What is Ion Exchange?

Ion exchange is a reversible process that involves swapping the ions of an insoluble solid with mobile counterions. It’s often used for water softening, demineralisation, or to remove regulated contaminants from drinking and industrial waters.


Most ion exchange systems use resin or natural zeolite materials to reversibly capture undesirable ions. They’re designed to be easily refreshed with a brine solution so the system always has access to desirable ions.

Water Treatment

Ion exchange can be used to remove pollutants, metals and radionuclides from aqueous solutions. Ion exchange resins are synthetic polymeric materials that contain ionic functional groups to which exchangeable ions are attached. They can also be natural or inorganic materials, such as montmorillonite clays, zeolites, and soil humus. They can be cation or anion exchangers, and some are amphoteric, able to exchange both cations and anions.

Water treatment processes such as water softening and water deionization use ion exchange to remove impurities from the water. These include positively and negatively charged molecules (ions), which determine the total dissolved solids (TDS) and conductivity of the water. The greater the number of ions present, the higher the TDS and conductivity.

Ion exchange is also used in liquid-phase desalination, where anions and cations in salt water are exchanged for carbonate anions and calcium cations respectively by passing the water through an ion exchange column. This produces fresh water without the need for membranes.

However, the duration of an ion exchange process is limited by scaling and clogging of the ion exchange resins, as well as the presence of contaminants that may damage or deplete the ion exchange resin. These include oil and grease, suspended solids, oxidants, and cations and anions in the water. It is necessary to regularly regenerate the resin, which can be costly.

Water Softening

Water softening is a common industrial method for removing the minerals that produce hard water, most notably calcium and magnesium. It involves passing water through a bed of natural or synthetic (polystyrene divinyl benzene is the most common) ion exchange resin that trades sodium ions for these hardness-producing ions. The magnesium and calcium ions bind to the negatively charged resin beads and are flushed away into a discharge tube. The resin is then regenerated by passing a concentrated solution of common salt slowly through the bed. This replenishes the supply of sodium ions and allows the process to start over.

In homes, ion exchange softeners are commonly used to treat hard water and prevent the formation of scales that can shorten appliance lifespans and clog pipes. The resin is filled with small, negatively-charged sand-like beads that bind to magnesium and calcium ions in the hard water. As the magnesium and calcium ions bind to the resin, they are displaced by the positively-charged sodium ions. Eventually, the resin will become depleted of its supply of sodium and must be regenerated using a brine solution.

While some people are concerned about adding sodium to their water, the amount that is released in this process is far less than the suggested daily intake of sodium. The ion exchange process is also efficient and creates significantly less waste water than other methods such as distillation or reverse osmosis.

Chemical Removing

Ion exchange is the reversible interchange of ions between a solution with soluble ionized substances and a solid. This process is used to remove impurities from liquids such as water, and replace them with pure substances. The devices in which ion exchange takes place are called demineralizers, and they usually consist of tanks or columns filled with ion exchange resins that can be either zeolite or polymer beads. Each resin bead contains permanently fixed ions of a certain type, which can either be cations or anions, with mobile counterions of the opposite charge that keep neutrality within the bead structure.

The ion exchange resin absorbs the unwanted cations or anions and sticks them to its surface, leaving the desirable ions free to pass through. The resin becomes depleted of desirable cations or anions over time, so it must be “regenerated” or refilled with desired ions to continue working effectively. The regeneration process involves washing the resin with a liquid that removes the undesirable ions, followed by flushing away the wash and refilling the resin with desirable ions.

DuPont Water Solutions offers a wide range of ion exchange resins to meet your specific chemical removal needs. In addition to standard cation and anion resins, we offer highly cross-linked resins with specialized functional groups designed for enhanced separation performance. For example, our Purolite S930 and S920 resins feature iminodiacetic and isothiouronium functional groups respectively. These unique functional groups enable them to effectively treat a variety of contaminants, including uranium, arsenic, nitrates, perchlorate and boron.

Ion Removal

Ion exchange can remove many dissolved contaminants from water, making it ideal for water treatment, chemical purification, and separation. The ion-exchange process occurs when a solution passes through or over an ion-exchange resin, which contains mobile or exchangeable ions that can accept ions from the solute solution. As the ions from the solution pass through the resin, the undesirable or harmful cations are removed from the solution and swapped with desirable cations, such as sodium. This process is commonly used in devices called demineralizers to remove impure ions from a fluid and to produce pure water.

Ion-exchange systems can be outfitted with a variety of different types of resins, depending on what contaminants are targeted. For example, if you are looking to eliminate strontium from your drinking water, you will need a system equipped with a cation resin. This is because strontium (Sr++) is an alkaline earth metal that dissolves in the water as the mineral strontianite and celestite, often found in bedrock aquifers. High levels of strontium in drinking water can have multiple negative health effects, including abnormal bone growth, skeletal problems, and cancer.

The resin material in ion-exchange resins can be made from synthetic organic materials that have ionic functional groups to which exchangeable ions are attached or from inorganic and natural polymeric materials. Over time, the resin will become depleted of desirable cations or anions, which is why these systems require regular regeneration.