Journal of Chemistry
Volume 2019 (2019), Article ID 6170967, 7 pages
https://doi.org/10.1155/2019/6170967

Research Article

Using Electrode Made of Carbon Nanotubes and Bismuth Oxide for the Determination of Metal Concentration by Anodic Stripping Voltammetry

Thi Thu Phuong Nguyen,¹ Xuan Gian Trinh,² and Dao Thi To Uyen3,4

¹Faculty of Chemical Technology, Hanoi University of Industry, 298 Cau Dien, Bac Tu Liem, Hanoi, Vietnam
²Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
³Center of Excellence for Green Energy and Environmental Nanomaterials (CEGrEEN), Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City 755414, Vietnam
⁴NTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City 700000, Vietnam

Correspondence should be addressed to Thi Thu Phuong Nguyen; thuphuongdhcn@yahoo.com

Received 13 March 2019; Revised 30 May 2019; Accepted 27 June 2019; Published 2 September 2019

Guest Editor: Nguayen Van Noi

Copyright © 2019 Thi Thu Phuong Nguyen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We have successfully manufactured a new electrode modified with bismuth oxide (Bi₂O₃) using carbon nanotubes (CNTs). The electrode was fabricated to detect cadmium (Cd), lead (Pb), and indium (In) by differential pulse anodic stripping voltammetry (DP-ASV). The electrode surface was studied by scanning electron microscopy (SEM), and the reduction and oxidation processes were studied by cyclic voltammetry (CV) techniques. Operational parameters such as electrode size, bismuth concentration, and electrolytic background were optimized. The DP-ASV method used fabricated electrodes with a linear response range from 1.5–20 μg·L⁻¹ with Cd(II) and Pb(II) and 2.5–20 μg·L⁻¹ with In(III); low detection limit (LOD) of 0.22 μg·L⁻¹ with Cd(II), 0.65 μg·L⁻¹ with In(III), and 0.26 μg·L⁻¹ with Pb(II); and good repeatability with relative standard deviations (RSD) of 2.65%, 2.51%, and 3.34% with Cd(II), Pb(II), and In(III), respectively (n = 8). The electrode can be used to test the content of Cd(II), In(III), and Pb(II) in water.

1. Introduction

The Cd and Pb determination is very important because of their toxic effects to environment and humans [1]. Several compounds of In can cause cancer and are toxic [2]. Some analytical methods such as atomic absorption spectrometry (AAS) [3–8], inductively coupled plasma-mass spectrometry (ICP-MS) [9–11], and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [12–16] have been applied to analyse Cd(II), In(III), and Pb(II). These techniques have high selectivity and sensitivity, but they are very expensive and time-consuming. The electroanalytical method not only has high accuracy and sensitivity but also is low cost, has good repeatability, and allows in situ measurement [17]. Carbon paste electrodes (CPEs) have been widely applied in analytical chemistry in some recent years [18–21]. Nowadays, CNTs are also used in CPEs because they have high mechanical strength, electrical conductivity, and surface area [22–25]. The electrochemical analysis methods using various working electrodes (WEs) have been widely used, in which mercury electrodes have been most commonly used. However, mercury is very toxic. Bismuth is less toxic than mercury, and it has some similar electrochemical characteristics of mercury, so it has been used a lot to replace mercury in electrochemical analysis[26–35]. In this study, we manufactured a new modified CNT electrode with Bi₂O₃ as replacement for mercury electrodes, and this electrode is used as a WE in electrochemical analysis equipment for the simultaneous analysis of Cd(II), In(III), and Pb(II). The method was tested successfully on water samples. In addition, the results were compared with those of graphite furnace atomic absorption spectroscopy (GF-AAS). The accuracy of the method is evaluated by using the sediment certified reference material (CRM) and graphite furnace atomic absorption spectrometry (GF-AAS).

2. Experimental

2.1. Chemicals and Reagents

CH₃COONa, KI, KCl, NaNO₃, paraffin oil, CH₃COOH (100%, m/v), and HNO₃ (65%, m/v) were purchased from Merck (Germany). Multiwalled carbon nanotubes (MWCNTs 95%, diameter × length: 10–35 nm × 1–10 μm). Bismuth oxide (Bi₂O₃, grain size < 10 μm) was provided by Sigma-Aldrich. Working standards for Pb, Cd, and In were prepared using standard solutions supplied by Merck (standard solutions of 1000 ppm for Cd, Pb, and In).

2.2. Apparatus

The Metrohm 797 VA Computrace (Switzerland) with the working electrode (WE) was the CPE modified with Bi₂O₃ using CNTs. For AAS measurements, a PerkinElmer 3300, USA, was used. For SEM measurements, a field emission SEM S-4800, Hitachi, Japan, was used.

2.3. Fabrication of Electrodes

CNTs (heated at 700°C for 15 min) and parafﬁn oil (6 : 4, w/w) were mixed with an agate mortar and pestle, and then they were transferred into test tubes with the help of ultrasonic agitation for 2 h. By continuous mixing of carbon paste with Bi₂O₃ by mixing similar to the abovementioned method, the modified carbon nanotube paste was obtained. This modified carbon nanotube stuff was packed into a Teflon tube. Electrode surface was cleaned with a filter paper.

2.4. Procedures

The analytical solution was added into the electrolyte beaker. The measurement conditions were as follows: the deposition potential (Edep) −1.2 V, deposition time (tdep) 120 s, speed 15 mV per second, and pulse amplitude (ΔE) 50 mV·s⁻¹. After a rest time, 20 s, the DP-ASV was saved. The DP-ASV of blank solution was saved with the similar measurement conditions. Before the test, oxygen was removed from the analytical solution by exposing to pure nitrogen gas for 5 minutes.

3. Results and Discussions

3.1. Influence of Electrode Diameters

In stripping voltammetry analysis, all preconcentration and stripping processes happen on the surface of the WE, so the electrode diameter has a great influence on the limit diffusion line. So, we study the influence of different electrode diameters (diameters are 1.8, 2.2, 2.5, 3, 3.5, and 4.0 mm) on the peak (Ip) of metal ions. The results are shown in Figure 1. At 3 mm electrode diameter, Ip of all 3 ions are high, the peak is balanced, and the repeatability is good. Therefore, we chose 3 mm as the optimum electrode diameter.

Figure 1: Effect of the electrode diameters at a modified CPE with 5% Bi₂O₃ (w/w) in the supporting electrolyte mixture of 0.1 M KI and acetate buffer. Conditions: Edep = −1.2 V; tdep = 120 s; ΔE = 50 mV·s⁻¹; speed = 15 mV·s⁻¹.

3.2. Influence of Bi₂O₃

The simultaneous analysis of Cd(II), Pb(II), and In(III) by DP-ASV using a CPE modified with 1%, 3%, 5%, and 8% (w/w) Bi₂O₃ was researched. The CPE modified with 5% (w/w) Bi₂O₃ produced the highest Cd, In, and Pb peaks; resolution is high, and repeatability is good. Therefore, the CPE modified with 5% (w/w) Bi₂O₃ was used for the further studies.

3.3. Influence of Edep and tdep

Influence of Edep on the Ip of Cd(II), Pb(II), and In(III) has been investigated with Edep from −0.9 to −1.4 V (Figure 2). The effect of tdep in the range from 30 s to 300 s was also researched (Figure 3). The best stripping signal was obtained at Edep −1.2 V and tdep 120s. From these results, Edep and tdep of −1.2 V and 120 s were selected for the further studies.

Figure 2: DP-ASV of Cd, In, and Pb at different Edep values. Conditions: supporting electrolyte mixture of 0.1 M KI and acetate buffer; tdep = 120 s; ΔE = 50 mV·s⁻¹; speed = 15 mV·s⁻¹.

Figure 3: DP-ASV of Cd, In, and Pb at different tdep. Conditions: supporting electrolyte mixture of 0.1 M KI and acetate buffer; Edep = −1.2 V; ΔE = 50 mV·s⁻¹; speed = 15 mV·s⁻¹.

3.4. Cyclic Voltammetry (CV)

The reduction and oxidation processes were studied by potential scanning from −1.2 to 0.3 V and continuous scanning from 0.3 V to −1.2 V at a speed of 15 mV per second. The results are shown in Figure 4. In Figure 4, the anodic scan resulted in a peak potential of Bi which was −0.12 V that reflects the oxidation of the metallic bismuth.

Figure 4: Cyclic voltammograms at a modified CPE with 5% Bi₂O₃ (w/w) in the supporting electrolyte mixture of 0.1 M KI and acetate buffer. Conditions: Edep = −1.2 V; tdep = 120 s; ΔE = 50 mV·s⁻¹; speed = 15 mV·s⁻¹.

3.5. Morphological Surface Characterization

Figure 5 shows the scanning electron micrograph (SEM) surface image of the CPE containing 5% Bi₂O₃ (w/w) before and after reduction at Edep −1.2 V with tdep of 120 s. After the electrochemical reduction, the electrode surface was changed because Bi₂O₃ was converted to Bi at −1.2 V for 120 s following the reaction [36]

(a)

(b)

Figure 5: Scanning electron micrographs of the CPE modified with 5% Bi₂O₃ (w/w) (a) before and (b) after electrochemical reduction at −1.2 V for 120 s in the supporting electrolyte mixture of 0.1 M KI and acetate buffer solution. Other conditions are as in Figure 1.

According to the SEM image, the electrode surface has got a porous structure, and the surface is relatively uniform. Therefore, it is supposed to be beneficial for the stripping analysis.

3.6. Influence of Supporting Electrolytes

The influence of the background electrolytes, acetate buffer, mixture of NaNO₃ and acetate buffer, KCl and acetate buffer, and KI and acetate buffer, on the Ip of metals were studied. The results are shown in Figure 6. According to [37], simultaneous measurement of Cd(II), Pb(II), and In(III) is possible if there is a difference of peak potential of at least 100 mV. In Figure 6(a), the resolution between Cd and In signals in acetate buffer as well as the mixture of NaNO₃ and acetate buffer and KCl and acetate buffer solution is not good. Amongst these mentioned electrolytes, the supporting electrolyte mixture of 0.1 M KI and acetate buffer is the best choice with the best resolution, and largest peak current is for Cd(II), Pb(II), and In(III) (Figure 6(b)). Therefore, a medium containing 0.1 M KI and acetate buffer was selected.

(a)

(b)

Figure 6: DP-ASV of Cd, Pb, and In supporting electrolytes: (a) acetate buffer (pH = 4.5); (b) mixture of 0.1 M KI and acetate buffer (pH 4.5). Conditions: [Cd(II)] = 5 μg·L⁻¹; [Pb(II)] = 5 μg·L⁻¹; [In(III)] = 10 μg·L⁻¹. Other conditions are as in Figure 1.

3.7. Linear Response Range, LOD, and Reproducibility

The DP-ASV at different concentrations was recorded. The results of linear range are shown in Figure 7(a), and the calibration curves are shown in Figures 7(b)–7(d). The linear response ranges were 1.5–20 μg·L⁻¹ with Cd(II) and Pb(II) and 2.5–20 μg ·L⁻¹ with In(III).

(a)

(b)

(c)

(d)

Figure 7: (a) DP-ASV of Cd(II), Pb(II), and In(III) and the calibration curve of (b) Cd(II), (c) In(III), and (d) Pb(II). Conditions: [Pb(II)]; [Cd(II)] = 1.5, 6, 10, 15, and 20 μg·L⁻¹; [In(III)] = 2.5, 7, 11.5, 15, and 20 μg·L⁻¹; supporting electrolyte mixture of 0.1 M KI and acetate buffer. Other conditions are as in Figure 1.

The CPE modified with Bi₂O₃ using MWCNTs also demonstrated LOD (S/N = 3) of 0.22 μg·L⁻¹ with Cd(II), 0.26 μg·L⁻¹ with Pb(II), and 0.65 μg·L⁻¹ with In(III) (at tdep = 120 s). The method has a fine reproducibility with RSD of 2.65%, 3.34%, and 2.51%, respectively (n = 8) (with the concentration of Cd(II), Pb(II)), and In(III) to be 10 μg·L⁻¹).

Various Bi precursor-modified and Bi₂O₃-modified carbon electrodes for analysis of Cd(II), Pb(II), In(III) are shown in Table 1.

Table 1: Various Bi precursor-modified and Bi₂O₃-modified carbon electrodes for analysis of Cd(II), Pb (II), and In(III).

Electrode material
Voltammetric
Studied ions
LOD
Linear range
Reference

Bi₂O₃/SPE
SW-ASV
Pb
5 μg·L⁻¹ (at pH = 1.2)
5–150 μg·L⁻¹ (at pH = 1.2)
[28]

10 μg·L⁻¹ (at pH = 4.5)
10–150 μg·L⁻¹ (at pH = 4.5)

Cd
2.5 μg·L⁻¹ (at pH = 1.2)
5–150 μg·L⁻¹ (at pH = 1.2)

5 μg·L⁻¹ (at pH = 4.5)
10–150 μg·L⁻¹ (at pH = 4.5)

Bi₂O₃/SPE
SW-ASV
Pb
2 μg·L⁻¹
50–300 μg·L⁻¹
[29]

Bi₂O₃/SPE
DP-ASV
Pb
1.0 μg·L⁻¹

[30]

Cd
2.0 μg·L⁻¹

Bi₂O₃/SPE
SW-ASV
Pb
2.3 μg·L⁻¹
20–100 μg·L⁻¹
[31]

Cd
1.5 μg·L⁻¹
20–100 μg·L⁻¹

Bi₂O₃/SPE
SW-ASV
Pb
1.1 μg·L⁻¹
0–80 μg·L⁻¹
[32]

Cd
2.1 μg·L⁻¹
0–160 μg·L⁻¹

Bi₂O₃/SPE
SW-ASV
Pb
5 μg·L⁻¹

[33]

Cd
5 μg·L⁻¹

BONPs-IL/CPE
SW-ASV
Pb
0.21 μg·L⁻¹
3–30 μg·L⁻¹
[34]

Cd
0.15 μg·L⁻¹

BiOCl/MWCNT-GCE
SW-ASV
Pb
1.9 μg·L⁻¹

[35]

Cd
4 μg·L⁻¹

Bi₂O₃/MWCNT-CPE
DP-ASV
Pb
0.26 μg·L⁻¹
1.5–20 μg·L⁻¹
This study

Cd
0.22 μg·L⁻¹
1.5–20 μg·L⁻¹

In
0.65 μg·L⁻¹
2.5–20 μg·L⁻¹

SPE: screen printed electrode; GCE: glassy carbon electrode; SW-ASV: square wave anodic stripping voltammetry; BONP: bismuth oxide nanoparticle; IL: ionic liquid; BiOCl: bismuth-oxychloride.

3.8. Influence of Ions

The influence of several ions K⁺, Na⁺, Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺, and Cu²⁺ at concentration range from 0.1 to 100 mg·L⁻¹ on the peak of Cd(II), Pb(II), and In(III) was examined. No signal changes were observed on the Ip of Cd(II), Pb(II), and In(III) (changes in peak currents < 10%). The influence of cetyltrimethylammonium bromide and Triton X-100 surfactants in range from 0.1 to 10 mg·L⁻¹ concentrations was also studied. A decrease in the Ip value was found for Cd(II), Pb(II), and In(III) determination on increasing the amount of the surfactant. Nevertheless, this surfactant interference was reduced by the UV irradiation with a UV lamp at 254 nm wavelength in 90 min.

3.9. Determination of CRM

The accuracy of the method was studied by analysis of sediment CRM (MESS-2). The results are shown in Table 2. The results of these five trials were not significantly different from the certified values following Student’s t test (texp = 2.52 greater than t (0.05, 4)).

Table 2: Cd(II), Pb(II), and In(III) determination in CRM by the DP-ASV method (using the CPE modified with Bi₂O₃) and μ value of CRM.

Ion
(μg/g)
μ (μg/g)

Pb(II)
20.5
21.9

In(III)
<LOD
<LOD

Cd(II)
<LOD
0.24

Note. is the average value of the DP-ASV method (5 measurements).

3.10. Determination of River Water Samples

Water pollution in some rivers of Hanoi, Vietnam, has been a serious problem, with high concentration of metal ions and other ions [38, 39]. So, we took some river samples in Hanoi to analyse Cd(II), In(III), and Pb(II). The samples were acidified by 10% HNO₃, and then the solution was filtered using the 0.45 μm membrane filter. The sample was treated with UV light for 90 minutes at wavelength 254 nm.

After filtering, 10.0 mL of the solution was taken, and the mixture of 0.1 M KI and acetate buffer was added. Content analysis of Cd, In, and Pb in 3 different river samples by DP-ASV was performed using the manufactured CPE. The content of Pb(II) in the samples ranged from 1.45 ± 0.01 μg·L⁻¹ to 1.93 ± 0.20 μg·L⁻¹. The content of Cd(II) and In(III) in samples was smaller than the limit of quantitative. The results of Pb(II) in this sample using the DP-AAS method with CPE modified with Bi₂O₃ using MWCNTs were compared with GF-AAS and are shown in Table 3. The results of these five measurements were not significantly different from the certified values.

Table 3: Pb(II) content in water samples identified by the DP-ASV method (using the CPE modified with Bi₂O₃ using CNTs) and the GF-AAS method.

Samples
GF-AAS
DP-ASV
Recovery (%)
texp

1
1.86
1.93
96.37
2.52

2
1.62
1.76
108.64
2.35

3
1.47
1.45
98.64
1.68

Note. All concentrations are expressed in μg·L⁻¹; the result of the DP-ASV method is the average value of 5 measurements.

4. Conclusions

The CPE modified with 5% (w/w) Bi₂O₃, 3 mm diameter, using CNTs was applied for simultaneous analysis of the concentration of Cd(II), Pb(II), and In(III) by the DP-ASV method. The CPE modified with 5% (w/w) Bi₂O₃ are not only easy to make and easy to use, but also environmentally friendly. The method has good accuracy, low detection limit, and good repeatability.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conﬂicts of interest regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the Hanoi University of Industry and Chemical Analysis Laboratory, Institute of Chemistry, Vietnam Academy of Science and Technology, for providing support to this work.

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