Environmental Engineering Reference
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decreased as the pH exceeded 7 due to the fact that at a higher pH, more oxygen-
containing groups on the CNTs surface are ionized and thus they adsorb more water
(Peng et al., 2003). The formation of water clusters blocks the access of THM molecules
to the adsorption site, resulting in less adsorption of THMs. Otherwise, the adsorption of
THMs onto CNTs is primarily an exothermic process, in which a decrease in THM
adsorption with an increase in temperature was observed (Lu et al., 2006b). The
adsorption isotherms of THMs were well fitted with the Langmuir and Freundlich
models.
In comparison with PACs, CNTs exhibited a superior capability for adsorption of
CHCl 3 , whereas the larger molecule CHBr 3 is preferentially adsorbed onto PACs (Lu et
al., 2005). This may be due to the fact that activated carbon is hydrophobic, and thereby,
has a greater affinity for the adsorption of relatively non-polar molecules.
10.3.4.2 Aromatic Hydrocarbons and Polycyclic Aromatic Hydrocarbons (PAH)
Xylenes. Chin et al. (2007) used SWNTs for the adsorptive removal of o-xylene
and p-xylene from aqueous solutions. The different adsorption behaviors of o-xylene
and p-xylene on CNTs provided useful information about the interaction between CNTs
and xylenes. It was found that p-xylene was more preferentially adsorbed onto as-grown
SWNTs, due to the fact that adsorption of aromatic compounds on unfunctionalized
CNTs is based on π-π stacking (Chin et al., 2007). The methyl groups of a p-xylene
molecule push the π electrons towards each other (i.e., to the center of the ring), whereas
those of an o-xylene molecule push the π electron toward the same side of the ring. In
other words, the
electrons in a p-xylene molecule are homogeneously distributed on the
benzene ring, whereas π electrons in an o-xylene molecule are disproportioned.
Therefore, the hybridization of π electrons between SWNTs and p-xylene is more
favorable, resulting in a higher adsorption capacity for p-xylene than for o-xylene. For
the same reason, the adsorption of p-xylene onto CNTs is less dependent on the solution
pH; the adsorption of p-xylene on both as-grown and HNO 3 -purified CNTs is only
slightly affected by a solution pH in the range of 3-9. Since the ionization of the surface
groups upon varying pH does not significantly affect the adsorption of p-xylene onto
CNTs, it implies that π-π stacking may still be the predominant interaction for the
adsorption of p-xylene.
π
In contrast, the adsorption of o-xylene onto both as-grown and purified CNT is
significantly dependent on the solution pH. The adsorption capacity of the as-grown
CNT for o-xylene increased dramatically when the pH value reached 9. While a high
adsorption capacity of the purified CNTs (115 mg/g) for o-xylene was observed at pH 3,
it decreased sharply as pH increased to 5 (63 mg/g), and then gradually increased until
the pH reached 9 (Chin et al., 2007). The increase in the adsorption capacity of CNTs for
o-xylene when the pH value is raised to 9 contradicts the “water cluster adsorption
 
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