Environmental Engineering Reference
In-Depth Information
Not only were CNTs oxidized by using concentrated nitric acid, they were
functionalized in the presence of other oxidants such as KMnO 4 , NaOCl, and H 2 O 2 .
Oxidation of CNTs by KMnO 4 and H 2 O 2 achieved little enhancement in terms of the
specific surface area, whereas HNO 3 oxidation displayed a larger increase when
compared to as-grown CNTs (from 122-154 m 2 /g) (Li et al., 2003b). On the other hand,
the amounts of carboxyl and lactone groups on the HNO 3 -treated CNTs are higher than
those of KMnO 4 - and H 2 O 2 -treated CNTs, with the amount of phenolic groups on CNTs
in the following order: H 2 O 2 -treated CNTs < HNO 3 -treated CNTs < KMnO 4 -treated
CNTs. Apparently, H 2 O 2 -treated CNTs having the lowest amount of groups exhibited
the lowest adsorption capacity among the three oxidized CNTs. However, KMnO 4 -
treated CNTs displayed the highest loading amount of Cd 2+ , although CNTs treated with
HNO 3 contained a higher amount of functional groups. This was attributed to the
adsorption role of the residual MnO 2 particles supported on the CNTs (Li et al., 2003b).
This phenomenon provides a hint for modifying CNTs by impregnation with metal
oxide(s), which will be further discussed below.
Using sodium hypochlorite solutions to simultaneously purify and oxidize CNTs,
researchers obtained oxidized CNTs having 7-8 times more total acidic surface sites
than the raw CNTs (Lu and Chiu, 2006; Lu et al., 2006a; Lu and Liu, 2006). These
CNTs became more hydrophilic, resulting in a higher adsorption capacity for zinc(II)
and nickel(II). One surprising result in these studies was the sharp decrease in the
specific surface area after the purification process (from 590 to 423 m 2 /g for SWNTs
and from 435 to 297 m 2 /g for MWNTs), which was attributed to the fact that CNTs
become shorter and the confined space among isolated CNTs becomes smaller (Lu and
Chiu, 2006). Wu (2007b) compared the effects of HNO 3 and NaOCl modification on
CNTs in the adsorption of Cu 2+ , and pointed out that HNO 3 and NaOCl modification
significantly increased the average pore diameter and pore volume of CNTs. The
oxidizing ability of NaOCl-modified CNTs is attributed to be higher than that of HNO 3 -
modified CNTs, thereby a much higher loading amount of Cu 2+ could be achieved on
NaOCl-modified CNTs. Wu (2007b) claimed that the adsorption capacity of NaOCl-
modified CNTs for Cu 2+ (47.39 mg/g) was higher than that of any other known
adsorbents.
One should remember that by using different synthesis processes, CNTs can be
produced with different morphologies, structures, and amounts of functional groups on
their surface, and consequently their adsorption behavior is not likely to be identical. For
example, four MWNTs were prepared by the chemical vapor deposition method (Li et
al., 2006), all employing different starting hydrocarbons, temperatures, and catalysts in a
horizontal (Hor) or vertical furnace (Vert); symbolized as Xylene-Fe-800 o C-Hor,
Benzene-Fe-1150 o C-Vert, Propylene-Ni-750 o C-Hor, and Methane-Ni-650 o C-Vert,
respectively. After being treated in concentrated nitric acid, the porosity and specific
surface area of these oxidized CNTs are in the following order: Xylene-Fe-800 o C-Hor <
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