Simultaneous trace-levels determination of Hg(II) and Pb(II) ions in various samples using a modified carbon paste electrode based on multi-walled carbon nanotubes and a new synthesized Schiff base

Analytica Chimica Acta 746 (2012) 98–106

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Simultaneous trace-levels determination of Hg(II) and Pb(II) ions in various samples using a modified carbon paste electrode based on multi-walled carbon nanotubes and a new synthesized Schiff base Abbas Afkhami a,∗ , Hasan Bagheri b , Hosein Khoshsafar a , Mohammad Saber-Tehrani c , Masoumeh Tabatabaee d , Ali Shirzadmehr a a

Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran Department of Chemistry, Takestan Branch, Islamic Azad University, Takestan, Iran c Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran d Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 A new chemically modified carbon paste electrode was constructed and used.  A new Schiff base and multi-walled carbon nanotube was used as a modifier.  The electrochemical properties of the modified electrode were studied.  The electrode was used to the simultaneous determination of Pb2+ and Hg2+ .

a r t i c l e

i n f o

Article history: Received 16 February 2012 Received in revised form 6 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: Simultaneous determination Square wave voltammetry Modified carbon paste electrodes Heavy metals

a b s t r a c t A modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTs) and 3(4-methoxybenzylideneamino)-2-thioxothiazolodin-4-one as a new synthesized Schiff base was constructed for the simultaneous determination of trace amounts of Hg(II) and Pb(II) by square wave anodic stripping voltammetry. The modified electrode showed an excellent selectivity and stability for Hg(II) and Pb(II) determinations and for accelerated electron transfer between the electrode and the analytes. The electrochemical properties and applications of the modified electrode were studied. Operational parameters such as pH, deposition potential and deposition time were optimized for the purpose of determination of traces of metal ions at pH 3.0. Under optimal conditions the limits of detection, based on three times the background noise, were 9.0 × 10−4 and 6.0 × 10−4 ␮mol L−1 for Hg(II) and Pb(II) with a 90 s preconcentration, respectively. In addition, the modified electrode displayed a good reproducibility and selectivity, making it suitable for the simultaneous determination of Hg(II) and Pb(II) in real samples such as sea water, waste water, tobacco, marine and human teeth samples. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the release of various harmful heavy metal ions into the environment has attracted great attention worldwide because of their toxicity and widespread use. Mercury and lead

∗ Corresponding author. Tel.: +98 811 8272404; fax: +98 811 8272404. E-mail address: [email protected] (A. Afkhami). 0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2012.08.024

are identified as the highly toxic elements [1,2]. Although mercury is not an abundant chemical element in nature, it has become widespread as a result of many industrial and agricultural applications [3–5]. Due to its high toxicity and very high bioaccumulation factor (up to 106) in the food chain, the monitoring of mercury in natural waters is very important. Also, the widespread presence of lead in the human environment comes from anthropogenic activities. The most important sources of lead exposure are industrial emissions, soils, car exhaust

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gases and contaminated foods. Water and food samples can contain high levels when presence in near lead sources [2,6]. Consequently, the development of rapid, simple and accurate method with high sensitivity for the determination of mercury and lead at (ultra)trace levels in environmental and biological materials is of particular significance. Several analytical techniques such as spectrophotometry, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), atomic absorption spectrometry (AAS), atomic fluorescence spectrometry and neutron activation analysis have been developed for the determination of these elements [3,4,7]. However, most of the above methods require several time consuming manipulation steps, sophisticated instruments and special trainings. Particularly, electrothermal AAS (ET-AAS) has shown satisfactory detection limit for determination of lead at nano-molar levels in water samples. Unfortunately, the influence of the salt content for the determination of metals in samples with saline matrices, as well as the lack of multi elemental analysis and the incapacity for speciation studies has reduced their application to water samples [8]. Furthermore, cold vapor atomic absorption spectroscopy (CV-AAS) is the most widely used method due to its simplicity, relatively low cost of operation, high sensitivity and selectivity for Hg determination. However, CV-AAS is not straightforwardly applicable to some environmental, clinical, or biological samples in view of low analyte content and matrix of the sample [7,9]. For these reasons, simultaneous determination of various metal ions at trace and ultra trace concentrations in real matrices is one of the main aims of new selective, highly sensitive and environmentally safe analytical methods. Electrochemical methods are selective and sensitive methods used for the simultaneous determination of inorganic and organic compounds. Stripping voltammetry became very useful due to its remarkable sensitivity, its multi-element, low cost technique and stability to on-line measurements. Such a technique may be certainly a good alternative to spectroscopy, since it allows carrying out a multicomponent determination and it does not need too expensive equipments. Studies in analytical electrochemistry have shown great interest in this area and various modified electrodes have been constructed for this purpose [10–12]. The development and application of chemically modified electrodes (CMEs) have received considerable attention in recent years. CMEs are characterized by purposefully altering their surface characteristics to display new qualities that can be exploited for analytical purposes. The huge success of CMEs arises most often from the remarkable and sometimes unique properties of the modifiers [13–16]. Among these electrodes, carbon-paste electrodes (CPEs), due to the ease of their construction, easy renewability of the surface, wider potential window of −1.4 to +1.3 V (versus SCE) according to experimental conditions and compatibility with various types of modifiers, have been widely used as suitable matrices for the preparation of the modified electrodes. Further, they show relatively residual currents 10 times lower than the solid graphite or noble metal electrodes [17–19]. For improving the electrochemical performance of electrodes, chemical or physical modification of their surfaces has performed with specific species. CPEs are easy to modify and there have been continuous efforts toward the preparation of suitable designed modified electrodes for improving both sensitivity and selectivity of detection. The modifying agents can be directly added to the carbon paste or adsorbed on the electrode surface during the measurement, which is termed in situ modification. The operation mechanism of such chemically modified carbon paste electrodes (CMCPEs) depends on the properties of the modifier used for important selectivity towards the target species [20,21].

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Nowadays, due to the unique properties of carbon nanotubes (CNTs) such as ordered structure with high aspect ratio, ultralight weight, high mechanical strength, high electrical conductivity, high thermal conductivity, metallic or semi-metallic behavior and high surface area, they have been widely used for the development of chemically modified electrodes [22–24]. The combination of these characteristics makes CNTs unique materials with capability to promote electron transfer reaction and improve sensitivity in electrochemistry, and thus they are widely used to prepare modified electrodes. Moreover, extensive efforts have been devoted to design novel CNTs modified electrodes to improve the voltammetric determinations of organic [11,24,25] and inorganic compounds [22,26]. In comparison to the conventional CPEs, the carbon nanotube paste electrodes have shown a considerable enhancement in electrochemical signals leading to improvement of the detection limit in the voltammetric measurements. In this work we used a carbon paste electrode modified with multi-walled carbon nanotubes (MWCNTs) for the simultaneous determination of trace amounts of Hg(II) and Pb(II) by square wave anodic stripping voltammetry (SWASV). Selectivity and sensitivity of the determination of these metal ions by adsorptive stripping voltammetry was improved by their interactions with specific ligands [27]. The electrode is modified by introducing a Schiff base into the matrix of paste composite. 3-(4-methoxybenzylideneamino)2-thioxothiazolodin-4-one (L) is a new synthesized Schiff base Scheme 1. It is capable to form complexes with target ions. It is successfully used as a selective agent for the anodic stripping voltammetric determination of Hg(II) and Pb(II) at a carbon paste electrode modified with multiwalled carbon nanotubes. Such properties caused obtaining low detection limits in the voltammetric determinations with modified CPEs as working electrode. The created selectivity in this procedure makes the electrode very suitable for the detection of trace amounts of target analytes in various real samples. To the best of our knowledge, most of the published electrochemical studies have utilized glassy carbon electrodes or other kinds of electrodes for simultaneous determination of Hg(II) and Pb(II) (Table 1), and no study has reported on the simultaneous determination of Hg(II) and Pb(II) using carbon nanotube paste electrodes modified with the Schiff base [28–34]. Also, no paper has reported electrochemical determinations of mercury and lead in real samples such as sea water, waste water, tobacco, marine and human teeth samples. Thus, in this paper, we described initially the preparation and suitability of a 3-(4methoxybenzylideneamino)-2-thioxothiazolodin-4-one modified carbon nanotube paste electrode as a new sensor in the electrochemical determination of Hg(II) and Pb(II) in different samples in the presence of various species.

2. Experimental 2.1. Apparatus and chemicals All electrochemical experiments including cyclic voltammetry (CV), square wave voltammetry (SW) and other methods were performed using an Autolab Potentiostate/Galvanostate, Model 302 N. A conventional three-electrode system was used with a carbon paste working electrode (unmodified or modified), a saturated Ag/AgCl reference electrode and a Pt wire as the counter electrode. A magnetic stirrer (PAR-305) with a Teflon-coated magnet was used to provide the convective transport during the preconcentration step. The whole measurements were automated and controlled through the programming capacity of the apparatus. A CV-AAS on a Varian Spectra AA-220 atomic absorption spectrometer, VGA 77 Vapor Generator Accessory that was equipped with deuterium

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Table 1 Comparison of some characteristics of the different modified electrodes for the determination of Hg(II) and Pb(II). Electrode

Gold microwire electrode Montmorillonite-calcium modified CPE Modified CPE containing humic acid MES/gold nanoparticles modified gold electrodea SBA-15 silica organofunctionalized with 2-benzothiazolethiol graphite–polyurethane composite electrodes Ionic liquid-functionalized ordered mesoporous silica SBA-15 modified CPE CPE modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol L–MWCNT–CPE a b

Method

Linear range (␮mol L−1 )

Detection limit (␮mol L−1 )

Refs.

Pb(II)

Hg(II)

Pb(II)

Hg(II)

DPASV SWASV DPV SWASV

9.60 × 10−4 to 3.86 × 10−2 – N.R.b 4.83 × 10−3 to 4.82 × 10−1

3.49 × 10−4 to 1.49 × 10−2 – N.R. 4.98 × 10−3 to 4.98 × 10−1

9.60 × 10−4 1.40 × 10−3 5.00 × 10−3 7.54 × 10−4

3.48 × 10−4 5.20 × 10−3 8.00 × 10−3 6.54 × 10−4

[28] [29] [30] [31]

SWASV

N.R.

N.R.

0.09 × 10−3

0.60 × 10−3

[32]

DPASV

0.40–90.00

0.08–50.00

4.00 × 10−2

1.00 × 10−2

[33]

DPASV

0.30–7.00

2.00–10.0

4.0 × 10−2

4.0 × 10−4

[34]

SWASV

2.00 × 10−3 to 7.00 × 10−1

2.00 × 10−3 to 7.00 × 10−1

6.00 × 10−4

9.00 × 10−4

This work

Mercaptoethanesulfonate. Not reported.

lamp background correction was used for mercury determination. Mercury absorbance was measured by a mercury hollow cathode lamp that operated as the line source at a lamp current of 0.4 mA. The 253.7 nm resonance line was selected with a slit width of 0.5 nm. Also, a Varian model Spectra AA-220 atomic absorption spectrometer equipped with a GTA-100 graphite furnace atomizer, deuterium lamp as a background corrector was used for the determination of lead. An automatic sampler was employed for injecting the solution into the furnace. All experiments were performed using pyrolytically coated graphite tubes. Lead absorbance was measured by a lead hollow cathode lamp that operated as the line source at a lamp current of 10 mA. The 217.0 nm resonance line was selected with a slit width of 0.7 nm. A 10 ␮L sample was used with a 5 ␮L of NH4 H2 PO4 and Mg(NO3 )2 mixed matrix modifier. An Agilent 8453 diode array UV–vis spectrophotometer (Agilent, USA) equipped with 1.0 cm path length quartz cells was used to obtain absorbance spectra and absorbance curves. A Mettler balance (Toledo-AB104, Greifensee, Switzerland) was used for weighing the solid materials. A micropipetter (Eppendorf–Multipette® plus) was used for transferring the solutions throughout the present experimental work. A pH-meter, Model 713, with a glass electrode (Metrohm, Swiss), was used to determine pH values of the solutions. All the chemicals were of analytical grade, or better, and used as received. Unless otherwise stated, all the solutions were prepared with doubly distilled water (DDW). The 100.0 mg L−1 mercury and lead standard solutions were prepared by the dissolution of adequate amounts of nitrate salts in DDW and for mercury the solution was acidulated with nitric acid at pH 2.0. Multi-walled carbon nanotube (MWCNT) (purity more than 95%) with outer diameter between 5 and 20 nm, inner diameter between 2 and 6 nm and tube length from 1 to 10 ␮m was from Plasmachem GmbH (Germany). Paraffin oil and graphite powder were obtained from Merck Company and used as received. Britton–Robinson (B–R) universal buffer, acetate buffer, NH4 Cl buffer solution, KNO3 solution and phosphate buffer were prepared in DDW and were tested as supporting electrolytes. pH adjustments were performed with 0.01–1.0 mol L−1 HCl or NaOH solutions.

2.2. Synthesis of 3-(4-methoxybenzylideneamino)-2-thioxothiazolodin-4-one The Schiff base was synthesized according to the literature [35]. A solution of N-aminorhodanine (2 mmol) in (CH3 CN or CH3 OH,

10 mL) was treated with 4-methoxybenzaldehyde in a molar ratio of 1:1.5 and the resulting mixture was acidified by 37% hydrochloric acid (3 drops) or acetic acid (8 drops). The reaction mixture was refluxed for 8 h. The solid residue was filtered, washed with cold solvent (10 mL) to afford the product.

2.3. Pretreatment of multi-walled carbon nanotube materials and preparation of modified electrode A pretreatment of the CNTs is usually necessary to eliminate graphitic nanoparticles, amorphous carbon, metallic impurities, and/or to improve the electron transfer properties and/or to allow further functionalization [36]. The pretreatment consists in exposing the CNTs to an acidic solution of sulfuric, nitric or hydrochloric acid, or mixture of these acids at room temperature, under refluxing or under sonication for different times [36,37]. Following one of the purification methodologies, 500 mg of MWCNT was heated at 400 ◦ C using an air flow of 12 mL min−1 (quartz tubular reactor of 14 mm diameter), for 1 h. To eliminate metal oxide catalysts, the heated processed amount of MWCNTs was dispersed in 60 mL of 6.0 mol L−1 HCl for 4 h under ultrasonic agitation; filtered on a Whatman No. 42 filter paper and washed until the pH of the solution was neutral; and finally, dried. The unmodified CPE was prepared by mixing fine graphite powder with appropriate amount of paraffin and thorough hand mixing in a mortar and pestle (75:25, w/w), and a portion of the composite mixture was packed into the end of a polyethylene syringe (2.5 mm diameter). Electrical contact was made by forcing a thin copper wire down into the syringe and into the back of the composite. The MWCNT–CPE was prepared by mixing the unmodified mixture with MWCNT (10%, w/w) and then the resulted composite was dissolved in dichloromethane (for more homogeneity of the electrode composite components leading to an increase in the reproducibility after each electrode surface polishing). The mixture was stirred by a magnetic stirrer till the solvent evaporated completely. The prepared modified composite was then air dried for 24 h and was used in the same way as the case of the unmodified electrode. To prepare the modified electrode (L–MWCNT–CPE) a portion of L (12% w/w) was mixed with the above prepared MWCNT–CPE and the resulted composite was dispersed in dichloromethane. The homogenized composite was stirred by a magnetic stirrer till the solvent evaporated completely. Then, the prepared modified composite was air dried for 24 h. The resulting mixture was mixed with paraffin and transferred into the syringe. The paste was carefully packed into

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the syringe tip to avoid possible air gaps, which often enhance the electrode resistance. This composite has remarkable advantages, such as compactness, high mechanical stability, and low electrical resistance and renewal characteristics by simple polishing. The prepared CPEs were then immersed in the supporting electrolyte placed in the electrolysis cell and several sweeps were applied until a low background current was achieved. Also, the pastes surface was smoothed and rinsed with water before each measurement. 2.4. Analytical procedure The analysis of Hg(II) and Pb(II) using SWASV was carried out in a 20.0 mL aliquot (pH 3.0) using the following steps after purging with nitrogen for at least 5 min: (a) pre-conditioning step: potential of 0.9 V vs. Ag/AgCl for 45 s was applied before each measurement to ensure dissolution of the remaining deposits on the surface of the modified electrode; (b) the preconcentration step proceeded at −1.2 V vs. Ag/AgCl for 90 s; at the end of the preconcentration time, stirring was stopped and a 10 s rest period was allowed for the solution to become quiescent; (c) the square wave anodic stripping voltammograms were recorded when swept from −0.8 V to 0.7 V vs. Ag/AgCl. In the pre-conditioning and preconcentration processes, the detection solutions were stirred with a magnetic stirrer. The peak currents at about 0.29 and −0.43 V vs. Ag/AgCl for Hg(II) and Pb(II) were measured, respectively. All measurements were carried out at room temperature (25.0 ± 0.1 ◦ C). Calibration graphs were prepared for the peak current against Hg(II) and Pb(II) concentrations. Calibration graphs were prepared by plotting the net anodic peak currents vs. Hg(II) and Pb(II) concentrations in solutions. 2.5. Real samples preparation Persian Gulf water sample was collected in a 1.0 L glass bottle, acidified by 5 mL of nitric acid, kept in a refrigerator and filtered through a filter paper (Whatman No. 40) before use. A 40 mL of water sample was transferred to a round-bottom flask, 2.0 mL of H2 SO4 (98%), 0.5 mL of HNO3 (70%), 2.0 mL of K2 S2 O8 (5%) and 4 mL of KMnO4 (5%) were added and refluxed at 80 ◦ C for 2 h [38]. This solution was cooled, neutralized with sodium hydroxide and diluted to 100 mL in a volumetric flask. Then 10 mL of this solution was treated by the recommended procedure described in Section 2.4. Petrochemical wastewater samples were collected in a 2.0 L PTFE bottle and filtered through a filter paper (Whatman No. 40) before use. The real metallurgy wastewater sample was collected from the Powder Metallurgic Factory (Tehran, Iran), which had red-brown color and without any obvious suspended materials. It was diluted with HNO3 and filtered through a filter paper (Whatman No. 40) before use. Then 10 mL of this solution was treated by the recommended procedure described in Section 2.4. The proposed method was applied to determine mercury and lead in tuna fish and shrimp samples. Tuna fish and shrimp were purchased from local fish market. 500 mg of dried samples was placed in a digestion vessel and 5 mL of HNO3 (70%) as well as 6 mL of H2 O2 (30%) were added. The vessel was immediately assembled, gently swirled and placed in the pre-heated oven at 180 ◦ C for about 1.5 h [38,39]. Then 6 mL of 1 mol L−1 of K2 S2 O8 was added and heated for 30 min. The digested samples were cooled at room temperature [39]. Appropriate amounts of 2 mol L−1 NaOH were added to neutralize the excess of HNO3 and then the pH was adjusted at the optimized value. Then the procedure given in Section 2.4 was performed. For the determination of Hg(II) and Pb(II) in tobacco samples, the samples (500 mg) were accurately weighted into the PTFE high-pressure microwave acid-digestion vessels, and 3.0 mL of

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Table 2 The overall stability constants of complexation between investigated metal ions with L at 25 ◦ C and ethanol solvent (N = 5). Metal ion

log ˇ

Ag(I) Cd(II) Co(II) Ni(II) Cu(II) Fe(II) Hg(II) Pb(II)

1.38 2.74 1.89 2.13 2.08 1.97 6.42 7.21

± ± ± ± ± ± ± ±

0.04 0.02 0.06 0.03 0.05 0.06 0.08 0.03

concentrated nitric acid plus 5.0 mL of 30% hydrogen peroxide were added. The vessels were sealed tightly and then positioned in the carousel of the microwave oven. The system was operated at full power for 8.0 min. The digest was evaporated to near dryness. The residue was dissolved with 5 mL 5% (m/v) nitric acid, and quantitatively transferred to a 50 mL volumetric flask for further analysis [40]. The healthy teeth without any restoration with metallic amalgam or filling were collected from Legal Medicine Organization of Iran from 2 adults who died on September 2011 less than 48 h after death. After pulling out, the teeth were rinsed with distilled water for eliminating traces of blood and saliva and have been then cut sagittally in the buccolingual direction with a low speed diamond saw microtome [41]. Carious substance was removed with a tungsten carbide bur from each tooth. One half of the teeth was rinsed in 0.1 mol L−1 HCl solution, then washed with DDW, allowed to dry and was stored at freezer until the end of the sampling process. Afterward, to simplify the crushing and grinding process, each tooth was wrapped in plastic film and was introduced into liquid nitrogen. It was ground in a chromium-steel micromill. The certain amounts of powder samples not exceeding 0.1 g were dissolved in 2 mL of 2 mol L−1 HCl and stored at 50 ◦ C for 6 days in closed and sealed polypropylene test tubes. The digested samples transferred and diluted to 10 mL and stored at 4 ◦ C for next step.

3. Results and discussion 3.1. Complexation investigation of Schiff base with metal ions The 3-(4-methoxybenzylideneamino)-2-thioxothiazolodin-4one with two sulfurs, one oxygen and two nitrogens donating Schiff base is insoluble in water. Consequently, we study the complexation of several metal ions in ethanol solution. Typical spectra were depicted in Fig. 1. As it was observed, significant apparent changes in the spectrum of ligand, on addition of metal ions indicate that, among the investigated cations the ligand has stronger interaction with Pb(II) and Hg(II) ions therefore, it may be used as an efficient modifier for the preconcentration of these ions (Fig. 1(I) and (II)). Also, Fig. 1(III) is useful to compare the absorbance changes of the ligand after the addition of target metal ions (Hg(II) and Pb(II)) with a metal, e.g. Ag(I), which it has lower affinity to ligand. It is clear due to form a relative weak complex (Ag with L), the absorbance changes of the L after addition of silver ions is lower than Fig. 1(I) and (II). Further analyzing the absorbance at maximum absorbance wavelength using a nonlinear least-squares curve-fitting program (Kinfit program) the stoichiometry and stability constant of L complexes with the investigated metal ions were obtained [42]. The results are given in Table 2. From the results observed for analytes at a mole ratio of 0.5, it can be immediately concluded that a 2:1 complex of [L2 M]2+ is formed in ethanol solution (Fig. 2). The unique nature of Schiff base donor is enhanced by the existence of widely spread ␲-conjugation system. The authors decided to use L as a

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Fig. 1. UV–vis spectra for titration of L (1.0 × 10−4 mol L−1 ) with (I) Hg(II), (II) Pb(II) and (III) Ag(I) (1.0 × 10−5 to 1.0 × 10−4 mol L−1 ) in ethanol (25 ◦ C), respectively.

suitable modifier for the selective determination of lead and mercury using carbon nanotube paste electrode. 3.2. Electrochemical behavior of Hg(II) and Pb(II)on the surface of various electrodes The CVs were obtained for the unmodified CPE, MWCNT–CPE in the presence of analytes and the L–MWCNT–CPE at pH 3.0 in the presence and absence of Pb(II) and Hg(II) in the positive going scan using a deposition potential of −1.2 V vs. Ag/AgCl for 90 s. There were no observable peaks for an unmodified CPE in the presence of 1.2 × 10−5 mol L−1 Pb(II) and Hg(II) (Fig. 3(I), curve a,) and for L–MWCNT modified CPE in the absence of Pb(II) and Hg(II) (Fig. 3(I), curve c). MWCNT–CPE and L–MWCNT–CPE in the presence of metal ions showed two anodic peaks at −0.48 V for Pb(II) and 0.31 V vs. Ag/AgCl for Hg(II) in the positive going scan (Fig. 3(I), curves b and d). The L–MWCNT–CPE also showed cathodic peaks at 0.620 and 0.100 V vs. Ag/AgCl correspond to Pb(II) and Hg(II), respectively, in the negative going scan (Fig. 3(I), d). As shown in Fig. 3(I), the electrode prepared with L and MWCNTs showed the larger and well-defined anodic and cathodic peak

currents as compared with the MWCNT–CPE (curves b and d). In one hand, the pretreatment of the MWCNTs was used to eliminate impurities as carbonaceous materials and metallic compounds and to introduce carboxyl and carboxylate groups at the ends and/or at the sidewall of the nanotube structure that can increase the conduction of electrons. On the other hand, in the case of L–MWCNT–CPE, obtaining maximum response for Hg(II) and Pb(II) can be explained on the basis of Pearson theory. According to Pearson theory, hard acids prefer to co-ordinate with hard bases and soft acids to soft bases. Hg(II) and Pb(II) ions are soft Lewis acids and as a rule, their interactions with surface functional groups (soft bases) would be favored [43]. Furthermore, the modification of electrode with Schiff base and MWCNTs lead to the improvement in the selective electrochemical response for target analytes. Fig. 3(II) shows the SWASV responses of 3.0 × 10−7 mol L−1 Pb(II) and Hg(II) on the surface of the three above mentioned electrodes. As can be seen, two sharp and well-resolved anodic peaks at -0.43 and 0.29 V appeared on the surface of L–MWCNT–CPE for Pb(II) and Hg(II), respectively. 3.3. Optimization of analytical conditions In order to obtain the optimum experimental conditions, some variables affecting the peak current with supporting electrolyte, pH, deposition potential, deposition time and the amount of modifier for a 3.0 × 10 −7 mol L−1 Hg(II) and Pb(II) solution were studied.

Fig. 2. Structures of L (A) and its probable interactions in 2:1 complex (B) with M2+ (analyte ions).

3.3.1. Supporting electrolyte and pH The metals have different electrochemical behaviors in different electrolytes. The effect of the nature of supporting electrolytes at different pH values on the voltammetric behavior of the proposed modified carbon paste electrode was investigated. Among five tested supporting electrolytes namely: Britton–Robinson buffer solution, phosphate buffer solution, acetate buffer solution, NH4 Cl buffer solution and KNO3 , the best sensitivity was obtained utilizing a 0.15 mol L−1 KNO3 solution. Voltammetric peaks were observed in all these electrolytes, however, when the measurements were performed in KNO3 solution, the largest stripping peak current, the lowest background current and the best shape of the peaks were obtained. Changes in the KNO3 concentration over the range 0.10–1.0 mol L−1 did not affect the current intensity and the peak potentials of either peak. Also, the effect of the solution pH was also studied in the pH range of 2–9. The voltammetric response was strongly pH dependent. The current peak intensities for Pb(II) and Hg(II) were greater in the KNO3 supporting electrolyte at pH 3.0. Hydrochloric acid has been widely recommended for the determination of several heavy metals by anodic stripping voltammetry [44]. The maximum anodic peak current was observed at pH 3.0 using HCl solution for pH adjustment. At higher pHs, the peak current decreased rapidly and the peak shape was destroyed, at excessively low pHs, the peak

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Fig. 3. (I) Cyclic voltammograms of 1.2 × 10−5 mol L−1 Pb(II) and Hg(II) with scan rate of 100 mV s−1 and (II) square-wave voltammograms of 3.0 × 10−7 mol L−1 Pb(II) and Hg(II) in 0.15 mol L−1 KNO3 (pH 3.0) on the surface of various electrodes. Curves (a) CPE (b) MWCNT–CPE in presence of analytes (c) L–MWCNT–CPE in absence of analytes and (d)) L–MWCNT–CPE in presence of analytes, Accumulation potential: −1.2 V; accumulation time: 90 s; scan rate: 50 mV s−1 ; SW frequency 25 Hz; pulse height: 40 mV.

current decreased because the degradation of L took place and it lost its ability of immobilizing Pb(II) and Hg(II). These reasons might be effective in this behaviour explanation, at pH 3.0, the peak current is high. Continuous increase of pH led to a decrease of the peak currents, which is due to the hydrolysis of Hg(II)and Pb(II) in basic solutions. Also, Zhang et al. reported that Hg2+ exists as the dominant species in the solution at pH ≤ 3.0 and Hg(OH)2 at pH > 5.0 whereas both the species exist in the pH range 3.0–5.0. In the presence of Cl− , the species such as HgCl2 , (HgCl2 )2 , Hg(OH)2 and HgOHCl are also present in small amounts between pH 4.0 and 6.0 [45]. At pH < 2.5, Pb(II) precipitates in the form of chloride. Taking account of the factors above, pH 3.0 was suitable for the supporting electrolyte. In addition to the better detectability obtained with the HCl medium, this medium would be particularly convenient considering that, in sample preparation, this acid is employed to redissolve the residues obtained after the digestion procedure. 3.3.2. Deposition potential The effect of the deposition potentials on metals stripping signals were studied with 3.0 × 10 −7 mol L−1 each of Hg(II) and Pb(II) with the deposition time of 90 s from −0.6 to −1.3 V vs. Ag/AgCl. When the deposition potential shifted from −0.6 to −1.3 V vs. Ag/AgCl, the stripping peak currents increased sharply, as displayed in Fig. 4 with the deposition potential becoming more negative, the reproducibility of stripping currents for Hg(II) and Pb(II) became

poor, because hydrogen evolution was beginning to be significant in the medium at such negative potentials [46]. Also, the metals deposited on the electrode surface might be damaged by the hydrogen bubble and lead to decrease in current signals at very negative potentials. Therefore, −1.2 V was selected for the following experiments due to good sensitivity. 3.3.3. Accumulation time The influence of the deposition time on the stripping peak currents was also studied from 30 to 150 s. As can be seen from Fig. 5, as the accumulation time increased from 30 to 90 s, the stripping peak currents of Hg(II) and Pb(II) increased linearly. However, the linear trend was not obvious with the further prolonging of the deposition time; only a slight increase of the stripping responses was found due to the saturation loading on the electrode surface. For longer accumulation times, currents were found to level off and reach to constant values owing to the saturation loading of active sites at the electrode surface. Furthermore, the long accumulation time might lead to the appearance of a new stripping peak which attributed to the formation of intermetallic compounds. In all experiments, we observed a small peak in the potential value at about +0.40 V, especially in high concentrations. The formation of intermetallic compounds between sample components is one major constraint on quantitative determinations using anodic stripping voltammetric methods. The effect, the nature of which is quiet well known and related to different interactions between elements, i.e., formation of a new compound on or in the modified carbon nanotube paste

22 20

30

18

25 20

14

Pb(II)

12

Hg(II)

15

Pb(II) Hg(II)

10

10

5

8 6

I/µA

I/µA

16

-0.5

-0.6

-0.7

-0.8

-0.9

-1

-1.1

-1.2

-1.3

-1.4

Accumulation potential (V vs. Ag/AgCl) Fig. 4. Effect of the accumulation potential on the peak heights for a solution containing 0.15 mol L−1 KNO3 (pH 3.0), 3.0 × 10−7 mol L−1 Pb(II) and Hg(II); accumulation time: 120 s, scan rate: 50 mV s−1 , SW frequency 25 Hz, pulse height: 40 mV.

0

20

40

60

80

100

120

140

160

accumulation time/s Fig. 5. Effect of the accumulation time on the peak heights for a solution containing 0.15 mol L−1 KNO3 (pH 3.0), 3.0 × 10−7 mol L−1 Pb(II) and Hg(II); accumulation potential: −1.2 V, scan rate: 50 mV s−1 , SW frequency 25 Hz, pulse height: 40 mV.

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Fig. 6. In 0.15 mol L−1 KNO3 (pH 3.0) and under optimum conditions (I) SW voltammograms of Pb(II) in the presence of Hg(II). Pb(II) concentrations (from a to h): 2.00 × 10−3 ; 1.00 × 10−2 ; 3.00 × 10−2 ; 6.00 × 10−2 ; 15.00 × 10−2 ; 3.00 × 10−1 ; 5.00 × 10−1 and 7.00 × 10−1 ␮mol L−1 . (II) SW voltammograms of Hg(II) in the presence of Pb(II). Hg(II) concentrations (from a to h): .00 × 10−3 ; 1.00 × 10−2 ; 3.00 × 10−2 ; 6.00 × 10−2 ; 15.00 × 10−2 ; 3.00 × 10−1 ; 5.00 × 10−1 and 7.00 × 10−1 ␮mol L−1 .

electrode, can reflect on the presence of additional signals, signal depression, etc. [33]. However, it did not have any obvious effects on the response of analytes. Thus, the accumulation time of 90 s was chosen in the following experiments. 3.3.4. Amount of Schiff base (L) The effect of the amount of L within the carbon nanotube paste electrode was evaluated. The use of L as modifier can greatly improve the sensitivity and selectivity of determinations, which is due to its functional sites. The peak current intensity was increased by increasing the amount of modifier, because the concentration of L on the surface of the modified electrode increased correspondingly. At 12% (w/w) of L, the largest peak current was obtained. However, the continuous increase of the amount of modifier caused a decrease in the peak current, because excessive L may result in a decrease in the conductivity of the electrode. So the best ratio of the L in carbon nanotube paste composition was 12% (w/w).

the background noise were 9.0 × 10−4 and 6.0 × 10−4 ␮mol L−1 for Hg(II) and Pb(II), respectively. When compared with the previously reported methods, the present method exhibited the better figures of merit and wide applications. As the results show that the anodic peak current is different for the oxidation of Hg and Pb while the initial concentration was the same. This can be due to one or more of following reasons. One can be attributed to higher affinity between modified electrode surface and Pb(II) in comparison with Hg(II), which was concluded in Fig. 1. Also it may be related to the difference between the diffusion coefficients of Pb(II) and Hg(II). Presence of various species of mercury

3.4. Simultaneous determination of Pb(II) and Hg(II) The SW voltammograms for different concentrations of Pb(II) and Hg(II) in the mixture were investigated when the concentration of one species changed, whereas the other was kept constant. The results are shown in Fig. 6. Examination of Fig. 6(I) shows that the peak current of Pb(II) increased with an increase in its concentration while the concentration of Hg(II) was kept constant (0.26 ␮mol L−1 ). Similarly as shown in Fig. 6(II), keeping the concentration of Pb(II) constant (0.20 ␮mol L−1 ), the oxidation peak current of Hg(II) was directly proportional to its concentration; while the oxidation peak current of Pb(II) did not change. It should be noted that, the change of concentration of one species did not have any influence on the peak current and peak potential of the other species. Indeed the changes are at the relative standard deviations (RSDs) level of individual voltammetric measurement (here about 4%). Calibration plots for the simultaneous determination of Hg(II) and Pb(II) on the modified carbon paste electrode were achieved by SWASVs under the optimal conditions. The SWASVs for different concentrations of Hg(II) and Pb(II) are illustrated in Fig. 7. If the concentrations of Pb(II) and Hg(II) increased synchronously, the peak currents at the modified carbon nanotube paste electrode increase accordingly as shown in Fig. 7. The resulting calibration plots are linear over the range 2.5 × 10−3 to 0.70 ␮mol L−1 for both Hg(II) and Pb(II). The calibration curves and correlation coefficients are y = 48.79X + 0.39, r = 0.9989 and y = 72.90X + 0.723, r = 0.9984 for Hg(II) and Pb(II) with a preconcentration time of 90 s at the deposition potential of −1.2 V vs. Ag/AgCl, respectively. Under the optimum conditions, the limits of detection based on three times

Fig. 7. SW voltammograms of different concentrations of Pb(II) and Hg(II) in 0.15 mol L−1 KNO3 (pH 3.0) and under optimum conditions. Concentrations of Pb(II) and Hg(II): (a) 2.00 × 10−3 ; (b) 1.00 × 10−2 ; (c) 3.00 × 10−2 ; (d) 6.00 × 10−2 ; (e) 15.00 × 10−2 ;(f) 3.00 × 10−1 ;(g) 5.00 × 10−1 and (h) 7.00 × 10−1 ␮mol L−1 .

A. Afkhami et al. / Analytica Chimica Acta 746 (2012) 98–106

105

Table 3 Results for Hg(II) and Pb(II) determination (␮mol L−1 ) in various water samples obtained using the optimum conditions (N = 5). Sample

Analyte

Added

Found

Recovery (%)

Atomic spectrometric methods

Sea water (Persian Gulf water)

Hg(II)

0.00 1.50 × 10−2 0.00 1.50 × 10−2

5.10 × 10−3 2.02 × 10−2 3.25 × 10−2 4.83 × 10−2

(±0.04)a (±0.02) (±0.03) (±0.03)

– 101.9 – 102.5

4.99 × 10−3 2.01 × 10−2 3.29 × 10−2 4.85 × 10−2

(±0.04) (±0.03) (±0.04) (±0.02)

0.00 1.50 × 10−2 0.00 1.50 × 10−2

3.47 × 10−3 1.82 × 10−2 5.65 × 10−2 7.18 × 10−2

(±0.03) (±0.02) (±0.04) (±0.03)

– 98.2 – 102.0

3.51 × 10−3 1.84 × 10−2 5.62 × 10−2 7.11 × 10−2

(±0.03) (±0.04) (±0.02) (±0.04)

0.00 1.50 × 10−2 0.00 1.50 × 10−2

1.78 × 10−1 1.94 × 10−1 5.31 × 10−2 6.84 × 10−2

(±0.02) (±0.02) (±0.04) (±0.03)

– 103.0 – 102.0

1.74 × 10−1 1.88 × 10−1 5.34 × 10−2 6.91 × 10−2

(±0.04) (±0.03) (±0.03) (±0.02)

Pb(II) Hg(II)

Petrochemical wastewater

Pb(II) Hg(II)

Metallurgy wastewater

Pb(II) a

Values in parentheses are S.D. based on five replications.

in solution like HgCl2 , (HgCl2 )2 , Hg(OH)2 and HgOHCl can affect the diffusion of mercury onto the electrode surface. Kinetics of the complexation of cations with Schiff base at the electrode surface can also be responsible for the accumulation of Pb(II) more than Hg(II). These cases can lead to higher peak current for Pb(II) than for Hg(II). 3.5. Repeatability, reproducibility and stability of electrode The repeatability and reproducibility of the electrode in the determinations of target analytes were tested. The repeatability was evaluated by performing 16 times determinations of same standard solutions (1.0 × 10−2 ␮mol L−1 ) Hg(II) and Pb(II). The RSDs were 1.7% and 2.1% for each of Hg(II) and Pb(II), respectively. In the case of reproducibility of electrodes, eight electrodes were prepared in a completely same manner. Then, the reproducibility was performed in the determination of 1.0 × 10−1 ␮mol L−1 Hg(II) and Pb(II) solution. The RSDs for the response of between electrodes was 3.3% and 3.5% for Pb(II) and Hg(II), respectively. Repetitive stripping voltammograms at the same modified electrode were recorded every day over half a month, and the maximum deviations obtained were 3.8% and 4.4%, respectively. The results indicate that the modified carbon nanotube paste electrode has good sensitivity, reproducibility and long-time stability.

3.6. Interference study In order to investigate the selectivity of the prepared electrode for simultaneous determination of Hg(II) and Pb(II), several species were checked as potential interfering species in their analysis. The common existing interferences of inorganic species were investigated to test the selectivity, as well as the influence of some other surface-active compounds. The tolerance limit was taken as the maximum concentration of foreign species that caused a relative error of approximately ±4% for the determination of 1.0 × 10−1 ␮mol L−1 Hg(II) and Pb(II) plus the potential interfering substances at pH 3.0. The interference study was conducted by placing the modified carbon paste into a solution containing target analytes at optimum conditions. It was found that 1000-fold for NH4 + , Ag+ , K+ , Mg2+ , Ca2+ , Mn2+ , Ba2+ , Cd2+ , Cu2+ , Co2+ , Ni2+ , Fe2+ , SCN− , Br− , NO3 − , PO4 3− , and SO4 2− , have no influences on the signal of 1.0 × 10−1 ␮mol L−1 of Hg(II) and Pb(II). Triton X-100, cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS) were selected as the model surface-active compounds to study the possible interferences with the stripping voltammetric measurements of the heavy metal ions. The SW responses resulting from the presence of these potentially interfering species were compared with those obtained for Hg(II) and Pb(II). It was clear that no interference occurred due to these species. This implies the

Table 4 Results for Hg(II) and Pb(II) determination (ng g−1 ) in tuna fish, shrimp, tobacco and human teeth samples obtained under the optimum conditions (N = 5). Sample

Analyte

Added

Found

Recovery (%)

Atomic spectrometric methods

Tuna fish

Hg(II)

0.00 10.00 0.00 10.00

93.78 (±0.11)a 104.0 (±0.12) 25.83 (±0.11) 35.71 (±0.10)

– 102.2 – 98.8

95.13 (±0.14) 104.26 (±0.10) 25.29 (±0.12) 35.69 (±0.15)

0.00 10.00 0.00 10.00

31.45 (±0.07) 41.60 (±0.06) 73.16 (±0.09) 83.27 (±0.06)

– 101.5 – 101.1

31.22 (±0.11) 41.79 (±0.09) 72.84 (±0.10) 83.11 (±0.09)

0.00 10.00 0.00 10.00

189.45 (±0.05) 199.26 (±0.04) 349.55(±0.04) 359.73(±0.04)

– 98.1 – 101.8

189.71 (±0.07) 200.04 (±0.07) 349.81(±0.06) 359.98(±0.05)

0.00 10.00 0.00 10.00

635.70 (±0.07) 645.93 (±0.08) 263.35 (±0.06) 273.29 (±0.05)

– 102.3 – 99.4

635.41 (±0.09) 645.87 (±0.11) 262.79 (±0.12) 273.25 (±0.08)

0.00 10.00 0.00 10.00

315.65 (±0.08) 325.58 (±0.06) 225.83 (±0.07) 235.97 (±0.04)

– 99.3 – 101.4

315.84 (±0.09) 325.71 (±0.07) 226.12 (±0.11) 236.20 (±0.09)

Pb(II)

Shrimp

Hg(II) Pb(II)

Tobacco

Hg(II) Pb(II)

Human teeth (1)

Hg(II) Pb(II)

Human teeth (2)

Hg(II) Pb(II)

a

Values in parentheses are S.D. based on five replications.

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possible direct application of L–MWCNT–CPE in real samples which contain common ions or species. 3.7. Analysis of real samples Monitoring the presence of analyte ions in sea and waste waters is an extremely important task to evaluate environmental and occupation exposure. Also, elevated their concentrations in teeth represent long-term exposure and significant accumulation, particularly in the kidneys and liver. Furthermore, recently some government laboratories reportedly detected traces of heavy metals in the tobacco and some food samples, a more than permitted amount that is announced by international standards. The general trend of modern analytical chemistry is towards the elaboration of simple, ecologically safe, sensitive, and selective methods for the determination of trace components. Therefore, in order to evaluate the applicability of the proposed method for the determination of Hg(II) and Pb(II) in real samples, its utility was tested by determining these compounds in wide variety of samples (Tables 3 and 4). The good recoveries of the samples indicate the applicability of the proposed method for the simultaneous determination of Hg(II) and Pb(II) in such real samples. The standard addition method was used for the analysis of prepared samples. Comparison of the results obtained by the proposed method with those obtained by atomic spectrometric methods (for Hg(II) CV-AAS and GF-AAS for Pb(II)) confirmed the accuracy of the results obtained by our proposed method, showing no significant difference between those methods. The data given in Tables 3 and 4 show the satisfactory results. The results confirm applicability of the proposed method for precise and accurate determination of the analyte ions in a wide variety of real samples with different complex matrices. 4. Conclusion A simple, fast, reproducible and direct procedure was used for the fabrication of a modified carbon paste electrode. MWCNTs and newly synthesized Schiff base as the effective modifier materials, was used to fabricate modified electrode for the simultaneous determination of Hg(II) and Pb(II). The prepared electrode has the advantages of high electrical conductivity and high resistance to interferences compared with unmodified CPE. High sensitivity and selectivity, and very low detection limits together with the ease of preparation and surface regeneration of the modified electrode, and reproducibility of the voltammetric responses makes the proposed modified electrode in question very useful for accurate determination of these ions in various real samples in nano-molar levels. Moreover, such a technique may certainly be a good alternative to CV-AAS and ETV-AAS, which, in the case of determination of the analytes in complex matrices, needs too expensive and complex system equipments. The obtained results agree well with those of CV-AAS and ETV-AAS. These made the system promising to be used in routine analytical applications. Acknowledgement This research is supported by the Bu-Ali Sina University Research Council and Center of Excellence in Development of Chemical Methods (CEDCM).

References [1] J.L. Manzoori, M.H. Sorouraddin, A.M. Haji Shabani, J. Anal. At. Spectrom. 13 (1998) 305–308. [2] A. Afkhami, M. Saber-Tehrani, H. Bagheri, T. Madrakian, Microchim. Acta 172 (2011) 125–136. [3] K. Leopold, M. Foulkes, P.J. Worsfold, Trends Anal. Chem. 28 (2009) 426–435. [4] N. Pourreza, H. Parham, A.R. Kiasat, K. Ghanemi, N. Abdollahi, Talanta 78 (2009) 1293–1297. [5] A. Afkhami, T. Madrakian, H. Siampour, Int. J. Environ. Anal. Chem. 86 (2006) 1165–1173. [6] A. Afkhami, M. Saber-Tehrani, H. Bagheri, J. Hazard. Mater. 181 (2010) 836–844. [7] H. Bagheri, A. Afkhami, M. Saber-Tehrani, H. Khoshsafar, Talanta 97 (2012) 87–95. [8] T. Oymak, S. Tokalıoglu, V. Yılmaz, S. Kartal, D. Aydin, Food Chem. 113 (2009) 1314–1317. [9] Y. Li, Y. Jiang, X.P. Yan, Z.M. Ni, Environ. Sci. Technol. 36 (2002) 4886–4891. [10] E. Sar, H. Berber, B. Asci, H. Cankurtaran, Electroanalysis 20 (2008) 1533–1541. [11] R.H. Ouyang, Z.Q. Zhu, C.E. Tatum, J.Q. Chambers, Z-.L. Xue, J. Electroanal. Chem. 656 (2011) 78–84. [12] A.A. Ensafi, M. Taei, T. Khayamian, Colloids Surf., B 79 (2010) 480–487. [13] Y. Oztekin, Z. Yazicigil, A. Ramanaviciene, A. Ramanavicius, Talanta 85 (2011) 1020–1027. [14] E. Shams, F. Alibeygi, R. Torabi, Electroanalysis 18 (2006) 773–778. [15] M. Ghiaci, B. Rezaei, M. Arshadi, Sens. Actuators B 139 (2009) 494–500. [16] L. Zhu, C.Y. Tian, R.L. Yang, J.L. Zhai, Electroanalysis 20 (2008) 527–533. [17] A. Afkhami, T. Madrakian, H. Ghaedi, Electrochim. Acta 66 (2012) 255–264. [18] M.H. Mashhadizadeh, M. Akbarian, Talanta 78 (2009) 1440–1445. [19] A. Afkhami, T. Madrakian, S.J. Sabounchei, M. Rezaei, S. Samiee, M. Pourshahbaz, Sens. Actuators B 161 (2012) 542–548. [20] S. Shahrokhian, M. Ghalkhani, M.K. Amini, Sens. Actuators B 137 (2009) 669–675. [21] S. Gutierrez-Fernandez, M.C. Blanco-Lopez, M.J. Lobo-Castanon, A.J. MirandaOrdieres, P. Tunon-Blanco, Electroanalysis 16 (2004) 1660–1666. [22] M.R. Ganjali, N. Motakef-Kazami, F. Faridbod, S. Khoee, P. Norouzi, J. Hazard. Mater. 173 (2010) 415–419. [23] A. Nojeh, G.W. Lakatos, S. Peng, K. Cho, R.F.W. Pease, Nano Lett. 3 (2003) 1187–1190. [24] H. Beitollahi, H. Karimi-Maleh, H. Khabazzadeh, Anal. Chem. 80 (2008) 9848–9851. [25] S. Shahrokhian, Z. Kamalzadeh, A. Bezaatpour, D.M. Boghaei, Sens. Actuators B 133 (2008) 599–606. [26] Y.H. Li, X.Y. Liu, X.D. Zeng, Y. Liu, X.T. Liu, W.Z. Wei, S.L. Luo, Sens. Actuators B 139 (2009) 604–610. [27] H.J. Wang, Stripping Analysis: Principles, in: Instrumentation and Application, VCH, Deerfield Beach, FL, 1985. [28] G.M.S. Alves, J.M.C.S. Magalhaes, P. Salaun, C.M.G. van den Berg, H.M.V.M. Soares, Anal. Chim. Acta 703 (2011) 1–7. [29] A.M. Beltagi, E.M. Ghoneim, M.M. Ghoneim, Int. J. Environ. Anal. Chem. 91 (2011) 17–32. [30] E.D. Jeong, M.S. Won, Y.B. Shim, Electroanalysis 6 (1994) 887–893. [31] X.H. Gao, W.Z. Wei, L. Yang, T. Yin, Y. Wang, Anal. Lett. 38 (2005) 2327–2332. [32] I. Cesarino, E.T.G. Cavalheiro, C.M.A. Brett, Electroanalysis 22 (2010) 61–68. [33] P.H. Zhang, S.I. Dong, G.Z. Gu, T.L. Huang, Bull. Korean Chem. Soc. 31 (2010) 2949–2954. [34] I. Cesarino, G. Marino, J. do Rosario Matos, E.T.G. Cavalheiro, Talanta 75 (2008) 15–21. [35] M. Tabatabaee, M.M. Heravi, M. Sharif, F. Esfandiyari, Eur. J. Chem. 8 (2011) 535–540. [36] F. Valentini, A. Amine, S. Orlandocci, M.L. Terranova, G. Palleschi, Anal. Chem. 75 (2003) 5413–5421. [37] T. Madrakian, A. Afkhami, M. Ahmadi, H. Bagheri, J. Hazard. Mater. 196 (2011) 109–114. [38] K. Helrich, Official Methods of Analysis, vol. 1, Association of Official Analytical Chemists (AOAC), 1990. [39] F.A. Silva, I.L. Alcantara, P.S. Roldan, Florentino, P.M. Padilha, Eclet. Quim. 30 (2005) 47–55. [40] W.Z. Yang, Q. Hu, J. Ma, L. Wang, G.G. Yang, G. Xie, J. Braz. Chem. Soc. 17 (2006) 1039–1044. [41] B. Jalevic, H. Odelius, W. Dietz, J.G. Noren, Arch. Oral Biol. 46 (2001) 239–247. [42] A. Afkhami, T. Madrakian, H. Tahmasebi, H. Keypour, H. Khanmohammadi, Phys. Chem. Liq. 46 (2008) 372–378. [43] A. Krishnan, T.S. Anirudhan, J. Hazard. Mater. 92 (2002) 161–183. [44] W. Wasiak, W. Ciszewska, A. Ciszewski, Anal. Chim. Acta 335 (1996) 201–207. [45] F.S. Zhang, J.O. Nriagu, H. Itoh, Water Res. 39 (2005) 389–395. [46] R. Pauliukaite, C.M.A. Brett, Electroanalysis 17 (2005) 1354–1359.