Electronic Supplementary Material
A nanoparticle sorbent composed of MIL-101(Fe) and dithiocarbamate-modified magnetite
nanoparticles for speciation of Cr(III) and Cr(VI) prior to their determination by
electrothermal AAS
Ali Saboori*
Young Researchers and Elite Club, North Tehran Branch, Islamic Azad University, Tehran,
Iran
* Corresponding author: Tel.: +98 9122978772 , E-mail address: [email protected] (Ali
Saboori)
Reagents and instrumentation
All reagents (FeCl3, (NH4)2Fe(SO4)2.6H2O, HCl, HNO3, EDTA, N-(2-Aminoethyl)-3(aminopropyl)trimethoxysilane (AEAPTMS), NaOH, 2-propanol, carbon disulfide (CS2), (3aminopropyl)-triethoxysilane (3-APS), benzene-1,4-dicarboxylic acid (H2BDC), toluene,
ammonium hydroxide (28% w/v), tetraethyl orthosilicate (TEOS), dimethylformamide (DMF),
ethanol, and methanol were of analytical grade and purchased from Merck (Darmstadt,
Germany) or from Fluka and were used without any purification. Standard solutions of 1000 mg
L-1 of Cr(III) and Cr(VI) were purchased from Merck. All working solutions were prepared using
deionized water.
A Varian (Varian Company, USA, www.varian.com) Spectra model AA 220 with a graphite
furnace atomizer (GTA-110) equipped with an ASC-6100 auto sampler was used for
determination of chromium concentration in the optimization studies and real sample analysis.
Deuterium background correction was employed in order to correct non-specific absorbance. An
chromium hollow cathode lamp with wavelength of 357.9 nm with a spectral bandwidth of 1.0
nm and a pyrolytic graphite coated graphite tube was used too. In all tests, the injection volume
was 20 μL. A 0.1% (w/v) Pd(NO3)2 solution was used as chemical modifier. The operating
conditions of the instrument are listed in Table 1S. Ar with 99.995% purity (Roham Gas
Company, Tehran, Iran) was employed as protective and purges gas. The pH of the solutions
were measured at 25 ± 1 ºC with a digital Metrohm 827 pH meter (Herisau, Switzerland,
www.metrohm.com) equipped with a combined glass-calomel electrode. A Bruker IFS-66 FT-IR
spectrophotometer (Karlsruhe, Germany, www.brukeroptics.com) was used for FT-IR spectra
recording. Scanning electron microscopy (SEM) was conducted using an SEM (KYKY-3200,
Beijing, China, Zhongguancun Beijing, China, www.kyky.com.cn)) instrument. Transmission
electron microscopy (TEM) analysis was conducted by employing a LEO 912AB electron
microscope (Leo Ltd., Germany, www.zeiss.com). Magnetic properties of nanocomposites were
measured by a vibrating sample magnetometer (VSM) model AGFM/VSM 117 3886 (Kashan,
Iran) with a magnetic field strength of 1 Tesla and at room temperature. The surface area and
pore volume of the nanocomposite and MIL-101(Fe) were determined by employing a
Micromeritics ASAP 2010 analyzer (Norcross , USA, www.micromeritics.com) at 77 K. BET
method was employed to calculate the surface area and pore volume. Elemental contents of
nanocomposite were determined by an elemental analyzer model Thermo Finnigan Flash EA112
(Okehampton, UK, www.thermoscientific.com).
Real sample pretreatment
Tea samples
Black and green tea samples were purchased from local supermarkets in Tehran, Iran. Briefly,
0.5 g of each sample was placed in a digestion cell and 8 mL HNO3 (65% w/w)-H2O2 (30% w/w)
solution in the ratio of 3:1 v/v were added to the sample. Digestion was carried out according to
following procedure: 2 min at 250 W, 2 min for 0 W, 6 min at 250 W, 5 min at 400 W, 8 min at
550 W, and ventilation for 8 min, respectively [19]. After digestion, the solution was cooled to
room temperature, filtered into a 500 mL volumetric flask and diluted with ultrapure water.
Water samples
River water sample (North of Tehran, Iran) and drinking water sample (Tehran, Iran) were
filtered into cleaned polyethylene bottles. Each sample was divided to two equal portions and the
pH of them was adjusted according to the opted experimental conditions before the extraction
procedure.
Reference material
A water standard reference material (SRM 1640 natural water) and GBW 07605 tea were
analyzed for validation of the extraction method. GBW 07605 tea was digested according to the
procedure mentioned for the tea samples.
Design of experiments approach
Various factors may influence the extraction efficiency of Cr species. To obtain precise optimum
value for each affecting factor, design of experiments (DOE) through response surface
methodology can be employed [1,2]. DOE approach reduces the number of test and required
time which terminates in the reduction of the overall required costs. Central composite design
(CCD) is one of the most widely used response surface method used for fitting a second-order
response surface and optimization study [3]. In this study, the StatGraphics plus 5.1 package was
used for designing of experiments, analyzing the data and calculating the predicted responses.
VSM and BET analyses
The
magnetic
property
of
Fe3O4
NPs,
Fe3O4@PAEDTC
NPs
and
MIL-
101(Fe)/Fe3O4@PAEDTC nanocomposite were studied by employing a magnetic property
measurement system at room temperature. As represented in Fig. 2S, the saturation
magnetization
intensity
of
Fe3O4
NPs,
Fe3O4@PAEDTC
NPs
and
MIL-
101(Fe)/Fe3O4@PAEDTC nanocomposite were 67, 42 and 29 emu g-1, respectively which are
enough for magnetic isolation with a conventional magnet in MSPE [4].
The BET analysis conducted to calculate the specific surface area and pore volume of MIL101(Fe) and MIL-101(Fe)/Fe3O4@PAEDTC nanocomposite. Surface and pore volume of MIL101(Fe) were 2310 m2 g-1 and 1.17 cm3 g-1, respectively while the specific surface area and pore
volume of MIL-101(Fe)/Fe3O4@PAEDTC were 1150 m2 g-1 and 0.70 cm3 g-1, respectively. This
decreas in surface area and pore volume values demonstrates the successful synthesis of MIL101(Fe)/Fe3O4@PAEDTC nanocomposite [10].
Effect of pH
The pH affects the surface charge of sorbents and complex formation between the metal species
and heteroatoms of the sorbent. In this regards, the influence of pH on the extraction efficiency
of Cr(III) and Cr(VI) ions was studied in the range of 1-7 (Fig. 3S). As depicted in Fig. 3b,
Cr(VI) ions retained quantitavely on MIL-101(Fe)/Fe3O4@PAEDTC surface at the pH range of
2-5, while Cr(III) ions sorbed into the sorbent surface at pH = 5.0. Besides, at pH =2.0 only
Cr(VI) is sorbed completely, while Cr(III) sorption percentage is negligible. Hence, pH = 2.0
was selected for the selective extraction of Cr(VI) ions and pH = 5.0 was selected for the
extraction of total chromium.
Sorption step
Central composite design was employed to study the effect of various factors such as
nanosorbent amount, and sorption time in the sorption step. The number of experimental runs
(N) can be obtained from the following expression [4]:
𝑵 = 𝟐𝒇 + 𝟐𝒇 + 𝑪𝟎
where f is the number of affecting factor and C0 is the number of experiment in center point. In
this study f and C0 were selected as 2 and 5 respectively that mean 13 trials should be performed.
The levels of each factor are presented in Table 2S (Electronic Supplementary Data). The Pareto
chart of main effects along with interaction effects, as result of analysis of variance (ANOVA), is
depicted in Fig. 4S. The vertical line on the Pareto chart determines statistically significant
effects. The bar exceeding beyond the vertical line are significant at 95% confidence level [5,6].
Based on the Pareto chart sorbent amount has the most significant positive effect on sorption
efficiency. The sorption of Cr(III) and Cr(VI) ions was increased as the sorption time value
increased. Nano-sized sorbents have large surface area to volume ratio and short diffusion route
compared to the conventional sorbents that lead to fast extraction kinetic and low consumption of
sorbent Response surface and two-dimensional contour plots (Fig. 4S) was employed for
analyzing concurrent effects of the sorption time and sorbent amount on the sorption efficiency.
Based on the results of CCD, the best conditions were sorbent amount of 14.8 mg and sorption
time of 6.5 min.
Optimization of elution step
At first effect of eluent type was studied by employing several acidic eluents including HCl,
HNO3, EDTA and mixture of them. The results exhibited that mixture of EDTA and diluted
HNO3 can recover Cr(III) and Cr(VI) ions without degradation of the nanocomposite. In the next
step four factors including EDTA concentration (mol L-l), HNO3 concentration (mol L-l), eluent
volume (mL) and elution time (min) were optimized by performing a CCD. CCD was selected
owing to its requirement to the least number of experiments (30 runs). The results of conducting
experiments were evaluated based on ANOVA and Pareto chart. The Pareto chart revealed that
all the factors have a significant effect on the extraction recovery (Fig. 5S). EDTA and eluent
volume has the greatest effect on the extraction recovery among the studied factors. Fig. 2Sb
(Electronic Supplementary Data) exhibits the simultaneous effect of the elution time and eluent
volume along with EDTA and HNO3 concentrations on extraction efficiency of the target
species. According to the results of CCD study, the best extraction conditions were selected as:
HNO3 concentration, 0.3 mol L-l; EDTA concentration, 0.6 mol L-l; eluent volume, 0.85 mL; and
elution time, 12.2 min.
Table 1S
Operating condition of ETAAS for determination of chromium.
Time (s)
Step
Temperature (ºC)
Argon gas flow rate (mL min-1)
Ramp Hold
Injection of modifier
55
5
20
220
Injection of sample
Drying
100
5
20
220
Drying
150
10
10
220
Ashing
1000
10
20
220
Atomization
2500
0
3
0 (read)
Cleaning
2700
1
5
220
Table 2S
Experimental variables along with their levels in central composite design (CCD).
Level
Lower Central
Sorption step
Elution step
Upper
-α
+α
A: Sorbent amount (mg)
7.0
13.5
20.0
1.9
23.1
B: Sorption time (min)
3.0
9.0
15.0
0.34
11.6
A: EDTA concentration (mol L-1)
0.30
0.55
0.80
0.05
1.05
B: HNO3 concentration (mol L-1)
0.20
0.35
0.50
0.05
0.65
C: Eluent volume (mL)
0.50
0.75
1.0
0.25
1.25
D: Elution time (min)
7.0
11.0
15.0
3.0
19.0
Table 3S
The recovery of Cr(III) and Cr(VI) ions in the presence of potentially interfering ions.
Potentially
Tolerable concentration ratio X/Cr
Recovery (%)
interfering ions
Clˉ
50000
95.1 ± 5.5
NO3̄
50000
97.4 ± 4.8
3AsO3
100
96.5 ± 5.0
AsO43100
94.0 ± 3.5
K+
50000
98.5 ± 6.0
+
Na
50000
96.7 5.4
2+
Mg
15000
99.1 ± 3.9
Ca2+
2000
98.0 ± 4.3
2+
Pb
250
96.3 ± 3.7
Co2+
200
95.1 ± 4.6
Zn2+
150
96.0 ± 3.0
2+
Cu
150
94.5 ± 3.4
Cd2+
100
96.5 ± 3.2
3+
Al
50
98.0 ± 2.1
Mn2+
300
97.2 ± 3.9
a
Recovery
b
standard deviation (n = 3)
Conditions: sample pH = 5.0, Cr(III) and Cr(VI) concentration = 100 ng L-1, sorption time = 6.5
min; eluent volume = 0.85 mL, 0.6 mol L-l EDTA in 0.3 mol L-l HNO3 solution, elution time =
12.2 min.
c
Concentration of potentially interfering ions.
Table 4S
Analytical features of MIL-101(Fe)/Fe3O4@PAEDTC nanocomposite and bare MIL-101(Fe) for
extraction of Cr(VI) ions.
a
b
Sorbent
LOD (ng L-1)
DLR
(ng L-1)
R2
RSD (%)
SC a
ERb (%)
MIL101(Fe)/Fe3O4@PAEDTC
1.0
3-300
0.9931
6.4
335
98
MIL-101(Fe)
19.0
50-1500
0.9948
7.0
59
22%
Sorption capacity (mg/g).
Extraction recovery.
Recovery (%)
100
80
60
40
20
0
1
2
Sorbent type
3
Fig. 1S: Effect of nanosorbent type on the extraction recovery of total chromium at pH = 5.0.
Fig. 2S: (a) The SEM micrographs of MIL-101(Fe) and (b) MIL-101(Fe)/Fe3O4@PAEDTC
nanocomposite. (C) Vibrating sample magnetometry curves of Fe3O4 NPs, Fe3O4@PAEDTC
NPs and MIL-101(Fe)/Fe3O4@PAEDTC nanocomposite.
Sorption (%)
100
90
80
70
60
50
40
30
20
10
0
Cr(III)
Cr(VI)
0
2
4
pH of sample
6
8
Fig. 3S: Effect of pH on the sorption efficiency; (1) MIL-101, (2) Fe3O4@PAEDTC and (3)
MIL-101/Fe3O4@PAEDTC.
(a)
(b)
Fig. 4S: (a) Pareto chart obtained from CCD in the sorption step. AA and BB are the quadratic
effects of sorbent amount and sorption time, respectively. (b) Response surface and twodimensional contour plot.
(a)
(b)
(b)
Fig. 5S: (a) Pareto chart obtained from CCD in the elution step. AA, BB, CC and DD are the
quadratic effects of EDTA concentration, HNO3 concentration, eluent volume and elution time,
respectively. (b) Some obtained response surface and two-dimensional contour plots.
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