Environmental Engineering Reference
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concentration of 30 mg/L. The longer time needed to attain equilibrium can be attributed
to the smaller pore of CNTs due to MnO 2 loading (the pore size of MnO 2 /CNT was 2.6
nm, compared to 10 nm in the case of CNTs used in Li's study). This was strongly
supported by the results of CNTs with a diameter of 15.68 nm in Wang's research. It
was found that the adsorption of Pb 2+ on these CNTs increased quickly with contact time
for the first 20 minutes, and reached equilibrium at various initial concentrations of Pb 2+
(from 20 to 40 mg/L) (Wang et al., 2007a). It can be said that the fast adsorption of Pb 2+
on CNTs is due to the large and uniform pores of CNTs, which reduce mass transfer
limitations as observed in porous materials such as activated carbon.
The adsorption of nickel(II) onto HNO 3 -treated CNTs reached equilibrium in 40
minutes (Chen and Wang, 2006) at Ni 2+ initial concentrations from 6-20 mg/L and onto
NaOCl-treated CNT (Lu and Liu, 2006) in 60 and 120 minutes at Ni 2+ concentrations of
10 and 60 mg/L. Specifically, it took only 20 minutes to attain equilibrium for
adsorption of Ni 2+ at an initial concentration of 200 mg/L (Kandah and Meunier, 2007).
It took a longer time (60 minutes) for adsorption of Zn 2+ onto NaOCl-treated CNTs to
reach equilibrium with initial Zn 2+ concentrations of 10-60 mg/L (Lu and Chiu, 2006).
In summary, adsorption time of heavy metal ions onto CNTs is shorter than that onto
activated carbon.
10.3.1.3 Adsorption Kinetics
Adsorption of Pb 2+ onto HNO 3 -treated CNTs (Li et al., 2005) and onto
MnO 2 /CNTs (Wang et al., 2007b), Ni 2+ onto HNO 3 -treated CNTs (Chen and Wang,
2006), and Zn 2+ on NaOCl-treated CNTs (Lu et al., 2006a) follows a pseudo-second-
order rate equation with a high correlation coefficient (R 2 > 0.995). It was found that the
sorption rate constantly increased with an increase in temperature (Lu et al., 2006a),
which was considered to be a result of a corresponding increase in the diffusion rate of
cations (e.g.,Zn 2+ ) due to the temperature increase. Using the Arrhenius equation, Lu et
al. (2006a) calculated the activation energy E a to be less than 15 kcal/mol.
10.3.1.4 Adsorption Isotherm
The adsorption isotherm of heavy metal cations including Pb 2+ , Zn 2+ , Cd 2+ , Co 2+ ,
Ni 2+ , and Cu 2+ onto CNTs can be well described by either the Langmuir or Freundlich
sorption isotherm equations. The adsorption isotherm for Pb 2+ , Co 2+ , and Cu 2+ onto
HNO 3 -treated CNTs (Li et al., 2005; Stafiej and Pyrzynska, 2007), Pb 2+ and Cu 2+ onto
iron oxide/CNTs (Peng et al., 2005a) are best fit using the Freundlich isotherm. In
contrast, the Langmuir isotherm was a better choice for describing the adsorption
isotherms of Pb 2+ onto MnO 2 /CNTs (Wang et al., 2007b), and Pb 2+ , Cu 2+ , and Cd 2+ onto
HNO 3 -treated CNTs (Li et al., 2003a; Wang et al., 2007a). It should be thus noted that
the Langmuir equation is valid for describing the adsorption on a homogeneous surface
 
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