Environmental Engineering Reference
In-Depth Information
Benzene-Fe-1150
o
C-Vert < Methane-Ni-650
o
C-Vert < Propylene-Ni-750
o
C-Hor,
whereas the quantities of carboxyl and phenols on the surface of the CNTs followed the
order: Benzene-Fe-1150
o
C-Vert < Xylene-Fe-800
o
C-Hor < Propylene-Ni-750
o
C-Hor <
Methane-Ni-650
o
C-Vert. In addition, the lead adsorption capacity of these CNTs
depends on the order of amounts of carboxyl and phenol groups on their surface:
Benzene-Fe-1150
o
C-Vert (11.2 mg/g) < Xylene-Fe-800
o
C-Hor (14.8 mg/g) <
Propylene-Ni-750
o
C-Hor (59.8 mg/g) < Methane-Ni-650
o
C-Vert (82.6 mg/g). It could
thus be concluded that MWNTs obtained from the synthesis method using methane as a
starting material together with accompanied conditions are the best. It is also true that in
comparison with MWNTs, SWNTs always show a higher potential for adsorption of
zinc(II) and nickel(II) (Lu and Chiu, 2006; Lu et al., 2006a; Lu and Liu, 2006). This can
be attributed to the higher surface area and pore volume of SWNTs, as well as their
larger amount of functional groups on the surface.
In order to enhance their removal efficiency for heavy metal cations, CNTs can
be coated with metal oxides. To this end, Peng et al. (2005a) coated iron oxide on nitric
acid-treated MWNTs to form a magnetic adsorbent that could be easily separated from
water, one with a recovery rate of above 98%. This magnetic composite showed very
high adsorption capability for Pb(II) (105.57 mg/g) and Cu(II) (45.44 mg/g) (Peng et al.,
2005a), which was far greater than most other adsorbents. Additionally, MnO
2
-coated
CNTs (Wang et al., 2007b) were also postulated as a promising adsorbent for lead with a
maximum adsorption capacity of 78.74 mg/g, which is 3 times higher than that of nitric
acid-treated CNTs. The authors indicated that a MnO
2
load of 30% is optimum for
effective removal of Pb(II).
In comparison with other conventional adsorbents, the maximum adsorption
capacities of Cu
2+
, Cd
2+
, Ni
2+
, Zn
2+
, and Pb
2+
on CNTs are higher than those for granular
activated carbon (GAC) or powder activated carbon (PAC), and a majority of other
conventional adsorbents (Rao et al., 2007). However, the maximum adsorption
capacities of Cu
2+
on iron slag and modified chitosan, Cd
2+
on algae and crab shell (Li et
al., 2003a), and Pb
2+
on crab shell, granular biomass, and algae (Li et al., 2003a) are
higher than those on CNTs (Rao et al., 2007). Furthermore, it was suggested that the
adsorption capacities of Ni
2+
, and Zn
2+
on CNTs are superior to other adsorbents (Rao et
al., 2007).
10.3.1.2 Effect of Contact Time
Li et al. (2005) reported that the adsorption capacity of Pb
2+
on CNTs increased
quickly for the first 10 minutes and then gradually reached equilibrium at 20, 50 and 60
minutes for initial Pb
2+
concentrations of 10, 20, and 30 mg/L, respectively. Similarly,
Wang et al. (2007b) found that the adsorption of Pb
2+
on MnO
2
/CNTs dramatically
increased for 15 minutes, then slowly reached a saturation state at 2 h for an initial Pb
2+
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