BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 22

Adsorption of single metal ions, dyes and organic pollutants on CNT-based
adsorbent composites is one of the most applications of this material. MWCNT/
iron oxide magnetic composites were prepared and used for adsorptions of several
metal ions. The CNTs were purified by using nitric acid, which results in modifi-
cation of the surface of the nanotubes with oxygen containing groups like carbon-
yl and hydroxyl groups. The adsorption capability of the composite is higher than
that of nanotubes and activated carbon. The sorption of ions such as Pb(II) and
Cu(II) ions on the composite were spontaneous and endothermic processes based
on the thermo dynamic parameters (ΔH, ΔS and ΔG) calculated from temperature
dependent sorption isotherms. Alumina-coated MWCNTs were synthesized and
reported for its utilization as adsorbent for the removal of lead ions from aqueous
solutions in two modes. With an increase in influent pH between 3 and 7, the per-
centage of lead removed increases. The adsorption capacity increases by increas-
ing agitation speed, contact time and adsorbent dosage. The reported composite
can be regenerated as it was confirmed by SEM and EDX analysis [58, 60].
Grafting of polymers to nanotubes has been realized via both“grafting-to”
and “grafting-from” approaches that mention before. The 'grafting-to' method
is based on attachment of premade end-functionalized polymer onto the tips and
convex walls of the nanotubes via chemical reactions such as etherification and
amidization. One advantage of this method is that the mass and distribution of
the grafted polymers can be more precisely controlled. However, initial grafted
polymer chains sterically prevent diffusion of additional polymer chains to the
nanotube surface resulting in low grafting density. The 'grafting-from' method is
based on immobilization of reactive groups (initiators) onto the surface, followed
by in situ polymerization of appropriate monomers to form polymer-grafted nano-
tubes. The advantage of this approach is that very high grafting density can be
achieved. But careful control of the amount of initiator and the conditions for the
polymerization reaction is required. Rather than the 'grafting-to' method used by
some researchers. They investigated the 'grafting-from' approach, which involves
the propagation of dendrimers from CNT surfaces by in situ polymerization of
monomers in the presence of CNT attached macro initiators [73].
The mobility and small size of the fugitive monomers, in contrast to that of
preformed dendritic polymers, are expected to improve the efficiency and yield of
the 'grafting-from' approach and the resulting debundling process. The number of
peripheral reactive groups can be precisely controlled by choosing the appropriate
synthetic generation. Functionalization of nanotubes with dendrimers represents
a particularly promising strategy to attach an in principle unlimited number of
functional groups onto the SWNT surfaces, and thus to significantly increase the
compatibility and reactivity of CNTs with thermosetting polymer matrices, such
as epoxy, bismaleimide, and cyanate ester. Finally, to achieve good alignment of
nanotubes to exploit their superior anisotropic mechanical properties, they applied
the reactive spinning process. Spinning has been widely used for thermoplastic