Environmental Engineering Reference
In-Depth Information
area and removes surface impurities [65,66]. Sennour et al. [67] have men-
tioned that thermal activated clays are used in the textile, oil, and sugar
industries. Excess heating collapses structure and reduces surface area
[39,65]. There have been many wastewater applications using acid and
thermal modified three-layer type clays [28,68-70].
Pillared clay is another modified clay which is prepared by intercalat-
ing natural clays with bulky polyoxycations such as Al (aliminium) or
Zr (zirconium). Calcination at high temperatures results in transform-
ing the intercalated polyoxycations into rigid oxide pillars with a highly
porous structure. Although the use of pillared clay is limited due to lack
of thermal stability [71], the removal of cationic and anionic dyes from
aqueous solutions by pillared bentonite and montmorillonite has been
reported [72,73].
Anionic and cationic surfactants can be used to increase dye adsorp-
tion and create so-called organoclay [74-76]. Anionic surfactant enhances
adsorption of basic dyes, and cationic surfactant enhances adsorption of
acid dyes. Excess surfactant may cause aggregation or dye solubilization. In
addition, large-scale production may not be practical due to the complex-
ity of the process [77] and high cost of surfactants [78,79].
9.3.1
Removal of Anionic Dyes by Expanding
hree-Layer Clays
Expanding three-layer clays, with their high cation exchange capacity, are
readily available, reusable and offer a low-cost alternative for dye removal
[80-85]. The three most important dye adsorption parameters are surface
charge, surface area and ion exchange.
However, as clays are generally hydrophilic, they are not effective adsor-
bents for the nonpolar organic compounds in water if not modified by sur-
factants. The pH of the solution determines surface charge and ionization/
dissociation of the adsorbate molecule [86,87], and H + and OH - are the
potential determining ions for clay minerals. Zeta ( ζ ) potential for natu-
ral bentonite increases in the negative direction with increasing pH and
the isoelectrical point (IEP) occurs in the pH range of 2-3 [49,50,88,121].
Above the IEP pH point, the surface charge is negative, and anionic dye is
repelled. For example, Özcan et al. [70] used Na-bentonite for adsorption
of anionic Acid Blue 193 while varying the pH from 1 to 11. Maximum
removal occurred at pH 1.5 and decreased at higher pH values as surface
charges became more negative [89,90].
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