Australian Zeolite

The Cation Exchange

Zeolites are also often referred to as molecular sieves. The term “molecular sieve” refers to the particular unique property of zeolites in their ability to selectively sort molecules based primarily on their size and electrochemical charge. The fact that the zeolites were formed from alkaline salts has imparted 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 and hold positively charged cations at ease through their open cage structure gives zeolites their great advantage to act as a 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 magnet attracting iron filings and holding them there.

The CEC is measured in milliequivalents per 100 grams [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 [i.e. 1+, 2+, 3+] cations and molecules that are needed to balance (neutralise) the cage’s negative charge.

The higher the negative charge the higher the CEC. The higher the CEC the stronger the attraction 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 Al, Pb²⁺, Hg²⁺, Ni²⁺, Cu²⁺, Ba, Fe³⁺,
Zn²⁺, Cr³⁺ and others such as Na, K, Ca²⁺, Mg²⁺ 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.