Environmental Engineering Reference
In-Depth Information
have a rough surface, making the adsorption of organic matter much easier, whereas the
heat treatment eliminates defects and the surface of graphitized CNTs becomes smooth.
This is confirmed by Fagan et al. (2004) who found that DCB weakly interacted with a
perfect SWNT structure, but this interaction was slightly stronger when the SWNT
surface had structural vacancies.
DCB adsorption and desorption from nanoscale fullerene were similar to those of
naphthalene (Cheng et al., 2005). Desorption of DCB from nanoscale C 60 exhibited
hysteresis, which could be described by a two-compartment desorption model.
Aniline and Resorcinol. The adsorption of resorcinol and other phenolic
derivatives on pristine MWNTs and HNO 3 -treated CNTs was investigated (Liao et al.,
2008); the adsorption of resorcinol on CNTs was faster than that on activated carbon.
Within the first minute, about 60% of the total amount was adsorbed when the
adsorption reached equilibrium with the CNTs. The uptake of resorcinol fluctuated very
little in the pH range between 4 and 8. However, when the pH was lower than 6, there
was a slight increase in the uptake of resorcinol as the pH value decreased. This slight
dependence of resorcinol uptake on pH showed the high stability of CNTs as an
adsorbent for resorcinol. In this case, the adsorption isotherm of resorcinol and
hydroquinone onto CNTs does not fit well with either the Freundlich or Langmuir
models, although the adsorption isotherm of other phenolic compounds such as phenol,
catechol, and pyrogallol can be described by both Freundlich and Langmuir models.
In another part of the study, the effect of adsorbents and adsorbates on the
adsorption affinities was investigated. Acid treated CNTs showed a lower adsorption
capacity for resorcinol when compared to pristine CNTs, though acid treated CNTs
possessed a higher specific surface area, a larger pore volume and a larger amount of
functional groups. This may be because that carboxylic groups on the surface of CNTs
acted as an electron, withdrawing groups localizing electrons from the π system of CNTs
that might interfere with and weaken the π-π dispersion forces between the aromatic ring
of resorcinol and the graphitic structure of CNTs. Moreover, acid treated CNTs
containing more surface groups are more negatively charged, which produces
electrostatic repulsion between the surface of CNTs and the resorcinol (C 6 H 4 O 2 2- ),
which then leads to a reduction of the uptake capacity. The number of hydroxyl groups
in the molecular structure of adsorbates was also found to affect the adsorption
capability. The amount adsorbed by CNTs increased with a corresponding increase in
the number of hydroxyl groups in the adsorbate molecule, which can be illustrated based
on the adsorbate uptake order: pyrogallol > catechol > phenol. In addition, meta-
hydroxyl containing compounds exhibited a much higher adsorption ability than ortho-
and para-hydroxyl containing compounds (the uptake order: resorcinol > hydroquinone
> catechol).
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