9.4.1 Carbon-Based Nanomaterials in Adsorption Technology
Nano-adsorbents have been used for removal of heavy metals, fluoride,
chlorophenols, dyes, hydrocarbons, and radionuclide (Schnorr and Swager 2011;
Ren et al. 2011). Adsorption technique has proven to be very efficient for removing
of contaminants from wastewaters. Conventional adsorption using activated carbon
as adsorbents has been widely used for this purpose (Li et al. 2020). Adsorption is
based upon surface chemistry, whereby the pollutants are selectively removed from
an aqueous solution by attaching the solute (adsorbate) into a solid surface (adsorbent). Compared to other alternative processes including coagulation, filtration,
precipitation, and oxidation, adsorption is economical, simpler in design, and higher
efficiency in removing hazardous pollutants. Researchers are in quest of the new
improved adsorbents with high adsorption capacity, high sensitivity, high selectivity, easy recyclable, and low cost (El-Din et al. 2017; Bandura et al. 2017;
Al-Jammal et al. 2020). Carbon-based nanomaterials possessing high surface-areato-volume ratio and good surface modification ability have inspired widespread
attention as a new type of adsorbents for the removal of various inorganic and
organic pollutants from different matrix (Schnorr and Swager 2011; Ren et al. 2011;
Chin et al. 2007). Large numbers of articles published on the adsorption of pollutants
using variety of carbon-based nanomaterials including CNTs, GN, and GO.
Bina et al. (2014) studied removal of benzene, toluene, ethylbenzene, and xylene
(BTEX) from aqueous solution by MWCNTs, SWCNTs, and hybrid carbon
nanotubes (HCNTs). Tubes of MWCNTs were opened as a sheet using hybrid of
MWCNTs and silica (HCNTs). This study showed that SWCNTs showed better
adsorption capacity for BTEX than the MWCNTs and HCNTs. The adsorption
capacity for MWCNTs and SWCNTs follows the following order:
xylene>ethylbenzene>toluene>benzene; for HCNT, the order is ethylbenzene >
xylene > toluene > benzene. The results of desorption study showed that BTEX
adsorbed on SWCNTs can easily be desorbed at 105 Æ 2
C and recycled.
Organic pollutant structure and adsorptive interactions with CNTs play important
role in their removal. Chen et al. (2007) evaluated adsorption of organic compounds
with varied physical-chemical properties (hydrophobicity, polarity, electron polarizability, and size) to three different types of CNTs (one SWNT and two MWNTs).
They found that the adsorption affinity correlated poorly with hydrophobicity but
increased in the order of nonpolar aliphatic < nonpolar aromatics
number of nitro functional groups.
For the purification of CNTs and to open up ends of CNTs, chemical oxidation
has been employed by HNO 3 , H 2 O 2 , KMnO 4 , or HCl (Chin et al. 2007). Purified
CNTs with opened ends have more adsorption sites, not only in the interstitial
channels of the CNT bundles but also inside the nanotubes. Chin et al. (2007)
studied the influences of nitric acid oxidation on the surface properties and the
adsorption capacity of SWCNTs. In order to eliminate the size effects on the
adsorption capacity, o-xylene and p-xylene were chosen as model adsorbates.
Using nitric acid oxidation, the internal and total surface areas of the SWCNTs
214
A. Singh et al.
Nano-adsorbents have been used for removal of heavy metals, fluoride,
chlorophenols, dyes, hydrocarbons, and radionuclide (Schnorr and Swager 2011;
Ren et al. 2011). Adsorption technique has proven to be very efficient for removing
of contaminants from wastewaters. Conventional adsorption using activated carbon
as adsorbents has been widely used for this purpose (Li et al. 2020). Adsorption is
based upon surface chemistry, whereby the pollutants are selectively removed from
an aqueous solution by attaching the solute (adsorbate) into a solid surface (adsorbent). Compared to other alternative processes including coagulation, filtration,
precipitation, and oxidation, adsorption is economical, simpler in design, and higher
efficiency in removing hazardous pollutants. Researchers are in quest of the new
improved adsorbents with high adsorption capacity, high sensitivity, high selectivity, easy recyclable, and low cost (El-Din et al. 2017; Bandura et al. 2017;
Al-Jammal et al. 2020). Carbon-based nanomaterials possessing high surface-areato-volume ratio and good surface modification ability have inspired widespread
attention as a new type of adsorbents for the removal of various inorganic and
organic pollutants from different matrix (Schnorr and Swager 2011; Ren et al. 2011;
Chin et al. 2007). Large numbers of articles published on the adsorption of pollutants
using variety of carbon-based nanomaterials including CNTs, GN, and GO.
Bina et al. (2014) studied removal of benzene, toluene, ethylbenzene, and xylene
(BTEX) from aqueous solution by MWCNTs, SWCNTs, and hybrid carbon
nanotubes (HCNTs). Tubes of MWCNTs were opened as a sheet using hybrid of
MWCNTs and silica (HCNTs). This study showed that SWCNTs showed better
adsorption capacity for BTEX than the MWCNTs and HCNTs. The adsorption
capacity for MWCNTs and SWCNTs follows the following order:
xylene>ethylbenzene>toluene>benzene; for HCNT, the order is ethylbenzene >
xylene > toluene > benzene. The results of desorption study showed that BTEX
adsorbed on SWCNTs can easily be desorbed at 105 Æ 2
C and recycled.
Organic pollutant structure and adsorptive interactions with CNTs play important
role in their removal. Chen et al. (2007) evaluated adsorption of organic compounds
with varied physical-chemical properties (hydrophobicity, polarity, electron polarizability, and size) to three different types of CNTs (one SWNT and two MWNTs).
They found that the adsorption affinity correlated poorly with hydrophobicity but
increased in the order of nonpolar aliphatic < nonpolar aromatics
number of nitro functional groups.
For the purification of CNTs and to open up ends of CNTs, chemical oxidation
has been employed by HNO 3 , H 2 O 2 , KMnO 4 , or HCl (Chin et al. 2007). Purified
CNTs with opened ends have more adsorption sites, not only in the interstitial
channels of the CNT bundles but also inside the nanotubes. Chin et al. (2007)
studied the influences of nitric acid oxidation on the surface properties and the
adsorption capacity of SWCNTs. In order to eliminate the size effects on the
adsorption capacity, o-xylene and p-xylene were chosen as model adsorbates.
Using nitric acid oxidation, the internal and total surface areas of the SWCNTs
214
A. Singh et al.