Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 40

population of the walls inside the opened nanotubes and formation of one-
dimensional chains in the grooves at the outer surface of the bundles. The
second step can be assigned to the filling of the remaining axial sites inside
the nanotubes and the completion of a quasi-hexagonal monolayer on the
outer surfaces of the bundles [22]. It is worth noting that the adsorption
reaches equilibrium much faster on external sites (grooves and outer sur-
faces) than on the internal sites (interstitial channels and inside the tube)
under the same temperature and pressure conditions.
The overall performance of CNTs in an adsorption treatment process
depends on several factors, the most important being the fraction of opened
and unblocked nanotubes. Opened CNT bundles provide more adsorption
sites than capped CNTs, resulting in fast sorption kinetics and increased sat-
uration capacity. In addition, CNTs are often found mixed with impurities
such as carbon-coated catalyst particles, soot, and other forms of carbons,
which can significantly decrease their adsorption efficiency. These impu-
rities can be removed by using several treatment methods like acid/base
treatment, heat treatment and so forth [23]. Furthermore, the surface func-
tionalities also influence the maximum adsorption capacity of CNTs. CNTs
mostly contain the oxygen functional groups such as hydroxyl (—OH), car-
bonyl (—CO), and carboxyl (—COOH) on their surface, formed during the
synthesis procedure and purification process, or can also be intentionally
generated by oxidation using various acids, ozone, or plasma [39-44]. These
functional groups can change the wettability of CNT surfaces, and make
them more hydrophilic and suitable for the adsorption of relatively low
molecular weight and polar compounds. They may also increase diffusion
resistance and decrease the surface area, which can reduce the accessibility
and affinity of CNT surfaces for some organic chemicals [45]. Meanwhile,
the functional groups can also block the access to the interior space of the
uncapped tubes. By subjecting the nanotubes to a high-temperature treat-
ment under vacuum, these chemical groups can be removed in order to
make the internal space of the tube accessible for adsorption [46,47].
2.2.2 Graphene
Graphene is the newest member in the family of carbon allotropes and
has emerged as the “celeb” material of the 21st century. Since its success-
ful isolation by the exfoliation of graphite in 2004 by Kostya Novoselov,
his fellow Nobel laureate Andre Geim and their group at the University
of Manchester, graphene has been attracting enormous attention in both
scientific and engineering communities. Graphene is a single atomic
layer of sp 2 hybridized carbon atoms, densely packed into an ordered