- Ion exchange resin
An ion-exchange resin is an insoluble matrix (or support structure) normally in the form of small (1-2 mm diameter) beads, usually white or yellowish, fabricated from an organic
polymersubstrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and released ions. The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion-exchange. There are multiple different types of ion-exchange resin which are fabricated to selectively prefer one or several different types of ions.
Most typical ion-exchange resins are based on crosslinked
polystyrene. The required active groups can be introduced after polymerization, or substituted monomers can be used. For example, the crosslinking is often achieved by adding 0.5-25% of divinyl benzeneto styreneat the polymerization process. Non-crosslinked polymers are used only rarely because they are less stable. Crosslinking decreases ion- exchange capacity of the resin and prolongs the time needed to accomplish the ion exchange processes. Particle size also influences the resin parameters; smaller particles have larger outer surface, but cause larger head lossin the column processes.
Besides being made as bead-shaped materials, ion exchange resins are produced as membranes. The membranes are made of highly cross-linked ion exchange resins that allow passage of ions, but not of water, are used for
There are four main types differing in their
* strongly acidic (typically,
sulfonic acidgroups, eg. sodium polystyrene sulfonateor polyAMPS)
* strongly basic, (quaternary
aminogroups, for example, trimethylammonium groups, eg. polyAPTAC)
* weakly acidic (mostly,
* weakly basic (primary, secondary, and/or ternary
aminogroups, eg. polyethylene amine)
There are also specialised types:
chelating resins( iminodiacetic acid, thiourea, and many others)
In this application, ion-exchange resins are used to replace the
magnesiumand calciumions found in hard waterwith sodiumions. When the resin is fresh, it contains sodium ions at its active sites. When in contact with a solution containing magnesium and calcium ions (but a low concentration of sodium ions), the magnesium and calcium ions preferentially migrate out of solution to the active sites on the resin, being replaced in solution by sodium ions. This process reaches equilibrium with a much lower concentration of magnesium and calcium ions in solution than was started with.
The resin can be recharged by washing it with a solution containing a high concentration of sodium ions (e.g. it has large amounts of common salt (NaCl) dissolved in it). The calcium and magnesium ions migrate off the resin, being replaced by sodium ions from the solution until a new equilibrium is reached.
This is the method of operation used in
dishwashers that require the use of ' dishwasher salt'. The salt is used to recharge an ion-exchange resin which itself is used to soften the water so that limescaledeposits are not left on the cooking and eating utensils being washed.
In this application, ion-exchange resins are used to remove
poisonous (e.g. copper) and heavy metal (e.g. leador cadmium) ions from solution, replacing them with more innocuous ions, such as sodiumand potassium. Note that this is why it is not recommended to prepare artificial milk( infant formula) with softened water, since increased levels of sodium and potassium in the diet can be harmful to nursing infants.
Few ion-exchange resins remove
chlorineor organic contaminants from water - this is usually done by using an activated charcoalfilter mixed in with the resin. There are some ion-exchange resins that do remove organic ions, such as MIEX (magnetic ion-exchange) resins. Domestic water purification resin is not usually recharged - the resin is discarded when it can no longer be used.
Production of high purity water
Water of highest purity is required for electronics, scientific experiments, production of superconductor, and nuclear industry, among others. Such water is produced using ion-exchange processes or combinations of membrane and ion-exchange methods. Cations are replaced with hydrogen ions using cation-exchange resins; anions are replaced with hydroxyls using anion-exchange resins. The hydrogen ions and hydroxyls recombine producing water molecules. Thus, no ions remain in the produced water. The purification process is usually performed in several steps with "mixed bed ion-exchange columns" at the end of the technological chain.
Ion-exchange in metal separation
Ion-exchange processes are used to separate and purify
metals, including separating uraniumfrom plutoniumand other actinides, including thorium; and lanthanum, neodymium, ytterbium, samarium, lutetium, from each other and the other lanthanides. There are two series of rare earthmetals, the lanthanides and the actinides, both of which families all have very similar chemical and physical properties. Ion-exchange is the only practical way to separate them in large quantities.
A very important case is the
PUREXprocess (plutionium-uranium extraction process) which is used to separate the plutoniumand the uraniumfrom the spent fuel products from a nuclear reactor, and to be able to dispose of the waste products. Then, the plutonium and uranium are available for making nuclear-energy materials, such as new reactor fuel and nuclear weapons.
The ion-exchange process is also used to separate other sets of very similar chemical elements, such as
zirconiumand hafnium, which incidentally is also very important for the nuclear industry. Zirconium is practically transparent to free neutrons, used in building reactors, but hafnium is a very strong absorber of neutrons, used in reactor control rods.
chemistryion-exchange resins are known to catalyze organic reactions. See for instance self-condensation.
Ion-exchange resins are used in the manufacture of fruit juices such as orange juice where they are used to remove bitter tasting components and so improve the flavor. This allows poorer tasting fruit sources to be used for juice production.
Ion-exchange resins are used in the manufacturing of
sugarfrom various sources. They are used to help convert one type of sugarinto another type of sugar, and to decolorize and purify sugarsyrups.
Ion-exchange resins are used in the manufacturing of pharmaceuticals, not only for catalyzing certain reactions but also for isolating and purifying pharmaceutical
active ingredients.Three ion-exchange resins, sodium polystyrene sulfonate, colestipol, and cholestyramine, are used as active ingredients. Sodium polystyrene sulfonateis a strongly acidic ion-exchange resin and is used to treat hyperkalemia. Colestipol is a weakly basic ion-exchange resin and is used to treat hypercholesterolemia. Cholestyramineis a strongly basic ion-exchange resin and is also used to treat hypercholesterolemia. Colestipol and cholestyramineare known as bile acid sequestrants.
Ion-exchange resins are also used as
excipients in pharmaceutical formulations such as tablets, capsules, and suspensions. In these uses the ion-exchange resin can have several different functions, including taste-masking, extended release, tablet disintegration, and improving the chemical stability of the active ingredients.
* [http://www.reskem.com/pages/resin.php Information on Ion Exchange Resins] including Resin equivalency charts for various brands, technical data sheets and brochures from Res-Kem Corp.
* F. Helfferich, Ion Exchange, McGraw Hill, New York, 1962 (Bible of the subject).
* Ion Exchangers (K. Dorfner, ed.), Walter de Gruyter, Berlin, 1991.
* C. E. Harland, Ion exchange: Theory and Practice, The Royal Society of Chemistry, Cambridge, 1994.
* Ion exchange (D. Muraviev, V. Gorshkov, A. Warshawsky), M. Dekker, New York, 2000.
* [http://www.ionexchange.books.kth.se A. A. Zagorodni, Ion Exchange Materials: Properties and Applications, Elsevier, Amsterdam, 2006.]
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