Ion Exchange

In 1850, Thomas and Way performed some of the first scientific research that indicated the existence of an ion exchange process. In their experiment, a solution of ammonium sulfate was passed through soil. The filtrate collected was composed of calcium sulfate instead of ammonium sulfate (KELLER 2005). The importance of this discovery (in ion exchange terms) was not fully understood until later in that decade, when it was found that this reaction was reversible. Ion exchange was then primary used to soften water.

The presence of calcium and/or magnesium in water results in water being considered “hard”. Calcium and magnesium ions in water react with heat, metallic plumbing and chemical agents such as detergents to decrease the effectiveness of nearly any cleaning task. Hard water can be softened using an ion exchange softening process (SKIPTON 2008).

Ion exchange processes can also remove various charged atoms or molecules (ions) such as nitrates, fluoride, sulphates, perchlorate, iron and manganese ions as well as toxic metals (radium, uranium, chromium, etc.) from water.

The most typical application of ion exchange is the preparation of high purity water for industrial applications, water softening, recovery or removal of metals in the chemical industry.

(Adapted from NEUMANN and FATULA 2009)

Synthetic and industrially produced ion exchange resins consist of small, microporous beads that are insoluble in water and organic solvents. The most widely used base-materials are polystyrene and polyacrylate. The diameter of the beads is in the range of 0.3 to 1.3 mm. The beads are composed of around 50% water, which is dispersed in the gel-structured compartments of the material.

Since water is dispersed homogenously throughout the bead, water-soluble materials can move freely in and out. To each of the monomer units of the polymer, so called “functional groups” are attached. These functional groups can interact with water-soluble species, especially with ions. Ions are either positively charged (cations) or negatively charged (anions). Since the functional groups are also charged, the interaction between ions and functional groups is exhibited via electrostatic forces. Positively charged functional groups interact with anions and negatively charged functional groups interact with cations.

The binding force between the functional group and the attached ion is relatively weak. The exchange can be reversed by another ion passing across the functional group. This process can be repeated continually, with one exchange reaction following another.

(Adapted from SKIPTON 2008)

The main component of ion exchange equipment is a microporous exchange resin, which is supersaturated with a loosely held solution. For water softening, this is usually done with sulfonated polystyrene beds that are supersaturated with sodium to cover the bed surface. As water passes through this resin bed, ions attach to the resin beads releasing the loosely held solution into the water.

After a time, the beds become saturated and the exchange resin must be regenerated or recharged. To regenerate, the ion exchange resin is flushed with a salt brine solution. The sodium ions in the salt brine solution are exchanged with the ions, which are flushed out with wastewater.

(Adapted from SKIPTON 2008)

Maintenance of water softening equipment is somewhat dependent on the type of softener. Some degree of monitoring or managing the regeneration process is generally required. Adequate backwashing of the resin bed is important to ensure the regeneration of the unit. However, regeneration creates wastewater.

The costs for ion exchange systems are very variable depending on scale and region. Moreover, costs depend on pretreatment requirements, discharge requirements and utilisation.

People on restricted sodium diets due to health reasons should account for increased intake through softened water. Drinking and cooking with softened water is often avoided by having a cold water line to the kitchen tap that bypasses the water softener. This provides hard water for drinking cooking and other uses. It is not recommended to repeatedly use softened water for plants, lawns or gardens due to the sodium content.