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4.1 Basic Principles of Adsorption and Ion Exchange 253

4.1.2 Ion-exchange materials

According to the defven by Helfferich (1962), ion exchangers are insoluble solid inition gi
materials, which carry exchangeable cations or anions. When the ion exchanger is in con-
tact with an electrolyte solution, these ions are exchanged with an equialent amount of v
other ions of the same sign. Cation and anion exchangers are the materials that carry
cations and anions, respectie the ability of both cation and anion . Some materials ha ely v v
xchangers”
ferich,
exchange and are called “amphoteric ion e (Helf 1962). There are a
number of different natural and synthetic materials that show ion-exchange properties. The
most important materials are described in the follo wing sections.
Naturally occurring ion e xc s er hang

Natural inorganic ion exchangers Many natural minerals, such as clays (e.g. bentonite)
and zeolites (e.g. clinoptilolite), are capable of ecrystalline alu- , er v we xchanging ions. Ho
minosilicates are principally the common natural minerals with cation-exchange proper-
xchangers,
ties (Hellferich, 1995). Amongst natural inorganic ion e zeolites and clays are
the most common materials. Other natural aluminosilicate materials, such as green sand,
are also found in specific waste-treatment applications.
ork structure with pores (channels) and w Zeolites have an open, three-dimensional frame
interconnecting cavities in the alumosilicate lattice. In the basic characteristics able 4.8, T
of the most important zeolite species of commercial use are presented.
v Clays hae a loose layer structure (Figure 4.6). Characteristic minerals are montmoril-
lonite and beidellite. Aluminosilicates such as montmorillonite, kaolinite, and feldspar can
act as cation and anion e xchangers.
v Natural inorganic ion exchangers hae the follo antages: wing disadv
• low exchange capacity
• low mechanical durability
• low abrasion resistance
• pore size cannot be adjusted
• clay minerals tend to conert to a colloidal form v
• difficult to size mechanically
• may be partially decomposed into acids or alkalis.

Table 4.8
Basic characteristics of some important zeolites (Hanson, 1995)

Species Formula Internal Density Ion-exchange
porosity (g/cm 3 ) capacity
(%) (meq/g)
Phillipsite (K,Na) 10 [(AlO 2 )(SiO 2 ) ]·20H 2 O 31 2.15–2.20 3.87
22
Chabazite Ca 2 [(AlO 2 ) (SiO 4 2 O 8 )]·13H 2 O 47 2.05–2.10 3.81
Mordenite a Na 8 [(AlO 2 ) (SiO 8 2 ) ]·24H 2 O 28 2.12–2.15 2.29
40
Clinoptilolite b Na 6 [(AlO 2 ) (SiO 6 2 ) ]·24H 2 O 34 2.16 2.54
30
a Figure 4.4.
b Figure 4.5.