Environmental Engineering Reference
In-Depth Information
cavities or inter-wall spaces, due to the impurities in the CNTs and restricted spaces
(0.335 nm) of the MWNTs. In order to explain the presence of adsorption-desorption
hysteresis in the case of fullerene, a deformation-rearrangement mechanism was
proposed. Spherical monomers of fullerene may result in aggregates (Cheng et al., 2004),
in which the closed interstitial spaces may be produced in or between small aggregates.
Subsequent rearrangement of the small aggregates and/or penetration of adsorbate
molecules into the closed interstitial spaces between the small aggregates during
adsorption could lead to molecular entrapment and hysteresis (Yang and Xing, 2007).
However, CNTs cannot form similar closed interstitial spaces in their aggregates due to
their length.
Fullerene (C 60 ) in the forms of large aggregates, small aggregates, and thin film
was used to adsorbe naphthalene from water (Cheng et al., 2004). Large-aggregate
fullerene had a diameter between 20 and 50
m, while that of small-aggregate fullerene
was between 1 and 3 μm. Distribution coefficients calculated for the sorption of
naphthalene on three forms of fullerene were less than that on activated carbon with the
following order: C 60 large aggregates < C 60 thin film < C 60 small aggregates. The
enhanced dispersal of fullerene could significantly affect the adsorption of naphthalene
by several orders of magnitude. The desorption of naphthalene from fullerene was very
limited; over a period of 60 days, only about 11% of the total naphthalene was desorbed
from C 60 small aggregates. This may be due to the irreversible desorption of naphthalene
from fullerene, as confirmed by the existence of adsorption-desorption hysteresis. The
adsorption-desorption of naphthalene from dispersed C 60 samples [C 60 large aggregates,
C 60 small aggregates (Cheng et al., 2004), and nanoscale C 60 particles (Cheng et al.,
2005)] was found to exhibit strong hysteresis. These experimentally determined
adsorption-desorption hysteresis can be described by a two-compartment desorption
model: first, adsorption to the external surfaces that are in contact with water, and
second, adsorption to the internal surfaces within the aggregates.
μ
10.3.4.3 Dyes and Natural Organic Matter (NOM)
Adsorption of various dyes onto MWNTs has been examined to determine the
main factor controlling the interaction between dyes and the MWNT surface, and the
adsorption affinities between them. Liu et al. (2008) compared a series of aromatic dye
molecules with different structural characteristics; they divided these dye molecules into
three categories in terms of morphology: pseudo planar molecules, non-planar molecules,
and azo-group-containing molecules, and in terms of their charge: positive, negative,
and neutral. The adsorption capacity order is: planar polynuclear compounds > planar
non-polynuclear compounds > non-planar compounds. This morphology-dependent
adsorption revealed that π-π stacking is the main driving force responsible for the dye-
MWNT interaction, and was also confirmed in the interaction between xylenes and
CNTs (Chin et al., 2007). It was illustrated that the planar molecules could easily
 
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