Non-conventional Adsorbents for Dye Removal - Green Chemistry for Dyes Removal from Wastewater - page 365

Clays invariably contain exchangeable ions on their surface and play an
important role in the environment by acting as a natural scavenger of mol-
ecules by taking up cations and/or anions either through ion-exchange
or adsorption, or both. The prominent ions found on the clay surface are
Ca 2+ , Mg 2+ , H + , K + , NH 4 + , Na + , and SO 4 2- , Cl - , PO 4 3- , NO 3 - . These ions can
be exchanged with other ions easily without affecting the structure of clay
mineral. Clays are classified by the differences in their layered structures
and there are several classes such as smectites (montmorillonite, sapo-
nite), mica (illite), kaolinite, serpentine, pylophyllite (talc), vermiculite
and sepiolite [51]. Because of their low cost, abundance in most continents
of the world, high adsorption properties and potential for ion-exchange,
clay materials are strong candidates as adsorbents [52]. Indeed natural clay
materials possess a layered structure and are considered as host materi-
als. From the literature data, the adsorption capabilities mainly result from
a net negative charge on the structure of minerals. This negative charge
gives clay the capability to adsorb positively charged species. Their adsorp-
tion properties also come from their high surface area and high poros-
ity. Among the clays, montmorillonite has the largest surface area and the
highest cation exchange capacity. Its current market price is considered to
be 20 times cheaper than that of activated carbon [14]. In recent years, there
has been an increasing interest in utilizing clay minerals like montmoril-
lonite, bentonite, kaolinite, diatomite and Fuller's earth for their capacity
to adsorb not only inorganic but also organic molecules. Clay minerals
exhibit a strong affinity for both heteroatomic cationic and anionic dyes as
reported in Table 10.2. However, the adsorption capacity for basic dye is
much higher than for acid dye because of the ionic charges on the dyes and
character of the clay. The adsorption process is mainly dominated by ion-
exchange. This means that the adsorption capacity can strongly vary with
the pH. In addition, some clays such as bentonite require activation by acid
washing before they exhibit adsorptive properties.
Good removal capability of clay materials to uptake dye was previously
demonstrated by Bagane and Guiza [53], Harris et al. [54], Ghosh and
Bhattacharyya [55], Espantaleon et al. [56], Al-Ghouti et al. [57], Shawabkeh
and Tutunji [58], Atun and Hisarli [59], Ozdemir et al. [60], Özcan et al.
[61], and Tsai et al. [62], and more recently by Demirbas and Alkan [63],
Feddal et al. [64], and Vanaamudan et al . [65]. For instance, the removal
performance of Fuller's earth and CAC for Basic Blue 9 was compared by
Atun and Hisarli [59]. They showed that the adsorption capacity is greater
on Fuller's earth than on CAC. Fuller's earth is efficient because it contains
variable amounts of dioctahedral smectites, natural zeolites and other sepi-
olites. Shawabkeh and Tutunji [58], studying the adsorption of Basic Blue 9