Novel Carbon-Based Nanoadsorbents for Removal of Synthetic Textile Dyes from Wastewaters - Green Chemistry for Dyes Removal from Wastewater - page 67

2.3.2.2 GrapheneOxide
GO has also proven to be an excellent adsorbent for removal of synthetic
dyes from aqueous systems. For instance, Yang et al. [118] demonstrated
the removal of Methylene Blue from its aqueous solution by using GO as
adsorbent. Almost complete removal of Methylene Blue could be achieved
at initial dye concentrations of less than 250 mg L -1 . The dye removal effi-
ciency increased with an increase in pH and ionic strength. Lower temper-
atures and the presence of dissolved organic matter favored the adsorption
process. Removal of Methylene Blue by GO has also been explored by sev-
eral other researchers [119,122].
Ramesha et al. [120] evaluated the sorption capacity of GO for the
uptake of three different cationic dyes, viz., Methylene Blue, Methyl
Violet, and Rhodamine B, from their aqueous solutions. A significantly
high adsorption capacity for all three of the dyes was exhibited by GO.
However, the sorption preference increased in the order Methylene Blue >
Methyl Violet > Rhodamine B. Such behavioral pattern was attributed to
the fact that Methylene Blue and Methyl Violet were positively charged,
whereas Rhodamine B had both positive and negative charges associ-
ated with its structure and hence the electrostatic interactions between
Rhodamine B and GO were considerably weaker. In the same study, the
utilization of GO for the removal of an acidic dye, Orange G, was also
investigated by Ramesha et al. [120]. However, GO showed a poor binding
affinity for Orange G. The two sulfonic groups of Orange G made it nega-
tively charged, resulting in electrostatic repulsion between the dye and the
adsorbent, and hence no significant removal was observed. Sharma and
Das [123] have also tested GO for the removal of Methyl Green dye. An
adsorption capacity in the range of 4.82-7.61 mmol g -1 was recorded at
different pH (4.0-9.0).
A study on the use of modified GO for removing Acridine Orange
from its aqueous solution was conducted by Sun et al. [121]. hat
research attempted to improve the effectiveness of GO as an adsorbent
through in situ reduction with sodium hydrosulfite. Parallel adsorption
tests under similar experimental conditions, carried out with pristine
GO and in situ reduced GO, showed that the latter had a much higher
adsorption capacity (3333 mg g -1 ) than the former (1428 mg g -1 ). To
identify the mechanism of enhancement in adsorption capacity, the
structure and morphology of GO before and after the reduction were
studied and compared using modern characterization techniques
(SEM, FTIR, AFM, XPS, and TGA). It was concluded that the enhance-
ment was probably due to the reduction of carbonyl groups to hydroxyl
groups.