Environmental Engineering Reference
In-Depth Information
applications. In this chapter, we describe two types of such carbon-based nanoadsorbents:
a hierarchical web of ACFs/CNFs and carbon micro-/nanoparticles, which have shown
tremendous potential as adsorbents for luorides and arsenic.
A hierarchal web of carbon micro-/nanoibers is prepared using micron-sized ACFs as
a substrate. CNFs are grown on ACFs by catalytic chemical vapor deposition (CVD). The
use of ACFs as a substrate to grow CNFs obviates the need for removing the substrate in a
postsynthesis step, and a hierarchal carbon web of microibers and nanoibers prepared in
this manner can be directly used in end applications. Figure 36.2 describes the tip-growth
mechanism for growing a CNF on an ACF by CVD.
Briely, a web of ACFs/CNFs is prepared by impregnation of ACFs with a suitable tran-
sition metal (Ni/Fe/Cu/Zn/Ag) catalyst, calcinations at high temperature, reduction by
hydrogen to convert metal oxides to their metallic state, growing of CNFs by CVD, and
subsequent sonication of the prepared web to dislodge the metal nanoparticles from the
tips [38-42]. The inal step opens up the tip and allows the solute to diffuse into the pores
and to be adsorbed on the interior surface of the material. Most important, a hierarchal
web prepared in this manner may be reimpregnated with another type of metal catalyst
(different from the parent catalyst used for growing the CNF), which may be required for
certain end applications. Thus, there are two categories of distinct conditioning when pre-
paring a web of carbon micro-/nanoibers for different end applications. In one category,
the metal catalyst, for example Fe, has two functions: (i) to grow the hierarchal structure
and (ii) to remove contaminant solutes, such as As, in the wastewater [41]. In the other cat-
egory, the parent catalyst, for example Ni, must be removed after growing the CNFs, and
the web must be reprocessed by impregnating it with another type of metal, such as Al
[40]. In the latter category, the sequence of steps for the incorporation of metals within the
micro-/nanopores of the carbon web is qualitatively similar to that for the parent catalyst.
However, the operating conditions are distinctively different. In either of the two catego-
ries, the hierarchal web may also be functionalized with surface functional groups for
enhancing its adsorptive/catalytic eficiency. For example, basic groups, such as hydroxyls
or ketones, may be incorporated to remove the solutes, which are acidic, or vice versa.
For the synthesis of carbon micro-/nanoparticles, the polymeric precursors are irst
synthesized as beads, using suspension polymerization, and then carbonized and acti-
vated by physical activation using steam or CO
2
to develop micropores and mesopores in
the beads. Iron is incorporated into the polymeric beads in an intermediate step during
C
x
H
y
x
C +
y
H
2
Nanometal
C
C
Nanometal
Nanometal
ACF substrate
FIGURE 36.2
Growing CNF on ACF by CVD using hydrocarbon source.
