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34 2. Adsorption, Ion Exchange, and Catalysis

ions is due to the properties of the structure of the materials. The exchanger consists of a
so-called matrix, with positie or negatie excess charge. This excess charge is localized v v
in specific locations in the solid structure or in functional groups. The charge of the
v
matrix is compensated by the so-called counterions, which can moe within the free
space of the matrix and can be replaced by other ions of equal charge sign (Helfferich,
1995).
The pores sometimes contain not only counterions but also solvent. When the exchanger
is in contact with the liquid phase, the solvent can trael through the exchanger and cause v
“swelling” to an e xtent that depends on the kind of counterions. Some electrolytes can also
penetrate into the exchanger along with the solvAs a result, there are additional coun- ent.
terions, the so-called coions, which hae the same charge sign as the f v ix ed ions.
Normally, an exchanger has many open areas of variable size and shape that are
altogether called “pores.” Only a fe ganic e xchangers contain pores of uniform cross w inor
section. So, the exchangers exhibit a three-dimensional network of channels with irre gu-
lar size.
Although ion exchange is similar to sorption since a substance is captured by a solid
in both processes, there is a characteristic difference between them: ion exchange is a
stoichiometric process in contrast to sorption (Helfferich, 1995). It means that in the
v
ion-exchange process, for eery ion that is remoed, another ion of the same sign is
v
released into the solution. In contrast, in sorption, no replacement of the solute takes
place.
Ion exchange can be seen as a reversible reaction involving chemically equivalent quan-
ybal,
tities (T 1980; Perry and Green, 1999). The w-softening reaction Ca ater 2 (aq)
re
2Na (s) Ca 2 (s) 2 2Na (aq) constitutes a characteristic example of cation exchange.
However, the characterization of an ion exchange as a “chemical process” is rather mis-
leading. Ion exchange is in principle a redistribution of ions between two phases by diffu-
sion , and chemical factors are less significant or een absent. The absence of any actual v
chemical reaction explains why the heat eed in the course of an ion exchange is usu- v olv
gligible,
ally very small to ne often less than 2 kcal/mol (Helf 1995). Only when an ferich,
ion exchange is accompanied or followed by a reaction such as neutralization can the
” whole phenomenon be characterized as “chemical. A characteristic example is in chelat-
ing resins where the ion exchange is followed by a chemical reaction and bond formation
between the incoming ion and the solid matrix.
v
Ion remoal by solids could ine more phenomena, as for example in inor ganic
v
olv
natural materials where ion uptake is attributed to ion exchange and adsorption processes
or even to internal precipitation mechanisms (Inglezakis et al ., 2004).
2.1.3 Catalysis

Catalysis is one of the most important technologies in our world. It is used e v ely in xtensi
industries for production and in waste treatment for the remoal of pollutants. Even our v
body constantly uses catalysis in biological processes. Enzymatic catalysis is necessary for
all living matter. Most essential of all catalytic processes is photosynthesis, which is seen
in most of the simplest and earliest e ed life forms. olv v