Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 53

solutions. The dye decolorization potential increased with increasing
adsorbent dose and reaction temperature, while it decreased with an
increase in initial dye concentration. The equilibrium adsorption data of
Direct Yellow 86 were best fitted to the Freundlich isotherm, while that
of Direct Red 224 to the Dubinin-Radushkevich isotherm. The MWCNTs
exhibited a stronger binding affinity for Direct Red 224 than Direct Yellow
86. Thermodynamic analyses indicated that the adsorption of direct dyes
onto MWCNTs was endothermic and spontaneous.
The feasibility of MWCNTs for removal of the reactive dye Procion
Red MX-5B was investigated by Wu et al. [82]. Both the Langmuir and
the Freundlich models were found suitable to describe the equilibrium dye
adsorption data, suggesting that monolayer sorption as well as heteroge-
neous energetic distribution of active sites on the surface of the adsorbent
were possible. The adsorption process followed the pseudo-second-order
kinetics with activation energy of 33.35 kJ mol -1 , indicating physisorption
of Procion Red MX-5B by MWCNTs. The MWCNTs have also been suc-
cessfully applied to the adsorption of other reactive dyes including Reactive
Red M-2BE and Reactive Blue 29 [85,90].
Geyikci [89] studied the adsorption characteristics of Acid Blue 161 onto
MWCNTs as a function of reaction time, pH and initial dye concentration.
The maximum adsorption yield (91.68%) was obtained at the conditions
of pH 3.0, initial dye concentration = 50 mg L -1 , adsorbent dose = 0.1 g L -1 ,
temperature = 298 K and contact time = 1 h. The adsorption kinetics was
found to obey the pseudo-second-order mechanism. The intraparticle dif-
fusion was not the sole rate-controlling step. Adsorption equilibrium data
fitted well to the Freundlich isotherm model as well as the Temkin model.
MWCNTs exhibited a significantly high adsorption capacity of 1000 mg g -1
for Acid Blue 161.
The efficacy of MWCNTs to adsorb anthraquinone dyes, Alizarin
Red S and Morin, from wastewaters has also been investigated [92]. The
maximum monolayer adsorption capacity was found to be 161.29 and
26.24 mg g -1 for Alizarin Red S and Morin, respectively. The adsorption
process followed pseudo-second-order kinetics with involvement of the
intraparticle-diffusion mechanism.
Azo dyes have also been successfully removed from aqueous systems
using MWCNTs as adsorbent. Yao et al. [87] investigated the adsorption
of Methyl Orange onto MWCNTs from aqueous solutions by performing
simple batch adsorption experiments. The MWCNTs exhibited a fairly high
adsorption capacity of approximately 52.86 mg g -1 with an adsorbent load-
ing of 0.3 g L -1 . The adsorption followed pseudo-second-order kinetics and
the equilibrium data fitted well with the Langmuir isotherm. Additionally,