Why Is Zeolite Used In The Purification Of Water?

Before we look into zeolite, we should first know that water which does not lather easily with soap is called hardwater.

Hardness in water is due to the presence of carbonates and biocarbonates of Calcium or Magnesium (when it is temporary hardness) or sulphates and chlorides of Calcium and Magnesium (when it is permanent hardness).

Temporary hardness of water can be removed easily by boiling water or by adding washing soda (Sodium Carbonate-hydrated).

Permanent hardness cannot be removed by merely boiling or adding washing soda.

A complex salt called sodium aluminium silicate (zeolite) is used to remove permanent hardness in water.

Nowadays this material is made synthetically. Permotit or zeolite is packed into a column and the hard water is allowed to flow through it. Double decomposition occurs and calcium aluminium silicate is formed. Eventually the permotit is completely converted into its calcium salt. It can be made fit for use again by pouring a strong solution of common salt through the column.

Sodium Aluminium Silicate (zeolite)+ Calcium Sulphate (salt causing hardness of water) Calcium Aluminium Silicate+ Sodium Sulphate (zeolite)

After adding salt water:-

Calcium Aluminium Silicate + Sodium Chloride (common salt) Sodium Aluminium silicate (zeolite) + Calcium Chloride

In the above reaction the tower is revived and reset for use again, thus the zeolite is not wasted.

The principle of the permutit process has been greatly extended to modern synthetic ion-exchange resins. These can be used to remove all metallic and acidic ions from water and provide a cheap and convenient way of making high purity water purer even than the best distilled water.

Another application is in the preparation of drinking water from sea water.

Published in The Hindu on Feb 7, 2002.