Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 336

9.3 The Expanding Three-Layer Clay Minerals and

Dye Adsorption

Clay minerals are hydrous aluminum silicates with large interlayer spaces.
These spaces can hold significant amounts of water and other substances
and can allow swelling and shrinking [31]. Clay minerals are classified into
four types by Grim and Güven [32]: two-layer types; three layer types; reg-
ular mixed-layer types; and the chain structure types.
Three-layer type clay minerals have a sheet structure with two layers of sil-
ica tetrahedrons and one central dioctahedral or trioctahedral alumina layer.
These types of clay minerals are divided into two categories: the nonexpand-
ing illites; and the expanding montmorillonites, smectites and vermiculites.
It is the expanding clays that are most used for dye adsorption [31].
Montmorillonite is a 2:1 layered silicate with negative charge due to
ionic substitution in its structure. This charge is balanced by exchange-
able cations such as Na + or Ca 2+ present in the interlayer space [23,33].
The Na, Ca, Mg, Fe, and Li-Al silicates in the smectite group include
Na-montmorillonite, Ca-montmorillonite, saponite (Mg), nontronite (Fe),
and hectorite [34]. Smectites are the dominant clay mineral in bentonite,
and montmorillonite is the most common smectite [35]. These clay miner-
als are altered or metamorphic forms of glassy igneous material, usually a
tuff or volcanic ash [36]. Montmorillonite has received considerable recog-
nition as an adsorbent because of its high adsorption capacity [29,37-44].
Vermiculite is another expanding, three-layer type, clay mineral that is
derived by alteration or weathering from black mica, chlorite, illite, and is a
secondary metamorphic mineral containing magnesium, iron, aluminum
and silicate [45,46].
Montmorillonite, vermiculite and bentonite are the clay miner-
als most often mentioned in studies to remove dyes from wasterwater
[29,30,37-40,47,48]. Modified or activated forms enhance dye adsorption
capacities [37,49-55]. Activation methods include acid activation [56],
treatment with surfactants [57], thermal treatment [58], polymer addition,
pillaring by different types of poly (hydroxo metal) cations [59], and graft-
ing of organic compounds [60].
Acid activation enhances surface area and average pore volume due to
the decomposition of the crystalline structure [56,61,62]. In addition, acid
activation can also change the chemical properties such as cation exchange
capacity and the surface acidity [63], but excessive acid decreases surface
area [64]. Another favorable method, thermal activation, increases surface