3.1 Characterization
FT-IR analyses confirm that DES was successfully loaded onto mGO (Fig. 1). The FT-IR spectrum of mGO was shown in the black spectrum. The stretching frequencies of the O-H band is illustrated at 3121 cm−1 [22]. The absorbance at 1586 cm−1 is attributed to aromatic C=C. Stretching vibration of C-O groups caused duple bonds at 1117 and 1042 cm−1 [22]. Peaks lower than 700 characterizes bond of Fe–O [23]. The green spectrum shows the FT-IR spectrum of UreaLiCl-mGO. The bands of 3400 cm−1, 1600 cm−1 and1400 cm−1 are associated to stretching frequency of N-H, deformation frequency of N-H and stretching frequency of N-C, respectively. The sorption peaks of Li-O and Fe-O were seemed at the wavelengths of 1300 cm−1 and 700 cm−1, respectively [23].
Plot of identified phase mGO is shown in Fig. S1. The X-ray diffraction (XRD) pattern of UreaLiCl-mGO is plotted in Fig. 2a. The diffraction peaks at 30.70 °, 35.97 °, 43.74 °, 53.94 °, 57.55 ° and 63.32 °characterized magnetite Fe3O4 (JCPDS cards No. 01-075-0449). Carbon structure was identified regarding peaks at 11.23 °, 21.64 °, 30.70 °, 35.97 °, 43.74 °, 53.94 °, 57.55 °, 63.32 ° and 75.08 ° (JCPDS cards No. 01-079-1715). The comparison between XRD patterns of mGO and UreaLiCl-mGO displays that the process of modification has no consequence on mGO purity and structure. The broad peaks are related to the presence of UreaLiCl. According to the Scherrer formula (Equation 1), the crystal size of the total product decreased from 17 nm to 6 nm, due to applying ultrasonic waves in the process of preparation [13].
\(d\left( {\mathop A\limits^{0} } \right)=\frac{{k\lambda }}{{\beta \operatorname{Cos} \theta }}\) (Eq. 1)
In this formula, λ is wavelength of incident beam (1.5406 Å), β is FWHM of peak in radian, θ is diffraction angle and k is the Scherrer constant (0.9).
Nanographs of mGO and of UreaLiCl-mGO are illustrated in Fig. 2b and Fig. 2c, respectively. Magnetic nanoparticles of Fe3O4 are structured as nanosphericals. Fe3O4 nanoparticles are located onto GO sheets homogenously and no agglomeration is detected, approving well-controlled conditions of synthesis including pH, time and amount of materials. In addition, DES covers mGO completely. The morphology of Fe3O4 is unchanged but ultrasonic waves decreased the average size of particles from 50 nm to about 27 nm [13].
AGFM curve of UreaLiCl-mGO is plotted in Fig. 2d. Saturation magnetization (Ms) of nanocomposite is about ±25 electromagnetic units (emu g−1). This amount is more than the minimum magnetic amount (±16 emu g−1) of magnetic adsorbent [24]. Meantime the component has superparamagnetic property.
3.2 Optimization of the method
3.2.1 Effect of pH
pH is one of the effective parameters on adsorption process. In alkaline pH, heavy metals form oxo or hydroxo complexes with OH− and in acidic pH, H+ ions occupy the active negative sites of adsorbents. In both condition the affinity of adsorbents to adsorb heavy metals decreases. To investigate the effect of pH, pH of a series of samples was adjusted from 2 to 9 and the procedure was applied according to section 2.4. The results are plotted in Fig. 3. From pH=4 to pH=6, the recovery stayed maximum. Before pH=4, H+ occupied the active sites of adsorbent and after pH=6, Pb(II) ions were precipitated as Pb(OH)2 and PbO, so recovery decreased. All subsequent works for separation and preconcentration of Pb(II) were done in pH of 5 ± 0.5.
3.2.2 Effect of equilibrium time and amount of adsorbent
The effect of contact time on Pb(II) adsorption onto UreaLiCl-mGO was investigated at different intervals in the range of one to 30 minutes. After gathering UreaLiCl-mGO by a magnet, the supernatant was removed and the settlement was eluted by HNO3 (1 mL, 2 mol L−1). The recoveries are steady after 15 minute so it was selected as the optimum agitation time for all the subsequent experiments.
A series of UreaLiCl-mGO ranging from 1.0 × 10−2 g to 5.0 × 10−2 g was tested and 1.0 × 10−2 g provided satisfying recovery in Pb(II) preconcentration. Low amount of the adsorbent confirms high affinity of UreaLiCl-mGO toward Pb(II) adsorption.
3.2.3 Selecting the best eluent and time of desorption
Elution step was optimized by investigating type and volume of eluents (Table 1). By increasing HNO3 concentration, the adsorption efficiency increased and HNO3 of 2 mol.L−1 was selected as the best eluent. Adsorbent collapsed in the presence of HCl, which is related to its negative consequence on Fe3O4 and DES. Meantime, 0.5 mL, 1 mL, 1.5 mL and 2 mL of HNO3 were tested and 0.5 mL was optimized volume of eluent.
Table 1 Effect of eluent type (1 mL) on the preconcentration recovery of Pb2+ (n = 3)
Adsorption conditions: T = 25 °C, C0 = 5.0 × 10-5 g⋅L−1, V = 50 mL, pH = 5 ± 0.5, amount of sorbent = 1.0 × 10-2 g, adsorption time = 30 min, desorption time = 5 min.
Recovery (%)
|
Eluent
|
67.57 ± 0.03
|
HNO3 (1 mol L-1)
|
98.94 ± 0.12
|
HNO3 (2 mol L-1)
|
96.52 ± 0.09
|
HNO3 (5 mol L-1)
|
34.38 ± 0.18
|
HCl (1 mol L-1)
|
27.29 ± 0.28
|
HCl (2 mol L-1)
|
7.04 ± 0.16
|
HCl (5 mol L-1)
|
Effect of desorption time was tested in the range of 1 - 5 minutes in room temperature. After 3 minutes, the recovery was maximum, so 4 minutes was selected as the optimum desorption time.
3.3 Swelling property of UreaLiCl-mGO
UreaLiCl-mGO (1.0 × 10−2 g) was added into a solution of Pb(II) (50 mL, 50 × 10−6 g L−1) at pH of 5 ± 0.5. After 15 minutes shaking, the UreaLiCl-mGO was collected by an external magnetic and weighted. The swollen ratio (g/g) was calculated according to Equation 2:
\(W=\frac{{{W_s} - {W_d}}}{{{W_d}}}\) (Eq. 2)
W (swelling ratio at time t), Ws (weight of swollen UreaLiCl-mGO) and Wd (weight of dry UreaLiCl-mGO) are 6.6, 7.0 × 10−2 g and 1.0 × 10−2 g, respectively. The ion adsorption causes swelling of adsorbent, illustrating high affinity of UreaLiCl-mGO to adsorb Pb(II) [10, 25].
3.4 Reusability and durability of UreaLiCl-mGO
In order to check the reusability of UreaLiCl-mGO, several consecutive preconcentration cycles of Pb(II) were examined according to general procedure. No significant variation was recorded up to seven cycles; however, preconcentration recovery decreased after this cycle. This weakness is related to the negative effect of HNO3 onto the adsorbent. H+ ions occupy the sites of adsorbent, which decrease the tendency of the adsorbent toward Pb(II).
Oxidation of magnetic adsorbents is one limit in MSPE method, which confines the applicability of magnetic adsorbents to a short time after their preparation. This feature is named durability and could be achieved by repeating the preconcentration procedure during several days. The performance of the adsorbent was checked during 6 months and the recovery has no important changes. During these months, the adsorbent was kept at room condition and preconcentration procedure was conducted as mentioned, which confirms stability of UreaLiCl-mGO against oxidation. This resistibility is related to presence of DES, which prevents Fe3O4 nanoparticles from collapse.
3.5 Effect of interference ions and selectivity of UreaLiCl-mGO
Pb(II) could be adsorbed onto UreaLiCl-mGO via electrostatic interaction, which is not a selective force and may be affected in the presence of other metals [23]. In addition, some onions and Pb(II) may form complexes, and decline the preconcentration ability of UreaLiCl-mGO. This feature was tested in the presence of various concentrations of different cations and anions in test solutions of Pb(II) (50 mL, 5.0 × 10−5 g L−1). The signals were compared with that of a solution containing only Pb(II) and recoveries were in the range of 95 - 105%. In concentration levels higher than those usually present in real samples, the method was not impaired (Table 2).
Table 2 The effect of interfering ions on recovery of Pb2+ preconcentration (n = 3)
Ions
|
Ratio of coexisting ions
|
Recovery
(%)
|
Na(I)
|
10000
|
97.12 ± 0.02
|
K(I)
|
10000
|
101.65 ± 0.05
|
Ca(II)
|
800
|
98.48 ±0.19
|
Mg(II)
|
700
|
97.19 ± 0.08
|
Cd(II)
|
50
|
99.19 ± 0.11
|
Cr(II)
|
600
|
96.10 ± 0.16
|
Pd(II)
|
300
|
100.03 ± 0.21
|
Zn(II)
|
100
|
101.27 ± 0.09
|
Al(III)
|
500
|
102.49 ± 0.09
|
Co(II)
|
20
|
99.98 ± 0.13
|
Cu(II)
|
30
|
100.27 ± 0.17
|
Ni(II)
|
100
|
98.17 ± 0.14
|
SO42-
|
1000
|
101.48 ± 0.27
|
Cl-
|
200
|
104.25 ± 0.15
|
NO3-
|
1000
|
99.59 ± 0.09
|
CO32-
|
2000
|
100.19 ± 0.16
|
Selectivity was checked via comparing the performances of UreaLiCl-mGO to adsorb Pb(II) (5.0 × 10−5 g L−1) and other heavy metals (5.0 × 10−5 g L−1) at the same time (Table 3). The ground reason for selective adsorption of Pb(II) onto UreaLiCl-mGO is their strong interaction in comparison with interactions of other heavy metals. Due to the small ionic radius of Pb(II), this ion has high charge density and is a hard ion; accordingly, it occupies the negative sites of adsorbent stronger and faster.
Table 3 Selectivity on the preconcentration of Pb2+
Ions
|
Recovery
(%)
|
Cd(II)
|
80.45 ± 0.11
|
Cr(II)
|
84.36 ± 0.04
|
Cu(II)
|
90.25 ± 0.08
|
Mn(II)
|
10.30 ± 0.16
|
Pb(II)
|
95.45 ± 0.21
|
Pd(II)
|
40.35 ± 0.18
|
Zn(II)
|
76.35 ± 0.10
|
3.6 Sample analysis
Four different seed oils including sesame, hemp, sunflower seeds and grapeseed were tested as real samples. The preparation method of samples was expressed in section of sample preparation. The concentrations of the analyte were determined in both the spiked and unspiked samples according to the following equation.
\(R\% =\frac{{{C_1} - {C_2}}}{{{C_3}}} \times 100\) (Eq. 3)
Where, C1, C2, C3 and R% are spiked portion, unspiked portion, the concentration of the analyte and relative recovery, respectively. According to data, the method is highly applicable to preconcentrate Pb(II) in seed oil (Table 4).
Table 4 Analytical results of Pb2+ determination onto UreaLiCl-mGO (n = 3)
Sample
|
Spiked
(g L-1)
|
Found
|
Recovery
(%)
|
Sesame
|
0
|
1.32
|
-
|
|
1.0 × 10-5
|
11.56
|
102.10
|
|
5.0 × 10-5
|
53.76
|
104.75
|
|
15 × 10-5
|
148.56
|
98.17
|
Hemp
|
0
|
5.37
|
-
|
|
1.0 × 10-5
|
14.96
|
97.33
|
|
5.0 × 10-5
|
52.67
|
95.12
|
|
15 × 10-5
|
152.67
|
98.26
|
Sunflower Seeds
|
0
|
2.56
|
-
|
|
1.0 × 10-5
|
11.95
|
95.14
|
|
5.0 × 10-5
|
48.56
|
95.11
|
|
15 × 10-5
|
152.76
|
100.13
|
Grapeseed Oil
|
0
|
0.7
|
-
|
|
1.0 × 10-5
|
10.56
|
98.69
|
|
5.0 × 10-5
|
50.85
|
100.29
|
|
15 × 10-5
|
152.76
|
101.36
|
3.7 Analytical figures of merit
Under optimal conditions, LDR (Fig. S2), LOD, LOQ, RSD and PF are 5.0 × 10−6 – 23 × 10−5 g L−1, 99 × 10−8 g L−1, 33 × 10−7 g L−1, 1.3% and 100, respectively. Respective data are provided in electronic supplementary materials (ESM).
UreaLiCl-mGO is compared with some new adsorbents (Table 5). UreaLiCl-mGO has significant improvements in adsorbent amount, RSD and LDR. Additionally, UreaLiCl-mGO is applicable in semi-neutral pH, which decreases the usage amount of solutions to pH adjustment. More importantly, UreaLiCl-mGO is a green, safe and earth-friendly adsorbent.
Table 5 Comparison of analytical features of the SPE method in present research and several reported method for determination of Pb2+ (FAAS was used for all detections)
Method
|
Amount
(g)
|
LOD
(g L-1)
|
RSD
(%)
|
PF
|
LDR
(g L-1)
|
Reference
|
SPE
|
12 × 10-3
|
1.71 × 10-6
|
1.81
|
110
|
8 × 10-6 -500 × 10-6
|
[26]
|
SPE
|
-
|
0.025 × 10-6
|
1.6
|
125
|
0.08 × 10-6 -16 × 10-6
|
[27]
|
MSPE
|
150 × 10-3
|
0.28 × 10-6
|
1.6
|
90
|
1 × 10-6 - 500 × 10-6
|
[28]
|
SPE
|
-
|
0.13 × 10-6
|
3.7
|
150
|
63 × 10-6 - 500 × 10-6
|
[29]
|
SPE
|
60 × 10-3
|
6.5 × 10-6
|
2.5
|
50
|
20 × 10-6 - 120 × 10-6
|
[30]
|
MSPE
|
1.0 × 10-2
|
99 × 10-8
|
1.3
|
100
|
5.0 × 10-6 -23 × 10-5
|
This work
|
3.8 Adsorption isotherm
Adsorption isotherm investigates the amounts of analyte adsorbed per unit mass of adsorbent and the nature adsorption of analyte onto adsorbent [31]. According to Fig. S3a, by increasing the initial concentration of adsorbate, the amount of Pb(II) taken by unit mass of UreaLiCl-mGO increased. Four different isotherm models including Langmuir (Fig. S3b), Freundlich (Fig. S3c), Temkin (Fig. S3d) and Dubinine-Radushkevich (Fig. S3e) were considered to investigate the adsorption model [31, 32]. Freundlich, a model of multilayer adsorption, is fitted with the system (Table S1). Respective data are given in Electronic Supplementary Materials.
3.9 Adsorption kinetic
The relation between time and adsorption of Pb(II) onto UreaLiCl-mGO is investigated through kinetic of adsorption. Two models of Pseudo-First-Order (Fig. S4a) and Pseudo-Second-Order (Fig. S4b) were studied (Table S2). The Pseudo-First-Order model is fitted with the system. The pseudo-first-order kinetics equation describes the adsorption in solid–liquid systems based on the sorption capacity of solids. It assumes that one ion is adsorbed onto one unoccupied adsorption site on the UreaLiCl-mGO [33, 34]. Respective data are given in Electronic Supplementary Materials (ESM).
3.10 Adsorption thermodynamic
The relation between temperature and adsorption of Pb(II) onto UreaLiCl-mGO is studied through thermodynamic of adsorption. The temperature has positive effect onto adsorption capacity of UreaLiCl-mGO (Fig. S5a). The positive values of ΔH and ΔS show endothermic nature of adsorption and more agitation in sample solution. Negative values of ΔG confirm spontaneous adsorption of Pb(II) onto UreaLiCl-mGO [35, 36]. The calculated data are achieved according to Fig. S5b. They are summarized in Table S3. Respective data are given in Electronic Supplementary Materials (ESM).