Ion Exchange: Theory and Practice (RSC Paperbacks) - Softcover

Harland, C E

Softcover

ISBN 10: 0851864848 ISBN 13: 9780851864846

Publisher: Royal Society of Chemistry, 1994

Synopsis

This revised volume covers the practical applications of ion exchange and the synthesis of organic ion exchange resins, which span 60 years of development. It covers the development of ion exchange materials and their utilization in industrial processes. Key applications in such areas as water purification, hydrometallurgy and chromatography are described and supported by chapters on the related scientific fundamentals governing equilibria and kinetics. 22 experiments are detailed, which not only compliment a chapter dedicated to the characterization of organic exchangers, but also serve to illustrate several pure and applied principles related to ion exchange phenomena.

About this title

Excerpt. © Reprinted by permission. All rights reserved.

Ion Exchange

Theory and Practice

By C. E. Harland

The Royal Society of Chemistry

Copyright © 1994 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-484-6

Contents

Preface, v, 
Boxes, xiii, 
Acknowledgements, xv, 
Chapter 1 Discovery and Structure of Solid Inorganic Ion Exchange Materials, 1, 
Chapter 2 The Development of Organic Ion Exchange Resins, 21, 
Chapter 3 Structure of Ion Exchange Resins, 39, 
Chapter 4 Properties and Characterization of Ion Exchange Resins, 49, 
Chapter 5 Ion Exchange Equilibria, 90, 
Chapter 6 The Kinetics and Mechanism of Ion Exchange, 134, 
Chapter 7 Some Basic Principles of Industrial Practice, 166, 
Chapter 8 Water Treatment, 179, 
Chapter 9 Non-water Treatment Practices, 238, 
Chapter 10 Some Engineering Notes, 261, 
Appendix, 277, 
Subject Index, 280, 

CHAPTER 1

Discovery and Structure of Solid Inorganic Ion Exchange Materials

THE PHENOMENON

An ion exchange reaction may be defined as the reversible interchange of ions between a solid phase (the ion exchanger) and a solution phase, the ion exchanger being insoluble in the medium in which the exchange is carried out. If an ion exchanger M-A+, carrying cations A+ as the exchanger ions, is placed in an aqueous solution phase containing B+ cations, an ion exchange reaction takes place which may be represented by the following equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)

For reasons which will be considered later, the anion in solution does not necessarily take part in the exchange to any appreciable extent. The equilibrium represented by the above equation is an example of cation exchange, where M- is the insoluble fixed anionic complement of the ion exchanger M-A+, often called simply the fixed anion. The cations A+ and B+ are referred to as counter-ions, whilst ions in the solution which bear the same charge as the fixed anion of the exchanger are called co-ions. In much the same way, anions can be exchanged provided that an anion-receptive medium is employed. An analogous representation of an anion exchange reaction may be written:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.2)

Further development of a physical model for the exchanger phase is best left until Chapter 2 when synthetic ion exchangers will be considered in more detail, but the previous equations illustrate the essential difference between ion exchange and other sorption phenomena. The main fact is that electroneutrality is preserved at all times in both the exchanger and solution phases, and this in turn requires that counter-ions are exchanged in equivalent amounts. The most important features characterizing an ideal exchanger are:

1. A hydrophilic structure of regular and reproducible form.

2. Controlled and effective ion exchange capacity.

3. Rapid rate of exchange.

4. Chemical stability.

5. Physical stability in terms of mechanical strength and resistance to attrition.

6. Consistent particle size and effective surface area compatible with the hydraulic design requirements for large scale plant.

Manufacturers of modern ion exchange materials have progressed a long way towards meeting all these requirements when compared with the prototype materials described below. The cost to industry of modern ion exchange resins is high, varying typically from £1000 to £4000 per m3. Therefore exchanger properties which minimize the volumes required (e.g. high exchange capacity), or which prolong resin life (e.g. physical and chemical stability), are important considerations. It therefore follows that continued efforts to improve the exchanger characteristics listed above play an important part in the activities of resin manufacturing companies.

INORGANIC MATERIALS

References to ion exchange phenomena have been attributed to Old Testament scribes, and later to Aristotle, but the first descriptions in modern scientific terminology have been credited to two English soil chemists, H. S. Thompson and J. T. Way in the mid-nineteenth century. They observed 'base' or cation exchange between calcium and ammonium ions on some types of soil. Upon treating a column of soil with a solution of ammonium sulfate it was found that most of the ammonia was absorbed, whilst the calcium contained originally in the soil was released and passed out of the column. Further studies furnished many sound conclusions as to the nature of ion exchange reactions, some of the more important ones being:

1. Exchange involved equivalent quantities of ions.

2. Certain ions were more easily exchanged than others.

3. The temperature coefficient for the rate of exchange was small.

4. The aluminosilicate fractions of soils were responsible in the main for the exchange although these components rarely took part in the exchange itself.

5. Materials possessing exchange properties could be synthesized from soluble silicates and aluminium sulfate.

The equivalence law governing the phenomenon was established in the early scientific history of the subject, as also was the fact that some ions were more easily exchanged than others; in other words ion exchangers showed greater selectivity or affinity for different ions. That an exchanger could be chemically synthesized proved to be of the utmost importance; it is for this reason that ion exchange studies and applications have reached such an advanced state today.

The ion exchange capacity of an exchanger is a measure of its total content of exchangeable ions, and is conventionally expressed in terms of the total number of equivalents of ion per kilogram (milli-equivalents per gram) of the exchanger in its dry state and in a given univalent ionic form. As will become evident when describing practical applications, the operating exchange capacity of an exchanger is invariably less than its total capacity. Also because of the presence of 'colloidal humus' in natural soils, exchange capacity data was difficult to systematize and to reproduce in relation to the inorganic minerals which were present. Consequently further studies were carried out utilizing the separated microcrystalline aluminosilicates, or 'clay fractions' of the soil which were obtainable in quite a pure form. These experiments proved that the main exchange agents were indeed contained in the finest or clay-like fractions of soils. Our knowledge of the structure and classification of such materials has shown that some of the inherently finely divided clay materials are directly responsible for the exchange characteristics observed; however, the phenomenon is not purely a property of particle size.

Why the clay minerals should possess an appreciable exchange capacity became more fully understood with the establishment of the crystal structures of the various types, for which most of the early credit is due to W. L. Bragg, L. Pauling, and others. Therefore before the ion exchange relationships of such materials can be fully understood a general appreciation of their structures is essential.

Geologically and genetically, clay minerals are difficult to define simply and adequately, but broadly they are layer lattice silicates of secondary origin. In the same classification are the micas, talc, chlorites, and serpentines which are not strictly clay minerals. In this context, secondary origin means that mineral formation has arisen from the weathering of primary or igneous rock, e.g. granites and basalts.

The basic structural unit making up the layer lattice silicates is the silica tetrahedron, (SiO4)4-. When three oxygen atoms of every tetrahedron are linked to similar units a continuous sheet structure is formed which is capable of indefinite extension in two directions at right angles; as a consequence the important physical property of minerals within this group is their plate-like character. One oxygen atom of each tetrahedral unit is not satisfied electrically and requires to be linked to external cations in order to establish electrical neutrality within the lattice. In most structures of this type the silica units are arranged in the form of hexagonal rings, each of which is surrounded by six similar ones, so that bonding takes place by the silica tetrahedra sharing three corners, as shown in Figure 1.1.

Each silica unit in the hexagonal sheet is linked to others through an oxygen atom. The Si-O-Si bond angle can vary, thus giving rise to different conformations of the ring structure, but the majority are based on a bond angle of about 141° 34'. As a result, the single oxygen atoms of each silica tetrahedron which are unsatisfied electrically are oriented in the same spatial direction.

Electrical neutrality in layer lattice silicates is maintained by condensing a sheet hydroxide structure with the sheet of silica tetrahedra (see Figure 1.2). The two types of structure which can take part in combinations of this nature are the hydroxides of divalent elements such as magnesium and those of trivalent elements such as aluminium. In both cases the cations are in six-fold co-ordination with anionic units, but whereas these are entirely hydroxide ions in the case of the pure hydroxide forms (namely brucite and gibbsite respectively) in the layer lattice silicates both hydroxide and oxygen ions are involved. The resulting layer lattice is theoretically electrically balanced within itself, and although the structure is capable of indefinite extension in two dimensions by ionic/covalent bonds, no similar continuity is possible in the direction at right angles to the basal plane. A further condensation of a silica layer in an inverted form above the hydroxide layer can also occur, thus increasing the size and complexity of the layer and giving rise to other forms of layer lattice silicate minerals.

There are two major subdivisions of layer lattice silicates: a single layer type based on a condensation of a hydroxide layer structure with one silica plane and a double layer unit in which a further inverted silica plane completes a sandwich-like structure above the hydroxide unit. Each layer lattice is theoretically complete within itself and although similar layers can stack above each other there can be no formal inter-layer ionic or covalent bond formation.

Single Layer Lattice Silicates

Trivalent cations. By far the most common mineral group of this category is that of the kaolin minerals or Kandite group. The cations involved are solely aluminium which are each linked to three hydroxyl units in one layer and to two oxygens and one hydroxyl in the other.

The structure is illustrated in Figure 1.3. The kaolin layer is electrically neutral but extension in the c-crystallagraphic direction is possible through hydrogen bonding. This weak linkage results in a plate-like or flaky crystal habit.

Because several spatial stacking arrangements are possible there are several kaolin minerals, each with the same chemical composition, namely Al2Si205(OH)4, but with different properties. Nacrite, dickite, kaolinite, halloysite, and livesite are well recognized species. No positive evidence has so far been published linking other trivalent cations with a single layer lattice structure, but it has been suggested that iron(III) can replace aluminium in part in the kaolin lattice.

Divalent Cations. Both magnesium and iron(II) can take part in single layer lattice silicate structures although the former is more common. The cations are also in octahedral co-ordination but in order to preserve electrical balance within the lattice, all three of the octahedral sites over each silica hexagonal ring are occupied, as against two in the equivalent kaolin lattice. Divalent cations thus form a trioctahedral series whereas the trivalent cation minerals are termed a dioctahedral series. The typical magnesium structure is antigorite Mg3Si2O5(OH)4, but there are other minerals of this group which probably differ by virtue of the layer stacking isomerism and in the degree of substitution of ion(II). Chrysotile, the common asbestos mineral, chamosites, and some chlorites are typical examples.

Double Layer Lattice Silicates

As in the single layer group, two characteristic types of unit are to be found in the double layer silicate minerals based on the valency of the counterbalancing cations. A dioctahedral series, based on pyrophyllite Al2,(Si2O5)2(OH)2 and a trioctahedral group with talc Mg3(Si2O5) 2, (OH)2, as the type minerals are well established. These structures are depicted in Figure 1.4. In marked contrast with minerals of the previous group, extensive elemental substitution within the double layer lattice is the rule rather than the exception. Not only does replacement of ions of identical charge and similar size occur but more complex substitution giving rise to charge deficiencies is quite common.

In micas, for example, aluminium ions replace silicon in the outermost tetrahedral layers of the lattice thus giving rise to a unit charge deficiency for every replacement. Electrical neutrality is then achieved by incorporating alkali or alkaline earth cations between the individual structural layers. Some typical structural formulae are:

KAl2,(AlSi3)O10(OH)2 – dioctahedral muscovite mica K(Mg, Fe)3(AlSi3)O10(OH)2 – trioctahedral biotite mica

Montmorillonite minerals or smectites are based on a layer lattice in which the ionic replacement is mainly in the central octahedral layer. Once again the ionic substitution introduces a charge deficiency, typically Mg2+ for Al3+. Counterbalancing hydrated cations occupy inter-layer positions, but because the charge deficiencies are situated in the centre of the layer, and are generally smaller in amount, the binding forces are less rigid than those in micas. The extra ions are thus less firmly held and are readily exchanged. Many montmorillonite minerals are known of both dioctahedral and trioctahedral types. Typical montmorillonite and mica structures are shown in Figures 1.5 and 1.6. Other minerals with a double layer structure are chlorites and vermiculites, where lattice charge deficiencies are counter-balanced by either hydroxide layers or hydrated cations between the layers.

All the clay minerals are of relatively small crystallite size, possessing a high specific surface area with many broken edge and surface bonds. In addition, the double layer group minerals frequently contain unbalanced electrical forces within the lattice, all of which have an influence on the overall electrical field surrounding the particles when they are suspended in a polar liquid medium. It is just such complex surface charge properties that play an important role in determining our understanding of such processes as the clarification and filtration of natural water supplies using coagulants such as aluminium sulfate.

ION EXCHANGE PROPERTIES OF THE ALUMINOSILICATES

Single Layer Lattice Silicates

The ideal constitution of the kaolin layer represents an electrically neutral unit, with rarely any isomorphous substitution of cations of different charges within the lattice. Consequently, kaolinite and related minerals would not be expected to show a large cation exchange capacity, and indeed this is usually the case. That a small but varying exchange capacity does occur may be attributed to two principal causes.

1. Broken Bonds. The naturally occurring single layer lattice clay minerals do not constitute a perfectly crystalline state, and around the edges of the silica–alumina layers, broken bonds give rise to unsatisfied negative charges which may be balanced by adsorbed cations. The size of the clay mineral particles and the number of lattice distortions play a part in determining the extent of ion exchange capacity, but it remains a matter of some conjecture whether true cation exchange is manifested by extensive breakdown of the crystal lattice (Tables 1.1 and 1.2).

2. The hydrogen of exposed hydroxyls. A further contribution to the exchange capacity of clay minerals is made by the hydrogens of exposed hydroxyl groups which may be replaced by exchangeable cations. This cause of exchange capacity is of particular significance for clay minerals where there exists an exposed sheet of hydroxyls on one side of the basal cleavage plane. Exposed hydroxyl groups may also exhibit a slight but reversible affinity for exchangeable anions. Isomorphous substitution in the kaolin lattice may occur, but it is not a major contributory factor to the ion exchange capacity unlike the case with some double layer lattice silicates.

Double Layer Lattice Silicates

Some of these minerals possess high cation exchange capacities greatly in excess of that attributable to surface area, crystal fracture and edge effects. The reason for this difference in behaviour arises out of a third and major cause of exchange capacity, namely isomorphous substitution resulting in charge deficiencies in the layer lattice. The montmorillonites typify such minerals of this type. The description of these minerals has been given earlier when it was stated that the counterbalancing cations are held rather loosely between the layer planes (see Figure 1.5). Because isomorphous substitution occurs in the octahedral layer the resulting positive charge deficiency is relatively delocalized with respect to the inter-lamellar plane. Although the cations may effect some kind of inter-layer bonding they are nevertheless readily exchanged by means of simple reversible diffusion between external solution and inter-lamellar sites.

(Continues...)

Excerpted from Ion Exchange by C. E. Harland. Copyright © 1994 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 1994
• Language: English
• ISBN 10: 0851864848
• ISBN 13: 9780851864846
• Binding: Paperback
• Edition number: 2
• Number of pages: 302