Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 293

A thermodynamic study on various types of CNTs showed both the dependence
of aniline adsorption on the surface chemical characteristics of adsorbents and
temperature (Xie et al., 2007). First, the adsorption isotherm of aniline obeyed the
Freundlich model rather than the Langmuir model. The absence of a plateau in aniline
adsorption isotherms infers a multi-layer adsorption on the CNT samples. Adsorption of
aniline on CNTs is spontaneous because of the negative free energy change, in
conjunction with a positive entropy change. The enthalpy change is negative in the case
of pristine MWNTs and KMnO 4 -treated MWNT samples, whereas a positive enthalpy
change was observed on citric acid-treated MWNTs and nitric acid-treated MWNTs.
This suggests that nitric acid and citric acid increase the amount of carboxylic groups on
the surface of MWNTs, which induces the endothermic chemical adsorptions.
Nitro-Substituted Aromatics. As mentioned above, CNTs exhibited a potential
application for adsorption of various benzene derivatives such as chloro-, amino-, and
hydroxyl-benzene. However, this does not mean that all benzene derivatives can be
adsorbed onto CNTs. Meyer et al. (2001) examined the adsorption of a series of
aromatics, including 1,2-dinitrobenzene, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 3-
nitrotoluene, 3,4-dinitrotoluene, 2,4-dinitrotoluene, 1-bromo-4-nitrobenzene, 4-
nitroacetophenone, 4-nitrobenzaldehyde, methyl 4-nitrobenzoate, 4-nitrobenzoic acid,
3,5-dinitrobenzoic acid, 4-nitrocinnamic acid, 2-cyclohexen-1-one, 3-nitrobenzophenone,
1-nitronaphthalene, 2-nitronaphthalene, 1-naphthol, 2-nitro-1-naphthol, 1-naphthylacetic
acid, phthalide, 2-nitrofluorene, nitrobenzene, 4-nitrotoluene, and 4-nitrophenyl acetate
on carbonaceous materials (Meyer et al., 2001). Even though a small amount of anionic
surfactant sodium dodecyl sulfate (1% in water, or 35 mM) was used to solubilize an
aromatic compound and permit wetting of the carbon surfaces, only nitrobenzene, 4-
nitrotoluene, and 4-nitrophenyl acetate were selectively adsorbed onto aligned, randomly
oriented, and oxidized carbon nanotubes, graphite, and C 60 fullerene (Meyer et al., 2001).
The quantity adsorbed onto the surface area (1 cm 2 ) of these nanotube samples was
calculated to be 0.02 mg of nitrobenzene or 4-nitrotoluene over a period of seven days.
10.3.5 Preconcentration Using CNTs and Fullerene
CNTs and fullerene were used as solid-phase extraction (SPE) adsorbents for the
preconcentration of metal ions and organic pollutants for analytical purposes. However,
the promising potential of CNTs and fullerene for preconcentration exhibits not only the
capability of these materials in analytical applications but also their ability for
contaminant separation. These materials can be used as adsorbents for solid-phase
extraction, implying that they definitely can be utilized to sorb and remove contaminants
in a column setting. Furthermore, CNTs can be regenerated by washing with appropriate
solvents or solutions. The high recovery values in such SPEs indicate that the adsorbents
can sorb and completely separate pollutants from water samples.