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An ion exchange is a chemical process in which ions are exchanged between two electrolyte solutions or between an electrolyte solution and a solid matrix structure known as an ion-exchange polymer. When the electrolyte solution reacts with the polymer, or resin, ions from the solution are trapped and attached to the solid substrate, which in turn releases different ions of the same charge. Cation exchange occurs when positively charged particles are exchanged, while anion exchange occurs when negatively charged particles are exchanged.
Ion exchange may occur in natural settings through aluminosilicate minerals known as zeolites, which are found in clay. The level of cation exchange that occurs in soil can be used as an indicator of the ability of the soil to retain nutrients. It is also an important factor in determining the purity of groundwater.
In industrial settings, ion exchange takes place through use of synthetic organic polymers. An ion exchange polymer may be fabricated to specifically filter out certain ions from a solution. There are four main types of ion exchange polymers that have different ion affinities and chemical properties: strong base anion resins, weak base anion resins, strong acid cation resins, and weak acid cation resins.
In addition to these four groups, other types of ion exchange resins exist. Chelating resins are important in filtering out toxic heavy metal cations, such as lead and mercury. These resins are used in wastewater and groundwater treatment.
Strong acid resins can be used to remove calcium and magnesium cations from water in a process known as water softening. The calcium and magnesium ions from the solution are exchanged for sodium or hydrogen ions from the resin. Household products such as laundry detergent employ zeolites for water softening to prevent the interference of calcium and magnesium ions with the cleaning action of soap.
Deionization of water is another common application of ion exchange. In deionization, mineral salts such as sodium and calcium are removed from water and replaced with hydrogen cations or hydroxide anions, depending on the type of resin used. This process produces a purified form of water comparable to distilled water, which is useful in laboratory and industrial processes where high purity of water is required.
Industrial ion exchange reactions are usually carried out using columns. These vessels contain a resin bed, which holds the ion exchange resin while the solution is passed through it. The flow can be controlled by valves and instrumentation attached to the column. As the solution is passed through the resin, ion exchange occurs until all bonding sites in the resin are filled, producing a purified solution.
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