Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 54

the adsorption process was found to be endothermic and thermodynami-
cally favorable. Adsorption of Methyl Orange by MWCNTs has also been
studied by Zhao et al. [86]. Ghaedi et al. [91] reported the removal of
Arsenazo III using MWCNTs as adsorbent. The effects of pH, temperature,
initial dye concentration, adsorbent dose, and contact time on the decolor-
ization efficiency were systematically studied. More than 96% dye removal
was recorded at pH 2. Kinetic studies showed that adsorption of Arsenazo
III onto MWCNTs obeyed the pseudo-second-order model and that intra-
particle diffusion was not the sole rate-controlling step.
Adsorption of cationic dye (Methylene Blue) and anionic dye (Acid Red
183) on MWCNTs was examined by Wang et al. [83] in single and binary
dye systems. The MWCNTs showed higher adsorption capacity towards
Methylene Blue in both the systems. The authors explained that Methylene
Blue, being a planar molecule, could easily approach MWCNTs via a face-
to-face conformation, which is favorable for π-π interactions between the
conjugated aromatic chromophore skeleton and the nanotubes. On the
contrary, non-planar molecules like Acid Red 183 are kept apart from
MWCNTs due to the spatial restriction, resulting in low π-π interactions
with the MWCNTs. Similarly, the adsorption of Reactive Blue 4 and Acid
Red 183 on MWCNTs was investigated in single and binary dye systems by
Wa n g et al. [93]. In single dye systems, adsorption of Reactive Blue 4 and
Acid Red 183 followed a pseudo-second-order kinetic mechanism, and the
equilibrium data could be well described by the Langmuir isotherm. From
the viewpoint of structure and properties of the dyes and MWCNT, the
adsorption was attributed to strong electrostatic attraction, as illustrated in
Figure 2.6. On the other hand, in binary dye systems, MWCNTs could only
adsorb Reactive Blue 4 and did not show any binding selectivity for Acid
Red 183. Such adsorption pattern was due to the competition between the
dyes for the same electrophilic sites and MWCNT's preference for adsorb-
ing planar molecules (Figure 2.6).
A comparative study of MWCNTs and activated carbon as adsorbents
for the removal of the textile dye Safranine O from aqueous solutions was
performed by Ghaedi et al. [84]. The equilibrium isotherm data best fitted
to the Langmuir isotherm model with the maximum dye sorption capacity
of MWCNTs being 43.48 mg g -1 and that of activated carbon 1.32 mg g -1 . In
another study, the comparative adsorption of cationic Methylene Blue and
anionic Orange II dyes from aqueous solution by using MWCNTs and car-
bon nanofibers (CNF) as adsorbents was explored in batch experiments by
Rodríguez and coworkers [88]. The adsorption of Methylene Blue onto CNF
was slightly higher than adsorption onto MWCNTs, while for Orange II, the
adsorption capacity of CNF was considerable lower than that of MWCNTs.