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Electroanalytical method for determination of lead(II) in orange and apple using kaolin modified platinum electrode
Chemosphere 76 (2009) 1130–1134
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Electroanalytical method for determination of lead(II) in orange and apple using kaolin modified platinum electrode M.A. El Mhammedi a,b,*, M. Achak c, M. Bakasse d, A. Chtaini b a
Université Hassan 1er, Faculté Polydisciplinaire, Kouribga, Morocco Equipe d’Electrochimie et des Matériaux Inorganiques, Université Sultan Moulay Slimane, Faculté des Sciences et Techniques, BP 523, Beni-Mellal, Morocco c Laboratoire d’Hydrobiologie et d ’Algologie, Faculté des Sciences Semlalia, Université Cadi Ayyad, Marrakech, Morocco d Equipe d’Analyse des Micro-Polluants Organiques, Faculté des Sciences, Université Chouaib Doukkali, BP 20, El Jadida, Morocco b
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 27 March 2009 Accepted 6 April 2009 Available online 19 May 2009 Keywords: Platinum electrode Kaolin Lead Square wave voltammetry
a b s t r a c t This paper reports on the use of platinum electrode modified with kaolin (K/Pt) and square wave voltammetry for analytical detection of trace lead(II) in pure water, orange and apple samples. The electroanalytical procedure for determination of the Pb(II) comprises two steps: the chemical accumulation of the analyte under open-circuit conditions followed by the electrochemical detection of the preconcentrated species using square wave voltammetry. The analytical performances of the extraction method has been explored by studying the incubating time, and effect of interferences due to other ions. During the preconcentration step, Pb(II) was accumulated on the surface of the kaolin. The observed detection and quantification limits in pure water were 3.6 109 mol L1 and 1.2 108 mol L1, respectively. The precision of the method was also determined; the results was 2.35% (n = 5). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Today, many heavy metals (such as lead, arsenic, cadmium or mercury) constitute a global environmental hazard (Mejare and Bulow, 2001). Major concerns regarding the toxic effect of heavy metal have led to increasing needs to monitor trace metals in a variety of matrices. Traditionally, such trace-metal measurements have been carried out in the central laboratory, in connection with time-consuming sampling, transportation and storage steps, and bulky atomic spectroscopy instrumentation. In this context, chemically modified electrodes (CMEs) appear as suitable alternative or complementary tools. Electrochemical analysis has always been recognized as a powerful tool for measuring trace metals (Marino et al., 2003; Gonzalez et al., 2002; Dragoe et al., 2006). Its remarkably high sensitivity is attributed to the accumulation step, during which the target metals are accumulated onto the working electrode. Various materials have been used as working electrodes for detection of heavy metals (Prior et al., 2006; El Mhammedi et al., 2009). The expensive electrodes, such as the static mercury drop electrode (SMDE) or a mercury-coated rotating disk electrode have been accepted as the widely used electrode material
* Corresponding author. Address: Université Hassan 1er, Faculté Polydisciplinaire, Kouribga, Morocco. Tel.: +212 68858296; fax: +212 23485201. E-mail address: [email protected] (M.A. El Mhammedi). 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.04.017
choice because of high reproducibility and sensitivity (Economou and Fielden, 2003). However, the toxicity of mercury and its compounds makes its less and less popular. Intensive research efforts have thus been devoted to the development of alternative electrodes, with a performance approaching that of mercury-based ones (Zejli et al., 2006; Ghiaci et al., 2007). Bismuth film electrodes (BFEs) have become an attractive topic as a potential replacement for mercury electrodes (Baldo and Daniele, 2004). Bismuth is an environmentally friendly element, with very low toxicity, and a widespread pharmaceutical use. However, the determination of copper using bismuth film electrodes has been relatively ignored due to the similar stripping potentials of copper and bismuth with only a few reports in the open literature (Prior et al., 2006). Kaolin, efficient natural adsorbents (Seki and Yordakoç, 2005; Tsai and Lai, 2006), are the main components of soils and possess a negative charge that is compensated for by exchange cations (Akçay and Yurdakoç, 2000). Consequently, it is important to study the determination of lead(II) with kaolin minerals modified platinum electrode (K/Pt) based on the accumulation process. This work was aimed at evaluating the potential analytical applications of kaolin (K) modified platinum electrodes (K/Pt) for the determination of Pb(II) within the nanomolar concentration range using square wave voltammetry. In order to assess the practical utility of the method for real matrices analysis, preliminary data concerning the effect of other metals as possible interferent, are also reported.
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All reagents used were analytical reagent-grade and double-distilled water was used throughout. Commercial kaolin was supplied by ECESA (Lugo, Spain). Pb(NO3)2 was dissolved into Bi-distilled deionized water (BDW) to form 103 mol L1 stock solutions. Working standards for calibration were prepared by diluting the primary stock solution with (BDW). Graphite powder was supplied from (Carbone, Lorraine, ref 9900, France). All electrochemical experiments were performed in 0.2 mol L1 KNO3 as supporting electrolyte. pH was adjusted by 0.1 H2SO4 or NaOH. All chemicals were of analytical grade and were used without further purification.
Electrochemical impedance spectroscopy (EIS) measurements were carried out with a same electrochemical system described above before and after immersion in solution containing lead(II) ions. Impedance spectra were obtained in the frequency range 100 kHz to 10 MHz with 10 points per decade at 0 V after 30 min of immersion in non-de-aerated solutions. A sine wave with 10 mV amplitude was used to perturb the system. The impedance diagrams are given in the Nyquist representation. After accumulation step, the electrodeposit layer was analyzed at carbon paste electrode (CPE) by cyclic voltammetry. The carbon paste was prepared by hand mixing of high purity graphite powder with kaolin in weight ratio 1:1. The method of standard additions was used for analysis of real samples, by spiking with appropriate amounts of standard lead(II) solution. All measurements were taken at room temperature.
2.2. Electrode preparation
3. Results and discussion
Prior to all measurements, the platinum plate was polished with emery paper up to 1200 grade, washed thoroughly with double-distilled water, degreased with AR grade ethanol, acetone and dried at room temperature. The equipment for electrodeposition of kaolin consisted of a glass cell containing the kaolin suspension, a platinum substrate electrode and a platinum counter electrode. The separation distance between electrodes was 2 cm. Electrodepoistion process was carried out at a constant voltage of 4.0 V for 10 h. After the electrodeposition process, the coated platinum substrate was removed from the suspension and was dried at room temperature. Scanning electron microscopy (SEM) was used to observe the morphology of samples.
3.1. Adsorptive and voltammetric characteristics of the lead(II)modifier complex
2. Experimental 2.1. Reagent
2.3. Apparatus Electrochemical measurements were performed with a voltalab potentiostat (model PGSTAT 100, Eco Chemie B.V., Utrecht, The Netherlands) driven by the general purpose electrochemical systems data processing software (voltalab master 4 software) connected to Pentium III computer run under windows 98. A conventional three electrodes system comprising a kaolin modified platinum electrode, a platinum wire counter electrode and an Ag/ AgCl (in saturated with KCl) reference electrode was used in all experiments. All potentials reported were referred to the Ag/AgCl electrode. Solutions were deoxygenated with high purity nitrogen for five minutes prior to each experiment and it was performed under a nitrogen atmosphere. The pH-meter (Radiometer Copenhagen, PHM210, Tacussel, French) was used for adjusting pH values. The coated specimen was examined using a scanning electron microscope (SEM, Jeol JSM-5500).
Fig. 1 shows the scanning electron micrograph of electrodeposited film prepared from kaolin particles onto platinum surface. The layer obtained appeared dense and uniform. The porosity of coating, especially the porosity connected with the substrate is very important factor for fixing lead onto kaolin/Pt surfaces. It is obvious that this substrate can react with metals and provide a fixed substrate for metals for the electrochemical redox reactions. Therefore the modified platinum electrode has been prepared and applied for electrocatalytic determination of Pb(II). Construction of kaolin modified platinum electrode made an opportunity for determination of Pb(II). Cyclic voltammogram in 0.2 mol L1 KNO3 (at the pH 5.6) (Fig. 2) shows cathodic and anodic peaks, which makes it possible to determine this cation. The cyclic voltammograms were obtained for the unmodified and the modified platinum electrode in the presence of lead(II) and without lead(II). There were no redox peaks in the CV of the modified platinum electrode without Pb(II) (Fig. 2a). The modified platinum electrode interacting with of lead(II) showed an anodic peak at 0.5 V versus calomel reference electrode (Fig. 2b). Although the cyclic voltammetry technique is limited for determination of lead(II) at unmodified platinum electrode, but in the presence of kaolin modifier the peak current was rapidly increased and therefore the detection limit decreased.
2.4. Procedure The platinum electrode modified with kaolin was immersed in a cell containing 20 mL of the lead sample solution to get the chemical deposition. Meanwhile the solution was stirred by a 1.2 cm magnetic strirrer bar (rotating about 600 rpm) at open circuit. After the accumulation step, the electrode was removed from the preconcentration cell, rinsed with water and placed in the measurement cell containing the supporting electrolyte (0.2 mol L1 KNO3). An initial potential (1.0 V versus Ag/AgCl) was applied for 10 s then the anodic stripping voltammogram was recorded in the differential pulse mode, yielding an anodic oxidation at the potential raging from 800 to 400 mV using a step potential of 25 mV; amplitude 5 mV and duration 5 s at scan rate 1 mV s1.
Fig. 1. Scanning electron micrograph of kaolin/platinum.
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of an increase in the surface coverage by the lead(II), which led to an increase in the electrode conductivity. The observed decrease of the charge-transfer resistance means also that the modified electrode becomes more conductive, which can be explained by the presence of lead on the electrode surface. The increase in the Cdl, which can result from a increase in local dielectric constant and/or an decrease in the thickness of the electrical double layer, suggested that the lead(II) function by accumulation at the electrode/solution interface. 3.3. Voltammetric analysis of electrodeposit In order to confirm the result obtained in the impedance spectroscopy part of this work regarding the K/Pt/Pb(II) interaction, we have performed some voltammetric measurements. As shown in Fig. 4, a voltammetric curve of the K(layer)-CPE preconcentrated for 10 min in HClO4 (0.1 M) with a scan rate of 100 mV s1. It may be noted the presence of a cathodic peak P1 observed at 604 mV, and an anodic peak, P2 recorded at 510 mV, the peak potentials were attributed to lead(II) behaviour in HClO4 0.1 M. An acidic medium was selected as suitable for relegate of Pb(II) according to Eq. (1). Pb(II) species leached out from the kaolin at the electrode/solution interface can be detected directly by reduction Eq. (2).
Kaolin PbðIIÞ ! kaolin þ PbðIIÞ PbðIIÞ þ 2 e ! Pbð0Þ
ð1Þ ð2Þ
3.4. Optimization of experimental conditions Fig. 2. Cyclic voltammogram of: (a) kaolin/Pt electrode before any contact with lead. (b) Kaolin/Pt electrode after incubation with lead(II) species during 30 min and (c) Pt electrode after preconcentration in lead solution; supporting electrolyte is 0.2 mol L1 KNO3, pH 5.6; the scan rate was 100 mV s1. [Pb(II)] = 4 103 mol L1.
3.2. EIS measurements EIS was carried out on a kaolin modified platinum electrode (K/ Pt) surface in 0.2 mol L1 KNO3 in the absence and presence of lead(II) at 0 V at 298 K after 30 min of immersion (Fig. 3). The calculated charge-transfer resistance (Rct) and the double layer capacitance (Cdl) were 30.62 kX cm2 and 207.8 pF cm2 for the starting electrode and 21.6 kX cm2 and 235.3 pF cm2 for the lead exposed electrode respectively, corresponding to the increased ionic contents. The charge-transfer resistance (Rct) values decreased in the presence of lead(II). On the other hand, the double layer capacitance (Cdl) values increased. This situation was the result
Fig. 3. Impedance spectra at 0 V (a) kaolin modified platinum electrode and (b) Kaolin/Pb(II). Conditions are as described in Fig. 2.
Optimum conditions for the electrochemical response were established by measuring the peak current in dependence on all parameters in both the preconcentration and the measurement steps. The dependence of the variation of peak current on the changes in the accumulation time was examined in a 4.2 105 mol L1 Pb(II) solution (pH 5.0). Anodic currents increased rapidly as the preconcentration time increased from 0.0 to 40.0 min, afterward, the anodic currents increased slowly, probably owing to the interaction sites between the Pb(II) ion and the active sites of the electrode surface should be saturated after 25.0 min. The influence of pH on the determination of lead(II) was also investigated. As the pH increased from pH 2.0 to 9.0, the peak
Fig. 4. Cyclic voltammogram in HClO4 (0.1 M) for kaolin layer, at carbon paste electrode (CPE) (a): before preconcentration step (b): after preconcentration in 3 104 mol L1 Pb(II), pH 5.6.Vb = 100 mV s1, between 0.2 and 1.2 V.
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The DL and QL values obtained by the proposed procedure are similar to those previously obtained with electroanalytical techniques (Akkermans et al., 1998; Saterlay et al., 1999) indicating that the method could be employed to analyze lead(II) in natural samples. The stability of kaolin modified platinum electrode was tested by the use of this modified electrode for reading peak current (after heating the electrode at 100 °C in each determination) over a period of 22 days. Statistical treatments of the results show that using the testing electrode, the maximum relative standard deviation was 2.73%. The results obtained with kaolin modified platinum electrode were compared with those obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for different spiked tap water (Table 1). The quantification of these samples for both techniques was carried out by the standard addition method. Lead(II) concentrations determined by two techniques are in very good agreement and no significant differences at the 97.8% confidence level were found. 3.6. Interferences
Fig. 5. Square-wave voltammograms in 0.2 mol L1 KNO3, pH 5.2, tp = 25 min, at kaolin/Pt of lead(II); (a) 0.09 106, (b) 0.2 106, (c) 0.54 106, (d) 0.8 106, (e) 0.9 106, (f) 0.95 106, (g) 0.99 106, (h) 1.05 106, (i) 1.07 106, (j) 1.15 106, (k) 1.2 106 mol L1.
current increased at first because kaolin can slowly dissolve in acidic solution and lose its ability of immobilizing Pb(II). Then the peak current reached a maximum value around pH 5.2. Continuous increasing of pH led to a decreasing of peak current, which is due to the hydrolysis of lead(II) in basic solution. 3.5. Calibration graph For the purpose of quantitative analysis, a calibration curve for Pb(II) with concentrations ranging over four orders was obtained by spiking the standards directly into distilling water and extracted under the optimal conditions. Linearity was observed over the range of 9.0 108–1.2 106 mol L1 with a correlation coefficient R2 of 0.99 (Fig. 5). The detection limit (DL, 3r) and quantification limit (QL, 10r) for Pb(II) were calculated from five times standard deviation of blank/slope of the calibration graph (Miller and Miller, 1988) were 3.6 109 mol L1 and 1.2 108 mol L1, respectively. The precision of this method was determined by analyzing standard solution at 1.2 106 of Pb(II) for five times in continuous, and the relative standard deviation (RSD) was 2.35%. These are indicative of the good precision and accuracy of the methodology and the possibility of its applications in complex samples.
The potential interference in the present system was investigated (Fig. 6). The interference is due to the competition of other heavy metal ions for the chelating agent and their subsequent co-extraction with lead. Several common metal ions were subjected to SWV under the optimal conditions previously described. All the cations were submitted to the same open-circuit preconcentration procedure described above. The kaolin modified platinum electrode was immersed in a mixture of Cd (II), Cu(II), Ag(I), Pb(II) and Hg(II) (5.0 106 mol L1 each one). These ions were chosen because they might reasonably be expected to exhibit redox activity in roughly the same potential range as Pb(II) at kaolin modified platinum electrode and exist in real samples. The results show that these ions have not interferences on the determination of Pb(II). 3.7. Analytical applications The suitability of the proposed methodology for the analysis of lead(II) in complex samples was checked by determining a trace of lead in orange and apple samples artificially spiked. For the orange fruit samples, the extract of the juices were extracted and had their pH values adjusted to 5.2 with NaOH solution. The square wave responses were also evaluated and demonstrated that possible interference exerts detectable effects on the recovery experiments. These values indicate that the process can be attributed to the adsorption of the matrix components at the electrode surface or to interactions between lead(II) and components of the juice. For apple, 20 g of representative samples were mixed and homogenized in 20 mL dionized water. The samples were used as received in preparing the supporting electrolyte (adding 0.2 mol L1 K2NO3) and the analytical curves were again obtained by SWV experiments under the optimized conditions. The recovery studies were realized by adding an appropriate volume of lead(II) standard solution (1.0 106 mol L1) to preconcentration cell. The recovery efficiency was calculated and the results obtained, in triplicate, were related to interference effects of the constituents of each sample. The results of relative standard deviations and
Table 1 Determination of lead(II) in the spiked tap water sample. Sample
Pb(II) added (107 M)
Pb(II) found (HAP/Pt) (107 M)
Pb(II) found (ICP-AES) (107 M)
RSD (%)
Tap water
10
9.87
9.55
2.04 (n = 5)
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for 25.0 min preconcentration time at open circuit. The K/Pt presented here has a wider linear range and a lower detection limit. The use of SWV is faster and more sensitive than other, conventional, techniques. Besides, the use of kaolin modified platinum electrode enables direct analysis of orange and apple samples without preparation or cleaning of the sample. This extra advantage could reduce the cost of the analysis and the time taken, hence resulting in improvements in analytical sensitivity. References
Fig. 6. Cyclic voltammogram after exposure to a solution containing Cd(II), Ag(I), Cu(II), Hg(II), and Pb(II) of concentration 5.0 106 mol L1.
Table 2 Results obtained from the linear regression curves for the determination of lead(II) at kaolin/Pt in orange and apple samples. Parameters
R2 Slope (A mol1) Standard deviation 1010 (A) Relative standard deviation (%) Recovery (%) (SWV) Recovery (%) (ICP-AES)
Peaks Orange
Apple
0.992 0.971 64.70 1.75 97.81 96.76
0.995 0.958 242.9 2.20 95.32 94.03
recovery percentage were considered satisfactory (Table 2). The proposed methodology has therefore proven to be applicable for use in complex samples. The interferences coming from the apple samples, resulting probably from sucrose, which can inhibit the adsorption process of lead(II) onto kaolin platinum electrode. 4. Conclusions Square wave voltammetry analysis utilizing the kaolin modified platinum electrode for the determination of lead dissolved in aqueous solutions has been demonstrated. The voltammetric response was linear in the concentration range of 9.0 108–1.2 106 M
Akçay, G., Yurdakoç, K., 2000. Removal of various phenoxyalkanoic acid herbicides from water by organo-clays. Acta Hydrochim. Hydrobiol. 28 (6), 300–304. Akkermans, R.P., Ball, J.C., Rebbitt, T.O., Marken, F., Compton, R.G., 1998. Sonoelectroanalysis: application to the detection of lead in wine. Electrochim. Acta 43 (23), 3443–3449. Baldo, M.A., Daniele, S., 2004. Anodic stripping voltammetry at bismuth-coated and uncoated carbon microdik electrodes. Anal. Lett. 37, 995. Dragoe, D., Spataru, N., Kawasaki, R., Manivannan, A., Spataru, T., Tryk, D.A., Fujishima, A., 2006. Detection of trace levels of Pb2+ in tap water at boron-doped diamond electrodes with anodic stripping voltammetry. Electrochim. Acta 51, 2437–2441. Economou, A., Fielden, P.R., 2003. Mercury film electrodes: developments, trends and potentialities for electroanalysis. Analyst 128, 205. El Mhammedi, M.A., Achak, M., Chtaini, A., 2009. Ca10(PO4)6(OH)2-modified carbonpaste electrode for the determination of trace lead(II) by square-wave voltammetry. J. Hazard. Mater. 161, 55–61. Ghiaci, M., Rezaei, B., Kalbasi, R.J., 2007. High selective SiO2–Al2O3 mixed-oxide modified carbon paste electrode for anodic stripping voltammetric determination of Pb(II). Talanta 73, 37–45. Gonzalez, P., Cortınez, V.A., Fontan, C.A., 2002. Determination of nickel by anodic adsorptive stripping voltammetry with a cation exchanger-modified carbon paste electrode. Talanta 58, 679–690. Marino, G., Bergamini, M.F., Teixeira, M.F.S., Cavalheiro, E.T.G., 2003. Evaluation of a carbon paste electrode modified with organofunctionalized amorphous silica in the cadmium determination in a differential pulse anodic stripping voltammetric procedure. Talanta 59, 1021–1028. Mejare, M., Bulow, L., 2001. Metal-binding proteins and peptides in bioremediation and phytoremediation of heavy metals. Trends Biotechnol. 19, 67. Miller, J.C., Miller, J.N., 1988. Basic statistical methods for analytical chemistry, part I. Statistics of repeated measurements. Analyst 113, 1351–1356. Prior, C., Lenehan, C.E., Walker, G.S., 2006. Enhanced resolution of copper and bismuth by addition of gallium in anodic stripping voltammetry with the bismuth film electrode. Electroanalysis 18, 2486. Saterlay, A.J., Agra-Gutiérrez, C., Taylor, M.P., Marken, F., Compton, R.G., 1999. Sonocathodic stripping voltammetry of lead at a polished boron-doped diamond electrode application to the determination of lead in river sediment. Electroanalysis 11 (15), 1083–1088. Seki, Y., Yordakoç, K., 2005. Paraquat adsorption onto clays and organoclays from aqueous solution. J. Colloids Interf. Sci. 287, 1–5. Tsai, W.T., Lai, C.W., 2006. Adsorption of herbicide paraquat by clay mineral regenerated from spent bleaching earth. J. Hazard. Mater. 134, 144–148. Zejli, H., Temsamani, K.R., Hidalgo-Hidalgo de Cisneros, J.L., NaranjoRodriguez, I., Sharrock, P., 2006. Electrochemical micro-extraction of lead(II) at a hydroxylapatitemodified platinum electrode. Electrochem. Commun. 8, 1544– 1548.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Electroanalytical method for determination of lead(II) in orange and apple using kaolin modified platinum electrode M.A. El Mhammedi a,b,*, M. Achak c, M. Bakasse d, A. Chtaini b a
Université Hassan 1er, Faculté Polydisciplinaire, Kouribga, Morocco Equipe d’Electrochimie et des Matériaux Inorganiques, Université Sultan Moulay Slimane, Faculté des Sciences et Techniques, BP 523, Beni-Mellal, Morocco c Laboratoire d’Hydrobiologie et d ’Algologie, Faculté des Sciences Semlalia, Université Cadi Ayyad, Marrakech, Morocco d Equipe d’Analyse des Micro-Polluants Organiques, Faculté des Sciences, Université Chouaib Doukkali, BP 20, El Jadida, Morocco b
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 27 March 2009 Accepted 6 April 2009 Available online 19 May 2009 Keywords: Platinum electrode Kaolin Lead Square wave voltammetry
a b s t r a c t This paper reports on the use of platinum electrode modified with kaolin (K/Pt) and square wave voltammetry for analytical detection of trace lead(II) in pure water, orange and apple samples. The electroanalytical procedure for determination of the Pb(II) comprises two steps: the chemical accumulation of the analyte under open-circuit conditions followed by the electrochemical detection of the preconcentrated species using square wave voltammetry. The analytical performances of the extraction method has been explored by studying the incubating time, and effect of interferences due to other ions. During the preconcentration step, Pb(II) was accumulated on the surface of the kaolin. The observed detection and quantification limits in pure water were 3.6 109 mol L1 and 1.2 108 mol L1, respectively. The precision of the method was also determined; the results was 2.35% (n = 5). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Today, many heavy metals (such as lead, arsenic, cadmium or mercury) constitute a global environmental hazard (Mejare and Bulow, 2001). Major concerns regarding the toxic effect of heavy metal have led to increasing needs to monitor trace metals in a variety of matrices. Traditionally, such trace-metal measurements have been carried out in the central laboratory, in connection with time-consuming sampling, transportation and storage steps, and bulky atomic spectroscopy instrumentation. In this context, chemically modified electrodes (CMEs) appear as suitable alternative or complementary tools. Electrochemical analysis has always been recognized as a powerful tool for measuring trace metals (Marino et al., 2003; Gonzalez et al., 2002; Dragoe et al., 2006). Its remarkably high sensitivity is attributed to the accumulation step, during which the target metals are accumulated onto the working electrode. Various materials have been used as working electrodes for detection of heavy metals (Prior et al., 2006; El Mhammedi et al., 2009). The expensive electrodes, such as the static mercury drop electrode (SMDE) or a mercury-coated rotating disk electrode have been accepted as the widely used electrode material
* Corresponding author. Address: Université Hassan 1er, Faculté Polydisciplinaire, Kouribga, Morocco. Tel.: +212 68858296; fax: +212 23485201. E-mail address: [email protected] (M.A. El Mhammedi). 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.04.017
choice because of high reproducibility and sensitivity (Economou and Fielden, 2003). However, the toxicity of mercury and its compounds makes its less and less popular. Intensive research efforts have thus been devoted to the development of alternative electrodes, with a performance approaching that of mercury-based ones (Zejli et al., 2006; Ghiaci et al., 2007). Bismuth film electrodes (BFEs) have become an attractive topic as a potential replacement for mercury electrodes (Baldo and Daniele, 2004). Bismuth is an environmentally friendly element, with very low toxicity, and a widespread pharmaceutical use. However, the determination of copper using bismuth film electrodes has been relatively ignored due to the similar stripping potentials of copper and bismuth with only a few reports in the open literature (Prior et al., 2006). Kaolin, efficient natural adsorbents (Seki and Yordakoç, 2005; Tsai and Lai, 2006), are the main components of soils and possess a negative charge that is compensated for by exchange cations (Akçay and Yurdakoç, 2000). Consequently, it is important to study the determination of lead(II) with kaolin minerals modified platinum electrode (K/Pt) based on the accumulation process. This work was aimed at evaluating the potential analytical applications of kaolin (K) modified platinum electrodes (K/Pt) for the determination of Pb(II) within the nanomolar concentration range using square wave voltammetry. In order to assess the practical utility of the method for real matrices analysis, preliminary data concerning the effect of other metals as possible interferent, are also reported.
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All reagents used were analytical reagent-grade and double-distilled water was used throughout. Commercial kaolin was supplied by ECESA (Lugo, Spain). Pb(NO3)2 was dissolved into Bi-distilled deionized water (BDW) to form 103 mol L1 stock solutions. Working standards for calibration were prepared by diluting the primary stock solution with (BDW). Graphite powder was supplied from (Carbone, Lorraine, ref 9900, France). All electrochemical experiments were performed in 0.2 mol L1 KNO3 as supporting electrolyte. pH was adjusted by 0.1 H2SO4 or NaOH. All chemicals were of analytical grade and were used without further purification.
Electrochemical impedance spectroscopy (EIS) measurements were carried out with a same electrochemical system described above before and after immersion in solution containing lead(II) ions. Impedance spectra were obtained in the frequency range 100 kHz to 10 MHz with 10 points per decade at 0 V after 30 min of immersion in non-de-aerated solutions. A sine wave with 10 mV amplitude was used to perturb the system. The impedance diagrams are given in the Nyquist representation. After accumulation step, the electrodeposit layer was analyzed at carbon paste electrode (CPE) by cyclic voltammetry. The carbon paste was prepared by hand mixing of high purity graphite powder with kaolin in weight ratio 1:1. The method of standard additions was used for analysis of real samples, by spiking with appropriate amounts of standard lead(II) solution. All measurements were taken at room temperature.
2.2. Electrode preparation
3. Results and discussion
Prior to all measurements, the platinum plate was polished with emery paper up to 1200 grade, washed thoroughly with double-distilled water, degreased with AR grade ethanol, acetone and dried at room temperature. The equipment for electrodeposition of kaolin consisted of a glass cell containing the kaolin suspension, a platinum substrate electrode and a platinum counter electrode. The separation distance between electrodes was 2 cm. Electrodepoistion process was carried out at a constant voltage of 4.0 V for 10 h. After the electrodeposition process, the coated platinum substrate was removed from the suspension and was dried at room temperature. Scanning electron microscopy (SEM) was used to observe the morphology of samples.
3.1. Adsorptive and voltammetric characteristics of the lead(II)modifier complex
2. Experimental 2.1. Reagent
2.3. Apparatus Electrochemical measurements were performed with a voltalab potentiostat (model PGSTAT 100, Eco Chemie B.V., Utrecht, The Netherlands) driven by the general purpose electrochemical systems data processing software (voltalab master 4 software) connected to Pentium III computer run under windows 98. A conventional three electrodes system comprising a kaolin modified platinum electrode, a platinum wire counter electrode and an Ag/ AgCl (in saturated with KCl) reference electrode was used in all experiments. All potentials reported were referred to the Ag/AgCl electrode. Solutions were deoxygenated with high purity nitrogen for five minutes prior to each experiment and it was performed under a nitrogen atmosphere. The pH-meter (Radiometer Copenhagen, PHM210, Tacussel, French) was used for adjusting pH values. The coated specimen was examined using a scanning electron microscope (SEM, Jeol JSM-5500).
Fig. 1 shows the scanning electron micrograph of electrodeposited film prepared from kaolin particles onto platinum surface. The layer obtained appeared dense and uniform. The porosity of coating, especially the porosity connected with the substrate is very important factor for fixing lead onto kaolin/Pt surfaces. It is obvious that this substrate can react with metals and provide a fixed substrate for metals for the electrochemical redox reactions. Therefore the modified platinum electrode has been prepared and applied for electrocatalytic determination of Pb(II). Construction of kaolin modified platinum electrode made an opportunity for determination of Pb(II). Cyclic voltammogram in 0.2 mol L1 KNO3 (at the pH 5.6) (Fig. 2) shows cathodic and anodic peaks, which makes it possible to determine this cation. The cyclic voltammograms were obtained for the unmodified and the modified platinum electrode in the presence of lead(II) and without lead(II). There were no redox peaks in the CV of the modified platinum electrode without Pb(II) (Fig. 2a). The modified platinum electrode interacting with of lead(II) showed an anodic peak at 0.5 V versus calomel reference electrode (Fig. 2b). Although the cyclic voltammetry technique is limited for determination of lead(II) at unmodified platinum electrode, but in the presence of kaolin modifier the peak current was rapidly increased and therefore the detection limit decreased.
2.4. Procedure The platinum electrode modified with kaolin was immersed in a cell containing 20 mL of the lead sample solution to get the chemical deposition. Meanwhile the solution was stirred by a 1.2 cm magnetic strirrer bar (rotating about 600 rpm) at open circuit. After the accumulation step, the electrode was removed from the preconcentration cell, rinsed with water and placed in the measurement cell containing the supporting electrolyte (0.2 mol L1 KNO3). An initial potential (1.0 V versus Ag/AgCl) was applied for 10 s then the anodic stripping voltammogram was recorded in the differential pulse mode, yielding an anodic oxidation at the potential raging from 800 to 400 mV using a step potential of 25 mV; amplitude 5 mV and duration 5 s at scan rate 1 mV s1.
Fig. 1. Scanning electron micrograph of kaolin/platinum.
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of an increase in the surface coverage by the lead(II), which led to an increase in the electrode conductivity. The observed decrease of the charge-transfer resistance means also that the modified electrode becomes more conductive, which can be explained by the presence of lead on the electrode surface. The increase in the Cdl, which can result from a increase in local dielectric constant and/or an decrease in the thickness of the electrical double layer, suggested that the lead(II) function by accumulation at the electrode/solution interface. 3.3. Voltammetric analysis of electrodeposit In order to confirm the result obtained in the impedance spectroscopy part of this work regarding the K/Pt/Pb(II) interaction, we have performed some voltammetric measurements. As shown in Fig. 4, a voltammetric curve of the K(layer)-CPE preconcentrated for 10 min in HClO4 (0.1 M) with a scan rate of 100 mV s1. It may be noted the presence of a cathodic peak P1 observed at 604 mV, and an anodic peak, P2 recorded at 510 mV, the peak potentials were attributed to lead(II) behaviour in HClO4 0.1 M. An acidic medium was selected as suitable for relegate of Pb(II) according to Eq. (1). Pb(II) species leached out from the kaolin at the electrode/solution interface can be detected directly by reduction Eq. (2).
Kaolin PbðIIÞ ! kaolin þ PbðIIÞ PbðIIÞ þ 2 e ! Pbð0Þ
ð1Þ ð2Þ
3.4. Optimization of experimental conditions Fig. 2. Cyclic voltammogram of: (a) kaolin/Pt electrode before any contact with lead. (b) Kaolin/Pt electrode after incubation with lead(II) species during 30 min and (c) Pt electrode after preconcentration in lead solution; supporting electrolyte is 0.2 mol L1 KNO3, pH 5.6; the scan rate was 100 mV s1. [Pb(II)] = 4 103 mol L1.
3.2. EIS measurements EIS was carried out on a kaolin modified platinum electrode (K/ Pt) surface in 0.2 mol L1 KNO3 in the absence and presence of lead(II) at 0 V at 298 K after 30 min of immersion (Fig. 3). The calculated charge-transfer resistance (Rct) and the double layer capacitance (Cdl) were 30.62 kX cm2 and 207.8 pF cm2 for the starting electrode and 21.6 kX cm2 and 235.3 pF cm2 for the lead exposed electrode respectively, corresponding to the increased ionic contents. The charge-transfer resistance (Rct) values decreased in the presence of lead(II). On the other hand, the double layer capacitance (Cdl) values increased. This situation was the result
Fig. 3. Impedance spectra at 0 V (a) kaolin modified platinum electrode and (b) Kaolin/Pb(II). Conditions are as described in Fig. 2.
Optimum conditions for the electrochemical response were established by measuring the peak current in dependence on all parameters in both the preconcentration and the measurement steps. The dependence of the variation of peak current on the changes in the accumulation time was examined in a 4.2 105 mol L1 Pb(II) solution (pH 5.0). Anodic currents increased rapidly as the preconcentration time increased from 0.0 to 40.0 min, afterward, the anodic currents increased slowly, probably owing to the interaction sites between the Pb(II) ion and the active sites of the electrode surface should be saturated after 25.0 min. The influence of pH on the determination of lead(II) was also investigated. As the pH increased from pH 2.0 to 9.0, the peak
Fig. 4. Cyclic voltammogram in HClO4 (0.1 M) for kaolin layer, at carbon paste electrode (CPE) (a): before preconcentration step (b): after preconcentration in 3 104 mol L1 Pb(II), pH 5.6.Vb = 100 mV s1, between 0.2 and 1.2 V.
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The DL and QL values obtained by the proposed procedure are similar to those previously obtained with electroanalytical techniques (Akkermans et al., 1998; Saterlay et al., 1999) indicating that the method could be employed to analyze lead(II) in natural samples. The stability of kaolin modified platinum electrode was tested by the use of this modified electrode for reading peak current (after heating the electrode at 100 °C in each determination) over a period of 22 days. Statistical treatments of the results show that using the testing electrode, the maximum relative standard deviation was 2.73%. The results obtained with kaolin modified platinum electrode were compared with those obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for different spiked tap water (Table 1). The quantification of these samples for both techniques was carried out by the standard addition method. Lead(II) concentrations determined by two techniques are in very good agreement and no significant differences at the 97.8% confidence level were found. 3.6. Interferences
Fig. 5. Square-wave voltammograms in 0.2 mol L1 KNO3, pH 5.2, tp = 25 min, at kaolin/Pt of lead(II); (a) 0.09 106, (b) 0.2 106, (c) 0.54 106, (d) 0.8 106, (e) 0.9 106, (f) 0.95 106, (g) 0.99 106, (h) 1.05 106, (i) 1.07 106, (j) 1.15 106, (k) 1.2 106 mol L1.
current increased at first because kaolin can slowly dissolve in acidic solution and lose its ability of immobilizing Pb(II). Then the peak current reached a maximum value around pH 5.2. Continuous increasing of pH led to a decreasing of peak current, which is due to the hydrolysis of lead(II) in basic solution. 3.5. Calibration graph For the purpose of quantitative analysis, a calibration curve for Pb(II) with concentrations ranging over four orders was obtained by spiking the standards directly into distilling water and extracted under the optimal conditions. Linearity was observed over the range of 9.0 108–1.2 106 mol L1 with a correlation coefficient R2 of 0.99 (Fig. 5). The detection limit (DL, 3r) and quantification limit (QL, 10r) for Pb(II) were calculated from five times standard deviation of blank/slope of the calibration graph (Miller and Miller, 1988) were 3.6 109 mol L1 and 1.2 108 mol L1, respectively. The precision of this method was determined by analyzing standard solution at 1.2 106 of Pb(II) for five times in continuous, and the relative standard deviation (RSD) was 2.35%. These are indicative of the good precision and accuracy of the methodology and the possibility of its applications in complex samples.
The potential interference in the present system was investigated (Fig. 6). The interference is due to the competition of other heavy metal ions for the chelating agent and their subsequent co-extraction with lead. Several common metal ions were subjected to SWV under the optimal conditions previously described. All the cations were submitted to the same open-circuit preconcentration procedure described above. The kaolin modified platinum electrode was immersed in a mixture of Cd (II), Cu(II), Ag(I), Pb(II) and Hg(II) (5.0 106 mol L1 each one). These ions were chosen because they might reasonably be expected to exhibit redox activity in roughly the same potential range as Pb(II) at kaolin modified platinum electrode and exist in real samples. The results show that these ions have not interferences on the determination of Pb(II). 3.7. Analytical applications The suitability of the proposed methodology for the analysis of lead(II) in complex samples was checked by determining a trace of lead in orange and apple samples artificially spiked. For the orange fruit samples, the extract of the juices were extracted and had their pH values adjusted to 5.2 with NaOH solution. The square wave responses were also evaluated and demonstrated that possible interference exerts detectable effects on the recovery experiments. These values indicate that the process can be attributed to the adsorption of the matrix components at the electrode surface or to interactions between lead(II) and components of the juice. For apple, 20 g of representative samples were mixed and homogenized in 20 mL dionized water. The samples were used as received in preparing the supporting electrolyte (adding 0.2 mol L1 K2NO3) and the analytical curves were again obtained by SWV experiments under the optimized conditions. The recovery studies were realized by adding an appropriate volume of lead(II) standard solution (1.0 106 mol L1) to preconcentration cell. The recovery efficiency was calculated and the results obtained, in triplicate, were related to interference effects of the constituents of each sample. The results of relative standard deviations and
Table 1 Determination of lead(II) in the spiked tap water sample. Sample
Pb(II) added (107 M)
Pb(II) found (HAP/Pt) (107 M)
Pb(II) found (ICP-AES) (107 M)
RSD (%)
Tap water
10
9.87
9.55
2.04 (n = 5)
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for 25.0 min preconcentration time at open circuit. The K/Pt presented here has a wider linear range and a lower detection limit. The use of SWV is faster and more sensitive than other, conventional, techniques. Besides, the use of kaolin modified platinum electrode enables direct analysis of orange and apple samples without preparation or cleaning of the sample. This extra advantage could reduce the cost of the analysis and the time taken, hence resulting in improvements in analytical sensitivity. References
Fig. 6. Cyclic voltammogram after exposure to a solution containing Cd(II), Ag(I), Cu(II), Hg(II), and Pb(II) of concentration 5.0 106 mol L1.
Table 2 Results obtained from the linear regression curves for the determination of lead(II) at kaolin/Pt in orange and apple samples. Parameters
R2 Slope (A mol1) Standard deviation 1010 (A) Relative standard deviation (%) Recovery (%) (SWV) Recovery (%) (ICP-AES)
Peaks Orange
Apple
0.992 0.971 64.70 1.75 97.81 96.76
0.995 0.958 242.9 2.20 95.32 94.03
recovery percentage were considered satisfactory (Table 2). The proposed methodology has therefore proven to be applicable for use in complex samples. The interferences coming from the apple samples, resulting probably from sucrose, which can inhibit the adsorption process of lead(II) onto kaolin platinum electrode. 4. Conclusions Square wave voltammetry analysis utilizing the kaolin modified platinum electrode for the determination of lead dissolved in aqueous solutions has been demonstrated. The voltammetric response was linear in the concentration range of 9.0 108–1.2 106 M
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