Ion exchange

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Revision as of 12:30, 31 August 2007

A process used to remove dissolved solids from aqueous solution by electrostatic adsorption into ion resins. It is mainly used for purification purposes, but can also be implemented as a technique for valuable metals recovery from wastes and hydrometallurgical processes.


Ion exchange is used for separation and purification purposes and is most widely used in water purification and water softening. Ion exchange is generally used for treatment of solutions with low concentrations (10 ppm or less). For more concentrated solutions, methods like solvent extraction are more efficient and economical. Ions are exchanged between a solid organic resin (exchangers) and metal ions in aqueous solution.

Cation exchange: R-H+ + A+ → R-A+ + H+

Anion exchange: R+Y- + B- → R+B- + Y-

R = organic resin A+ = positively charged metal ion B- = negatively charged metal ion complex

The organic resins consist of a functional group bound to a solid structure usually consisting of crosslinked polystyrene.

Cation exchangers are classified into strong acid cation exchanger and weak acid cation exchangers depending on the type of functional group attached to it.

  • Strong acid exchangers have a sulphonyl group (-SO3H) as functional group which can also be in the salt form (-SO3Na). Active over the entire pH range and are eluted and regenerated with strong acids like hydrochloric acid (HCl) or sulphuric acid (H2SO4).
  • Weak acid exchangers have a carboxylic group (-COOH) as functional group. The weak acid exchangers are not active at pH values below 4-6 and have lower capacities than the strong acid exchangers.

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

  • Strong base exchangers have a quaternary ammonium group (-NR3+OH-) as functional group. Active over the entire pH range and regeneration is achieved with strong bases like (NaOH).
  • Weak base exchangers commonly have a tertiary amine group (-NR2) as functional group. The weak base exchangers are not active at pH values above 7 and are mainly used for sorption of strong acids. Regeneration is achieved with weak bases like ammonia (NH2) or sodium carbonate (Na2CO3).

The ion exchange operation is divided into the following sub-processes of sorption, elution eventually followed by regeneration.

  • Sorption: The aqueous solution containing the metal ions is passed slowly through a column containing the resin for ion exchange. The metal ions bind to the resin.
  • Elution (stripping): The metal ions are subsequently stripped from the loaded resin with a small volume of eluant. The eluant will replace and hence also release the metal ions from the resin into the aqueous solution.
  • Regeneration: Depending on type of ion exchanger and stripping agent used the ion exchanger sometimes has to be regenerated, for example a strong acid cation exchanger has to be protonated in order to be in the active state.

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