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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: aminulislam.ch@amu.ac.in

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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