, isotopic exchange in heterogenous systems and many other areas of solid state chemistry.

Towards the concluding stage of discussion on synthetic ion exchangers, it is important to point out that very few inorganic exchangers have been used on commercial scale. The main reason seems to be that among the useful ones, the majority are based on metals which are costly. The other deterent seems to be that their regeneration power is not as good as that of their organic counterparts.

SAQ 14 What are the main advantages of majority of inorganic ion exchangers over their organic counterparts?

...

...

SAQ 15 Which is the most thoroughly studied class of inorganic ion exchangers? Which particular compound has received the maximum attention?

...

...

31

Ion Exchange Chromatography 9.11 APPLICATIONS

Ion exchange is one of the very powerful tools for separations. It has a very broad spectrum of applications and the simple property of exchanging the ions has been very intelligently exploited for various purposes of separation, enrichment, recovery and decontamination in various areas of science and technology. In the limited space of discussion available here, it may not be possible even to simply list the applications and therefore, it is difficult to accommodate a detailed explanation about them. But for the purposes of clarity in presentation in a concise form, the entire range of applications is being subdivided into the following heads: i) Separation of metal ions and anions ii) Separation of organics iii) Separation of ionized from nonionized iv) Separation of actinide elements v) Miscellaneous applications.

These are briefly presented below:

9.11.1 Separation of Metal Ions and Anions One of the largest uses of ion exchangers is their capability to separate metal ions. Ion exchangers show natural selectivity among cations but it can be enhanced by proper choice of aqueous medium, type of ion exchanger and eluting agent. The literature is full of examples where separations of topical interest are conveniently achieved.

The choice of different cation exchangers for the separation of different cations looks logical but it may be important to point out that almost equal number of separation of metals are achieved on anion exchangers. The metals on anion exchangers are not separated as cations but as anionic metal complexes. In this context, the best example is the separation of metals as anionic chlorocomplexes. The ease of formation of these complexes and their stability determine the selectivity on the anion exchangers. There is a full periodic table type chart available for the sorption behaviour of different metal ions on Dowex-1(strong anion exchanger) in the complete range of acidity of hydrochloric acid. The data given therein have been very useful in designing metal ion separations. As a typical example Co(II) and Ni(II) are separated from hydrochloric medium on a strongly basic anion exchanger (Dowex-1). The separation is based upon the fact that Co(II) but not Ni(II) forms an anionic chlorocomplex (probably CoCl 3 ) in 9M HCl presumably because of instability of the chlorocomplex. The retained Co(II) is washed from the column by water because the complex is decomposed and cobalt is recovered as cobaltous chloride.

There are numerous example like this. The extremely high selectivity of the quaternary ammonium type anion exchange resins for the metallic anionic complexes is one of the most amazing examples of ion exchange chromatography. The industrial application of cation exchangers for water softening is well-known where mainly the Ca2+, Mg2+ and other heavy metal ions are removed by the sodium form of the exchanger. For complete deionization of water, it is passed through a cation exchanger and then an anion exchanger or a mixed bed of both types of ion exchangers.

The best illustration of potential of ion exchange to separate closely similar metal ions is the separation of rare earths. A cation exchange resin alone can provide separation since the affinities of the lanthanide ions for the resin vary inversely with their hydrated radii and these, in turn, vary inversely with the crystallographic radii. Thus, the order of elution is LuLa. The similarity of various rare earths necessitates the use of complexing agents to increase the separation factor. The use of carboxylic acids

32


such as citric, glycolic and tartartic, adjusted to appropriate pH, as eluting agents markedly enhances the separation. Among the other useful ligands employed as eluting agents are -hydroxyisobutyric acid, EDTA, and 2-hydroxy-EDTA.

The rich deposits of the metals are exhausting and the metallurgists have to depend upon low grade reserves and other leaner sources like the metal wastes. For the recovery of metal from low grade ores or the metal wastes, the matrices are first leached with various reagents. The resulting leach liquors besides the metal of interest contain several ionic impurities. The ion exchange technique has proved to be very useful in increasing the metal values of the leached liquors. Besides this, the resins are used to recover metal values in the tailings of other hydrometallurgical operations. Ion exchange resins have also been employed for the upgrading of impure concentrates. In the metallurgy of uranium, it is quantitatively recovered from the leach liquor by means of anion exchange. Ion exchange has been used for the recovery of gold. Another example of the successful use of ion exchange is in the recovery of chromium from electroplating waste.

The separations like molybdenum from rhenium, zirconium from hafnium and niobium from tantalum have been achieved using ion exchange chromatography and the conditions developed are used in metallurgical operations.Several anions interfere in the estimation of various cations and vice-versa. In such situations, ion exchange chromatography is confined to inorganics.

9.11.2 Separation of Organics Ion exchange is equally effective for the separation of organic molecules like amino acids, sugars, nucleic acid and peptides. The purification of several important organic molecules have been achieved by ion exchange chromatography. The use of carboxylic acid cation exchange resin for recovering and purifying the antibiotic, streptomycin, is one of the classic examples of industrial use of ion exchange chromatography. It should be kept in mind that streptomycin is a cation.

9.11.3 Separation of Ionized from Nonionized The separation of ionized materials from nonionized or slightly ionized materials when both are present in water is accomplished by a process known as ion exclusion. The process utilizes conventional ion exchange resins. If we look at a column of ion exchange resin, it contains three phases; the solid network of resin beads, the liquid inside the beads (resin liquid) and the liquid surrounding the beads (interstitial liquid). Most low molecular weight solutes diffuse freely in and out the resin liquid phase. However, the organic non-ionic solutes tend to exist at the same concentration in both the resin liquid and the interstitial liquid phases.

The ionic materials, because of Donnan membrane effect, exist at a considerably lower concentration in the resin liquid than in the interstitial phase. Thus, if a solution containing ionic and nonionic substances are fed to the column and the column is rinsed with water, the ionic solution will reach the bottom first because it has to essentially displace the interstitial liquid. The non-ionic solution must displace the interstitial liquid and the liquid inside the beads. Thus, the nonionic material will emerge out of the column after the ionic solute has passed out of the column.

Hence, ion exclusion offers a method of deionizing or removing majority of ionic constituents from organic products without the use of heat, electricity and chemical regenerants. Typical examples of applications of ion exclusion are separation of acids and salts from glycerine, alcohol and amino acids, the separation of strongly ionized and weakly ionized materials such as acetic and mineral acids, mono-, di- and triethanolamine and mono-, di- and trichloroacetic acid.

33


9.11.4 Separation of Actinide Elements Similar to the separation of lanthanides ion exchange chromatography has played a major role in the separation of actinides, especially trans plutonium elements. Glen T. Seaborg used ion exchange equipment to identify each element of the 5f series beyond any doubt by the sequence of their appearance analogues to the sequence of the corresponding 4f elements. The primary valency of all the actinides is +3 similar to the lanthanides which also exhibit decreasing ionic radii (cf lanthanide contraction). In practice, solution containing all the actinides is sorbed on the top layer of a column containing acidic cation exchanger. Individual actinides are eluted from the resin bed by passage of an eluent solution through the column of resin as shown in Fig. 9.3.

Fig. 9.3: Elution of tripositive actinide and lanthanide ions

The elution is accomplished by the use of another metal ion (i.e. M3+) which shifts the equilibrium of the exchange reaction through competition with M+ for position in the resin. Alternatively, it is also achieved by adding a complexing anion to the solution which, by reducing the concentration of free metal ions, also shifts the equilibrium to the left. In Fig. 9.2 is shown the elution sequence for the separation of the lanthanide and actinide cations from a column of cation exchange resin using a complexing agent. It was considered as the big triumph for ion exchange chromatography for the separation of various actinides which was otherwise considered as impossible.

9.11.5 Miscellaneous Applications The mosaic of applications of ion exchange will be incomplete if we do not mention the applications from other areas of science. Here too, it is difficult to enumerate even some of them because they are too many to list. Thus, only a few of them are being

34


mentioned here. Ion exchange resins are polymeric materials that may be considered insoluble acids and bases. They can be used to promote reactions which can be catalysed by conventional acids and bases. Some advantages of solid substantially insoluble ion exchange catalysts are as follows: i) ease of separation by filtration or decantation, ii) reduction of cost because the catalyst can be used repeatedly usually without

regeneration, iii) increased product yield and efficiency, and iv) elimination of corrosion problem. The major disadvantage of using ion exchange resins as catalyst appear to be thermal and chemical stability limitations. In some cases, this particular problem is resolved by use of inorganic ion exchangers. Some of the examples of ion exchange catalyzed reactions are acetal formation, alcohol dehydration, aldol condensation, esterification and ester hydrolysis.

The utilization of ion exchange in food processing has been quite successful in both beverage and canning industries. In the bottling of carbonated beverages, the presence of carbonate and bicarbonate in water supplies has to be removed. They neutralize the citric and phosphoric acids added to carbonated beverages. The treatment of raw wines and whiskeys with ion exchange resins is of considerable interest. The anion exchange resins remove aldehyde and catalyze several esterification reactions thereby improving the taste and bouquet of the product.

Ion exchange resins have been scanned as therapeutic agent or additives in host of medical disorders or ailments. A purified highly subdivided weak base anion exchanger has been quite successfully used as antacid in peptic ulcer therapy and other gastrointestinal disorders. The use of cation exchangers as means of removing sodium from the body in the treatment of edemas and hypertension has been very encouraging.

Ion exchange finds a very useful application in agricultural science in the form of formulations for plant nutrients. Ion exchange formulations containing nitrogen, phosphorus and potassium in addition to the minor nutrients that may be deficient have been found useful in fortifying a wide variety of potting soils. Such a fortified soil may retain a large supply of nutrient without injury to plant. The need for frequent fertilization is eliminated, nutrition is continuous and self-regulatory.

Before we conclude this section, it may be mentioned that preceding text simply acts as pointer to the great scope of ion exchangers in different areas of science and technology.

SAQ 16 Give two important examples of use of ion exchange from organic chemical technology.

...

...

SAQ 17 How does ion exchange help in improving the quality of alcoholic drinks?

...

...

35

Ion Exchange Chromatography 9.12 SUMMARY

The unit begins with a historical background of the process of ion exchange and focuses on the point that the most important breakthrough in the field was the discovery of ion exchange resins. The cation, anion and amphoteric exchangers are known. The basic features of ion exchange mechanism are discussed. There are organic and inorganic ion exchangers but the former dominate the field. Some important features of synthesis of ion exchange resins are discussed.

There are two main routes of synthesis-condensation polymerization, and addition polymerization. Because of certain advantages, the addition polymerization has taken an edge over the condensation polymerization route. An idea is given about the trade names of different types of ion exchangers assigned by the manufacturers and the information generally provided on the labels.

The ion exchange polymers are very complex materials and some of their characteristics have to be properly understood before they are put to use. The different properties discussed in detail are moisture content, particle size, cross-linkage, capacity, distribution ratio, equivalency of exchange and resin selectivity.

The different methods used for operating ion exchangers are elaborated. The mechanism operating during the uptake of cations in mixed aqueous organic media is explained. This is followed by discussion on chelating resin and synthetic inorganic ion exchangers. The important features, classes, advantages and drawbacks of these types of exchangers are discussed.

The unit concludes with a discussion on applications. There are too many applications. But for the purposes of clarity of presentation, only a few representative ones are cited and they are discussed under different groups.

9.13 TERMINAL QUESTIONS 1. What are the important characteristics of a useful ion exchanger?

2. What are the broad parameters on which the chemical, thermal, and mechanical stability and ion exchange behaviour of the resins depend?

3. What are the advantages if the particle size of the ion exchange resin bead is decreased? Is there any serious drawback in decreasing the particle size?

4. Name the factors on which the selectivity of an ion exchanger for an ion depends.

5. What are the apparent selectivity coefficients for a mono-divalent and mono- trivalent exchange? What are the implications of the term?

6. Suggest the best possible ion- exchange based method for the following: i) Separation of K+ from Na+

ii) Separation of Co2+ from Mg2+ iii) Recovery of Cs137 from highly radioactive fission product waste. iv) Separation of HCl from CH3COOH. State only the broad outlines.

36


7. What are the broad categories of synthetic inorganic ion exchangers? Which one of these shows anion exchange properties?

8. What particular property is responsible for the separation of lanthanides by ion exchange chromatography? How is the separation potential of this technique enhanced?

9. What is the principle involved in ion-exclusion method?

10. What are the advantages of using ion-exchange resins as catalysts?

9.14 ANSWERS Self Assessment Questions 1. The characteristic difference between adsorption and ion exchange is that the

later is a stoichiometric process. Every ion removed from the solution is replaced by an equivalent amount of another ionic species of same sign. However, in adsorption solute may be taken up without any species being replaced.

2. 3 XCl + Na3PO4(aq) X3PO4 + 3 NaCl(aq)

Here, X represents the structural unit of the ion exchanger and PO 34 is present in the solution.

3. The two distinct classes of aluminosilicates are one with three-dimensional network and the other with loose layered structure.

4. The organic resinous exchangers deteriorate fast at high temperature, under high dose of ionizing radiations and in highly oxidizing media.

5. Yes, there is some justification because of the following two reasons: i) The mechanism of transfer in both cases is ion exchange ii) The column loaded with liquid ion exchanger ( extraction

chromatography) behaves more or less in a way similar to that shown by a column packed with solid ion exchanger.

6. The addition polymeric resins are chemically and thermally more stable than the condensation polymers. Moreover, in addition polymerization, the degree of cross linking and particle size is easy to control.

7. The role of divinylbenzene in the synthesis of styrene-divinylbenzene polymeric resin is as a cross-linking agent. The degree of cross-linking can be adjusted by varying the amount of divinylbenzene content.

8. The size of the swollen bead will depend on the type of functional group and the degree of cross linking of the polymer.

9. If the degree of cross-linking is decreased, the resin swells more and thus, the diffusion of ions within the resin becomes faster. This, in turn, gives faster equilibrium rate particularly for large ions.

10. The D/Kd value depends on the composition of the solution. This includes pH, ionic strength, type and molarity acid and presence of water miscible organic solvents and other ions.

37


11. i) FALSE ii) TRUE

iii) FALSE

12. From the decreasing trend in the Kd value data with increasing THF content, it can be proposed that the anionic chlorocomplexes of both the metal ions are not being available to the solid anion exchanger. They are being held as ion association complex by the protonated organic solvent. The prominent mechanism is

(Organic solvent) + H+ ( Protonated organic solvent) (Protonated organic solvent) + ( Anionic complex of the metal)

lexation compIon associ)lexmetal comp( Anionic solvent) d organic (Protonate

The force of attraction between the two is increasing with increasing organic solvent, i.e. lowering of dielectric constant of the medium.

13. The two main limitations of chelating resins are as given below: i) By introduction of certain groups, the mobility of counter ions is reduced;

hence, the kinetics of exchange is slowed down. ii) It is difficult to regenerate the resin.

14. The main advantages of synthetic inorganic ion exchanger over their organic counterparts are

i) higher thermal and radiation stability ii) higher chemical stability in highly oxidizing media, and iii) unusual selectivity for particular ions.

15. The most studied class of inorganic ion exchanger is acidic salts of polyvalent metal ions and zirconium phosphate has received maximum attention.

16. The two examples are as follows: i) separation of amino acids, and ii) recovery and purification of streptomycin.

17. The quality of alcoholic drinks is improved by anion e