Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 39

to this unique combination of properties, CNTs are expected to have strong
interactions with both organic and inorganic contaminants. There are basi-
cally four possible sites in CNT bundles for the adsorption of different pol-
lutants (see Figure 2.2) [36,37]:
i. Internal sites : These sites are found within the hollow inte-
rior of individual tubes and are accessible only if the caps are
removed and the open ends are unblocked.
iii. Interstitial channels : These sites are found in the interior
space between individual nanotubes in the bundles and are
easily accessible to the adsorbate species.
iii. External groove sites : Grooves are present on the periphery
of a nanotube bundle and the exterior surface of the outer-
most nanotubes, where two adjacent parallel tubes meet.
iv. Outside surface : The adsorbate can also bind to the curved
surface of individual tubes on the outside of the nanotube
bundles.
A consensus is that adsorption on close-ended CNTs first takes place
in the grooves between adjacent tubes on the perimeter of the bundles,
followed by the adsorption on the convex external walls [22]. For puri-
fied opened CNT bundles, the first step of adsorption proceeds by the
( 4 )
( 3 )
2
( 1 )
Figure 2.2 Different adsorption sites on a homogeneous bundle of partially open-ended
SWCNTs: (1) internal, (2) interstitial channel, (3) external groove site, and (4) external
surface. Sites 1 and 2 comprise the internal porous volume of the bundle, whereas
sites 3 and 4 are both located on the external surface of the bundle. Reprinted from [38];
Copyright 2006, with permission from the American Chemical Society.