Environmental Engineering Reference
In-Depth Information
Xu and Boyd [101] pointed out that large hexadecyltrimethylammo-
nium (HDTMA) molecules are initially adsorbed by cation exchange in
the interlayer, which causes extensive clay aggregation and loss of surface
area. As loading increases, HDTMA is adsorbed on the external surfaces of
aggregates. In addition, recent investigations have shown that surfactants
may block the fine pores and decrease the specific surface area of bentonite
[105]. However, these investigations also show that the specific surface area
has no effect if maximum adsorption density of any dye is way below the
monolayer coverage. Surface area is not as important as other parameters.
Contrary to some surfactant modification processes, acid and thermal
modification tends to increase the surface area. Acid activation decreases
overall average pore size with a concomitant increase in the surface area
[68,70]. Thermal activation similarly enhances the surface area [99,106].
Recently, Toor and Jin [25] modified natural bentonite by thermal acti-
vation (TA), acid activation (AA) and combined acid and thermal acti-
vation (ATA) for the removal of anionic dye, Congo red (CR). More dye
was removed because interlayer spaces collapsed, creating a more tightly
bound structure and an increase in surface area (Table 9.4).
9.3.2
Removal of Cationic Dyes by Expanding
hree-Layer Clays
The adsorption mechanism of cationic dyes on natural and modified three-
layer clays has also been investigated by many researchers. Cationic dyes
are known as basic dye and carry a positive charge in their molecule [21].
Positive cationic dyes are attracted to the negative clay surfaces [39,72],
and above the IEP the surface of the expanding three-layer clays are nega-
tive due to successive deprotonation of positively charged groups and the
underlying negative charge density. Adsorption increases with increasing
pH. Tahir and Naseem [88] showed that bentonite removal of cationic
malachite green oxalate increases from 29% to 91% with an increase in pH
Table 9.4 Characteristics of acid, thermal and acid + thermal modified
bentonites [25].
Characteristics
Pore size (Å)
Surface area (m 2 g -1 )
Raw bentonite
50.28 ± 0.004
25.7 ± 0.008
Acid activated bentonite (0.5 M HCl)
53.78 ± 0.12
75.5 ± 0.10
hermal activated bentonite
51.65 ± 0.10
34.6 ± 0.06
Acid and thermal activated bentonite
56.13 ± 0.09
84.6 ± 0.12
 
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