What Is Ion Exchange?

Ion exchange is a process that involves the transfer of negatively or positively charged ions. These ions can either be anions or cations.

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When a resin material is depleted of desirable ions, it must be “regenerated”. This involves washing the resin with water to wash away undesirable ions and replacing them with desirable ions.

Resins

Ion exchange resin is a special fabricated polymeric material used to remove dissolved impurities from water. They are available in a wide variety of shapes, sizes, and structures based on the process used for their fabrication. They are essentially gels that act as small beads with numerous surface pores that trap ions. The resins are fabricated from polymeric materials such as styrene or less frequently, acrylic. Most ion exchange resins are crosslinked with divinyl benzene for increased stability and are chemically treated after polymerization to provide the desired ionic specificity.

Ions (positive or negative) bound to the resin’s functional groups can be replaced with other ions with equal specificity by washing the resin in solutions containing the desirable ions. The impurities are then removed from the water and the resins regenerated in a similar manner to return them to their original ionic form.

When used in water treatment, ion exchange resins are usually arranged in a uniform mixture of cation and anion resins (typically 2 parts cation and 3 parts anion). The resins are held in a vessel that can be closed off at the top and bottom with screens or slotted cylinders to prevent the resin from spilling out of the vessel and contaminating the environment.

In its water-swollen state, the ion exchange resin is quite spherical with a typical bulk density of 1.1-1.5. The physical properties of the resin can be further modified based on the type of resin and its application.

Regeneration

Ion exchange is used to remove unwanted elements or substances from a liquid, such as water. This can be done to remove hardness (also known as softening), or regulated contaminants, such as uranium and perchlorate, from drinking or industrial water. The resin that is chosen for an application is based on the water chemistry that needs to be treated, and what is desired in the final water.

Ion Exchange resins can only hold so many cations or anions, and they eventually become saturated with undesirable ions and depleted of desirable ions. When this occurs, the system will perform regeneration, washing out the unwanted ions and refilling the resin with desirable ions. This allows the resin to continue operating effectively.

The regenerant chemical for cation resins is typically sodium hydroxide. The regenerant is introduced into the ion exchange vessel and washed over the resin bed. A slow rinse is used to drive off the anions and other ions that have been adsorbed to the resin. The resin is then ready for a new service cycle.

For anion systems, a strong base regenerant is often used, such as a solution of sodium silicate. The resin bed is usually preheat before the introduction of the regenerant caustic, to prevent the formation of gel and enhance silica removal.

Controls

The ion exchange system must be designed with control features that ensure proper operation. A centralized control console provides the operator with easy access to all functions and enables quick and accurate adjustments. The controller has a built-in electronic sensor for monitoring system pressure and temperature to prevent overflow or underflow of the resin. The controller also provides automatic calibration of the IC pump for accurate injection, even at low flows. The controller also features an underdrain system that retains the ion exchange resin in the tank, evenly collects service flow and backwash flow, and distributes both to the tank drain for efficient rinse cycles.

The Ion Exchange process is often used in water treatment applications to remove undesirable ionic contaminants such as calcium and magnesium from the water. This is done by exchanging the contaminant with another ionic substance, such as sodium or potassium.

Ion exchange can be used to separate biomolecules based on their isoelectric point (pI). The amino acids in proteins carry either a positive or negative charge. Depending on the pH of the buffer used, the amino acid ions will bind to either the cation or anion exchange resins. By using a pH gradient, the protein can be separated according to its pI and eluted from the column. This type of separation is known as ion exchange chromatography or IEX chromatography.

Application

Ion exchange is used in laboratory and production environments to separate or purify a wide variety of charged or ionizable molecules including proteins, peptides, enzymes, antibiotics, DNA, vitamins and more. The process starts with the solubilization of a protein sample using an appropriate buffer, such as tris, pyridine, acetate, citrate or phosphate. This will allow the molecule to be tightly bound to the column and eluted during separation. Anionic, cationic or zwitterionic detergents can also be used to help with this process for proteins that have low aqueous solubility.

The resin is saturated with desirable ions and as water passes through the system, these ions are released and replaced with undesired ions. This causes the resin to become depleted and it will need to be regenerated. During regeneration, the undesirable ions are flushed away and the resin is filled with desired ions. Then the procedure can be repeated as needed.

The ion exchange process is most commonly used to remove unwanted ionic substances from water. It is a popular way to soften water and can be a good option for those who have to limit their sodium intake due to health reasons. However, the softened water is often not suitable for use in gardens or lawns because of high levels of sodium in the water. In addition, ion exchange can be used to remove acidic contaminants from water and make it more suitable for drinking or cooking.