| By virtue of its physical and chemical properties, natural zeolites have two outstanding properties: 1) adsorption and; 2) cation exchange capacity .
Adsorption is a kind of adhesion which takes place at the surface of a solid (e.g. natural zeolite) in contact with another medium (e.g. feeds) resulting in the accumulation or increased concentration of molecules (e.g. ammonia) from that medium in the immediate vicinity of the surface. Selective adsorption capacity is achieved by making sure that the size of the empty spaces and open channels is just big enough to adsorb the target molecules ( Figure 4 ). Figure 4 - Selective adsorption capacity
Natural zeolites vary in their crystalline pore sizes. Although natural zeolites Zeolite is a generic term for a large group of hydrous aluminosilicates that are analogous in composition to the feldspars. They are characterized by their ability to lose and gain water reversibly and to exchange some of their constituent cations without major changes of structure. Zeolite minerals are commonly found as secondary minerals in cavities and fissures in basic volcanic rocks, particularly basalts, and in veins and other late stage hydrothermal environments. They are known as authogenic minerals in tuffaceous sandstones and other sedimentary rocks and are formed from fine grained volcanic ash (an alkaline earth aluminosilicate glass) being carried by the wind from an erupting volcano and deposited onto the land surface or into shallow freshwater or saline, alkaline lakes or into the sea close to the source of eruption. After deposition, the non-crystalline volcanic ash then hydrates (i.e. combines with water) and reacts with the surrounding lake, marine or ground water and then transforms into micrometer size crystals of zeolites, probably by a dissolution-reprecipitation mechanism. Zeolites are used commonly in aquaculture, petroleum refineries, agriculture and as additives in animal feeds supplements.
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