ION EXCHANGE
Zeolite cages
are occupied by ions such as sodium or potassium; cations that counter the
negative charge of the alumina-silica framework. These ions are readily
exchanged in external solutions. This property has
been intensively studied whereby the introduction of other cations, usually
by ion exchange, has been used to modify the actions of the zeolites. These efforts have allowed almost every element on the
periodic table to be introduced into a zeolites framework (Dyer, 63).
A simple example of ion exchange between a zeolite and solution is:
Na+ + K+
<-> Na+ + K+
Where the species in bold is inside
the zeolite frame.
These stoichiometric reactions can
be characterized by the construction of ion exchange equilibrium.
Ion Exchange Equilibrium
Consider the case of a system between
cation A, initially in solution, and B, initially in a zeolite. These cations have an equivalent valence charge.
These substances are allowed to equilibrate
under the simplification of isonormality (solutions are at equal normality). At the point of equilibrium, solution and solid phases
are analyzed to determine the distributions of A and B between the phases.
An isotherm can now be plotted which
records the equivalent fraction of the entering ion in solution, AS,
against that in the zeolite, AZ.
The fraction of A in solution is given
by:
AS = ZAmA/ZAmA
+ ZBmB
mA,B
are the concentrations of the respective ions in solution.
Similarly, the equivalent fraction
in the zeolite is:
AZ = ZAMA/ZAMA
+ ZBMB
MA,B
are the concentrations of the ions in solid phase.
A plot of the idealized isotherm shapes
is shown in figure 1.
If the solid phase has equal preference for A or B then the isotherm would
be a straight line as shown by curve 1 in the figure 1 (ref 2). If A is
selectively preferred, a line such as 2 is given, while a preference for
species B yields curve 3.
This description of zeolites is important
for several reasons. First, it provides quantitative
information about the effectiveness of a zeolite’s performance. Secondly, a wealth of thermodynamic and kinetic information can be extracted from this as explained by Dyer. Practically, a series of selectivity data can be gained
from the zeolites that relate its selectivity to different ions. For example, synthetic zeolite Y shows the ion preference
series: Cs>Rb>K>Na>Li, whereas for synthetic
zeolite X: Na>K>Rb>Cs>Li. These zeolites
possess the same structure but differ in the overall charge on the cage.
Practically Speaking
The creation of these isotherms allows
the exploitation of zeolites in several applications. 
For example, there is a constant need
to remove protein breakdown products from wastewater. Some
areas of the world where naturally occurring zeolites clinoptilolite, figure
2 (3), and phillipsite are found; they have been employed in the effective
removal of ammonia and ammonium ions from aqueous effluent (4). Specifically, a plant in Japan is currently using clinoptilolite in a wastewater
plant with a capacity of up to 500 m3/day (2, 83).
Table 1 lists some other practical
uses of clinoptilolite to treat liquid effluents (2)
The Japanese have also employed clinoptilolite
to control soil pH and moisture content.
The control of pH is related to the
ability of the zeolite to function as a slow-release agent to improve nitrogen
retention in the soil. Adding a zeolite to soil adds
a high capacity selective exchanger (figure 3). Data
for 
potassium-ammonium ion exchange capabilities for clinoptilolite are shown
in figure 4 (2).
Finally, Emadi, et al. have reported
that clinoptilolite is much more effective than carbon in removing ammonia
from aquatic fish environments. This has important
consequences since the ammonia concentration in aquatic environments is
vital to several physiological factors of fish such as growth rate, oxygen
consumption, and disease resistance
