thumb|right||Ion exchange resin beadsIon-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification. In many cases ion-exchange resins were introduced in such processes as a more flexible alternative to the use of natural or artificial zeolites.
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 divinylbenzene to styrene at 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 loss in 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 electrodialysis.
There are four main types differing in their functional groups:
There are also specialised types:
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 limescale deposits are not left on the cooking and eating utensils being washed.
Few ion-exchange resins remove chlorine or organic contaminants from water - this is usually done by using an activated charcoal filter 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.
A very important case is the PUREX process (plutonium-uranium extraction process) which is used to separate the plutonium and the uranium from 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 zirconium and 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.
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.
Category:Polymers Category:Water Category:Synthetic resins Category:Polyelectrolytes
cs:Ionex et:Ioniidid it:Resina a scambio ionico nl:Ionenwisselaarhars ja:イオン交換樹脂 pl:Jonit pt:Resina de troca iônica ru:Иониты th:เรซินแลกเปลี่ยนอิออน uk:Іоніти zh:離子交換樹脂This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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