Ion exchange

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Ion exchange is a process used to remove dissolved ions from solution by electrostatic sorption into ion exchange materials (most commonly into ion exchange resins). The removed ions are replaced with equivalent amount of other ions of the same charge. Ion exchange is most commonly used for purification purposes, but is also widely implemented in separation and extraction of valuable substances. Deionisation of water and water softening can be named as a most common application. However, the spectrum of applications varies from large scale extraction of metals in hydrometallurgical processes to laboratory purification of highly valuable proteins.
Ion exchange is a process used to remove dissolved ions from solution by electrostatic sorption into ion exchange materials (most commonly into ion exchange resins). The removed ions are replaced with equivalent amount of other ions of the same charge. Ion exchange is most commonly used for purification purposes, but is also widely implemented in separation and extraction of valuable substances. Deionisation of water and water softening can be named as a most common application. However, the spectrum of applications varies from large scale extraction of metals in hydrometallurgical processes to laboratory purification of highly valuable proteins.
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[[Image:CatIonExchange061010.png|thumb|left|500px|Cat ion exchange]]
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[[Image:Simple cation exchange with description 071111.png|thumb|left|500px|Cat ion exchange]]
== Process ==
== Process ==

Revision as of 16:49, 11 November 2007

Ion exchange is a process used to remove dissolved ions from solution by electrostatic sorption into ion exchange materials (most commonly into ion exchange resins). The removed ions are replaced with equivalent amount of other ions of the same charge. Ion exchange is most commonly used for purification purposes, but is also widely implemented in separation and extraction of valuable substances. Deionisation of water and water softening can be named as a most common application. However, the spectrum of applications varies from large scale extraction of metals in hydrometallurgical processes to laboratory purification of highly valuable proteins.

Cat ion exchange
Cat ion exchange

Contents

Process

Metal ions initially contained in an aqueous solution are exchanged with ions initially contained in a solid material (most of then in an organic ion exchange resin). Such process is called cation exchange and can be illustrated by reaction

Image:IonExchangeEq1.gif (1)

where R = ion exchanger; A+ = positively charged metal ion; bars indicate phase of the ion exchanger. A similar process involving anions is called anion exchange:

Image:IonExchangeEq2.gif (2)

where B- and Y- = anions or negatively charged metal ion complexes. Chemical selectivity of reactions (1) and (2) is desirable but is not a requirement. Contrary to many other chemical separations, reactions (1) and (2) can be successfully used even if they are shifted to "wrong" direction. To achieve an efficient separation, column techniques are applied.


Ion exchange materials

There is a wide variety of organic and inorganic ion exchange materials. The organic materials (for example ion exchange resins) can be both cation- and anion exchangers. Only cation exchange inorganic materials (for example zeolites) are known. Organic resins consist of functional groups bound to different polymeric frameworks (most commonly to crosslinked polystyrene). Inorganic materials are negatively charged porous structures with exchangeable cations located in internal voids. Similarly to conventional acids, cation exchangers are classified into strong and weak cation exchangers depending on the type of functional group attached to the polymer.

  • Most typical strong acid exchangers contain sulfonic groups (-SO3-). Such materials are active over the entire pH range.
  • Most of weak acid exchangers have carboxylic groups (-COOH). The weak acid exchangers are not active at pH values below 4-6 (this value significantly differs for different materials). However, they often have higher ion exchange capacities than the strong acid exchangers and have other specific advantages.

Anion exchangers are classified in the similar way into strong base anion exchanger and weak base anion exchangers.

  • Strong base exchangers have quaternary ammonium groups (-NR3+). They are active over the entire pH range.
  • Weak base exchangers have primary (-NH2), secondary (-NRH), and/or tertiary (-NR2) amine groups. The weak base exchangers are not active at alkaline pH. However, they are advantageous in many practical cases.


Conventional ion exchange operations

Most commonly the ion exchange is performed in cyclic operations. Each cycle is divided into the following main sub-processes: sorption, elution, and, eventually, regeneration. In most techniques solutions are consequently pumped through a column loaded with the ion exchange resin.

  • Sorption: The solution containing the targeted ions is passed slowly through the column. The ions bind into the resin. Ions initially contained in the exchanger are released.
  • Elution (stripping): The target ions are subsequently stripped from the loaded resin with a small volume of an eluent. The eluent replaces and hence also releases the target ions from the resin into the solution phase.
  • Regeneration: Depending on type of the ion exchanger and the stripping agent, the ion exchanger sometimes has to be regenerated. For example, if the sorption step uses a cation exchanger loaded with H+ ions but the elution leaves Na+ ions in the exchanger phase, the material has to be protonated. A strong acid could be applied in order to convert (regenerate) the exchanger in the initial state.


Applicability

A huge success of ion exchange in area of water purification and water softening "fired back" in a certain sense. The technique is commonly perceived as suitable almost only for water purification. However, it can be successfully applied almost for any separation of ionic, ionisable, or locally chargeable substances. The technique is robust allowing successful separations even in non-optimised chemical systems. This is one of main advantages in comparison with competing techniques, such as solvent extraction.

The main limitation of the method is rather economical. The separation process is inexpensive if operates with low concentration of ions. Increase of the concentration results in the cost increase. Thus, competing techniques (membrane separations, solvent extraction, etc) could be considered (but not necessary preferred) for treatment of concentrated solutions. As a rule of thumb, treatment of solutions with metal ion concentrations below 10 ppm is most efficient with ion exchange. However, there are economically successful applications to solutions and even slurries containing more then 3M of target ions.


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