Ion exchange

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Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic ion exchangers.

Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). 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.

Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are:

• H⁺ (proton) and OH⁻ (hydroxide)
• Single charged monoatomic ions like Na⁺, K⁺, or Cl⁻
• Double charged monoatomic ions like Ca²⁺ or Mg²⁺
• Polyatomic inorganic ions like SO₄²⁻ or PO₄³⁻
• Organic bases, usually molecules containing the amino functional group -NR₂H⁺
• Organic acids, often molecules containing -COO⁻ (carboxylic acid) functional groups
• Biomolecules which can be ionized: amino acids, peptides, proteins, etc.

Ion exchange is a reversible process and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.

[edit] Applications

Most typical example of application is preparation of high purity water for electronic and nuclear industries; i.e. polymeric or mineralic insoluble ion exchangers are widely used for water softening, water purification, water decontamination, etc.

Ion exchange is a method widely used in household (laundry detergents and water filters) to produce soft water. This is accomplished by exchanging calcium Ca²⁺ and magnesium Mg²⁺ cations against Na⁺ or H⁺ cations (see water softening).

Ion exchange chromatography is a chromatographical method that is widely used for chemical analysis and separation of ions. For example, in biochemistry it is widely used to separate charged molecules such as proteins.

Ion exchangers are used to enrich and purify precious metals, including uranium, lutetium, neodymium, samarium, and thallium.

An important area of the application is extraction and purification of biologically produced substances such as amino acids and proteins.

Industrial and analytical ion exchange chromatography is another area to be mentioned.

Ion exchangers are used in nuclear reprocessing and the treatment of radioactive waste.

Ion Exchange is also widely used in the food & beverage, hydrometallurgical, metals finishing, chemical & petrochemical, pharmaceutical, sugar & sweeteners, ground & potable water, nuclear, softening & industrial water, semiconductor, power, and a host of other industries.

[edit] Others

In soil science, cation exchange capacity is the ion exchange capacity of soil for positively charged ions. Soils can be considered as natural weak cation exchangers.