Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 338

Baskaralingam et al. [83] failed to remove very much Acid Red 151 using
Na-bentonite, and the maximum 5% uptake decreased with increasing pH.
Similar trends for bentonite were observed for adsorption of Procion Navy
Hexl [38], Congo red [39], Supranol Yellow 4GL [73], and Sulfacid bril-
liant pink [91]. Similarly, Zohra et al. [80] mentioned that the anionic dye
(Benzopurpurin 4B) did not adsorb onto natural Na-bentonite because of
the similar surface charge.
The ability of Ca-bentonite to remove anionic dye from aqueous solu-
tions has also been studied. Ca-Bentonite has a permanent negative charge
due to the isomorphous substitution of Al 3+ for Si 4+ in the tetrahedral layer
and Mg 2+ for Al 3+ in the octahedral layer [73,83]. The high surface calcium
content also serves to neutralize any OH − and decreases the effect of pH
on adsorption. Lian et al. [92] selected natural Ca-bentonite as a cheaper
bentonite adsorbent for the removal of Congo red from aqueous solutions.
Their results showed that adsorption is dependent on concentration, but
changed only slightly (10% decrease) in the pH range of 5-11.
Natural bentonite is sometimes reasonably efficient as an adsorbent
for anionic dye in water [53,54]. However, surface-modified bentonite is
much more efficient and has a greater potential to provide an alternative
to activated carbon [84,95,96]. Surface modification includes heat treat-
ment [58], acid activation [63], treating with cationic surfactants [78] and
polymer modification [60,93]. Introducing quaternary ammonium salts
imparts hydrophobicity and makes an “organoclay” with an affinity for
organic compounds [94], and organoclays have been widely used in waste-
water treatment processes [84,95,96]. Table 9.3 lists adsorption capacities
of organo-bentonites for selected anionic dyes.
Literature data show that the adsorption capacities of the expanding
three-layer clay increases upon modification [28,38,50,51,63,82,83]. For
example, natural and modified bentonites using dodecyl trimethylammo-
nium bromide (DTMA) for the removal of Acid Blue 193 were compared by
Özcan et al. [70]. The adsorption capacity of DTMA-bentonite (740.5 mg/g)
is about 11-fold higher than that of raw Na-bentonite (67.1 mg/g) at 20 C.
Recently, Akl et al. [37] also showed similar results for hexadecyl trimethyl
ammonium bromide (CTAB)-modified bentonite, where 37.05 mg/g and
210.104 mg/g of Congo red were removed. In the modification process, the
alkyl chain length of surfactants affected the adsorption capacity of ben-
tonite. Ma et al. [50] further emphasized that higher adsorption capacity
is obtained using long-chain surfactants. Decreasing the surfactant carbon
chain length from C16 to C8 during modification decreases the adsorption
capacities of Orange II from 298.39 mg/g for C16-bentonite to 44.08 mg/g
for C8-bentonite.