Anion-exchange

Basic Analytical Chemistry

Title: What are ion exchange resins? Write an essay on the applications and use of ion exchange resin.

Anion-exchange resinis a wide variety of insoluble syntheticpolymers, usually in the small beads form of white or cream colour, which contains sites that are positively or negatively charged whereby it can exchange ions attached to it with the other ions in the surrounding medium. The resins have structure of pores which is greatly developed on the surface. This structure acts as sites where ions are easily trapped and released. The ions trapping process only occur when other ions are released at the same time. This process is therefore called as ion exchange process.

Depending on the type of the functional group, ion exchangers can be divided into mainly several types which is strong acidic, strong basic, weak acidic and weak. Ion exchangers which contain sulpho- and phospho-acidic groups and those that contain tetraammonium basic groups are strong acidic and strong basic exchangers, respectively, whereas those that contain phenolic and primary amino groups are weak acidic and weak basic exchangers, respectively. Ion exchangers with carboxyl groups and tertiary amino groups are at the medium position between strong and weak acidic and basic exchangers, respectively.

Furthermore, there is another specialised type of ion exchange resin which is the heavy-metal-selective chelating resin. This type of resin is similar to the weak acid cation exchanger. As it can form stable complex when combines with heavy metal cation, therefore it has a high selectivity for heavy metal cations. The active site in this type of ion exchange resin is EDTA compound which is permanent on the resin. The active site is usually balanced by a cation such as Na+ to form Res-EDTA-Na. The Na+ ions can then be exchanged with heavy metal cations in a certain solution.

Ion exchange resins are widely used in different applications. One of such applications is water treatment. There are two major types of water treatment which are water softening and demineralisation. Water containing hard metal ions such as calcium and magnesium are known as hard water. It is soften using a water softening process. In this process, hard metal ions are removed by passing the water through a column containing cation resins in the sodium. These hard metal ions are replaced by an equivalent quantity of sodium. Regeneration process is done by passing a solution containing high concentration of the cations, which usually is the sodium chloride solution down through the resin to replace the ions that have attached to the resin. The resins are then rinsed with water to remove the regenerant solution.

The second type of water treatment is demineralisation. Demineralisation is done by using two types of resins.We know that all the dissolved matter in natural water supplies is in the form of charged ions. Firstly, the water is passed through a bed of cation exchange resin in a column similar to that described for softeners. In the process, cations in the water are attached the resin while hydrogen ions are detached from resins. Therefore the effluent comprises very weak mixture of acids. Later, the water will be passed through a second column which contains anion exchange resin in the hydroxide form. Now, anions in the water are exchanged for hydroxide ions which react with the hydrogen ions to form water. Water that has gone through demineralization is also called demonized water as hydrogen ions and hydroxide ions are removed.

Another use of ion exchange resins is in the metal separation process. This ion exchange process is widely used in industry to separate and purify metal. Normally, it is used to separate lanthanides and actinides as this two series have similar physical and chemical properties. One of the most important cases is the PUREX process whereby plutonium and uranium are separated from the fuel products of nuclear reactors.

Ion exchange resin is also used in treatment of radioactive waste. In this process, a measured quantity of the ion exchange medium is mixed with the liquid waste in a appropriate container. The mixture is then allowed to equilibrate for a specific time either with or without stirring. At the end of the reaction time, the ion exchange medium is removed from the liquid using conventional separation techniques such as decantation or filtration. The treatment can be repeated as necessary by adding more ion exchange media.

References:

1. http://en.wikipedia.org/wiki/Ion_exchange_resin

2. http://www.remco.com/ixidx.htm

3. http://nzic.org.nz/ChemProcesses/water/13D.pdf

4. www-pub.iaea.org/MTCD/publications/PDF/TRS408_scr.pdf

5. http://www.remco.com/ixidx.htm

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