During the last few decades, ion exchange materials have evolved from laboratory tool to industrial products with significant technical and commercial impact. The current article briefly summarizes the history of the development of the ion exchange materials. The article defines the ion exchange materials and their types. It signifies the kinetics involved in the ion exchange process with description of factors affecting the rate of ion exchange. The paper suggests about steps targeting to enhance the rates of the ion exchange process.
1.1. Historical Background. Whenever an ion is removed out of an aqueous solution and is replaced by another ionic species, it is generally referred to as “ion exchange.” It is an ancient technique, in cultural heritage. Renaissance pottery of Mediterranean basin consisting of glass thin film had heterogeneous distribution of silver and copper nanoparticles of varied sizes, deposited on ceramic substrate. The copper/silver deposition was obtained by putting a mixture of copper and silver salts and oxides, together with vinegar, ochre, and clay, on the surface of a previously glazed pottery. Then the whole system was heated to about 600∘C in a reducing atmosphere produced by the introduction of smoking substances in the kiln. In these conditions, an ion exchange process between metal ions and alkali ions in the glass was induced. Later on, the metal ions followed by a reduction aggregated, remaining trapped within the first layer.
Even before the sunup of the human civilization, the ion exchange technique can be traced back to the Holy bible, mentioning the preparation of drinking water by “Moses” from brackish water.
Although, the exchange of cations as a phenomenon was first discovered slightly more than hundred years ago, two English agricultural chemists, Thompson and Way, noted that certain soils had a greater ability than others to absorb ammonia from fertilizers. They found that complex silicates in the soil performed an ion exchange function. They were able to prepare materials of this type in the laboratory from solutions of sodium aluminate and sodium silicate. In 1906, Rober Gans used materials of this type for softening water and for treating sugar solutions. The first organic ion exchangers were synthesized in 1935; Adams and Holmes found that the crushed phonograph records exhibit ion exchange properties. In the middle 1940’s, ion exchange resins were developed based on the copolymerization of styrene crosslinked with divinylbenzene. These resins were very stable and had much greater exchange capacities than their predecessors. The polystyrene-divinylbenzene-based anion exchanger could remove all anions, including silicic and carbonic acids. This innovation made the complete demineralization of water possible.
The discovery of ion exchangers was predetermined by the overall progress in fundamental science in the middle (Michael Faraday, concept of ions) and late (Svante Arrhenius, theory of electrolytic solutions) eighteenth century. These ideas were crucial for the scientific discovery of ion exchangers, because such materials, both organic and inorganic, are essentially polyelectrolytes.This means that they can be considered as consisting of two ions of opposite charge. Contrary to conventional electrolytes, ions of one charge are fixed to a polymeric (organic ion exchangers) or crystalline (inorganic ion exchangers) structure. Like any ionic compound, an ion exchange material can dissociate and participate in ion exchange reactions. However, dissociation does not result in dissolving of the material. This causes many specific phenomena and many possibilities to design heterogeneous systems with ionic properties.
1.2. Ion Exchange Materials. Ion exchange is a chemical reaction in which free mobile ions of a solid, the ion exchanger, are exchanged for different ions of similar charge in solution. The exchanger must have an open network structure, either organic or inorganic, which carries the ions and which allows ions to pass through it.
An ion exchanger is a water-insoluble substance which can exchange some of its ions for similarly charged ions contained in a medium with which it is in contact; this definition is all embracing. Referring to a “substance” rather than a compound includes many exchangers—some of them are natural products which do not have a well defined composition. The term “medium” acknowledges that ion exchange can take place in solution both aqueous and nonaqueous, in molten salts, or even in contact with vapours. The definition is not limited to solids, since some organic solvents which are immiscible with water can extract ions from aqueous solution by an ion exchange mechanism.
The definition also indicates something about the process of ion exchange. Basically it consists of contact between the exchanger and the medium in which the exchange takes place. These are usually a solid ion exchanger and an aqueous solution.The fact that ions are exchanged implies that the exchanger must be ionized, but only one of the ions in the exchanger is soluble.That ion can exchange, while the other, being insoluble, cannot do so.
If we represent the ion exchanger as M+X−to show that it is ionized—M+ being the soluble ion—and imagine it placed in a solution of the salt NY, which ionizes to give the ions N+ and Y−, the exchange reaction can be written as follows:
M+ X− + N+ + Y− → N+ X− + M+ + Y−.
This resembles a simple displacement reaction between two salts MX and NY. Since the ion Y− takes no part in the reaction it can be omitted from both sides of the equation, which then simplifies to
M+ X− + N+ → N+ X− + M+ .
In this example cations are exchanged, similarly an analogous equation can be written for an ion exchange when the insoluble ion in the exchanger is the cation.
There is no strict difference between ion exchange resin and chelating resins because some polymers can act as chelating or nonchelating substance depending on the chemical environment. Ion exchange resembles sorption because both are surface phenomenon, and in both cases a solid takes up a dissolved species. The characteristic difference between these two phenomena is in stoichiometric nature of ion exchange. Every ion removed from the solution is replaced by an equivalent amount of same charge. In sorption on the other hand a solute is usually taken up nonstoichiometrically without being replaced.
Conventionally, ion exchange materials are classified into two categories, cation exchangers and anion exchangers, based on the kind of ionic groups attached to the material.
Cation exchange materials containing negatively charged groups like sulphate, carboxylate, phosphate, benzoate, and so forth are fixed to the backbone material and allow the passage of cations but reject anions, while anion exchange materials containing positively charged groups like amino group, alkyl substituted phosphine (PR3 +), alkyl substituted sulphides (SR2+), and so forth are fixed to the backbone materials and allow passage of anions but reject cations. There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins or passing the treated solution through several different ion exchange materials. There is one other class of ion exchanger known as chelating ion exchanger. Many ions accept lone pair of electrons from ligands establishing covalent like bonds, called coordination bonds. Depending on the number of coordination bonds, the ligand is called monodentate, bidentate, or polydentate. Coordination interactions are highly specific. An example of a coordination compound is the coordination of a metal ion with ethylene diamine tetra acetic acid (EDTA).
Organic structure of functional groups often contains nitrogen, oxygen, and sulphur atoms, which are electron donor elements.
1.3. Types of Ion Exchange Material. Based on origin the ion exchange materials can be classified as natural and synthetic (Figure 1).
Figure 1: Classification of ion exchangers.
1.4. Natural Organic Products. Several natural organic materials possess ion exchange properties or can be given to them by simple chemical treatment. Plant and animal cells act as ion exchangers by virtue of the presence of carboxyl groups of amphoteric proteins. These carboxyl groups, (–CO2H), and phenolic groups, (–OH), are weakly acidic and will exchange their hydrogen ions for other cations under neutral or alkaline conditions. The humins and humic acids found in natural soil “humus” are examples of this class of exchanger; the partially decayed and oxidized plant products contain acid groupings. Several organic products are marketed which are based on treated cellulose, either in fibre form for use in ion exchange columns or as filter papers for ion exchange separations on paper. Many ion exchangers have been prepared from other natural materials such as wood, fibres, peat, and coal by oxidation with nitric acid or, better still, with concentrated sulphuric acid when the strongly acid sulphonic acid group, (–SO3H) is introduced into the material. The latter process was particularly successful with sulphonated coals. These can exchange in acid solution, because the exchanging group itself is ionized under these conditions, whereas the weaker carboxylic and phenolic groups are not. All these materials have certain disadvantages; however, they tend to colour the solutions which are treated, and their properties are difficult to reproduce because of the difficulty of controlling the treatment they are given.
1.5. Natural Inorganic Products. Many natural mineral compounds, such as clays (e.g., bentonite, kaolinite, and illite), vermiculite, and zeolites (e.g., analcite, chabazite, sodalite, and clinoptilolite) exhibit ion exchange properties. Natural zeolites were the first materials to be used in ion exchange processes. Clay materials are often employed as backfill or buffer materials for radioactive waste disposal sites because of their ion exchange properties, low permeability, and easy workability. Clays can also be used in batch ion exchange processes but are not generally suited to column operation because their physical properties restrict the flow through the bed.
1.6. Modified Natural Ion Exchangers. To improve exchange capacity and selectivity, some naturally occurring organic ion exchangers are modified; for example, cellulose based cation exchangers may be modified by the introduction of phosphate, carbonic, or other acidic functional groups.
The sorption parameters of natural materials can be modified by a chemical and/or thermal treatment; for example, by treating clinoptilolite with a dilute solution of acids or some salts, a more selective form of sorbent can be developed for a particular radionuclide.
In Japan natural minerals treated with alkaline solutions under hydrothermal conditions have been proposed for the sorption of caesium and strontium from solution. These treatments have provided materials with distribution coefficients of 1000 to 10000. Good results have been reported for the removal of caesium and strontium by neoline clays modified with phosphoric acid.
1.7. Synthetic Organic Ion Exchangers. In 1935, two chemists at the National Chemical Laboratory in Teddington, Adams and Holmes, demonstrated that organic ion exchange resins could be synthesized in a manner similar to the well identified resin “Bakelite,” which was prepared by Baekeland in 1909. “Bakelite” is a hard, insoluble condensation resin polymer and can be easily made by heating together phenols and formaldehyde, in the presence of acid or base with the elimination of water. Basically the reaction takes place in two stages.
The repeat of the pervious reactions will generate a threedimensional structure containing weakly acidic phenolic –OH or OR groups.
Adams and Holmes showed that these exchanged their hydrogen ions in alkaline solution; they also prepared cation exchanger which worked in acid solution by introducing strongly acidic sulphonic acid groups, –SO3H, into the structure.
By a suitable modification of their synthesis they were also able to introduce basic groups derived from amines and so prepare synthetic anion exchangers. For the first time it became possible to prepare under controlled conditions both cation and anion exchange resins, the behaviour and stability of which were considerably in advance of other materials.
Until the development of modern techniques of high polymer chemistry, the condensation resins were used successfully. In 1944 d’Alelio, in the United States, produced superior materials. D’Alelio’s resins, as well as most of their descendants, were based on a regular three-dimensional network formed by polymerizing the benzene derivative styrene. The double bond in the side chain may be opened and styrene units linked end to end to give the polymer chain.
Here, the repeating unit based on the styrene molecule occurs millions of times. Styrene may also be regarded as a derivative of ethylene, in which one hydrogen atom has been replaced by a phenyl group, –C6H6. Just as ethylene may be polymerized to polyethylene (polythene), styrene readily polymerizes to polystyrene. The chains may be linked together into two- and three-dimensional structures if styrene is mixed with a small proportion of divinyl benzene which resembles styrene in its properties but has two unsaturated side chains through which it can polymerize.
D‘Alelio prepared such resins by emulsion polymerization, using suitable emulsifiers, from which the resin settles as spherical beads. The polymer itself does not work as ion exchanger, but it can be sulphonated just as can the condensation resins, introducing the strongly acidic group, like – SO3H, into the benzene rings of the polymer. The hydrogen ions of this group will exchange with other cations in solution.
The second of these reactions gives a quaternary ammonium salt, which is strongly ionized, and is readily converted to the corresponding base. The anion readily exchanges with other anions at all pH values. The cation forms part of an insoluble polymer chain or network, so that the exchanger as a whole is insoluble in water.
1.8. Synthetic Inorganic Ion Exchangers
1.8.1. Synthetic Zeolites. Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth metals, having infinite, three-dimensional atomic structures. They are further characterized by the ability to lose and gain water reversibly and to exchange certain constituent atoms, also without major change of atomic structure. Along with quartz and feldspar minerals, zeolites are three-dimensional frameworks of silicate (SiO4) tetrahedra in which all four corner oxygens of each tetrahedron are shared with adjacent tetrahedra. If each tetrahedron in the framework contains silicon as its central atom, the overall structure is electrically neutral, as is quartz (SiO2). In zeolite structures, some of the quadricharged silicon is replaced by triply charged aluminum, giving rise to a deficiency of positive charge. The charge is balanced by the presence of singly-and doubly-charged atoms, such as sodium (Na+), potassium (K+), ammonium (NH4+), calcium (Ca2+), and magnesium (Mg2+), elsewhere in the structure. The empirical formula of a zeolite is of the type