A carbon paste electrode modified with kaolin for the ...

Applied Clay Science 43 (2009) 130–134

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

A carbon paste electrode modified with kaolin for the detection of diquat M.A. El Mhammedi a,⁎, M. Bakasse b, R. Najih a, A. Chtaini a a b

Equipe d'Electrochimie et des Matériaux Inorganiques, Université Sultan Moulay Slimane, Faculté des Sciences et Techniques, BP: 523, Beni-Mellal, Morocco Equipe d'Analyse des Micro-Polluants Organiques, Faculté des Sciences, Université Chouaib Doukkali, BP: 20, El jadida, Morocco

a r t i c l e

i n f o

Article history: Received 3 December 2007 Received in revised form 21 July 2008 Accepted 23 July 2008 Available online 29 August 2008 Keywords: Carbon paste electrode Kaolin Diquat Square wave voltammetry

a b s t r a c t A carbon paste electrode modified with kaolin was employed for the quantification of diquat from aqueous solutions. The new electrode (K-CPE) revealed interesting electroanalytical detection of diquat based on the adsorption of this herbicide onto kaolin under open circuit conditions. The diquat reduction peaks were observed around − 0.69 V and − 0.97 V respectively for peaks P1 and P2 (vs. SCE) in 0.1 mol L− 1 K2SO4 using square wave voltammetry. The response depends on the concentration of diquat in the bulk solution as well as the parameters involved in the preconcentration, pH and the measurement steps. The best results were obtained under the following optimized conditions: 5 min accumulation time, 5 mV pulse amplitude, 1 mV s− 1 scan rate in 0.1 mol L− 1 potassium sulfate. Using such parameters a linear dynamic range up 80 mol L− 1 diquat was observed with a detection and quantification limits of 4.21 × 10− 9 mol L− 1 and 3.55 × 10− 8 mol L− 1 respectively. The recovery (I%) was obtained by spiking natural water with 2 × 10− 5 mol L− 1 diquat. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Increasing use of pesticides and herbicides in agriculture, forestry, and domestic activities for controlling pests is polluting our water resources day by day. The leaching run-off from agricultural and forest lands; deposition from aerial applications and discharge of industrial wastewater are responsible for this water contamination (Gupta, 2004). The pesticides form a strong class of water pollutants, as they are sometimes nonbiodegradable. Pesticides are carcinogenic in nature. Therefore, toxicity of pesticides and their degradation products is making these chemical substances a potential hazard by contaminating our environment (Schultz et al., 2003). Diquat (1, 1′-dimethyl-4, 4′-bipyridilium dibromide) is one of the most widely used herbicides, and held the largest share of the global herbicide market until recently overtaken by glyphosate. This chemical type of herbicide – a bipyridyl – is shared with few other pesticides. Diquat is one of the most hazardous compounds for human health (Agriculture Canada, 1982); the repeated exposures may cause skin irritation, sensitization, or ulcerations on contact (Vanholder et al., 1981). It is a potential contaminant of waters due to its high solubility (about 620 g L− 1 at 25 °C) (Hazardous Substances Databank, 1988). Therefore, it is necessary to determine these hazardous materials in water. In the past, several methods have been developed to detect and quantify these compounds in waters and soils such as capillary electrophoresis (CE) (Carneiro et al., 2000; Vinner et al., 2001), mass spec-

⁎ Corresponding author. Tel.: +212 68858296; fax: +212 23485201. E-mail address: [email protected] (M.A. El Mhammedi). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.07.021

trometry coupled to either capillary electrophoresis (Nunez et al., 2002) or HPLC (Aramendia et al., 2006) and radioimmunoassay (Braithwaite, 1987). Each of these methods requires enrichment steps to determine concentrations in the range of 0.1 g L− 1. Recently, several interesting electrochemical determinations on modified electrodes have been developed using square wave voltammetry (El Mhammedi et al. 2008; El Mhammedi et al., 2007). Chemically modified electrodes are of increasing interest in modern electroanalysis, as they allow the accumulation of analytes on the electrode surface. They are also easily prepared by adding the modifier directly to a carbon paste electrode. This preconcentration step increases the sensitivity and selectivity of the determination. In this paper, we are going to study the application of kaolin as modifier in the construction of a carbon paste electrode for the determination of diquat at nanomolar concentrations. The advantages of using a CPE include the availability of a wide potential range for analysis, easily renewable surface, and simplicity of fabrication. 2. Experimental 2.1. Reagents Graphite powder was supplied from (Carbone, Lorraine, ref 9900, France). Diquat was obtained from (Sigma, St. Louis, MO, USA) and used as received. Commercial kaolin was supplied by ECESA (Lugo, Spain) and consisted of 95 wt.% kaolinite, 5 wt.% mica and traces of halloysites. All electrochemical experiments were performed in K2SO4 as supporting electrolyte. pH was adjusted by 0.1 H2SO4 or NaOH. All chemicals were of analytical grade and were used without further purification. Bi-distilled deionized water (BDW) was used throughout the work.

M.A. El Mhammedi et al. / Applied Clay Science 43 (2009) 130–134

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Fig. 1. X-ray diffraction diagram of the kaolin, K: kaolin, Q: quartz, C: calcite.

2.2. Apparatus

range was scanned from 10° to 70° with a step size of 0.02° 2θ per second.

All electrochemical experiments were performed with a potentiostat (model PGSTAT 100, Eco Chemie B.V., The Netherlands), equipped with a three-electrode system mounted on cell. The working electrode was a carbon paste or kaolin modified carbon paste. An SCE and a platinum electrode were used as reference and counter electrodes respectively. The pH-meter (Radiometer Copenhagen, PHM210, Tacussel, French) was used for adjusting pH values. Scanning electron microscopic (SEM) pictures of kaolin were obtained using a JEOL JSM 840 scanning electron microscope at 5 kV accelerating voltage. Infrared spectra for both as-prepared and calcined powders were obtained using an infrared Fourier Spectrometer (FT-IR, model Mattson Galaxy S-7000, USA). Each powder was mixed with KBr in the proportion of 1/150 (by weight) for 15 min and pressed into pellets using a hand press. X-ray diffraction studies on both as-prepared and calcined powders were carried out using a high resolution XRD: Cu Kα radiation, XPERT), (λ = 1.5406 nm) produced at 30 kV and 25 mA. The 2 Theta

2.3. Fabrication of the K-CPE Kaolin and graphite powder were mixed in different ratios by mass (m/m) with potassium sulfate. A certain quantity of graphite paste containing kaolin was packed firmly into the cavity (0.1256 cm2) of a plastic pipette tip. Electrical contact was established with bare carbon. The resulting electrode is hereby denoted as K-CPE. The carbon paste electrode alone (CPE) was prepared in a similar way. The surface of each electrode was wetted with distilled deionised water before use. Current densities were calculated from the geometric surface area of the electrode. 2.4. Electrochemical procedure The modified CPE was immersed in 20 ml 0.1 mol L − 1 K2SO4 containing a known amount of diquat solution at open circuit. The electrochemical cell was deaerated (with nitrogen, 99.99%) for ten minutes. Cyclic and square wave voltammetric experiments were

Fig. 2. Infrared spectra of the kaolin.

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performed. For cyclic voltammetry the potential range was from −0.3 to −1.2 V versus SCE and scan rate was 50 mV s− 1. The parameters square wave voltammetry measurements were: a step potential of 25 mV; amplitude 5 mV and duration 5 s at scan rate 1 mV s − 1. All measurements were carried out at room temperature. A calibration curve was obtained after optimization of the voltammetric conditions, by adding diquat to supporting electrolyte. The standard deviation of the mean current measured at the reduction potential of diquat for five voltammograms of the blank solution in pure electrolytes was used in the determination of the quantification limit (10σ/slope) and detection limit (3σ/slope) together with the slope of the straight line of the analytical curves. 2.5. Analysis samples For the square wave voltammetric determination of diquat ions in natural samples, 20 mL of a fresh sample was pipetted into the voltammetric cell and the voltammetric measurements were carried out promptly under the optimized conditions. The SW parameters were the same as described above. The quantification of the trace diquat was done by the standard addition. All peak currents and electric charges quoted are mean values of five replicate measurements. 3. Results

Fig. 4. Voltammetric curve for K-CPE preconcentred for 5 min in 1.9 × 10− 3 mol L − 1 diquat, scan rate: 50 mV s− 1; supporting electrolyte 0.1 M K2SO4 (pH 8.0) and 50% (w/w) of K/CP ratio.

(Moore and Reynolds, 1997). In the FTIR spectrum (Fig. 2) the characteristic bands of kaolinite appeared as reported in the literature (Percival et al., 1974). SEM of the kaolin particles showed dense surfaces (Fig. 3) and interparticle pores. 3.2. Voltammetric responses of diquat

3.1. Kaolin The XRD patterns (Fig. 1) showed kaolinite (K) as major clay mineral and quartz (Q). Some kaolins contained traces of calcite (C)

Fig. 4 shows the voltamogram of immobilized diquat onto kaolin in 0.1 mol L− 1 K2SO4 electrolyte. Two pairs of reversible peaks (P1,P4) and (P2,P3) were observed for K-CPE between −0.2 to −1.2 V probably due to DQ2+/DQ+. and DQ+./DQ0 respectively. For comparative response of the two electrodes (CPE and K-CPE), a square wave voltametry was employed since it is well recognized to be a more sensitive voltammetric technique than the normal cyclic voltametry. Very sharp and well-defined peaks (Fig. 5) were obtained at K-CPE between −0.2 to −1.2 V. A control experiment was performed under the same conditions in the absence of kaolin. Compared to CPE, K-CPE clearly showed higher sensitivity towards diquat in terms of an enhanced current density. 3.3. Optimization Supporting electrolytes have been known to enhance the electroanalytic response of kaolin modified electrodes towards the detection of diquat by increasing either the current response and/or lowering the detection potential. We studied the electrochemical response characteristics of the K-CPE in 0.1 mol L− 1 Na2HPO4, 0.1 mol L− 1 K2SO4

Fig. 3. Scanning electron micrograph of the kaolin.

Fig. 5. Square wave voltammetry of 9.5 × 10− 5 mol L− 1 diquat in 0.1 M K2SO4 (pH 8.0) at different carbon paste electrodes (a) CPE, (b) K-CPE (1:1); accumulation time: 10 min.


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Table 1 Results obtained from square wave voltametry curves of diquat in electrolytes considering peak 1 and peak 2 Electrolytes

iP1 (μA)

iP2 (μA)

Na2HPO4 (0.1 mol L− 1) K2SO4 (0.1 mol L− 1) HClO4 (0.1 mol L− 1)

23 31 11.5

12.3 19.8 6.7

and 0.1 mol L− 1 HClO4 solutions at different pH. Reduction peaks were observed in all these electrolytes. In K2SO4 the cathodic peak currents were higher, and better defined peak shapes were observed (Table 1). Hence, all subsequent experiments were carried out in 0.1 mol L− 1 K2SO4 as supporting electrolyte. The effect of the preconcentration time on the square wave voltammetric measurements was studied in solutions (pH 8.0) containing 1.2 × 10− 4 mol L − 1 of diquat at K-CPE (1:1). The current intensity increased suddenly to reach a maximum after 5 min of accumulation. Normally, the increase in the response current continued until a maximum signal level (presumably corresponding to either saturation or an equilibrium surface coverage) is attained. Hence, a 5 min preconcentration time was employed in all subsequent experiments. The current response of K-CPE (1:1) at constant diquat concentration (1.2 ×10− 4 mol L− 1) increased from to pH 5.4 until 9.5, with a maximum at pH 8.0 and decreased up to pH 12.0. The reduced sensitivity in acidic solution can be related to the slow dissolution of kaolinite in acidic solution. The peak potentials do not however seem to be affected by the concentration of protons, suggesting the absence of any protonation step. Therefore, only a 0.1 mol L− 1 K2SO4 (pH 8.0) was used in further studies. The dependence of the cathodic current response on the kaolin loading in the carbon paste at pH 8.0 was performed in 0.1 mol L− 1 K2SO4 containing 9.5 × 10− 5 mol L− 1 diquat (Fig. 6). The current response increased with increasing amount of kaolin until the mass of the modifier was about 25% of the total mass of the paste. The background current also increased and became too large if the concentration of modifier was higher than 25% of kaolin loading by mass. This result is probably due to the reduced conductivity of the modified electrode. Presumably, more kaolin at the electrode surface reduces the amount of conductive area of the activated graphite particles. Hence a 25% kaolin-modified carbon paste electrode was used throughout this work. 3.4. Calibration There was a steady increase in current response following addition of increasing concentrations of diquat in pH 8.0 potassium sulphate solutions with K-CPE (Fig. 7). A plot of current response (Ip) versus

Fig. 6. Influence of amount of modifier on peak current of 9.5 × 10− 5 mol L− 1 diquat in square wave voltammetry at K-CPE in 0.1 mol L− 1 potassium sulphate, the preconcentation time was 10 min.

Fig. 7. Square wave voltammograms at K-CPE electrode in 0.1 mol L− 1 K2SO4 for different concentrations of diquat; (a) 1.2× 10− 7 mol L− 1, (b) 8.7 × 10− 6 mol L− 1, (c) 2.19 × 10− 5 mol L− 1, (d) 4.75 × 10− 5 mol L− 1, (e) 7.95 × 10− 5 mol L− 1.

diquat concentration was linear up 80 mol L− 1, reaching a plateau at concentrations N80 mol L− 1 due to a possible decrease in the conductive area of the modifier. The regression equation of the linear plot for peaks P1 and P2 were Ip1 (μA) = 0.8646 [diquat] + 10.747 (r2 = 0.9909) and Ip2 = Ip2 (μA) = 0.574 [diquat] + 7.8448 (r2 = 0.9930). The value of relative standard deviation has been used for the determination of the detection limit (DL) and quantification limit (QL). For this value the DL and QL were 4.21 ×10− 9 mol L− 1 and 3.55×10− 8 mol L− 1 for P1 and 8.44×10− 9 mol L − 1 and 5.32×10− 8 mol L− 1 for P2. The precision of this methodology for the determination of diquat was evaluated for five successive measurements of the same samples containing 5.0 × 10− 7 mol L− 1 diquat. The deviation coefficients were 1.9% and 2.5% for the peaks 1 and 2. 3.5. Practical applications The analytical performance of the method was investigated to determine the diquat ions in spiked river water. The latter was collected in Oum Er Rbia river and analyzed without any pre-treatment in the preparation of supporting electrolyte by adding 0.1 mol L− 1 K2SO4. The quality of this river is generally good except the section situated in Kasba-Tadla (point 3) and Dar Ouled Zidouh (point 5) witch presents a high level of organic matter (Chemical oxygen demand, Biological oxygen demand) equal to (120 mg L− 1, 32 mg L− 1) and (33 mg L− 1, 3 mg L− 1) respectively. The determination of diquat with K-CPE was performed under the optimal conditions. The effect of five samples on the signal

Fig. 8. Signal of current (I %) in five points in the Oum Er Rbia river at K-CPE under the optimized conductions.

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corresponding to reduction of diquat was studied (Fig. 8). I (in %) was defined as I% ¼ Ip=Ipmax 100

ð1Þ

where Ip is the current of the peak in river water and Ipmax is the current of the peak in pure water. A satisfactory signal was obtained with 2 × 10− 5 mol L − 1 herbicide. The recovery was lower in Kasba-Tadla and Dar Ouled Zidouh water. The analytical sensitivity, defined by the slop of the calibration curves, was decreased. Probably due to the presence of organic matter which can inhibit the adsorption process of diquat at the modifier surfaces. Interference is possible because of the complexity of the matrix. Before use, the samples have been filtered through a membrane. 4. Conclusion This study shows that kaolin in a carbon paste electrode allowed to quantify diquat concentrations at nanomolar levels. A more sensitive electrochemical method for the determination of diquat in its raw material form and its commercial formulations in river water samples has been developed. This type of electrode offers certain advantages over conventional electrodes by ease of fabrication, activity, sensitivity and simplicity. Thus, K-CPE shows great promise for potential sensing applications, such as in amperometric sensors for flow-injection analysis. References Agriculture Canada, 1982. Guide to the chemicals used in crop protection. 7e édition. Publication no. 1093.

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