What Is Ion Exchange?

Background

Soluble chemical compounds, when dissolved in water, become ionized; that is, their molecules dissociate into positively and negatively charged components called ions. Consider common table salt, sodium chloride. In its solid form, this compound consists of one sodium atom (Na) and one chlorine atom (Cl) tightly coupled together (NaCl). When dissolved in water, however, the compound splits into two ions, Na+ and Cl-.

Absolutely pure water is a very good insulator; that is, it resists the flow of electricity through it. With ions present, however, electricity will flow through water. Positive ions tend to migrate towards the cathode, or negatively charged electrode and are called cations. Negative ions flow towards the anode, or positive electrode, and are called anions. Because of this effect, measurement of the resistance of water is a very good indication of how pure it is: the higher the resistance, the purer the water.

Ion Exchange

Contaminant ions can be removed from water in a process called ion exchange. As the name implies, contaminant ions are not merely removed from the water; instead, they are exchanged for another kind of ion. This process occurs in ion exchange resin. These resins are usually long chain hydrocarbons such as polystyrene. Attached to these long chain molecules are sites that, because of their chemical makeup, tend to attract and hold ions.

Resin is formed into small porous beads to increase the number of sites that are exposed to water and can participate in ion exchange. Initially, resin is loaded with a harmless ion, usually hydrogen (H+) for cations and hydroxide (OH-) for anions. As waste water passes through the resin, the contaminant ions in the water displace the harmless ions from the sites on the resin. This is because the resin has a greater affinity for the contaminant ion. Affinity for most resins is based loosely on ionic size and charge.

Although ion exchange sounds complex, practical implementation is very simple. Water, contaminated with some dissolved ionic species (dissolved metals, cyanide or some other pollutant), is passed through a column filled with ion exchange resin. Water is discharged from the bottom of the column with little or undetectable concentrations of contaminants.

Sites closest to the entrance fill up first. This means that resin at the top of the column will become loaded first. If the column were made of glass you could actually watch as the column loaded from the top down. Copper contaminants change sand-colored resin to blue, for example. When most of the sites in a resin column are occupied by a single type of ion, then the resin is said to be loaded. As the sites begin to fill up, chances decrease that a contaminant ion will encounter a site occupied by an ion it can displace. This means that the closer to loaded a column is, the more likely it is that contaminants will pass through the column (this is called leakage).

The contaminated solution must remain in contact a certain amount of time in order for all contaminant ions to find a site to exchange with a harmless ion. If the flow rate through the column is too high, reducing contact time with the resin, all contaminants may not be removed.

The ion exchange process percolates water through bead-like spherical resin materials (ion-exchange resins). Ions in the water are exchanged for other ions fixed to the beads. The two most common ion-exchange methods are softening and deionization.

Softening

Softening is used primarily as a treatment method to reduce water hardness for  household use as well as for use prior to  reverse osmosis (RO) processing. The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the “softened” water.

Deionization

Deionization (DI) beads exchange either hydrogen ions for cations or hydroxyl ions for anions. The cation exchange resins, made of styrene and divinylbenzene containing sulfonic acid groups, will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly, the anion exchange resins, made of styrene and containing quaternary ammonium groups, will exchange a hydroxyl ion for any anions (e.g., Cl-). The hydrogen ion from the cation exchanger unites with the hydroxyl ion of the anion exchanger to form pure water.

These resins may be packaged in separate bed exchangers with separate units for the cation and anion exchange beds. Or, they may be packed in mixed bed exchangers containing a mixture of both types of resins.  Deionization can be an important component of a total water purification system when used in combination with other methods  such as RO, filtration and carbon adsorption.