The structure of natural zeolite is very interesting and complex. The primary building units
(PBU) of zeolites are the SiO4and AlO4tetrahedra. They connect via oxygen ions into
secondary building units(SBU), which are then linked intoa three-dimensional crystalline
structure of zeolite. Substitution of Si by Al defines the negative charge of the zeolite
framework, which is compensated by alkaline and earth alkaline metal cations. Therefore
natural zeolites appear as cation exchangersbecause they have negative charge on the
surface. In the zeolite lattice, substitution is not limited to Si-Al substitution. Atoms of iron,
boron, chromium, germanium, and titanium may also substitute silicon. Water molecules
can be present in voids of large cavities and bonded to framework ions and exchangeable
ions via aqueous bridges. One of the most investigated zeolite in basic and applied research
is clinoptilolite. The characteristic way of linking of PBUs and the formation of unique
structural units ultimately results in the factthat these materials are highly porous with
channels and cavities in the structure that havecharacteristic pore sizes and shapes. In the
structure of clinoptilolite, there are three types of channels, of which two are parallel, and
made of ten and eight-membered rings of Si/AlO4
, while one, defined by eight-membered rings, is vertical. In these channels the hydrated cations can occupy the following places: I- cation (Na- and Ca-ions) is located in the 10-member ring channels (free diameters 0.44 x
0.72 nm); II- cation (Na- and Ca-ions) is located in the 8-member ring channels (free diameters 0.41 x 0.47 nm);
III- cation (K- ion) is located in the 8-member ring vertical channels (free diameters 0.40 x 0.55 nm); IV
- cation (Mg-ion) is located in the channel of 10- member rings and it is located in the center of the channel (Figure 2).