Environmental Engineering Reference
In-Depth Information
and nanocarbon forms such as CNTs and carbon onions, in native form and as composites,
have been used for adsorption-based water remediation.
15-17
Recently, graphenic materials
(and their composites) are inding huge application possibilities in this area, and studies
indicated toward their high adsorption eficiency; a summary is presented in this section.
Adsorption-based removal of thiophene by single-walled CNTs and graphene was
theoretically investigated, using periodic boundary conditions, van der Waals density
functional, and local density approximation, by Denis and Iribarne.
18
The study provided
some insights into the orientation of adsorbed molecules as well. Detailed investigation
about the adsorption of various organic pollutants such as 1,2,4-trichlorobenzene (TCB),
2,4,6-trichlorophenol (TCP), 2-naphthol and naphthalene (NAPH) on graphene and GO
was undertaken by Pei and coworkers.
19
A batch equilibration method and micro-Fourier
transform infrared spectroscopy were employed in the study. Nonlinear isotherms for
the adsorption of all four species indicated that in addition to hydrophobic interactions,
various other speciic interactions are involved in the adsorption process. Under alkaline
pH, 2-naphthol had higher adsorption capacity on graphene compared with acidic pH.
Higher π-electron density of anionic 2-naphthol than that of neutral 2-naphthol at alkaline
pH, facilitating the π-π interaction with graphene was reported to be the reason for the
enhancement. In the case of GO, adsorption capacity increased in the order NAPH < TCB <
TCP < 2-naphthol. The adsorption mechanism was found to be different in the case of
GO and graphene. Adsorption on graphene was mainly through π-π interactions. On the
contrary, TCP and 2-naphthol can form H-bonding between hydroxyl groups of TCP and
2-naphthol and O-containing functional groups on GO leading to high adsorption. The
removal eficiency of graphene toward various organic chemicals such as acrylonitrile,
p-toluenesulfonic acid (p-TA), 1-naphthalenesulfonic acid (1-NA), and methyl blue (MB)
was examined using RGO as the model graphenic material.
20
The study indicated that spe-
cies with more benzene rings will be adsorbed faster with maximum adsorption capacity
pointing toward the π-π stacking-based adsorption. The observed maximum adsorption
capacities of p-TA, 1-NA, and MB were ~1.43, ~1.46, and ~1.52 g/g respectively. Removal
of aromatic pollutants by sulfonated graphene was investigated by Zhao et al.,
21
taking
naphthalene and 1-naphthol as model pollutants where the kinetics and thermodynamics
of the adsorption were also investigated. Graphene demonstrated high adsorption capac-
ity of about 2.3-2.4 mmol/g for naphthalene and 1-naphthol.
22
The study indicated that
the adsorption involves stacking on the surface of graphene nanosheets (NS) with low
activation energy and thermodynamically the process was found to be spontaneous and
endothermic. Xu et al.
23
investigated the amputation of bisphenol A from aqueous solution
using graphene. A maximum adsorption capacity of 182 mg/g was reported, which was
explained on the basis of hydrogen bonding as well as π-π interactions. Graphene was
found to be a highly eficient adsorbent for the removal of various pesticides from water.
Graphene was found to adsorb material more than its own self-weight in certain cases.
An unprecedented adsorption of various pesticides such as chlorpyrifos, endosulfan,
and malathion with maximum adsorption capacities of ∼1200, 1100, and 800 mg/g, respec-
tively, was reported by Maliyekkal et al.
24
The adsorbent was found to be highly reusable,
and theoretical investigations pointed toward the adsorption being mediated by water
molecules. Figure 34.1a shows the transmission electron microscopic (TEM) image of the
RGO sample used in the study. The adsorption eficiency of RGO and GO (Figure 34.1b)
and the adsorption energy calculated using DFT calculations (Figure 34.1c) are also given.
The utility of GO and RGO for the removal of anionic and cationic dyes such as MB,
methyl violet, rhodamine B (RhB), and orange G from aqueous solutions was reported
by Ramesha et al.
25
The presence of a large variety of negatively charged functionalities
