Environmental Engineering Reference
In-Depth Information
chatterjee et al. [14] impregnated chitosan (cS) hydrogel beads with MWcNTs and studied the adsorption efficiency of the
modified adsorbent for congo red (cR) dye removal. cNT impregnation rendered the beads materially denser and the porosity
of the cS/cNT beads also increased due to the addition of cNT. The field-emission SEM (fE-SEM) image of the modified
adsorbent revealed that cNT was present both uniformly throughout the cS matrix and as small aggregates in the cS/cNT
beads. The energy-dispersive X-ray (EDX) analysis of the modified adsorbent after cR adsorption revealed higher S (wt.%) in
the cS/cNT beads (4.62%) than in the cS beads (1.83%), indicating that cNT impregnation increased the adsorption capacity
of cS/cNT beads. A negative effect of cNT concentration in cS beads was observed. An increase from 0.01 to 0.05 wt.% cNT
showed a decrease in adsorption capacity. It was attributed to the formation of larger aggregates of cNTs, which hindered the
access of cR to the adsorption site of cS and cetyltrimethylammonium bromide (cTAB). cTAB is a dispersant present in cS/
cNT beads that interacts with the hydrophobic moieties of cR and partly has ionic interaction with the anionic charge on cR.
The adsorption of cR onto cS/cNT was highly pH dependent, and maximum adsorption by the cS/cNT beads (423.1 mg/g)
occurred at pH 4.0. Since beads formation was carried out in a sodium hydroxide precipitation bath, the amine groups of cS
molecules were not protonated. An increase in initial pH 4.0-5.6 and from pH 5.0 to 6.0 during adsorption indicated a decrease
in H
+
concentration in the solution. However, such behavior was absent when the initial pH of the cR solution was neutral or
basic as protonation of amine groups did not occur at pH 7.0 and 9.0 due to absence of free H
+
in solution. The langmuir iso-
therm model showed a better fit with the experimental isotherm data than the freundlich isotherm model. The maximum
adsorption capacity of cS/cNT beads for cR was reported as 450.4 mg/g. The values obtained from pseudo-first-order, pseudo-
second order kinetic models and the intraparticle diffusion model showed that the mass transfer rate was fairly increased by the
addition of cNT. Bazrafshan et al. [15] studied the removal of reactive red 120 textile dye (RR-120) using SWcNTs. The
SWcNTs were synthesized by the catalytic cVD method. They were further functionalized by dispersing the former in nitric
acid. The adsorptive removal of RR-120 was influenced by pH. The optimum pH for the process was 5.0 as below and above
this pH adsorptive removal of the dye was found to decrease. Maximum adsorption capacity of SWcNTs for the dye was
reported as 426.49 mg/g, with 85.3% dye removal taking place at pH 5.0 with initial dye concentration of 50 mg/l. The proton-
ation of electron Π-rich regions on the surface of SWcNTs at lower pH values forms positive surface charge, increasing the
uptake of negative charged dye. However, with an increase in pH, the carboxylic groups ionized and caused an increase in neg-
ative surface charge density resulting in reduced removal of RR-120. The increased adsorbent dosage resulted in an increase in
the surface area and adsorption sites that helped in achieving maximum dye adsorption (88.28%, 882.83 mg/g). Adsorption
equilibrium was achieved in 180 min contact time. The BET model fitted well with the isotherm data.
In another study, resorcinol was taken as a model phenolic derivative and the efficiency of MWcNTs was assessed for the
adsorption of the former [16]. Nitric acid treatment was conducted to oxidize the MWcNTs. The kinetics of resorcinol onto
MWcNTs was fast. The experiments were not performed at pH values greater than 8.0 as in basic conditions resorcinol is
oxidized. Resorcinol uptake increased with the decrease in solution pH since its solubility decreases with the lowering of pH.
A comparison of the adsorption properties of treated and nontreated MWcNTs was performed. A reduction in the uptake
capacity of resorcinol was observed due to the electrostatic repulsion between negatively charged resorcinol and acid-treated
MWcNTs. Also, the carboxylic groups, those that are present on the surface of MWcNTs, act as electron withdrawing groups
localizing electrons from the Π system of MWcNTs and interfering and weakening the forces between the aromatic ring of
resorcinol and the graphite structure of MWcNTs. The hydroxyl group (located in meta-position) present in the structure of
resorcinol also positively influenced its adsorption. The potential of MWcNTs for herbicide adsorption has also been tested
by researchers [17]. Diuron and dichlobenil were the two representative herbicides studied, and the effect of lead on their
adsorption onto MWcNTs was observed. The MWcNTs with outer diameters of less than 8nm (MWcNTs8) exhibited
maximum adsorption capacity for both herbicides due to their large surface area and pore volume, and the experimental iso-
therm data was in agreement with the Polanyi-Manes model. It was also observed that an increase in oxygen content of
MWcNTs decreased the adsorption of the herbicides due to deprotonation of carboxylic groups at pH 6.0. Since MWcNTs
with outer diameters of 10-20nm and oxygen content of 1.52% (MWcNTs-O (1.52%)), and MWcNTs-O (2.66%) and
MWcNTs-O (7.58%), had quite similar diameters and surface areas, but different oxygen contents, they were chosen to
study the effect of lead on the sorption of herbicides. In case of Pb(II), in a single-solute system, MWcNTs-O (2.66%) and
MWcNTs-O (7.58%) showed Pb(II) adsorption capacities of 17.75 and 91.67 mg/g for the same Pb
2+
equilibrium concentration
implying that the surface O-containing groups improved the ion exchange capabilities of MWcNTs and, therefore, resulted in
higher Pb
2+
adsorption. The adsorption of diuron or dichlobenil on MWcNTs-O (7.58%) was suppressed significantly (~10-30
and ~15-30%, respectively) in the presence of Pb
2+
. But less effect of Pb
2+
on herbicide removal was noticed when MWcNTs-O
(2.66%) and MWcNTs-O (1.52%) were used as adsorbent. It was concluded that complexation of Pb
2+
with carboxylic groups
was responsible for the reduced herbicide removal. Also, hydrated lead cations occupied partial surface of MWcNTs-O
and shielded the hydrophobic and hydrophilic sites of MWcNTs, leading to inhibition in herbicides adsorption around the
metal-complexed moieties.
