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248 4. Adsorption and Ion Exchange

which in turn polymerizes to form the silica gel. The treatment of silica gel in olv es also v
v
washing, drying, and actiation. Silica gel is mainly used for water remoal (Noble and v
Terry, 2004).
Organically modified clays
Organically modified clays are mixtures of anthracite and bentonite clay or b. The ulk clay
surface of the clay is usually actiated after treating with a quaternary amine. These clays v
are employed only in the purification of gas streams.

Polymeric resin adsorbents
Polymeric resins found in liquid treatment can be classified into three main cate gories:

(a) the carbonized ion-exchange resins,
(b) the divinylbenzene (DB) adsorbents, and
(c) the post-cross-linked adsorbents.

Each type is produced by following a different procedure and has its own adv antages
and limitations. Carbonized resins have the advantage of being structured with the desired
pore distribution depending on the contaminant to be adsorbed. Divinylbenzene adsorbents
, can be used in environments with high relati since they are very hydrophobic e humidity v
materials. In general, the most important feature of polymeric resins is that the y adsorb the
xample,
pollutant without reacting with them. For e the adsorption on actiated carbon is v
generally an exothermic process that may lead to temperature eleation and bed-fire out- v
bursts. On the other hand, safer and controlled operation can be assured using polymeric
adsorbents, since they are much less reacti e materials. v
Zeolite molecular sie ves

Zeolite molecular sieves are composed of silicon and aluminum and can be natural or man-
v made minerals. Molecular siees are crystalline, hydrated aluminosilicates of (most com-
monly) sodium, calcium, potassium, and magnesium. The alumininosilicate portion of the
structure is a three-dimensional open framework consisting of a network of AlO 4 and SiO 4
tetrahedra linked to each other by sharing all of the oxygens (Sherman, 1978). Zeolites
may be represented by the empirical formula

n
R Al O SiO H O X Y
2/ n 2 3 2 2
where X is generally equal to or greater than 2, and n is the cation valence (Sherman,
1978). The zeolite frameork contains pore channels and interconnected voids and ca w vi-
ties, which are occupied by cations and water molecules. These cations are generally
mobile and can be e xchanged.
A typical synthetic zeolite is the zeolite ZSM-5 shown in Figure 4.2. This zeolite is
extensively used as a catalyst.
In Table 4.3, the basic characteristics of the most important zeolite molecular sie e v
species are presented. Zeolite micropore openings are of the same order of magnitude as