Non-conventional Adsorbents for Dye Removal - Green Chemistry for Dyes Removal from Wastewater - page 369

are useful as adsorbents for removing color, and in particular to interact
efficiently with acid dyes from wastewater. The authors obtained adsorp-
tion capacities of 57.47 mg and 212.8 mg of Acid Blue 40/g commercial
activated carbon and treated alunite, respectively. Among other inorganic
materials, silica beads also deserve particular attention [95-98,54,99],
considering the chemical reactivity of their hydrophilic surface resulting
from the presence of silanol groups. Their porous texture, high surface
area and mechanical stability also make them attractive as adsorbents for
decontamination applications. However, due to their low resistance toward
alkaline solutions their usage is limited to media of pH less than 8 [100].
Moreover, the surface of siliceous materials contains acidic silanol (among
other surface groups) which causes a strong and often irreversible non-
specific adsorption. For that reason, it is necessary to eliminate the neg-
ative features of these adsorbents. In order to promote their interaction
with dyes, the silica surface can be modified using silane coupling agents
with the amino functional group [96]. Phan et al. [99] also showed that
modified silica beads have a better potential for the removal of acid dyes
from colored effluents. Recent strategies involve silica monoliths either as
composites or biocomposites when they include immobilized living cells
or biopolymers (enzymes, alginates, etc.). Recently, Rodrigues et al. [101]
reviewed some of the most promising materials and pointed out their
advantages.
10.5 Zeolites
Adsorption provides an attractive alternative for the treatment of polluted
waters, especially if the adsorbent is inexpensive and does not require an
additional pretreatment step before its use. From these two points of view,
zeolites represent an interesting alternative. Natural zeolites are highly
porous aluminosilicates with different cavity structures. These abundant
and low-cost resources are becoming widely used as alternative materi-
als in areas where adsorptive applications are required due to their high
cation-exchange ability as well as their molecular sieve properties. The
general chemical formula of zeolites is M x/n [Al x Si y O 2(x + y) ]pH 2 O, where M is
(Na, K, Li) and/or (Ca, Mg, Ba, Sr), n is cation charge; x/y = 1-6, p/x = 1-4.
The characteristics of zeolites and their applications in water and waste-
water treatment have been reviewed by Ghobarkar et al. [103], and more
recently by Wang and Peng [104]. Zeolites consist of a wide variety of spe-
cies, more than 40 natural species such as clinoptilolite, mordenite, chaba-
zite, stilbite, and launmontite. However, the most abundant and frequently