Environmental Engineering Reference
In-Depth Information
in GO resulted in the high adsorption eficacy of GO toward cationic dyes. Anionic dyes,
on the other hand, showed negligible adsorption on GO. However, RGO was found to be
highly suitable for the adsorption of anionic dyes, again pointing toward the inluence of
the nature of functionalities present on graphene on the adsorption of various contami-
nants on graphene. Yang et al.
26
and Zhang et al.
27
also reported the removal of MB by GO
with high adsorption eficacy. A comparative study between the MB adsorption capacities
of three different carbonaceous materials, AC, GO, and multiwalled CNTs was reported
recently by Li et al.
28
The studies indicated that compared with CNTs, graphenic materials
are better candidates for this application. The mechanism of adsorption was explained on
the basis of the π-π electron donor-acceptor interaction and electrostatic attraction.
Graphene has been used as an adsorbent for inorganic anions and cations as well. GO was
found to be an excellent substrate for the adsorption of Cu
2+
from water.
29
Aggregation of GO
induced by Cu
2+
can lead to adsorption-aided removal of Cu with high eficiency. Compared
with AC, GO exhibited 10× enhanced adsorption capacity. Deng et al.
30
reported the adapt-
ability of functionalized graphene for the adsorption of Pb(II) and Cd(II). The as-prepared
graphene sheets had 30 wt%
PF
6
−
(since potassium hexaluorophosphate solution was used as
the electrolyte for the synthesis), and showed adsorption capacities in the order of 406.6 mg/g
(pH 5.1) for Pb(II) and 73.42 mg/g (pH 6.2) for Cd(II). Graphitic oxide is reported to be a highly
eficient adsorbent for arsenic and sodium with maximum adsorption capacities for arsenate,
arsenite, and sodium being 142, 139, and 122 mg/g, respectively.
31
Graphene sheets synthe-
sized by ionic liquid-assisted electrolysis were used for the adsorption of Fe
2+
from drinking
water recently.
32
Compared with GO, graphene synthesized via this method showed a 6-fold
increase in adsorption capacity (maximum adsorption capacity of 299.3 mg/g). A recent report
by Zhao et al.
33
suggested that GO can be an eficient adsorbent for the preconcentration of
U(VI) ions from large volumes of aqueous solutions. The study proposed that the process is
dominated by inner-sphere surface complexation and not by outer-sphere surface complex-
ation or ion exchange where the oxygen-containing functional groups on the surfaces of the
GO played an important role. This process also was found to be endothermic and spontane-
ous with a maximum capacity of 97.5 mg/g. A spontaneous, endothermic adsorption process
of phosphate onto graphene was studied by Vasudevan and Lakshmi.
34
A maximum adsorp-
tion capacity of 89.37 mg/g at an initial phosphate concentration of 100 mg/L was reported in
this study. Li et al.
35
demonstrated the utility of graphene for luoride removal from drinking
water. The process was found to be spontaneous endothermic in nature with a maximum
adsorption capacity of 17.65 mg/g.
Assembling graphenic materials into 3-D forms has also been found to be advantageous
for the decontamination of wastewater. Zhao et al.
36
recently developed a 3-D structure
made up of graphenic material (through a hydrothermal process involving thiourea)
called graphene sponge (GS) having tunable pore structure and surface properties that
can be used as eficient, low-cost, robust, and reusable adsorbent for contaminants from
water. High adsorption capacities against a variety of contaminants, including dyes, oils,
and other organic solvents, were demonstrated by the GS media. For MB and diesel oil, GS
can have an adsorption capacity of 184 and 129 mg/g, respectively. It was also proposed
that the dye adsorption performance of GS strongly depends on its surface charge con-
centration and speciic surface area. However, the oil adsorption capacity depends solely
on the speciic surface area. Bi et al.
37
also developed a similar material and used it as a
sorbent for oils and other organic solvents. High adsorption eficacy was demonstrated
by this material for petroleum products, fats, and toxic solvents such as toluene and chlo-
roform (up to 86× its own weight). Dong et al.
38
reported a two-step CVD process to pre-
pare a 3-D monolithic hybrid of graphene and CNTs for the selective removal of oils and
