Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 341

same trend was also observed elsewhere [51,70,79,80,81]. Even though the
electrostatic mechanism is very important, it is not the only mechanism for
dye adsorption in the system. Adsorbents can also interact with dye mol-
ecules via other mechanisms such as hydrophobic interaction and chemi-
sorption [83]. Talep et al. [97] observed this by using alginate-organophilic
montmorillonite composite to remove two acidic dyes (Acid Green B and
Direct Pink 3 B).
In addition, Ma et al. [50] maintain the idea that the main driv-
ing mechanism for the adsorption of acid dye is anion exchange. The
counter-ion bromide in the organo-bentonite is replaced by the dye anion
and the adsorption capacity of organo-bentonite is affected by the surfac-
tant alkyl chain length. When the longer alkyl chain surfactant is modi-
fied, bentonite gives higher adsorption capacity because inorganic cations
in bentonite exchange with surfactants to increase interlamellar spacing
and thus expose new sorption sites [83]. The exchange capacity of ben-
tonite increases with the amount of surfactant [50,102]. These results
support the study done by Li et al. [79]. They modified bentonite using
epicholorohyrin-dimethylamine polyamine (EPI-DMA) and showed that
the layer space is expanded due to the intercalation of EPI-DMA into the
clay layers, leading to a more hydrophobic and more positive surface. Their
results also showed that modified clay is suitable for adsorption of Disperse
Yellow SE-6GRL, Disperse Red S-R, Reactive Reddish Violet K2-BP and
Reactive Jade Blue K-GL. However, their study also indicates that rela-
tively large amounts of polymer are required to make negative clay surface
become positive.
Silva et al. [72] and Talep et al. [97] mentioned similar results for mont-
morillonite. Organic modification can significantly improve the clay's
adsorption capability towards anionic dyes. Adsorption is ascribed to the
binding between anionic groups (e.g., sulfonic groups) of the dye and the
positively charged surface of organoclays [76,83].
The above-mentioned mechanisms illustrate how modification changes
the surface charge of the expanding three-layer clays and increases their
active sites. The ion-exchange mechanism offers an exchange of the inor-
ganic cation with the organic surfactant cation. As a result, the adsorp-
tion capacity of organoclay increases compared to natural clay mineral
[70,94,103].
In addition to pH and modification or activation processes, surface area
and ion exchange mechanisms are used to explain adsorption of anionic
dye on bentonite. Some researchers mentioned that the adsorption capac-
ity of natural bentonite is hampered by its small surface area [84,104], and
modification of bentonite further decreases the surface area [37,50,83].