Environmental Engineering Reference
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approach the surface of MWNTs via a face-to-face formation, whereas the non-planar
molecules are kept apart from the MWNT surface due to spatial restriction, resulting in
low
interactions with the MWNTs. On the other hand, much more favorable
adsorption of positive molecules was observed, as compared to neutral molecules (Liu et
al., 2008).
π
-
π
A detailed investigation on the adsorption of Procion Red MX-5B dye on
MWNTs was conducted at various pHs and temperatures (Wu, 2007a). The adsorption
isotherm of the dye on MWNTs was described by the Langmuir isotherm. This implied
that dye adsorbs onto MWNTs as monolayer coverage and a homogeneous surface of
MWNTs can be assumed. Based on a calculation of the thermodynamic parameters,
adsorption of the dye onto MWNTs is spontaneous and thermodynamically favorable.
Moreover, the positive entropy change indicates that the degrees of freedom also
increased at the solid-liquid interface during the adsorption of the dye onto CNTs. The
positive enthalpy change promoted the endothermic process of the adsorption process,
thereby increasing the adsorption capacity with a corresponding increase in temperature.
The values of the enthalpy change (ΔH
o
) were 31.55 and 41.77 kJ/mol at pH 6.5 and 10,
respectively; suggesting that the adsorption of Procion Red MX-5B onto CNTs was a
physisorption process. This hypothesis was supported by the activation energy value of
33.35 kJ/mol at pH 6.5 (Wu, 2007a). In addition, adsorption kinetics of the dye obeyed
the pseudo-second-order kinetic model and the adsorption process involved intraparticle
diffusion, but it is not the only rate-controlling step.
An interesting concept was reported, in which caged MWNTs were used for the
elimination of ionic dyes (Fugetsu et al., 2004b). The authors prepared caged MWNTs
via the encapsulation of MWNTs in cross-linked alginate (ALG) microvesicles using
Ba
2+
as the bridging ion. This caged structure allowed MWNTs to be highly dispersed
with high uniformity, and to overcome difficulties related to high cost and high-pressure.
Moreover, these caged MWNT were highly biocompatible with the cells/animals studied
in vitro and in vivo. The vesicles containing MWNTS had a diameter between 400 and
600 μm. For cationic dyes (acridine orange: AO; ethidium bromide: ET), the adsorption
capacity was in the order: caged MWNTs > caged CNFs > caged ACTC (activated
carbon) > activated carbon. For anionic dyes (Eosin bluish: EOB; Orange G: OG),
adsorption capacity followed the order: CNTs > caged CNTs > ACTC > caged ACTC >
CNF > caged CNF. The cage affects the mass transfer of anionic dyes due to the
negatively charged carboxylic groups on the surface of the adsorbents. Here, MWNTs
showed the highest capability for eliminating both cationic and anionic dyes. This may
be because the hexagonally arrayed carbon atoms in the graphite sheets of MWNTs are
active sites for trapping the targets, and that MWNTs possess a large pore as compared
to activated carbon.
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