A novel method was developed for the simultaneous determination of Pb(II), Cd(II), and Zn(II) based on the cathodic stripping response at a bismuth film electrode associated with oxine as a chelating agent. The developed method provided a high and sharp electrochemical response compared with the method without oxine. A linear response of peak currents was observed for Pb(II), Cd(II), and Zn(II) concentration in the range from 2 ppb to 110 ppb. The detection limits of Pb(II), Cd(II), and Zn(II) were 0.45, 0.17, and 0.78 ppb, respectively. This method was successfully applied to the determination of Pb(II), Cd(II), and Zn(II) in lake-water and river-water samples. The metals were detected at the ultratrace level, showing the feasibility of the proposed method for environmental applications.
The release of different pollutants into the environment has increased significantly due to industrialisation. Among such pollutants, potentially toxic heavy metals, such as Pb(II), Cd(II), Hg(II), Ni(II), and Zn(II), are the most critical because they have a potentially damaging effect on human physiology and biological systems. Nevertheless, these metals have increasingly been used in industry in the production of anticorrosion coatings, pigments, alloys, and batteries [
Stripping voltammetry (SV) is a potential alternative for trace analyses due to numerous advantages such as faster analysis, higher selectivity and sensitivity, low cost, easy operation, and possibility to perform the analysis
In order to enhance the selectivity and sensitivity of SV, several procedures have been developed in which SV is preceded by an adsorptive collection of complexed metals with specific chelating agents onto the electrode surface. Cu(II), Cd(II), and Pb(II) are determined by means of SV combined with oxine (8-hydroxyquinoline) as a chelating agent [
In the present paper, we extended the analytical utility of the bismuth film electrode with the development of a new method for the simultaneous determination of Pb(II), Cd(II), and Zn(II) using stripping voltammetry in combination with oxine as a complexing agent. The facial procedure that involved the
All chemicals used in this study were of analytical reagent grade. Bismuth, lead, cadmium, and zinc standard stock solutions (1000 mg/L), sodium acetate (CH3COONa, 99%), acetic acid (CH3COOH, 99.8%), HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid), oxine (8-hydroxyquinoline), ammonia (NH3, 25%), sodium hydroxide (NaOH, 99%), and hydrochloric acid (HCl, 37%) were supplied from Merck company (Germany).
A three-electrode cell configuration was used for the voltammetric measurements. A glassy carbon electrode (2.8 mm diameter disk) was used as a working electrode, Ag/AgCl/KCl 3 M solution as a reference, and platinum wire as a counter electrode (CPA-HH5 Computerised Polarography Analyser, Vietnam).
Before use, the glassy carbon electrode (GCE) was polished with 0.2
There are several factors affecting the electrochemical properties of BiFE. A primary study showed that the concentration of bismuth and oxine, as well as pH of the solution, significantly affected the electrochemical signals. In the present study, a Box–Behnken design (BBD) was applied to optimise the conditions for the
Factors in BBD and their levels.
Bismuth concentration ( |
pH ( |
Oxine concentration ( | |
---|---|---|---|
Central level (0) | 600 | 6 | 581 |
High level (+1) | 1000 | 8 | 1016 |
Low level (–1) | 200 | 4 | 145 |
Based on the experimental data, a second-order polynomial model was obtained, which correlates the relationship between the responses and the studied variables. The relationship could be expressed as in equation (
The traditional optimisation approach, that varies one variable at a time, is based on the experience that does not guarantee the attainment of the true optimum of the conditions for preparing
Design matrix and responses for full factorial design.
Runs | Coded variable levels | Peak current | ||||
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Pb | Cd | Zn | |
1 | 0 | 0 | 0 | 1.92 ± 0.31a | 1.97 ± 0.44 | 1.96 ± 0.34 |
2 | 0 | +1 | +1 | 1.78 ± 0.52 | 1.99 ± 0.32 | 1.92 ± 0.31 |
3 | –1 | –1 | 0 | 0.91 ± 0.41 | 0.94 ± 0.33 | 0.93 ± 0.52 |
4 | 0 | –1 | –1 | 1.08 ± 0.25 | 1.02 ± 0.41 | 1.01 ± 0.11 |
5 | 0 | 0 | 0 | 1.95 ± 0.35 | 2.00 ± 0.15 | 2.00 ± 0.11 |
6 | –1 | 0 | –1 | 1.23 ± 0.41 | 1.26 ± 0.25 | 1.26 ± 0.21 |
7 | 0 | –1 | +1 | 1.16 ± 0.15 | 1.19 ± 0.17 | 1.18 ± 0.41 |
8 | +1 | 0 | –1 | 1.39 ± 0.25 | 1.43 ± 0.22 | 1.43 ± 0.25 |
9 | –1 | 0 | +1 | 1.43 ± 0.35 | 1.47 ± 0.38 | 1.46 ± 0.35 |
10 | 0 | +1 | –1 | 1.28 ± 0.32 | 1.32 ± 0.26 | 1.31 ± 0.32 |
11 | +1 | –1 | 0 | 1.07 ± 0.31 | 1.10 ± 0.31 | 1.10 ± 0.28 |
12 | +1 | +1 | 0 | 1.40 ± 0.42 | 1.44 ± 0.42 | 1.43 ± 0.43 |
13 | 0 | 0 | 0 | 1.98 ± 0.38 | 2.04 ± 0.44 | 2.03 ± 0.11 |
14 | –1 | +1 | 0 | 1.25 ± 0.26 | 1.29 ± 0.20 | 1.28 ± 0.18 |
15 | +1 | 0 | +1 | 1.59 ± 0.18 | 1.64 ± 0.35 | 1.63 ± 0.24 |
aPeak current is expressed as mean ± standard deviation (
The response variables and independent variables (coded) are related following the second-order polynomial equations:
The high values for the coefficient of determination (
Analysis of variance (ANOVA) for B-B design.
Terms | Coefficient |
|
Coefficient |
|
Coefficient |
|
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Constant | 1.95 | ≤0.001 | 2.00 | ≤0.001 | 2.00 | ≤0.001 |
|
0.08 | 0.003 | 0.08 | 0.061 | 0.08 | 0.042 |
|
0.19 | ≤0.001 | 0.22 | 0.001 | 0.21 | 0.001 |
|
0.12 | 0.001 | 0.16 | 0.006 | 0.15 | 0.005 |
|
–0.35 | ≤0.001 | –0.37 | 0.001 | –0.36 | ≤0.001 |
|
–0.44 | ≤0.001 | –0.44 | ≤0.001 | –0.45 | ≤0.001 |
|
–0.19 | ≤0.001 | –0.18 | 0.014 | –0.19 | 0.008 |
|
0.00 | 0.909 | 0.00 | 0.960 | 0.00 | 0.912 |
|
0.00 | 1.000 | 0.00 | 1.000 | 0.00 | 1.000 |
|
0.11 | 0.004 | 0.13 | 0.047 | 0.11 | 0.050 |
Regression |
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Lack of fit |
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The profile for predictive values in Minitab-16 was used for the optimisation process. The optimisation design matrix (Figure
Profiles for predicated values (a) and reliability function for peak current of Pb (b).
The electrochemical behavior of Pb(II), Cd(II), and Zn(II) in the HEPES buffer solution of pH 6.5 with and without oxine was studied using cyclic voltammetry (CV) in the potential range from 0 to 1.6 V. Figure
Cyclic voltammetric curves of Pb(II), Cd(II), and Zn(II) at GCE and BiFE with and without oxine (10 ppb Pb, 10 ppb Zn, and 5 ppb Cd in HEPES buffer pH 6).
Because protons participate in the electrode reaction of Pb, Cd, and Zn, the effect of pH on the voltammetric behavior of Pb, Cd, and Zn at BiFE was studied by means of SqW-AdSV in the pH range of 4.2–7.8 (under the conditions: 10 ppb Pb(II), 10 ppb Zn(II), and 5 ppb Cd(II); bismuth concentration: 640 ppb; oxine concentration: 726 ppb, accumulation potential: −1.6 V, accumulation time: 250 s, stripping and adsorptive potential: −0.7 V, stripping time: 10 s, cathodic scan rate: 0.2 Vs−1). It was found that the cathodic peak potentials (
These slopes of –0.050, –0.065, and –0.062 for Zn, Cd, and Pb, respectively, were closed to the theoretical value of –0.059/pH at 25°C expected from the Nernst equation. This indicates that the electrochemical process of each analyte took place with an equal number of protons and electrons.
Some information concerning the electrochemical mechanism can be provided from the relationship between the voltammetric signs and the scan rates (denoted as
The plots of
Based on the Laviron theory [
The plots of
The values of (1 −
Complexing reaction between metal (M) and oxine.
M(II) is Pb(II), Cd(II), and Zn(II). The reactions at BiFE in aqueous solutions were proposed as follows: The accumulation step at −1.6 V where M is Pb(II), Cd(II) and Zn(II); HOx is oxine. The stripping and adsorption step at BiFE at −0.7 V Square-wave voltammetric potential scan is applied for loading metals to BiFE. The reduction process involved two electrons and two protons as follows: Electrode surface cleaning step:
The interference of other substances on the electrochemical response of BiFE in the detection of Pb, Cd, and Zn was studied under the optimal conditions. Na2SO4, KHCO3, CaCl2, Mg(NO3)2, Cu(NO3)2, and Co(CH3COO)2 were chosen as interferents because they were commonly found with the target analytes in lake and river water. The relative error (RE) was taken as the relative deviation of the peak current measured with and without interferents. These substances did not show any interference with Pb(II), Zn(II), and Cd(II) detection (RE < 5%) (Tables
The SqW-AdSV replicate measurements at BiFE was performed with different analyte concentrations. The SqW-AdSV curves were measured in the mixtures of the same concentration of Pb, Cd, and Zn at 10 ppb, 20 ppb, and 30 ppb. Each SqW-AdSV signal was obtained by successive measurements for eight times (Figure
For the individual determination of the target analytes, the concentration of one of them varied from 2 to 110 ppb while keeping the other two constant at 20 ppb (data not shown). The cathodic peak current of each of the metal increased linearly with its concentrations ( In the case of simultaneous increase in the concentrations of the target analytes, the SqW-AdSV voltammograms obtained at BiFE with oxine in HEPES buffer solution pH 6.5 are shown in Figure
The LOD for the simultaneous measurement was 0.45, 0.17, and 0.78 ppb for Pb, Cd, and Zn, respectively. It is worth noting that this LOD for each species was nearly equal to that with the individual measurement (0.40 ppb for Pb(II), 0.13 ppb for Cd(II), 0.46 ppb for Zn(II)), indicating that the analytes did not interfere with each other in the determination. Compared with other methods, the developed one here was advantageous in terms of low detection limit and compatible linear range (Table
SqW-AdSV voltammograms using BiFE under optimal conditions in solutions containing different concentrations of Pb, Cd, and Zn (2–110 ppb each metal). Upper small figure is a linear regression line for Cd (red), Pb (blue), and Zn (black) (conditions: 0.01 M HEPES buffer solution pH 6.5; [oxine] = 726 ppb; [Bi(III)] = 640 ppb;
Comparison of LOD and linear range related to different electrodes and methods for determination of analytes.
Electrode | Method | Analyte | LOD (ppb) | Linear range (ppb) | Ref. |
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Hg-Bi/SWNTs/GCE | SqW-AdSV | Cd(II) | 0.98 | 10–130 | [ |
Pb(II) | 1.3 | ||||
Zn(II) | 2 | ||||
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BiFEs | SqW-AdSV | Cd(II) | 0.82 | 5.6–39.2 | [ |
Pb(II) | 1.64 | 4.1–41.4 | |||
Zn(II) | 0.08 | 2.6–39.2 | |||
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G/PANI/NME | SqW-AdSV | Cd(II) | 0.1 | 1–300 | [ |
Pb(II) | 0.1 | 1–300 | |||
Zn(II) | 1.0 | 1–300 | |||
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Ƴ-AlOOH@SiO2/Fe3O4/GCE | SqW-AdSV | Zn(II) | 2.6 | 2616–36624 | [ |
Cd(II) | 1.12 | 1120–15680 | |||
Pb(II) | 0.4 | 414–99456 | |||
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Bi/GCE | SqW-AdSV | Cd(II) | 0.49 | 0.05–100 | [ |
Pb(II) | 0.41 | 0.05–100 | |||
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BiFE | SqW-AdSV | Zn(II) | 0.78 | 2–110 | This work |
Cd(II) | 0.17 | 2–110 | |||
Pb(II) | 0.45 | 2–110 |
SWNT: single-walled carbon nanotube; G/PANI/NME: graphene-based polyaniline nanocomposite-modified electrode.
Reproducibility of the electrochemical response is of special interest for automatic monitoring of potentially toxic heavy metals. Hence, the response of BiFE was performed for a ten-day period by immersing the electrode in a solution of spiked water with 10 ppb Pb, 10 ppb Cd and 10 ppb Zn (10 measurements were performed during the working-day period). The electrode was stored in the spiked solution between each analysis at a potential of −0.15 V. The changes of average
The water from a lake and two rivers in Quang Binh province, Vietnam, namely, Cau Rao River, Nam Ly Lake, and Kien Giang River, was used to determine the concentration of lead, cadmium, and zinc using the proposed method (SqW-AdSV) and GF-AAS for the sake of comparison. The concentrations of the target metal are listed in Table
Pb, Cd, and Zn concentrations in real samples analysed by SqW-AdSV and GF-AAS.
Notation | SqW-AdSV | GF-AAS | ||||
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Pb (ppb) | Cd (ppb) | Zn (ppb) | Pb (ppb) | Cd (ppb) | Zn (ppb) | |
Cau Rao River | 15.0 ± 8.2a | 4.3 ± 0.7 | 13.6 ± 4.0 | 16.4 ± 4.7 | 3.9 ± 0.6 | 12.8 ± 3.6 |
Nam Ly Lake | 26.9 ± 5.1 | 5.1 ± 2.4 | 20.3 ± 3.8 | 27.1 ± 2.1 | 4.6 ± 0.3 | 18.3 ± 2.0 |
Kien Giang River | 21.7 ± 4.4 | 3.1 ± 1.5 | 7.3 ± 2.6 | 20.0 ± 5.3 | 3.5 ± 0.7 | 8.9 ± 1.7 |
aData are expressed as mean ± SD (standard deviation) (
SqW-AdSV voltammograms obtained for the water samples: (a) Cau Rao River, (b) Nam Ly Lake, and (c) Kien Giang River (the same conditions as in Figure
Using Box–Behnken design allowed optimising the preparation of BiFE with oxine as a chelating agent. The resulting electrode was successfully used for the simultaneous determination of Pb(II), Cd(II), and Zn(II) with a low detection limit, wide linear range, and good selectivity and without the interference of each other. A satisfied recovery and reproducibility of the proposed method were also obtained. This method was successfully applied to the determination of Pb(II), Cd(II), and Zn(II) in real water samples from lakes and rivers.
The data used to support the findings of this study are included within the article and within the supplementary information file.
The authors declare that they have no conflicts of interest.
Figure S1: effects of operational parameters. Figure S2: successive measurements of SqW-AdSV curves for eight times. Table S1: effects of Na2SO4 on the stripping peak current. Table S2: effects of KHCO3 on the stripping peak current. Table S3: effects of CaCl2 on the stripping peak current. Table S4: effects of Mg(NO3)2 on the stripping peak current. Table S5: effects of Co(CH3COO)2 on the stripping peak current. Table S6: effects of Cu(NO3)2 on the stripping peak current.