SIMULTANEOUS ARSENIC (III) AND LEAD (II) DETECTION

Environmental Engineering and Management Journal September 2014, Vol.13, No. 9, 2317-2323

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

SIMULTANEOUS ARSENIC (III) AND LEAD (II) DETECTION

FROM AQUEOUS SOLUTION BY ANODIC STRIPPING

SQUARE-WAVE VOLTAMMETRY

Anamaria Baciu1, Aniela Pop1, Florica Manea1, Joop Schoonman2

1“Politehnica” University of Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, Department of Applied Chemistry and Engineering of Inorganic Compounds and Environment, 6 V. Parvan Street, 300223, Timisoara, Romania

2Delft University of Technology, Department of Chemical Engineering, Materials for Energy Conversion and Storage, Julianalaan 136, 2626 BL, Delft, The Netherlands

Abstract Nano-Ag electrodeposited on carbon nanotubes-epoxy and carbon nanofibers-epoxy composites electrode materials as Ag-CNT and Ag-CNF electrodes were tested for simultaneous detection of arsenic (III) and lead (II) from aqueous solution. The electrochemical behaviour of As(III) and Pb(II) on both composite electrode offered useful detection information and Ag-CNF exhibited better electroanalytical performance. The detection scheme using Ag-CNF for simultaneous detection of As(III) and Pb(II) was proposed in this paper based on anodic stripping with square-wave voltammetry (ASSWV) involving two deposition steps characteristic to both species. Key words: arsenic (III), lead (II), simultaneous detection, anodic stripping square-wave voltammetry Received: March, 2014; Revised final: August, 2014; Accepted: September, 2014

Author to whom all correspondence should be addressed: e-mail: [email protected]

1. Introduction

Arsenic (As) and lead (Pb) are common trace elements that belong to the heavy metals class characterized by high toxic properties, which exhibit a negative impact on life quality. Their presence in the aquatic media is due to various natural and anthropogenic factors. Due to their toxic character, the maximum allowance concentration in water imposed by regulation is 10 gL-1 for each (US-EPA, 2000). Moreover, such elements tend to concentrate in all aquatic matrices and especially to biota component of the ecosystem entering in the trophic chain, with a great negative risk on the human health. Several well-known analytical methods have been used for arsenic determination, e.g., chemiluminescence (Fujiwana et al., 1991), chromatography (Gong et al., 2002), spectroscopic methods (Story et al., 1992). These methods are

expensive and require certain skills for operating (Hung et al., 2004). The electrochemical methods for heavy metals detection have been attracted considerable attention due their simplicity, rapidity and high sensitivity. In addition, these methods are very suitable for in-field determination. In particular, voltammetric methods are valid and very effective alternative for the simultaneous determination for the multicomponents analysis (Locatelli et al., 2001). The problem that arises by using the voltammetry consists of the detection of the electroactive components that exhibit very close potential oxidation which leads to the signal overlaping. In general, lead is considered as interference in arsenic detection by electrochemical methods, and in order to detect them simultaneously specific electrode material and procedure are required to be developed.

Baciu et al./Environmental Engineering and Management Journal 13 (2014), 9, 2317-2323

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In order to improve the analytical performances for arsenic detection, several electrochemical methods involving modified carbon-based electrodes by using nanoparticles (NP) have been reported (Dai et al., 2006; Ivandini et al., 2006; Mendez et al., 2009; Shin and Hong, 2010; Simm et al., 2005). The carbon-based composite electrodes involving epoxy matrix have been reported by our group as advanced electrode materials for various analytical application (Motoc et al., 2012; Motoc et al., 2013; Remes et al., 2012). Electrodes based on carbon nanotube materials are superior to the traditional carbon electrodes in terms of the electrocalalytic activities towards various reactions. The electrocatalytic effect is mainly attributed to carbon nanotubes (CNT) or carbon nanofibers (CNF) but the metallic catalyst present on their substrate due to its synthesis improved the electroanalytical signal (Ye et al., 2004). Many studies have been carried out to the fabrication of sensors modified with metal NP. The combination between CNT or CNF and metal nanoparticles is much interesting because metal reveal unique electrical, magnetic, and optical properties that can be improved by the mixing with CNT (Guascito, et al., 2011; Park and Gong, 2012). In particular, silver-decorated carbon nanotubes-epoxy composite has been reported for ibuprofen detection (Manea et al., 2012a). Also, metal-doped zeolite modified electrode have been reported to enhance the detection performance (Baciu et al., 2011; Baciu et al., 2012).

In this study, two types of nanostructured carbon composite electrodes, carbon nanotubes and carbon nanofibers were used as substrates for silver nanoparticles electrodeposition to detect simultaneously arsenic (As) and lead (Pb) from water. The electrochemical behaviour of the modified electrodes, namely Ag-CNT and Ag-CNF, in the

presence of individual As (III) and Pb (II) was studied by cyclic voltammetry based on anodic stripping procedure. To improve the electroanalytical performance for simultaneous detection of As (III) and Pb (II), anodic stripping with square-wave voltammetry (ASSWV) procedure was developed. 2. Experimental 2.1. Materials

The epoxy resin used in the study was Araldite®LY5052/ Aradur®5052 purchased from Huntsman Advanced Materials, Switzerland. Multi-walled CNTs and CNFs synthesized by catalytic carbon vapor deposition were produced by NanocylTM, Belgium. 2.2. Preparation of Ag-electrodecorated CNT and CNF composite electrodes

CNT was dispersed in tetrahydrofuran, 99.9% (THF, Sigma Aldrich) by ultrasonication according to the procedure reported previously by us (Remes et al., 2012). The compositions of the composite electrodes contain of 20 %, wt. CNTs and respective, 20%, wt. CNFs. The disc surface areas of 19.63 mm2 were obtained after filling PVC cylinders with composite materials. The nanostructured carbon composite electrode surface was decorated with silver by electrodeposition at a potential of -0.4 V/SCE for 60 s in the presence of 0.1 M AgNO3

solution. The schematic diagram for the Ag-CNT and Ag-CNF composite preparation is shown in Fig. 1. 2.3. Electrochemical measurements

Electrochemical measurements were performed in unstirred solutions using a computer controlled Autolab potentiostat/galvanostat PGSTAT 302 (EcoChemie, The Netherlands), with a standard three electrodes configuration.

Fig. 1. Schematically procedure of Ag-CNT and Ag-CNF composite preparation

Simultaneous Arsenic (III) and Lead (II) detection from aqueous solution by anodic stripping square-wave voltammetry

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The three-electrode system consisted of a

carbon-based composite working electrode, a platinum wire as counter electrode and a saturated calomel reference electrode (SCE). All experiments were carried out with a typical cell of 50 mL at room temperature (25°C).

3. Results and discussion 3.1. Cyclic voltammetry measurements

During the preliminary studies, the electrochemical behavior of arsenic (III) on carbon nanotubes epoxy (CNT), carbon nanofiber epoxy (CNF), silver-electrochemically decorated carbon nanotubes-epoxy (Ag-CNT) and silver-electrochemically decorated carbon nanofiber-epoxy (Ag-CNF) was studied by cyclic voltammetry (CV) taking into account the deposition step of arsenic (III) on the electrode surface by applying a preconditioning level before CV running. No response to the arsenic (III) presence was noticed on the cyclic voltammogram recorded on CNT and CNF composite electrode and for silver-decorated the signal for arsenic oxidation was recorded only after preconditioning step by maintaining the potential value at -0.4 V/SCE for 60 seconds to assure the arsenic deposition (the results are not shown here).

Taking into account the principle of the anodic stripping voltammetry, which imposed a preconditioning step for arsenic deposition on the electrode surface by its reduction process, the optimization of the deposition potential and time is required. For 3 mM As (III), the deposition potential was varied from -1 to -0.25 V/SCE at CNT composite electrode. Useful signals corresponding to the arsenic stripping process determined as the difference between the arsenic anodic stripping peak current and the background current, for each potential value at 60 seconds are presented in Fig. 2a.

The lowest useful signal was recorded for the deposition potential of -0.25 V/SCE, at which arsenic (III) reduction process just started and the maximum signal was reached for the potential of -0.4 V/SCE, corresponding to the reduction process peak, in according with CV results. More negative potential applying lead to useful signal reducing, due to hydrogen evolution that hampered the arsenic deposition through the bubble formation at the electrode surface.

From these data, the optimum deposition potential of -0.4 V/SCE was chosen for the subsequent anodic stripping voltammetry experiments. In addition, the deposition time was varied between 5 and 180 minutes to select the optimum one based on the useful response for arsenic detection. From Fig. 2b, the optimum deposition time is 120 seconds.

The optimum operating conditions for the deposition step prior to the all anodic stripping voltammetric experiments are the deposition potential of -0.4 V/SCE for the deposition time of 120 seconds.

3.2. Detection measurements

Square-wave voltammetry (SWV) is a technique that enhances the detection sensitivity and it was operated under 0.02 V step potential, 0.2V modulation amplitude and 10 Hz frequency for both composite electrodes.

Series of SWVs recorded at both composite electrodes at different As (III) concentrations ranges subsequently to As (III) electrodeposition at the potential value of -0.4 V/SCE for 120 seconds are presented in Figs 3a, c. The slopes of the linear dependences between the anodic stripping peak currents and As (III) concentrations allowed to determine the sensitivities reached under these working conditions (Figs. 3b, d).

(a)

(b)

Fig. 2. Useful signal corresponding to the 3 mM arsenic (III) anodic stripping peak recorded by CV at CNT in 0.09 M Na2SO4 + 0.01 M H2SO4 supporting electrolyte at: 60 seconds deposition time at various deposition potentials (a); -0.4 V

deposition potential at various deposition time (b); potential scan rate: 0.05 Vs-1


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The electroanalytical parameters achieved

by this technique are presented in Table 1. It can be noticed that Ag-CNF composite electrode exhibited the best performance for As (III) detection. Based on these results, the optimum detection scheme for arsenic (III) determination using Ag-CNF electrode can be proposed as: Step 1 - Deposition: As3++3e-As0, occurred at the potential value of -0.4 V/SCE for 60 seconds; Step 2 - Anodic stripping: As0 As3++3e-, occurred at the potential value of +0.09 V/SCE during square-wave voltammetry running (Manea et al., 2012b).

Taking into account the same principle of anodic stripping using square-wave, voltammetry technique (ASSWV) should be used for the detection of other heavy metals. The electrode characterized by the best performance for arsenic (III) detection was selected for testing in the simultaneous detection of arsenic (III) and lead (II) from the aqueous solutions using this technique. Though, it is very important to consider the specific deposition potential characteristics to each heavy metal.

Table 1. The electroanalytical parameters determined for arsenic (III) anodic stripping determination at silver electrodecorated

composite electrode (60 seconds Ag electrodeposition time) using SWV technique

(a)

(b)

(c)

(d)

Fig. 3. Square-wave voltammograms recorded at 0.02 V step potential, 0.2 modulation amplitude and 10 Hz frequency, between -0.25 and +0.25 V/SCE in 0.09 M Na2SO4+0.01 M H2SO4 supporting electrolyte (curve 1) and in the presence of 0.001-0.01 mM arsenic concentrations (curves 2-11) on the electrodes: Ag-CNT (a) and Ag-CNF (c); Calibration plots of the current densities vs.

arsenic (III) concentration recorded at: E= +0.03 V/SCE on Ag-CNT (b) and E=+0.009 V/SCE on Ag-CNF (d)

Electrode type

Potential value

V / SCE

Sensitivity mA /

mMcm-2

Correlation coefficient, R2

Relative standard deviation, RSD, %

The lowest limit of detection, LOD/ mM

Limit of quantification,

LQ/ M

Ag-CNT 0.03 181.081 0.996 0.019 4.38·10-5 1.46·10-4 Ag-CNF

0.09 218.381 0.996 0.301 1.85·10-5 6.18·10-4


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Thus, based on the literature data regarding

lead detection on silver electrode (Alvesa et al., 2011; Bas and Jakubowska, 2008; Billon et al., 2004; Kirowa-Eisner et al., 1999), and our results of the preliminary tests regarding the influence of the potential value, which varied ranged from -1 to -0.4 V/SCE on the useful signal for lead oxidation, the potential value of -0.7 V/SCE for 60 seconds was selected as optimum for lead deposition prior to the anodic oxidation stripping. In Figure 4 is shown a series of SWVs recorded at Ag-CNF at various lead concentrations ranged from 1 M to 10 M, and the calibration plots corresponding to the dependence of the anodic current densities versus Pb (II) concentration. In comparison with the sensitivity for arsenic detection, the sensitivity for Pb (II) detection is better. It must notice that by conditioning at -0.4 V/SCE that represents the optimum potential for arsenic deposition, no oxidation peak characteristics to stripping Pb (II) was noticed.

Based on these results, the detection scheme for lead determination using Ag-CNF electrode can be proposed as: Step 1 - Deposition: Pb2++2e- Pb0, occurred at the potential value of -0.7 V/SCE for 60 seconds; Step 2 - Anodic stripping: Pb0 +2e- Pb2+, occurred at the potential value of -0.25 V/SCE during square-wave voltammetry running. The above-presented results suggested us to test each deposition condition for the simultaneous detection of arsenic (III) and lead (II). Neither the potential value of -0.4 V/SCE characteristics to arsenic deposition, nor the potential value of -0.7 V/SCE characteristics to lead (II) deposition, were suitable for the simultaneous detection of arsenic (III) and lead (II). This led to try to modify the detection scheme, by introduction of a new deposition step. Thus, the following schemes were applied to detect simultaneously arsenic (III) and lead (II): Variant I:

Step 1- Deposition: As3++3e-As0, occurred at the potential value of -0.4 V/SCE for 60 seconds; Step 2 - Deposition: Pb2++2e- Pb0, occurred at the potential value of -0.7 V/SCE for 60 seconds; Step 3 - Anodic stripping: As3++3e-As0 and Pb0

+2e- Pb2+, occurred during square-wave voltammetry running. Variant II: Step 1 - Deposition: Pb2++2e- Pb0, occurred at the potential value of -0.7 V/SCE for 60 seconds; Step 2 - Deposition: As3++3e-As0, occurred at the potential value of -0.4 V/SCE for 60 seconds; Step 3 - Anodic stripping: As3++3e-As0 and Pb0

+2e- Pb2+, occurred during square-wave voltammetry running.

The SWVs recorded by the application of the electrodetection- variant I are presented in Figure 5a. Square-wave voltammograms were recorded continuously after deposition steps in 0.09 M Na2SO4 +0.01 M H2SO4 supporting electrolyte by continuous alternative adding of 0.02 mM arsenic (III) concentration and of 0.005 mM lead (II) concentration reached a mixture containing 0.14 mM As (III) and 0.035 mM Pb (II). It is very clear that the anodic stripping peak for lead (II) appeared at -0.4 V/SCE and the anodic stripping peak for arsenic (III) appeared at -0.25 V/SCE. The calibration plots of the current densities versus lead (II) and respective, arsenic (III) concentration for each potential value are shown in Fig. 5b. The electroanalytical parameters determined for arsenic (III) and lead (II) detection for each individual/simultaneous detection scheme are presented in Table 2. No reproducible results recorded by SWV were reached applying variant II.

Based on these results, it can be concluded that the proposed detection scheme for simultaneous detection of As (III) and Pb (II) from aqueous solution allowed detecting each ion but with lower sensitivity, especial for As (III) detection.

Fig. 4. a) Square-wave voltammograms recorded at Ag-CNF composite electrode under 0.02 V step potential, 0.2 modulation amplitude and 10 Hz frequency, between -0.5 and -0.1 V/SCE in 0.09 M Na2SO4+0.01 M H2SO4 supporting electrolyte (curve 1) and in the presence of 0.001-0.01 mM lead (II) concentrations (curves 2-11); b) Calibration plots of the current densities recorded

at: E= -0.25 V/SCE vs. lead (II) concentration


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Fig. 5. (a) Square-wave voltammograms recorded at Ag-CNF composite electrode under 0.02 V step potential, 0.2 V modulation amplitude and 10 Hz frequency, between -0.5 and 0 V/SCE in 0.09 M Na2SO4+0.01 M H2SO4 supporting electrolyte (curve 1) and

in the presence of: 2- 0.02 mM As, 3- mixture of 0.02 mM As and 0.005 mM Pb, 4-mixture of 0.04mM As and 0.01 mM Pb, 5- mixture of 0.06 mM As and 0.015 mM Pb, 7-mixture of 0.08mM As and 0.02 mM Pb, 9-mixture of 0.1 mM As and 0.025 mM

Pb, 11-mixture of 0.12 mM As and 0.03mM Pb, 13-mixture of 0.014 mM As and 0.035 mM Pb; (b) Calibration plots of the current densities recorded at: E= -0.4 V/SCE vs. lead (II) concentration (curve a) and E=-0.25V/SCE vs. arsenic (III)

concentration (curve b) Table 2. The electroanalytical parameters determined for individual and simultaneous arsenic (III) and lead (II) anodic stripping

determination at Ag-CNF composite electrode using square-wave voltammetry

Analyte Electrodetection

scheme/Potential value, V vs. SCE

Sensitivity mA /

mMcm-2

Correlation coefficient, R2

Relative standard

deviation, RSD, %

The lowest limit of

detection LOD/ mM

Limit of quantification,

LQ/ M

Individual/0.09 218.381 0.996 0.301 1.85*10-5 6.18*10-4 As (III)

Simultaneous/-0.2 79.642 0.989 2.455 0.01 0.03 Individual/-0.2 236.636 0.991 0.831 0.0003 0.001

Pb (II) Simultaneous/-0.4 161.238 0.989 0.824 0.002 0.006

No peaks corresponding to the stripping

simultaneously arsenic and lead appears for the variant II detection scheme, which informed that this scheme is not suitable for the simultaneous detection of arsenic and lead. 4. Conclusions

Two types of modified composite

electrodes, Ag-CNT and Ag-CNF, based on Ag electrodeposited on carbon nanotubes-epoxy and carbon nanofibers-epoxy composite were successfully obtained using two-steps synthesis method consisted of composite obtaining by two-roll mil procedure and Ag nanoparticles decoration of composite surface by electrodeposition. Both Ag-CNT and Ag-CNF allowed detection of individual arsenic (III) and lead (II) species from aqueous solution, Ag-CNF exhibited better performance.

A two-deposition steps based anodic stripping using square wave voltammetry procedure was developed for simultaneous detection of As(III) and Pb(II) at Ag-CNF composite electrode. The electroanalytical performance for this electrode informed about its great potential for the detection applications in real waters.

Acknowledgements This work was partially supported by the Romanian-Swiss Research Programme IZERZ0 - 142210/1 (24 RO-CH/01.01.2013) funded by SNSF and UEFISCDI, partially by the PNII-Ideas-165/2011.

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SIMULTANEOUS ARSENIC (III) AND LEAD (II) DETECTION