Environmental Engineering Reference
In-Depth Information
and forming a monolayer coverage, whereas the Freundlich equation is applicable to a
heterogeneous surface. Several reports have mentioned that both Langmuir and
Freundlich can well describe the adsorption isotherms of heavy metal ions on CNTs (Li
et al., 2002; Lu and Liu, 2006; Kandah and Meunier, 2007). Some other researchers
have found that the Langmuir isotherm can fit metal ion sorption onto CNTs a little bit
better than the Freundlich isotherm (Chen and Wang, 2006; Lu and Chiu, 2006; Lu et al.,
2006a; Wu, 2007b).
10.3.1.5 Effect of Ionic Strength
It was found that ionic strength has a negative effect on the adsorption of heavy
metal ions (Pb
2+
, Cu
2+
, Cd
2+
, and Ni
2+
) onto CNTs; the adsorption of Ni
2+
onto CNTs
decreased with an increase in ionic strength (Chen and Wang, 2006; Lu and Liu, 2006).
However, the effect became smaller for cases with ionic strength greater than 0.05 mol/L
of NaClO
4
. The adsorption of Pb
2+
, Cu
2+
, and Cd
2+
by CNTs also showed the same
phenomenon (Li et al., 2003a). This phenomenon can be attributed to two reasons. First,
heavy metal ions form electrical double layer complexes with CNTs, which favor
adsorption when the concentration of the competing salt decreases. Second, the ionic
strength of the solution influences the activity coefficient of heavy metal ions, which
limit their transfer to the CNTs' surface (Reddad et al., 2002). Wu (2007b) further
claimed that the slight decrease in adsorption of Cu
2+
on CNTs was observed when
solution ionic strength increased from 0 to 0.05 mol/L, giving a proof of the formation of
inner-sphere complexes between Cu
2+
and CNTs.
10.3.1.6 Effect of pH and Sorption Mechanism
To further understand the sorption mechanism, an important clue is the effect of
pH on the adsorption of heavy metal cations, in addition to the information already
obtained from the effects of ionic strength and other phenomena. The mechanisms by
which metal ions are sorbed onto CNTs are attributed to electron static interaction,
sorption-precipitation, and chemical interaction between the metal ions and functional
groups on the CNT surface. The major sorption mechanism can be illustrated based on
the schematic diagram shown in Figure 10.5. Heavy metal ions may adsorb onto CNTs
through the formation of complexes with oxygen-containing functional groups,
electrostatic interaction, or donor-acceptor binding.
The adsorption capacity of CNTs for Pb(II), Cu(II), Co(II), Mn(II), and Cd(II)
increased, with corresponding increases in the pH value from 2.0 to 9.0 (Li et al., 2002;
Stafiej and Pyrzynska, 2007), and even to pH 11 with Pb(II), Cu(II) (Li et al., 2003a)
and pH 12 with Cd(II) (Li et al., 2003b). It is known that divalent metal ions can be
present in water in the form of M
2+
, M(OH)
+
, M(OH)
2
, M(OH)
3
-
, etc. (Chen and Wang,
2006). At low pH values, the net surface charge of CNTs is positive, and cation
Search WWH ::
Custom Search