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,
zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers
that exchange positively charged ions or anion exchangers that exchange negatively charged
ions. 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+ and OH−
Single-charged monatomic ions like Na+, K+, and Cl−
Double-charged monatomic ions like Ca2+ and Mg2+
Polyatomic inorganic ions like SO42− and PO43−
Organic bases, usually molecules containing the amine functional group -NR2H+
Organic acids, often molecules containing -COO− functional groups
Biomolecules that can be ionized: amino acids, peptides, proteins, etc.
Along with absorption and adsorption, ion exchange is a form of sorption.
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.
Applications Ion exchange is 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. Most typical example of application is preparation
of high purity water for power engineering, 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
to produce soft water. This is accomplished by exchanging calcium Ca2+ and magnesium Mg2+
cations against Na+ or H+ cations. Another application for ion exchange in domestic water
treatment is the removal of nitrate and natural organic matter.
Industrial and analytical ion exchange chromatography is another area to be mentioned. 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. An important area of the application is extraction and purification
of biologically produced substances such as proteins and DNA/RNA.
Ion-exchange processes are used to separate and purify metals, including separating uranium
from plutonium and other actinides, including thorium, and lanthanum, neodymium, ytterbium,
samarium, lutetium, from each other and the other lanthanides. There are two series of
rare earth metals, the lanthanides and the actinides, both of whose families all have
very similar chemical and physical properties. Using methods developed by Frank Spedding
in the 1940s, ion-exchange used to be the only practical way to separate them in large
quantities, until the advent of solvent extraction techniques that can be scaled up enormously.
A very important case is the PUREX 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 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 exchangers are used in nuclear reprocessing and the treatment of radioactive waste.
Ion exchange resins in the form of thin membranes are used in chloralkali process, fuel cells
and vanadium redox batteries. Ion exchange can also be used to remove hardness from water
by exchanging calcium and magnesium ions for sodium ions in an ion exchange column.
Liquid phase ion exchange desalination has been demonstrated. In this technique anions
and cations in salt water are exchanged for carbonate anions and calcium cations respectively
using electrophoresis. Calcium and carbonate ions then react to form calcium carbonate,
which then precipitates leaving behind fresh water. The desalination occurs at ambient
temperature and pressure and requires no membranes or solid ion exchangers. Theoretical energy
efficiency of this method is on par with electrodialysis and reverse osmosis.
Other applications 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. In pollution remediation and geotechnical
engineering, ion exchange capacity determines the swelling capacity of swelling or Expansive
clay such as Montmorillonite, which can be used to "capture" pollutants and charged ions.
In planar waveguide manufacturing, ion exchange is used to create the guiding layer of higher
index of refraction. Dealkalization, removal of alkali ions from
a glass surface. Chemically strengthened glass, produced by
exchanging K+ for Na+ in soda glass surfaces using KNO3 melts.
See also Ion chromatography
Ion Alkali anion exchange membrane
Ion-exchange resin References
Friedrich G. Helfferich. Ion Exchange. Courier Dover Publications. ISBN 978-0-486-68784-1.
Ion Exchangers, Walter de Gruyter, Berlin, 1991.
C. E. Harland, Ion exchange: Theory and Practice, The Royal Society of Chemistry, Cambridge,
1994. Ion exchange, M. Dekker, New York, 2000.
A. A. Zagorodni, Ion Exchange Materials: Properties and Applications, Elsevier, Amsterdam, 2006.
External links Illustrated and well defined chemistry lab
practical on ion exchange from Dartmouth College Some applets illustrating ion exchange processes
A simple explanation of deionization Ion exchange, BioMineWiki