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Electronic Supporting Material
A graphene oxide decorated with triethylenetetramine-modified magnetite for separation
of chromium species prior to their sequential speciation and determination via FAAS
Aminul Islam*, Hilal Ahmad, Noushi Zaidi, Suneel Kumar
Analytical Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh,
India-202 002
This file includes the Electronic Supplementary information of mf-GO (Figures, Optimization experimental parameters and Tables).
*Corresponding author: Analytical Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh, India -202 002 Tel.: +91 9358979659; E-mail address: [email protected]
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Figure S1. SEM image of mf-GO.
Figure S2. (A) TEM image of mf-GO, and (B) High resolution view of selected area in TEM
image.
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Figure S3. FT-IR spectra of mf-GO.
Figure S4. TGA/DTA spectra of mf-GO shows the thermal stability upto 300 0C.
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0 10 20 30 40 50 600
20
40
60
80
100
120
Cr(III)Cr(VI)
Time (minutes)
% R
ecov
ery
Figure S5. Effect of stirring time on the sorption of Cr(VI) (pH 2 ± 0.1) and Cr(III) (pH 8 ± 0.1); on mf-GO (Experimental conditions: sample volume 50 mL; Cr(III/VI): 20 µg mL−1, sorbent amount 50 mg).
Optimization of experimental variables
Effect of pH
Chromium in aqueous solution can be present in various forms depends upon the pH of solution.
The abundance of toxic Cr(VI) in the forms of HCrO4− and Cr(III) as Cr3+ and Cr(OH)2+ were
reported to be at pH 2–7 and pH > 4, respectively. Therefore, pH of a sample solution plays an
important role in the speciation of chromium. A series of sample solutions (50 mL, 20 mg L−1)
were adjusted to a pH range of 1–10 using suitable buffer solution and the effect of pH on
chromium speciation onto both, functionalized mf-GO and the host magnetic graphene oxide
were studied and illustrated in Figure 2. At pH 2.0, the sorption of Cr(VI) was found to be a
maximum (16.4 mg g−1) whereas Cr(III) was not uptaken at all and in the pH range of 8–9,
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sorption of Cr(III) was observed to be maximum (9.6 mg g−1) with a minimum retention of
Cr(VI). Such maxima in the sorption of chromium species at pH 2 and 8 were not observed with
the host magnetic graphene oxide and the observed sorption capacities were inferior too. In order
to compare the extraction efficiencies of mf-GO for Cr(VI) and Cr(III) within a single sample, a
50 mL of model solution containing both chromium species {Cr(III) and Cr(VI) 10 mg L−1 each}
was stirred and was found to achieve 100% extraction for both species at their respective
optimum pH values. Hence, for subsequent experiments, pH 2±0.1 and pH 8±0.1 was selected as
the working pH for Cr(VI) and Cr(III) sorption, respectively.
Effect of contact time
To investigate the sorption kinetics of chromium species, 50 mg of mf-GO was stirred with 50
mL solutions containing 20 mg L−1 of chromium from 5 min to 1 h (at optimum conditions).
Figure S5 shows that the 90% of Cr(VI) was sorbed in about 5 min and then reached to
equilibrium in 10 min, while for Cr(III) the sorption increased remarkably at the beginning of the
experiment and gets saturated in 30 min. A further increase in the contact time does not shows
any increase in sorption. Hence, it was concluded that 10 and 30 min stirring of sample solutions
with sorbent for the speciation of Cr(VI) and Cr(III) respectively, was enough to reach the
saturation level which reflects better accessibility of the active sites tailored on the surface of mf-
GO.
Elution studies and reusability of mf-GO
For an ideal sorbent the sorption capacity, quantitative recovery of sorbed analyte and, the
potential reusability are considered to be the key parameters. Elution studies were accomplished
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by using different solutions namely HCl, HNO3, NaOH, NH3 and NH4NO3 with varying volumes
(1–10 mL) and concentrations (0.1–2.0 M). The 5 mL of 2 N NH3 for Cr(VI) and 5 mL of 2 M
NH4NO3 for Cr(III) (ESI Table S1) were found to be sufficient for quantitative recovery (>99%).
During elution no precipitate formation was observed for CrO42-, this may due to the presence of
trace amount of chromium or soluble hydroxide formation at high pH values. However, use of 2
M HCl and HNO3 as an eluent recovered analyte ion but simultaneously causes the leaching of
Fe3O4 nano particles from the mf-GO surface resulting in complete loss of magnetic separation
property. To explore the potential reusability, mf-GO was subjected to several loading and
elution cycles under optimized conditions and was found that use of optimum eluent prevents
any leaching of incorporated ligand and Fe3O4 nano particles and thus contributes to the
sustainability of the material. The sorbent can be regenerated successfully up to 45 successive
cycles without loss of uptake capacity.
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ESI Table S1 Effect of type, concentration and volume of eluent on the recovery of Chromium species.
ESI Table S2. Effect of
foreign ions on the recovery
and determination of 0.5 mg
L−1 chromium species using
MSPE/FAAS.
Concentration of Eluent
Volume (mL)
Recovery (%)
Chromium (VI)
1 N NaOH 345
707578
2 N NaOH 345
808592
1 N NH3 345
828692
2 N NH3 345
8795100
Chromium (III)
1 N NH3 3
4
5
50
56
60
2 N NH3 3
4
5
65
68
70
1M NH4NO3 3
4
5
78
82
85
2M NH4NO3 3
4
5
85
98
100
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Foreign ions Added as
Amount added (mgL−1)
%Recovery RSD (N=3)
Cr(VI) Cr(III) Cr(VI) Cr(III)
CO32- Na2CO3 230 100.4 98.5 0.86 1.25
SO42- Na2SO4 2200 97 98.2 1.43 0.75
PO42- Na2HPO4 2000 96.3 96.7 1.30 1.42
NO3- NaNO3 300 98.5 100.0 1.46 0.43
Cl- NaCl 7500 97.5 99.2 2.21 0.62
Br- NaBr 7500 98 100.5 2.37 1.71
Na+ NaCl 5000 97.2 96.5 0.77 0.68
K+ KCl 4000 101 99.8 0.34 0.87
Ca2+ CaCl2 600 102 97.4 0.86 0.95
Mg2+ MgCl2 1000 102.2 96 0.77 0.67
Cr6+ K2Cr2O7 750 - 101.3 1.59 0.91
Cr3+ CrCl3 1000 102 - 0.87 1.1
Zn2+ ZnCl2 25 100 98 0.82 0.73
Cd2+ CdCl2 25 101.4 99.2 1.16 1.72
Ni2+ NiNO3 25 101.2 97.2 1.35 2.01
Cu2+ CuNO3 25 98.8 96 1.21 2.43
Co2+ CoNO3 25 100.6 98 0.58 1.77