Ion exchange is a reversible exchange of ions between a solid ion exchanger and a liquid solution. The solid undergoes no structural change in the process. An ion exchanger, or ion exchange resin, can be a natural or synthetic substance which can exchange its own ions with the ions present in a solution which is passed through it. The ion exchanger is porous and allows for a solution to permeate through the material. As the solution passes through the ion exchanger and leaves as effluent, it contains ions different from those with which it began.
Ion exchange is usually associated with water treatment. The first industrial application of ion exchange technology was in 1905 when a sodium-aluminosilicate cation exchanger was used to soften water. As water passes through an ion exchange resin, harmful ions found in the water become attached to the resin, which releases its harmless ions into the water. The resin is usually composed of synthetic beads. Each bead is a polymer matrix containing ion exchange sites on the surface and within the matrix itself.
Anions can only be exchanged for other anions, and cations for other cations. The ion exchange resin that is used is therefore specific for the type of ion to be removed from the solution. An anion exchange resin has positively charged ion exchange sites with anions attached, and cation exchange resins have negatively charged ion exchange site with attached cations. The ion exchange resin usually originates with attached ions that have low affinities for the exchange sites. As the solution containing ions flows through the resin, the ions with the most affinity for the exchange sites will replace those with the lowest affinities. It is important, therefore, that the ion exchange resin contain ions with a lower affinity than those which need to be exchanged. For example, cation exchange resins usually come with sodium (Na) or hydrogen (H) ions attached to the exchange sites. Both of these ions have low affinities to the sites. Almost any cation which comes in contact with the resin will have a greater affinity and replace the hydrogen or sodium ions at the exchange sites. Anion exchange resins often use chloride (Cl) or hydroxyl (OH) ions because of their low affinities for the exchange sites as well.
Different ions can have different charges as well as affinities. A monovalent ion has a single charge, a divalent ion has two charges, and a trivalent ion has three charges. The charge on a resin bead must always remain neutral. This means that for every divalent ion of greater affinity which attaches to a bead, two of the ions must be released from the bead. For every trivalent ion which attaches to a bead, three of the ions must be released from the bead. In water treatment, the ion exchange resin which is used is usually more selective for divalent and trivalent ions, which is practical since the problematic ions are usually one of the two.
Ion exchangers are reversible, that is, they can be regenerated. Once a solution has passed through an ion exchange resin, the resin beads now contain the undesirable ion and the original ion which was attached to the bead can be found in the effluent. The unwanted ions can be removed and replaced by the original ions by passing a solution containing these ions in a much greater concentration than would be found in an untreated solution. For example, an ion exchange resin which originally contained Na+ ions could be regenerated after use by washing it with a concentrated solution of sodium chloride, NaCl. When a solution of such great concentration is passed through an ion exchange resin, the resin is more selective for the monovalent ions simply because of the great number of ions bombarding the resin beads. When so many collisions occur, it is very likely that the unwanted ions will be removed and replaced. The early ion exchange resins could not be regenerated. The original resins were "gel" type materials which were not very porous at all. Because they were not porous, there were not as many opportunities for the collisions to occur during regeneration, and the higher affinity ions could not be released. In the 1950s, macroporous ion exchange resins were developed with the ability to be regenerated, which eventually evolved into the ion exchange beads and membranes in use today.
There are many different ion exchange resins available for use. Which resin used depends on the ion that needs to be removed from solution. Water softeners, for example, are designed to remove the calcium (Ca2+) and magnesium (Mg2+) cations from hard water. The ion exchange resin in water softeners contain cations such as Na+ or H+. When the hard water passes through the water softener, the resin exchanges its Na+ or H+ ions for the Ca2+ and Mg2+ ions. Ion exchange resins containing OH/- or Cl- anions will exchange with anions such as HCO3-, CrO42-, NO3-, or SO42- in a solution. The first water softeners developed in the early 1900s only operated over a narrow pH range, in fact, only slightly basic water could be treated. The development of both cationic and anionic exchange materials did not occur until 1935.
Ion exchange resins have different abilities to exchange ions, called the ion exchange capacity of the resin. This is a numerical value which represents the quantity of ions that an ion exchange resin is able to release coupled with the quantity of ions it can acquire. This exchange capacity can be expressed as an absolute number (the total exchange capacity) or as an operational value (the practical exchange capacity). The total exchange capacity is measured from the time the solution begins to permeate through the resin until the resin cannot exchange any more ions. The practical exchange capacity is measured from the time the solution begins to permeate through the resin until the effluent reaches a set tolerable concentration of exchanged ions.
Certain conditions must be kept constant during the ion exchange process, especially if determining the ion exchange capacity of a resin, because the exchanging ability of a resin is a function of the affinity of the ions towards the exchanger. The contact time needs to be kept constant, for one. The contact time is the amount of water which passes through the exchanger with respect to the volume of the exchanger. The temperature needs to be kept constant, because temperature changes can affect the size of the pores in the ion exchange resin. The pH value of the water must be held constant as well, because the dissociation of the ions attached to the resin can be effected by changes in pH.
There are many applications of ion exchange in use today. Water treatment has already been mentioned as one of the leading uses of ion exchange resins. Other applications include the separation of antibiotic drugs from fermentation broths, recovery separation of products during laboratory procedures, and pollution control. Manufacturers utilize ion exchange to remove toxic substances from their waste waters before discharging the water into the environment. The use of ion exchange resins is widespread and growing quickly as more and more industries realize the ease with which it can be used.
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