1. What are zeolites?
Zeolites are hydrated alumino-silicate minerals containing aluminium, silicon and oxygen in their regular framework. They form in nature as a result of a chemical reaction between volcanic lava and saline water.
Zeolites have large open spaces (or cages) in their structure that form channels. These channels allow ions and molecules to easily pass in and out of the structure. This ability puts zeolites in the class of materials known as "molecular sieves."
Another unique property of zeolites is that they are one of the few “negatively-charged” minerals found in nature. The charge is balanced by the presence of single and double-valanced atoms, such as sodium (Na+), potassium (K+), calcium (Ca+2), and magnesium (Mg+2), elsewhere in the structure.
Zeolites act as molecular sieves, or filtering agents, that attract and trap positively charged atoms, ions and compounds, and can therefore remove them from a system.
Compositionally, zeolites are similar to clay minerals. More specifically, both are alumino-silicates. They differ, however, in their crystalline structure.
Most types of clays have layered crystalline structures (similar to a deck of cards) and are subject to shrinking and swelling as water is absorbed and removed between the layers. In contrast, zeolites have a rigid, 3-dimensional crystalline structure (similar to a honeycomb) consisting of a network of interconnected tunnels and cages.
Water moves freely in and out of these tunnels and cages but the zeolite framework remains rigid. Another special aspect of this structure is that the pore and channel sizes are nearly uniform, allowing the crystal to act as a molecular sieve.
2. What is clinoptilolite?
There are more than 50 natural and 150 synthetic types of zeolites. Natural clinoptilolite is the most widely used type due to its high absorption rate, cation exchange, catalysis and dehydration capacities. It can be utilised as a chemical sieve, feed and food additive, soil conditioner as well as liquid, gas and odour absorber. Suitability for such applications is due to its large amount of pore spaces, a high resistance to extreme temperatures, and chemically neutral basic structure.
3. What is Cation Exchange Capacity (CEC)?
Na+, K+, Ca+2 and Mg+2 ions in a zeolite structure are known as exchangeable ions, because they can be replaced (exchanged) easily with other cations in an aqueous solution, without affecting the alumino-silicate framework. This phenomenon is known as ion exchange, or more commonly cation exchange. The magnitude of such cation exchange, in a given zeolite, is known as its cation-exchange capacity (CEC) and is commonly measured in terms of moles of exchangeable cation per gram (or 100 grams) of zeolite.
4. What are some of the major applications for zeolites?
Zeolites can be used to perform many important tasks, such as ion exchange, filtration, odour removal, sieving chemicals, softening water and gas absorption.
In agriculture, zeolites are used to promote better plant growth by improving the effect of fertilisers. Zeolites retain water in their porous structure. This means that the plants always have a reservoir of water and nutrients readily available in the soil. The combination of these effects therefore reduces the overall amount of irrigation and fertiliser needed, because less water is lost due to filtration and evaporation, and less fertiliser is lost due to leakage.
In animal nutrition, clinoptilolites are mainly used to protect animals against mycotoxin intoxications, to eliminate heavy metals and radionuclides, prevent digestive tract diseases, improve utilisation of feed nutrients and also raise hygiene and sanitary conditions wherever animals are housed. It may be directly added to the feed or water of the animals for mycotoxins protection or used as litter to provide a dry hygienic environment.
Apart from agricultural and livestock applications, zeolites are used in industries which, for example, produce cement and bricks or building materials and paints with anticorrosive properties. They are also used to deal with defluorination of industrial wastes, recovery of wastes, cleanup of sewage, heavy metal and ammonium ion removal and medical applications to name but a few.