ION EXCHANGE RESINS AND ITS REGENERATION
Ion Exchange resins are small insoluble granular like substances which have acidic or basic ions that can be exchanged. The positive or negative ions fixed on these resins are replaced by ions of the same sign in solution in the liquid in contact with them during reversible chemical reaction. Resins are used in separation and purification in various water treatment process.
The Ion exchange resin matrix is made by cross-linking hydrocarbon chains with one another by polymerization. The cross-linking gives the resin polymer a strength. These functional groups give th ion exchange resin its separation capabilities, and vary significantly from one type of resin to the other . The most common types of resin compositions are:
Strong acid cation (SAC) exchange resins. : SAC resins are composed of a polystyrene matrix with a sulphonate functional group that is either charged with sodium ions for softening applications, or hydrogen ions for deionization .
Weak acid cation (WAC) exchange resins.: An acrylic polymer that has been hydrolyzed with either caustic soda or sulphuric acid to produce carboxylic acid functional groups. Due to their high affinity towards hydrogen ions , WAC resins are typically used to selectively remove cations associated with alkalinity.
Strong base anion (SBA) exchange resins. : SBA resins are composed of a polystyrene matrix that has undergone chloromethylation and amination to fix anions to exchange sites.
Weak base anion (WBA) exchange resins: WBA resins are unique in that they do not have exchangeable ions, and are therefore used as acid absorbers to remove anions associated with strong mineral acids.
Chelating resins. Chelating resins are the most common type of specialty resin, and are used for selective removal of certain metals and other substances. In most cases the resin matrix is composed of polystyrene, though a variety of substances are used for functional groups, including thiol, triethylammonium, and aminophosphonic, among many others.
How does ion exchange resin work?
Ion exchange is a reversible swapping of charged particles — or ions — with those of like charge. This occurs when ions present on an insoluble ion exchange resin matrix effectively exchanges places with ions of a similar charge that are present in a solution.
The resin functions because of its functional groups, which are essentially fixed ions that are permanently bound within the polymer matrix of the resin. These charged ions will readily bond with ions of an opposing charge in solution. These counter ions solution will continue to bond with the functional groups until a equilibrium is reached.
During an ion exchange cycle, the solution to be treated would be added to the ion exchange resin column and allowed to flow through the beads. As the solution moves through the resin beads , the functional groups of the resin attract any counter ions present in the solution. If the functional groups have a greater affinity towards new counter ions than those already present, then the ions in solution will dislodge the existing ions and take their place, bonding with the functional groups through shared electrostatic attraction.
What is resin regeneration?
Over time, contaminant ions bind with all available exchange sites in the Ion Exchange resin . Once the resin is exhausted, it must be regenerated for further use through what is called as a regeneration cycle. During a regeneration cycle, the ion exchange reaction is essentially reversed through the application of a concentrated regenerant solution. Depending upon the type of resin and the application , the regenerant may be acid, salt or caustic solution. As the regeneration cycle proceeds, the contaminant ions on ion exchange resins are dislodged ,with the ions present in the regenerant solution.
Strong acid cation resins (SAC)
Weak acid cation resins (WAC)
Strong base anion resins (SBA)
Weak base anion resins (WBA)
Specialty resin
