Environmental Engineering Reference
In-Depth Information
of the surfactants. The interaction leads to adsorption of the surfactants onto the surface of CNTs. Adsorption is believed
to become stronger if the hydrophobic region of the surfactant molecules contains aromatic groups, which increase their
π-π stacking with the graphitic CNT sidewalls [13, 26]. CNTs can also be noncovalently functionalized with a polymer
by allowing polymeric molecules to wrap around the sidewall of the CNTs based on the aforementioned physical interac-
tions. Polymer wrapping can be easily implemented by mixing the CNTs in a solution containing a polymer and forming
a supramolecular complex. Polymers are believed to have the tendency to wrap around CNTs in a helical geometry to
minimize the strain in their conformations [30].
The modifiable surface of CNTs demonstrates the adaptation of their tunable surface properties with viable applications and
the optimization of their surface characteristic for enhancing the performance of CNTs.
8.3
adsorption of heavy metals
Heavy metals naturally exist in the ecosystem. However, the introduction of heavy metals to water by anthropogenic sources
such as industrial and agricultural waste disposal has created a serious pollution issue that has drawn major global concern. The
persistence of heavy metals in water resources threatens human health even at relatively minor levels of exposure because they
are not degradable and possess toxicity. Therefore, removing heavy metals from the ecosystem has become an indispensable
task. Adsorption has been the most effective and economical method for removing heavy metals from an aqueous environment
over the past decades [31]. In addition to easy handling and flexibility in design, the reversibility of the adsorption process
makes the adsorbent reusable after regeneration through desorption, making the process cost-effective.
CNTs are recognized as a highly efficient adsorbent for removing heavy metals from water because of their large surface
active sites and controlled pore size distribution. Surface-modified CNTs are more popular for adsorption studies than pristine
CNTs because of the enhanced adsorption capability of surface-modified CNTs. One of the most common methods for modi-
fying the CNT surface is chemical oxidation. CNTs are treated with strong oxidizing agents to make possible the attachment of
oxygen-containing functional groups, which increase adsorption affinity toward heavy metal ions including lead (Pb(II)) [32-34],
zinc (Zn(II)) [35-37], nickel (Ni(II)) [38-41], cadmium (Cd(II)) [42], and copper (Cu(II)) [43]. Another CNT modification
method is functionalization with metal oxides [44-46], polymers [18, 19, 47, 48], and organic substances [49-52] to form a new
composite called CNT-based nanocomposites. These functional components serve as efficient anchors for heavy metals by
taking advantage of the large surface area of CNTs. The high adsorption of CNT/tri(2-aminoethyl)amine (TAA) toward Pb(II)
was reported by Cui et al. [49], who attributed the adsorption to the coordination between the nitrogen group in the TAA and
the metal ions. Shao et al. [48] also reported that the amide functional groups in mWCNT/poly(acrylamide) and mWCNT/
poly(
N
,
N
-dimethylacrylamide) serve as efficient adsorption sites for Pb(II). The adsorption performances of various types of
CNTs are listed in Table 8.1 [18, 33, 35, 37, 42, 43, 48, 49, 53, 54].
Adsorption of heavy metals onto CNTs involves complementary steps that may be attributed to physical adsorption,
electrostatic attraction, precipitation, and chemical interaction [55]. Among these, the major step is the chemical interaction
between heavy metals and the surface functional groups of CNTs, where adsorption occurs by sharing the lone electron pair
from the functional group with the heavy metal [25]. This finding is consistent with the study of Wang et al. [56], who reported
that 75.3% of the total Pb(II) adsorbed onto CNTs depends on the chemical interaction with an oxygen-containing functional
group to form a chemical complex. Such a dominant mechanism allows adsorption onto CNTs to reach equilibrium in a shorter
time than conventional porous-structure adsorbents where the adsorbate needs to diffuse from the exterior into the inner surface
[39]. The adsorption behavior of heavy metals on the CNT surface mainly follows the langmuir isotherm, where the heavy
metals are adsorbed onto the CNT surface in a monolayer and are localized at the adsorption site without any interaction with
other heavy metal ions [45, 51, 57].
The surface properties of the CNTs are prone to changes depending on pH, which affects the adsorption capacity. The
amount of heavy metal adsorbed can be correlated to pH called the “point of zero charge” (pH
PZC
), in which a net charge of the
adsorbent is equal to zero [55]. The adsorption phenomena in CNTs are favored at pH higher than pH
PZC
because the surface of
CNTs is more negatively charged and induces an electrostatic attraction, which promotes the adsorption of heavy metals that
usually appear as cation species. However, decreasing pH to less than pH
PZC
reduces heavy metal adsorption because the CNT
surfaces become positively charged and compete with H
+
for adsorption sites [35, 38, 58]. Previous reports [43, 59-61] have
indicated that the pH
PZC
of pristine CNTs lies between pH 4 and 6, whereas pH
PZC
decreases with modified CNTs. The modified
CNTs demonstrate better adsorption properties because the adsorption of CNTs is effective over a broader range of pH. In most
cases, low pH is used for the desorption of heavy metals from CNTs with acidic solutions such as HNO
3
and HCl [33, 51, 53]
instead of adsorption. Over 90% of the adsorption sites on CNTs can be recovered and reused [53, 62]. The adsorption
performance of CNTs is found to remain stable for over 10 cycles of adsorption and desorption [50, 53, 63-65].
