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Ion Exchange (IX) is commonly used across a variety of industries, especially chemical and petrochemical, power, mining and metals, food and beverage, pharmaceutical, municipal, semiconductor, and others. IX can be an efficient solution for a variety of applications, including water softening, purification, separation, and pretreatment to protect downstream equipment and improve operational performance.
IX systems separate ionic contaminants from solution through a physical-chemical process where undesirable ions are replaced by other ions of the same electrical charge. This reaction occurs in an IX column or vessel when a process or waste stream is passed through a specialized resin that facilitates the exchange of ions. A common example is a water softening IX system, where the goal is to remove scale-forming calcium or magnesium ions from solution.
When the solution is passed through an IX resin composed of concentrated sodium ions, the calcium and magnesium ions are effectively captured from solution and held by the resin while the sodium ions are released from the resin into the effluent stream.
What is included is a basic ion exchange system?
A well-designed IX system conforms to the conditions of a specific application in both physical design specifications and in the chosen IX resin material. Common components of a basic IX vessel include:
IX resins are the most critical factor in IX system design. The substances present in the feed stream, as well as other process conditions, will determine the geometric shape, size, and material used in the IX resin.

How Does Ion Exchange Work?

By definition, ions are charged atoms or molecules. When an ionic substance is dissolved in water, its molecules dissociate into cations (positively charged particles) and anions (negatively charged particles). Taking advantage of this characteristic, IX selectively replaces ionic substances based on their electrical charges. This is accomplished by passing an ionic solution through an IX resin that serves as a matrix where the ion exchange reaction can take place.
Most commonly, IX resins take the form of tiny, porous microbeads, though they are sometimes available as a sheet-like membrane. IX resins are fashioned from organic polymers, such as polystyrene, which form a network of hydrocarbons that electrostatically bind a large number of ionizable groups. As the process or waste stream flows through the IX resin, the loosely held ions on the surface of the resin are replaced by ions with a higher affinity for the resin material.
Over time, the resin becomes saturated with the contaminant ions, and must be regenerated or recharged. This is accomplished by flushing the resin with a regenerant solution. Typically consisting of a concentrated salt, acid, or caustic solution, the regenerant reverses the IX reaction by replenishing the cations or anions on the resin surface, and releasing the contaminant ions into the wastewater.

What Contaminants do Ion Exchange Systems Remove?

The most common application of IX is sodium zeolite softening, though other popular applications include high-purity water production, dealkalization, and metals removal. IX can be an extremely effective strategy for removal of dissolved contaminants, though IX resins must be carefully chosen based on the substances present in the feed stream, as listed below.

IX Resin Types and Common Applications

IX resins target contaminants for removal based on their electrical charges. With few exceptions, cationic resins swap out positive ions, while anionic resins swap out negative ions. Key IX resin types and common uses are outlined below, though there are many other resin and application options as well.

Cationic Resins

Cation exchangers can be classified as either strong acid cation (SAC) resins or weak acid cation (WAC) resins, both of which are extensively used for demineralization. SAC resins are also commonly used for softening, while WAC resins are used for dealkalization applications. Contaminants removed by cation resins typically include:
Calcium (Ca+2)
Chromium (Cr+3 and Cr+6)
Iron (Fe+3)
Magnesium (Mg+2)
Manganese (Mn+2)
Radium (Ra+2)
Sodium (Na+)
Strontium (Sr+2)

Anionic Resins

Anion exchangers can be classified as either strong base anion (SBA) resins or weak base anion (WBA) resins. SBA resins are frequently used for demineralization, while WBA resins are often used for acid absorption. Contaminants removed by anion resins typically include:
Carbonates (CO3)
Chlorides (Cl-)
Cyanide (CN-)
Nitrates (NO3)
Perchlorate (ClO-4)
Silica (SiO2)
Sulfates (SO4)
Perfluorooctanoic acid (PFOA)
Perfluorooctanesulfonate anion (PFOS)

Specialty Resins

While specialty IX resins are highly effective for specific industrial applications, their greater specificity generally means greater expense and narrower adoption than conventional IX resins. Chelating resins, for example, are used extensively for concentration and removal of metals in dilute solutions, such as Cobalt (Co+2) and Mercury (Hg and Hg+2). Similarly, magnetic ion exchange (MIEX) resins are often deployed for removal of natural organic matter from feed water.
IX Column Configuration
Depending upon the needs of a specific application and characteristics of a treatment train, IX systems may include one or more resins, and/or multiple IX columns. Examples of common IX system configurations are given below, though there are many other variations to suit specific applications.
Mixed bed units
Contain both cation and anion resins in a single IX column and are used to produce ultra-high purity water with a near-neutral pH and low rinse water requirements.
Twin (or two) bed systems
Include a paired set of IX columns, one of which contains SAC resin, and the other contains SBA resin. Two bed IX systems are used for softening applications that do not require stringent purity standards.
Split stream Dealkalization
Involves operation of two SAC beds operating in parallel, where feed water is divided between the two beds, with one operating in the sodium cycle as a softener that yields full alkalinity, and the other operating in the hydrogen form as a demineralizer that removes all alkalinity. The streams are then blended to achieve a target alkalinity.

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