Zeolite Cation Exchange

The Cation Exchange Capacity

Zeolites are also often referred to as molecular sieves. The term 'molecular sieve' refers to the particular unique property Zeolites in their ability to selectively sort molecules based primarily on their size and electrochemical charge. The fact that the Zeolites are formed from alkaline salts has impacted in them a natural, strong, negative charge, called its cation exchange capacity 'CEC'. Each deposit has its own different CEC that is peculiar to that particular Zeolite deposit.

The possibility to attract an dhold positively charged cations at ease through their open cage structure gives Zeolites their great advantage to act as natural sieves or filters for many heavy metal cations and other molecules that carry positive charges. To use a layman's explanation, it has an action similar to a magent attracting iron filings and holding them there.

The CEC is measured in milli-equivalents per 100gms (meq/100g). This measurement is expressing the negative charge of the structure (cage) so that there is a difference between the amount of the positive charged (ie 1+, 2+,3+) cations and molecules that are needed to balance (neutralise) the cages negative charge.

The higher the negative charge, the higher the CEC. The higher the CEC, the stronger the attration and the more cations and other positively charged molecules that can be held in the cages.

Natural Zeolites can accommodate a wide variety of cations. For example, the heavy metals aluminium, lead, mercury, nickel, copper, barium, iron, zinc, chromium. Others such as sodium, potassium, calcium, magnesium and many other molecules which carry a positive charge. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution.