AN0137353056;[ehoo]03jul.19;2019Jul08.10:52;v2.2.500
Voltammetry Determination of Pb(II), Cd(II), and Zn(II) at Bismuth Film Electrode Combined with 8-Hydroxyquinoline as a Complexing Agent
1. IntroductionA 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 [1]. Lead and cadmium are among the most toxic and hazardous. Lead is relatively harmful to human and living things [2]. Drinking water containing a high level of lead ions would cause serious disorders, such as nausea, convulsions, coma, renal failure, cancer, and subtle effects on the metabolism and intelligence [3]. Cadmium is accumulated in the human body, causing erythrocyte destruction, nausea, salivation, diarrhea, muscular cramps, renal degradation, and chronic pulmonary problems [4]. Zinc is considered as an essential trace element for human beings due to its relationship with the insulin production and because it plays the role of a catalyst for more than 200 enzymes [5]. An excessive consumption of Zn(II) (50 mg/day) can inhibit the absorption of copper (II) acquired from the human diet [6]. Therefore, the measurement of trace metal ions is very important for the environmental protection, food and agricultural chemistry, and high purity materials, and also for monitoring environmental pollution. Several sensitive techniques have been developed for the measurement of these metal ions. Flame atomic absorption spectrometry (FAAS) has widely been used for the determination of trace metal ions (Pb, Cd, and Cu) [7]. Biller and Bruland report the analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater using the Nobias-chelate PA1 resin and inductively coupled plasma mass spectrometry (ICP-MS) [8]. X-ray fluorescence analysis is employed to analyse Cu, Pb, As, Cd, Zn, Fe, Ni, and Mn [9]. These methods have excellent sensitivities and good selectivity; nevertheless several disadvantages such as long working time and high cost of instrument limit their applications.
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 situ [10, 11]. Mercury-based electrodes, such as mercury film electrodes and hanging mercury drop electrodes have traditionally been used in the stripping techniques due to their high sensitivity, reproducibility, purity of the surface, and possibility of amalgam formation. Hence, they have widely been accepted as the most sensitive electrodes for the determination of heavy metals [12]. However, the use of these techniques tends to decrease because the use of mercury affects the environment. Recently, bismuth film electrodes (BiFEs) have been developed as a successful alternative for "toxic" mercury electrodes and are, by now, widely recognised in a number of electroanalytical laboratories worldwide [13]. BiFEs have already been employed for the simultaneous or individual determination of many metal ions, such as Ni [14], Cr [15], Pb and Cd [16], and some organic compounds such as thiamethoxam [17], parathion [18], and others.
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 [19]. A number of trace metals in seawater are determined using cathodic stripping voltammetry with mixed 1igands (dimethylglyoxine and oxine) [20]. To the best of our knowledge, few papers have reported the simultaneous detection of Pb(II), Cd(II), and Zn(II) using stripping voltammetry at BiFE associated with the oxine ligand.
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 in situ deposition of the bismuth film onto the glassy carbon electrode with a concurrent oxine-assisted accumulation of the analytes, followed by an cathodic stripping scan was demonstrated.
2. Experimental
2.1. Materials
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).
2.2. Apparatus
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).
2.3. Preparation of BiFE and Measurement Procedure
Before use, the glassy carbon electrode (GCE) was polished with 0.2 μm alumina powder on a polishing pad. The electrode was then rinsed thoroughly with ethanol and dried naturally and ready to use. Prepare a solution of 0.1 M HEPES buffer solution containing Bi(III), Pb(II), Cd(II), Zn(II), and oxine, then a potentiostat switched on at an accumulation potential (Eacc.p = –1.6 V) (Figure S1(a)) for 240 s (Figure S1(b)) on the rotating glassy carbon disk electrode. During this step, Bi(III) was reduced to Bi to form an in situ Bi film on the GCE. The other metal ions (Zn, Pb, and Cd) were also reduced to corresponding metals depositing on GCE. Following the accumulation step, GCE stopped rotating, and the potentiostat switched on at the stripping and adsorption potential (Estr.p = –0.7 V) (Figure S1(c)) for 10 s (Figure S1(d)). The analyte metals were oxidised to corresponding metal ions. They immediately combined with the oxine ligand to form complexes on the electrode surface. Then, the voltammograms were recorded by applying a negative-going square-wave voltammetric potential scan prior to each measurement, and three "cleaning" scans at 0.3 V were carried out without the deposition step to completely remove the metal residues. The method is called square-wave adsorptive stripping voltammetry (SqW-AdSV).
2.4. Optimisation of Conditions for Preparing BiFE in Combination with Oxine
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 in situ preparation of BiFE with the peak current in the determination of the metals as the response. The three variables (factors) with their experimental levels are shown in Table 1.
Factors in BBD and their levels.
| Bismuth concentration (X1, ppb) | pH (X2) | Oxine concentration (X3, ppb) | |
|---|---|---|---|
| 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 (1).(1)y=b0+b1X1+b2X2+b3X3+b11X12+b22X22+b33X32+b12X1X2+b13X1X3+b23X2X3,where y is the predicted response value (peak current, Ip, of Pb, Cd, and Zn); X1, X2, and X3 are the real independent variables (Table 1); b0 is the intercept term; b1, b2, and b3 are the linear coefficients; b12, b13, and b23 are the cross-product coefficients; and b11, b22, and b33 are the quadratic-term coefficients. All the coefficients of the equation from the experimental design were subjected to multiple nonlinear regression analyses by using the Minitab-16 software.
3. Results and Discussion
3.1. Preparing BiFE with Oxine as a Chelating Agent by BBD Approach
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 in situ BiFE using oxine as a chelating agent. Conversely, the approach that relies on a rational experimental design, allowing the simultaneous variation of all the experimental factors, saves time and resources. Therefore, the experiments based on BBD were run in a random manner to minimise the effect of uncontrolled variables. Table 2 shows the experimental design matrix and responses (cathodic peak current, Ipc) derived from each experiment.
Design matrix and responses for full factorial design.
| Runs | Coded variable levels | Peak current Ipc (μA) | ||||
|---|---|---|---|---|---|---|
| x1 | x2 | x3 | 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 |
The response variables and independent variables (coded) are related following the second-order polynomial equations:(2)Ipc,Pb=1.95+0.08x1+0.18x2+0.12x3−0.35x12−0.44x22−0.19x32−0.00x1x2+0.00x1x3+0.11x2x3,(3)Ipc,Cd=2.00+0.08x1+0.22x2+0.16x3−0.37x12−0.44x22−0.18x32−0.00x1x2−0.00x1x3+0.13x2x3,(4)Ipc,Zn=2.00+0.08x1+0.21x2+0.15x3−0.36x12−0.45x22−0.19x32−0.00x1x2+0.00x1x3+0.11x2x3.
The high values for the coefficient of determination (R2) were found to be 0.994, 0.976, and 0.982 for Pb(II), Cd(II), and Zn(II), respectively, suggesting an excellent agreement between the experimental and estimated values. The positive and negative signs in each equation implied the positive and negative effect of the variables, respectively. The value of b0 reflects the average value of the peak current at the centre points (b0 = 1.95 μA, 2.00 μA, and 2.00 μA for Pb, Cd, and Zn, respectively). The values of the coefficients indicated the amplitude of the effect. For clarity, the values and signs of the effect, as well as their p values of the three peak currents, are presented in Table 3. Interestingly, the influence pattern of the effect of all the three variables was very similar in terms of the values and the signs. All the linear and quadratic effects were statistically significant (p