Environmental Engineering Reference
In-Depth Information
15.2
nAnoAdsorbents in WAter treAtment
Nanoadsorbents are nanoscale particles from organic or inorganic materials that have a high affinity to adsorb substances.
Because of their high porosity, small size, and active surface, nanoadsorbents not only are capable of sequestering contaminants
with varying molecular size, hydrophobicity, and speciation behavior, but they also enable manufacturing processes to consume
raw materials efficiently without releasing a toxic payload [8]. Nanoadsorbents not only work rapidly, but also have considerable
metal-binding capacities. They can also be chemically regenerated after being exhausted [9]. for these reasons, recently, scholarly
interest in nanotechnology has been growing rapidly worldwide.
15.2.1
carbon nanotubes
Due to the high specific area and large micropore volume, carbon nanotubes (cNTs) have been widely utilized for the adsorp-
tion of various aquatic contaminants such as organic compounds and inorganic ions in the past decade. cNTs include single-
walled carbon nanotubes (SWcNTs) and multiwalled carbon nanotubes (MWcNTs), and both can be used for the adsorptive
removal of various pollutants from water. The unique physicochemical and electrical properties of cNTs surpass those of its
prominent counterpart, activated carbon. Some of the studies describing the adsorptive removal of various contaminants by
cNTs are discussed herein: Adsorptive removal of zinc (Zn
2+
) using purified cNTs (MWcNTs and SWcNTs) has been exam-
ined by lu and chiu [10]. The cNTs were subjected to thermal treatment and treated with sodium hypochlorite. As reported in
other studies [11], purification resulted in the removal of metal catalysts and amorphous carbon from cNTs, and large quantities
of nanotube bundles with a hollow inner tube diameter were observed in the purified cNTs. The Brunauer-Emmett-Teller
(BET) analysis showed that SWcNTs possess higher surface area and pore volume than MWcNTs, but average pore diameter
of SWcNTs was lower than that of MWcNTs. The zeta potentials of purified cNTs were more negative than that of raw cNTs
due to the presence of negative functional groups on the surface of purified cNTs. Adsorption of Zn
2+
increased in the pH
range of 1-8. But low Zn
2+
adsorption occurred at low pH due to the competition between H
+
and Zn
2+
for the adsorption
sites. After an increase in Zn
2+
adsorption up to pH 8.0, adsorption remained constant in the pH range of 8.0-11.0. The Zn
2+
species prevails until pH 8.0, and the process governing Zn
2+
removal is adsorption. The kinetics of Zn
2+
adsorption onto the
cNTs was fast (60 min). The kinetics and isotherm study confirmed that the adsorption capacity of Zn
2+
onto purified SWcNTs
was higher than that onto MWcNTs, which was in agreement with the BET results. The maximum adsorption capacities of Zn
2+
calculated by the langmuir model were reported as 43.66, and 32.68 mg/g with SWcNTs and MWcNTs, respectively, at an
initial Zn
2+
concentration range of 10-80 mg/l. The isotherm data was better explained by the langmuir model than by the
freundlich model. Gupta et al. [12] studied cr(III) removal by combining the magnetic properties of iron oxide with adsorption
properties of MWcNTs. Acid treatment (HNO
3
) of MWcNTs was performed to modify the surface of the cNTs with carbonyl
and hydroxyl groups. furthermore, the MWcNTs/nano iron oxide composites were prepared using ferric chloride hexahydrate
and ferrous chloride. Scanning electron microscopy (SEM) images of the prepared composite exhibited an entangled network
of oxidized MWcNTs with clusters of iron oxides attached to them. The presence of maghemite (γ-fe
2
O
3
) and magnetite
(fe
3
O
4
) was confirmed by the X-ray diffraction (XRD) analysis of the composites. The additional adsorbing sites provided by
the oxygen atoms of iron oxide nanoparticles (NPs) on the surface of MWcNTs became available for the electrostatic interac-
tion with cr(III) and, thus, the prepared composite showed higher cr(III) capacity. The composite was effective for cr(III)
adsorption in the pH range above pH 3 and below pH 7 due to the presence of cr(OH)
2+
and cr(OH)
2
+
species of chromium.
Also, at pH values of 4.0-7.0, the net negative surface charge on the former allowed increased adsorption of chromium species
on MWcNTs. The results of the fixed bed experiments revealed that lower flow rate favored cr(III) removal due to the increased
contact time between cr(III) and adsorbent. An increase in fixed bed layers also revealed an increase in cr(III) uptake, which
was attributed to the availability of more adsorption sites. Several researchers have also modified cNTs to evaluate the efficiency
of the former with the unmodified cNTs for the removal of various contaminants. Konicki et al. [13] evaluated the performance
of magnetic MWcNT-fe
3
c nanocomposites (MMWcNTs-IcN) for the adsorption of the anionic dye Direct Red 23 (DR23),
which is present in various industrial effluents. chemical vapor deposition (cVD) was used to obtain the nanocomposites.
Thermogravimetric analysis (TGA) confirmed that the composites were composed of graphite and iron carbide (fe
3
c) phases.
Adsorption equilibrium was attained in 160-340 min depending on the initial dye concentration. The availability of a large
number of vacant sites resulted in faster adsorption in the initial stage. The driving force of concentration gradient increased with
an increase in the initial concentration of the dye and resulted in increased dye adsorption capacity. With an increase in pH (from
3.7 to 11.1), the adsorption capacity decreased due to the electrostatic interaction between the anionic dye and the partially neg-
atively charged nanocomposites. At lower pH, the positive surface charge was dominant due to the protonation of oxygen-
containing functional groups. Temperature also favored the adsorption of DR23 onto the nanocomposites. The kinetic data and
experimental isotherm data fitted well with the pseudo-second-order model and freundlich isotherm model, respectively.
