Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 284

10.3.1.10 Effect of Dose
The sorption percentage of Ni 2+ , Pb 2+ , Cu 2+ , and Cd 2+ increased with an increase
in CNT mass (Li et al., 2003a; Chen and Wang, 2006; Lu and Liu, 2006), which could
be due to the availability of more sorption sites. However, the adsorption capacity of
CNTs for Ni 2+ decreased with a rise in CNT mass (Chen and Wang, 2006; Lu and Liu,
2006). In contrast, the adsorption capacity of CNTs for Zn 2+ increased with an increase
in the CNT dosage (Li et al., 2003b).
10.3.2 Adsorptive Removal of Radionuclides
MWNTs were used as sorbents for radionuclide 243 Am(III) for the first time by
Wang et al. (2005). They found that HNO 3 -treated MWNTs can adsorb Am(III)
efficiently, with fast adsorption and slow desorption kinetics being observed. The
adsorption of Am(III) onto MWNTs increased very quickly when the pH of the solution
increased from 2 to 5, and then slowly increased to a pH of 10 (88% removal). This
result, together with the weakly dependent sorption of Am(III) to MWNTs based on
ionic strength, indicate that the sorption mechanism of Am(III) is surface complexation.
Moreover, the linear isotherm sorption of Am(III) showed that the sorption of Am(III)
onto MWNTs is far from saturated, although only a low amount of MWNTs was used
(0.26 g/L). The fast adsorption kinetics allowed the initial equilibrium to be reached in
5-7 minutes. After more than 2 months, about 90% of the Am(III) adsorbed still
remained bound to MWNTs, even though the system was in the presence of a chelating
resin, a strong binding adsorbent for Am(III). These results showed that Am(III) forms
kinetically stabilized chemicomplexation with MWNTs and does not rapidly desorb
from MWNTs (Wang et al., 2005); thus, implying that MWNTs are a promising sorbent
for the treatment of nuclear wastes.
In another study, the adsorption of Th(IV) onto oxidized MWNTs showed a fast
and reversible adsorption (Chen et al., 2007). In the first 15 minutes of the contact time,
the adsorption of Th(IV) was rapid, and it took 60 minutes to attain the equilibrium.
Unlike the adsorption of Am(III), the adsorption isotherm of Th(IV) onto MWNTs
followed well the Langmuir model. In addition, thermodynamical calculations indicated
that the adsorption process is spontaneous and endothermic. Furthermore, desorption
results showed that 40% of Th(IV) adsorbed can be desorbed from the surface of
MWNTs by adjusting the solution pH. Based on this result, MWNTs can be reused in
the nuclear waste management.
10.3.3 Adsorptive Removal of Anions
Adsorption of fluoride from water onto aligned carbon nanotubes and amorphous
alumina supported on carbon nanotubes was reported (Li et al., 2001; Li et al., 2003c).