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4.1 Basic Principles of Adsorption and Ion Exchange 255

Figure 4.6 The typical structure of clays.

Natural organic ion exchangers Some common natural organic ion exchangers are

• polysaccharides, such as cellulose and peat
• proteins, such as casein, k and collagen eratin,
• carbonaceous materials, such as charcoals, lignites, and coals.

However, only charcoals, coal, lignite, and peat are used in industrial applications.
v
fecti
Although they are not as efe as synthetic organic e their low cost mak es
xchangers,
them an appealing choice. Before selecting these materials, one should keep in mind that
• they exhibit low exchange capacities
• they tend to swell or conert into a colloidal form v
• they have a loose physical structure
• their physical properties are not uniform
• they are nonselectie v
• v they are pH-sensitie

hang er Synthetic ion es xc
Synthetic inorganic ion exchangers Zeolites : Synthetic zeolites (Figure 4.7) are a ail- v
wder
,
able as po pellet, or bead. The greatest advantage of synthetic zeolites against natu-
ral zeolites is that they can be manufactured with a wide variety of physicochemical
properties. The main disadvantages of synthetic zeolites are
• their high cost compared to natural zeolites
• they are pH-sensitie v
• their limited mechanical stability

Titanates and silico-titanates : The oxide and hydroxide of titanium are ef ely used v fecti
e sho wn in applications of removing metal ions from w. Early studies (since 1955) ha v ater
that hydrous titanium oxide is the most appropriate material for extracting uranium from
seawater, whereas titanates and hydrous titanium oxide are suitable for removing strontium.