Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 276

the zigzag and helical tubes are either metallic or semiconducting (Dresselhaus et al.,
1996). Semiconducting types of nanotubes exhibit field effect transistor behaviors at
room temperature and their development for nanoelectronic devices has been attempted
(Dai, 2002). In theory, metallic nanotubes can have an electrical conductivity that is
much higher than metals such as copper and silver. Apart from electronic properties,
carbon nanotubes are one of the strongest and stiffest materials known, in terms of
tensile strength and elastic modulus (Ajayan, 1999). Furthermore, nanotubes can be
considered as chemically inert materials owing to their strong C-C covalent bond (in the
lattice), which is one of the strongest in nature. However, oxidation treatment of
nanotubes oxidizes the tube caps and defect sites on the cylindrical region (Kondratyuk
and Yates, 2007), resulting in open tubes and the creation of functional groups such as
carboxyl, lactones, and phenolic groups. Therefore, oxidized nanotubes not only have a
high adsorption capacity (Kondratyuk and Yates, 2007) but also become soluble and can
be further functionalized for advanced applications (Sun et al., 2002).
Owing to these unique properties, carbon nanotubes have been applied as
electron field emitters, quantum nanowires, catalyst support, chemical sensors, and as
sorbents for hydrogen and other gas storage (Ajayan, 1999; Andrews et al., 2002; Dai,
2002; Sun et al., 2002). Recently, carbon nanotubes have also been shown as promising
adsorbents for contaminants in aqueous solutions. Contaminants, including heavy metal
ions, polycyclic aromatic hydrocarbons, dyes, and natural organic matters, are the focus
of the following sections.
10.2.3 Mesoporous Carbon
It is obvious that mesoporous carbons possess not only the unique properties of
activated carbon such as high surface area, high pore volume and hydrophobic surface,
but also their own characteristics. Owing to the structure of mesopores, large molecules
can easily approach and be adsorbed onto the surface of mesoporous carbon, at a
significantly higher rate than for activated carbon. The specific surface area, pore
volumes, and average pore diameters of mesoporous carbon can be up to 2000-3500
m 2 /g (Guo et al., 2003a; Guo et al., 2003b), 5.5 cm 3 /g, and 23 nm (Han et al., 2000),
respectively. Mesoporous carbon also has the potential to modify its surface
characteristics such that it can be impregnated with metal (Joo et al., 2001) or metal
oxides (Gu et al., 2007) and functionalized with organic groups (Jung et al., 2008). In
addition, the surface chemistry of mesoporous carbon depends on the starting materials,
preparation methods, and other parameters like temperature used during the synthesis
process. To this end, Darmstadt et al. (2002) reported that the oxygen element on the
ordered mesoporous carbon (OMC) decreases as the temperature increases, while the
polyaromatic order of the outer surface and the mesopore surface of OMC materials
increase during heat-treatment.