What Is Ion Exchange?

Ion exchange is a water treatment technique that can be used to remove undesirable ions from drinking and industrial waters. This can include softening and the removal of regulated contaminants.

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Ion exchange can be a costly process as resins must be regenerated with salt solutions, acids or lye and wastewater generated during the regeneration is also produced. To maximise the efficiency of an ion exchange system, it is recommended to regularly analyse and rinse the column.

How It Works

Ion exchange is a chemical process that removes undesirable ions from water and wastewater. Water processes like water softening and deionization use ion exchange to remove contaminants. The ions are removed by exchanging them with ions that have the same electrical charge. For example, magnesium and calcium ions in hard water are exchanged with sodium ions in a water softener system. Cation exchange resins can also remove contaminants like arsenic, nitrate, sulfate and fluoride.

Ion Exchange resins are composed of microporous beads that are either synthetically or industrially produced. The beads are coated with a negatively charged material, such as polyacrylate or polystyrene divinylbenzene, to which positively charged ions like calcium and magnesium will stick. These ions are then exchanged with other positively charged ions available on the surface of the resin, such as sodium. As the hardness minerals are swapped for sodium ions, they do not build up on surfaces as scale deposits.

When the resin is completely depleted of its desired cations or anions, it needs to be recharged. This is done by washing the resin with a liquid to remove unwanted ions and refilling it with desirable ions. This can be accomplished with a solution such as sodium hydroxide, potassium perchlorate or sulfuric acid. Once the resin is regenerated, it is ready for use again.

Resins

Ion exchange resins are specialized plastic materials that simultaneously trap and release ions to purify liquids. The resins consist of ionizable functional groups attached to a bead structure. These functional groups contain ionic sites that are charged – either positively (cations) or negatively (anion). The functional group’s polarity determines its affinity for oppositely-charged ions.

Ionic-exchange resins are used extensively to soften water for domestic use, as well as in industrial applications such as the production of semiconductors, high-purity chemicals and electronics manufacturing. The resins are also useful in the separation of rare-earth elements, isotopes and amino acids from natural waters and wastewater streams.

During the softening process, water containing scale-forming ions such as calcium and magnesium passes through a resin bed containing strong acid cation (SAC) resin in the sodium form. During the exchange, the ions are replaced with sodium ions from the salt washing solution that is used to recharge the resin. The sodium diffuses from the resin into the bulk water solution, which is then softened for household and commercial uses.

Gelular strong acid cation resins degrade when they are saturated with sodium ions, and their capacity decreases as their operating temperature increases. The resins can be regenerated by washing the resin with a solution containing large amounts of common salt, which dissolves the salt and replaces the saturated sodium ions on the cation exchange site.

Regeneration

Ion exchange is a reversible chemical reaction. However, ion exchange resins can only hold so many desirable cations and anions before they become saturated and depleted. This is when the system must perform regeneration, flushing away undesirable ions and replacing them with desirable ions.

Ion exchange water treatment is used to remove a variety of unwanted elements from drinking and industrial water. It can target a specific problem such as water hardness or regulated contaminants like arsenic, nitrate, lead, boron and perchlorate. The water chemistry of the source water is analyzed to determine what elements need to be removed.

As water passes through the ion exchange resin bed, the heavy, challenging ions are swapped with lighter mobile counterions and held within the resin’s porous bead structure. The lighter, desired ions then exit the resin bed with the water stream. The ion exchange process is commonly used in water filters and softeners, but it can also be found in other applications such as nitrate reduction for drinking water and deionization for industrial water.

The type of ion exchange resin used is determined by the application. For example, traditional salt-based water softeners use cation exchange resin to remove calcium and magnesium ions from the water by exchanging them for sodium ions. Other ion exchange applications include wine and fruit juice processing, whey demineralization and cane sugar decolorization.

Applications

Ion exchange has numerous laboratory and industrial applications. For example, it is used to separate and purify blood components, as well as for the detection of different renal diseases. It is also used for the separation of organic acids and biomolecules such as proteins and peptides.

The ions or molecules that are to be separated by ion exchange are first loaded onto the ion exchange resin. The resin then swells as it absorbs the ions. Afterwards, the ions or molecules with a stronger electrostatic charge are attracted to the oppositely charged ions in solution.

Once the ions or molecules are bound to the resin, they are eluted from the resin using an aqueous mobile phase that contains buffers and salts. The aqueous mobile phase can be varied in pH or ionic strength to allow the ions to leave at different times depending on how strong or weak their electrostatic attraction is with the resin.

The resin is then regenerated to remove the undesirable cations or anions and refill it with desirable ions. Since ion exchange is a reversible process, the desirable ions will replace the unwanted ones until the resin becomes saturated. The resin must be regenerated at regular intervals to maintain its effectiveness.