Educational Information

The exchange of ions is a phenomenon universally spread throughout nature. It's utilization by man for the purification of brackish water has been given credibility.

Various natural minerals, called zeolites, were found to have exchange characteristics suitable for the softening of water, which involves the exchange of calcium and magnesium for sodium.

These organic zeolites, though suitable for water softening reactions, cannot be used in the ion exchange of hydrogen ions. The first acid-proof cation exchange materials were produced by the sulfonation of coal. In 1936, the first organic ion-exchange resin was successfully synthesized, thus opening the way for complete deionization of water.

Ion exchange resins are, by definition, insoluble solids containing fixed cation and anions capable of reversible exchange with mobile ions of the opposite sign in the solutions with which they are brought into contact.

Two different systems are employed to express the performance of ion-exchange resins. The water treatment engineer speaks in terms of kilograms per cubic foot, grains per gallon and parts per million. The chemist works with chemical equivalents, whereas the engineer usually converts everything to calcium carbonate (CaCO3) equivalents.

In order to work with ion exchange in water treatment, on should become familiar with the system of expressing concentrations and capacities as calcium carbonate equivalents. Ion exchange resin performance and data (for applications in water treatment) are nearly always presented as calcium carbonate equivalents.

A reasonably accurate water analysis is required to design an ion exchange installation for water treatment. It is common practice to state the amount of dissolved impurities as the cations and anions actually determined by analysis. For ion exchange work, it is highly convenient to have all values expressed as calcium carbonate equivalents. When a water analysis has been converted to calcium carbonate equivalents the sum of the total cations must equal the sum of the total anions.

The hardness in water is composed of the alkaline earth cations-calcium and magnesium. The sum of these two is the total hardness. The total hardness is then broken down into two categories, the carbonate or temporary hardness and the non-carbonate or permanent hardness. Temporary hardness is the most common and is responsible for the deposition of calcium carbonate scale in pipes and equipment.

Non-carbonate hardness will react with the carbonate alkalinity, frequently found in soap and detergent formulas, to form insoluble deposits.

Soaps react with hardness in water to form insoluble and unsightly scum, particularly the familiar "bathtub ring".

The softening of water by ion exchange is relatively simple, but highly effective. The process consists of passing water through a column of cation exchange resin in the sodium form. The sodium on the resin replaces the objectionable calcium and magnesium ions.