Trace determination of cadmium in water using anodic stripping voltammetry at a carbon paste electrode modified with coconut shell powder
© Rajawat et al.; licensee Springer. 2014
Received: 17 March 2013
Accepted: 8 January 2014
Published: 8 May 2014
Increasing awareness on the environmental impact of heavy metals has increased a considerable interest in the determination of metals in natural water bodies. The present paper describes the development and electrochemical application of carbon paste electrode modified with fibrous part of coconut shell for the determination of cadmium in water samples.
Determination was carried out using anodic stripping voltammetry. It is a two-step process. First, the metal ions get accumulated at the electrode surface at open-circuit potential, followed by a potential scan for voltammetric determination of cadmium.
Different parameters affecting the determination of Cd (II) were optimized and are as follows: HCl as stripping solvent, acetate buffer of pH 5 as accumulating solvent, and 15-min accumulation time. Triton X-100, cetyltrimethylammonium bromide, and sodium dodecyl sulfate were used as representative for neutral, cationic, and anionic surfactants, respectively, to see the effect of surface active macromolecules. Interference caused by other metal ions on the determination of cadmium was also studied.
The method shows the development of a sensor for the sensitive determination of cadmium with limit of detection at 105 μg L−1. This technique does not use mercury and, therefore, has a positive environmental benefit.
KeywordsPlant-modified carbon paste electrode Cocos nucifera Cadmium Stripping voltammetry
Cadmium is classified as one of the priority pollutants which entered water streams through various industrial operations (Pan et al. ). It is ranked seventh by the Environmental Protection Agency in ‘Top hazardous substances priority list’. Cadmium can easily be dissolved and transported by water (Li et al. [2009a, b]). However, due to anthropogenic activities, its content can be elevated at the site of the action. High concentrations of cadmium ions can injure human health and pollute the environment. It is carcinogenic to human by damaging human immune and central nervous systems and causes diseases such as renal dysfunction and liver damage. Hence, the identification of cadmium-polluted sites is needed by society (Eshaghi et al. ).
Different analytical methods for the determination of Cd (II) ions have been reviewed several times (Sneddon and Vincent ; Ferreira et al. ; Pyrzynska ; Davis et al. ). Among them is stripping voltammetric determination of cadmium using mercury-based electrodes which is one of the very sensitive analytical methods available. But, due to different issues related to its harmful effects and disposal, it is strongly recommended to replace mercury with another electrode material. Recently, modified carbon paste electrodes can be a better substitute of mercury-based electrodes due to its simplicity of preparation, the versatility of chemical modification, rapid renewal of the electrode surface, and sensitivity equivalent to that of mercury-based electrodes (Roa et al. ; Sar et al. ; Heitzmann et al. ; Lu et al. ; Li et al. [2009a, b]; Bagheri et al. ). Thus, modified carbon paste electrodes (MCPEs) and related sensors using different types of modifiers (chemicals, enzymes, and extracts) have been developed (Chow and Gooding ; Heitzmann et al. ; Ensafi et al. ; Portaccio et al. ).
In comparison to the conventionally used MCPEs, plant-modified carbon paste electrodes represent a green approach in the environmental perspectives. The use of plant agricultural wastes as a modifier in carbon paste electrodes is due to the high metal hyper-accumulating properties in certain plants (Rajawat et al. [2013b]; Mojica et al. [2006, 2007]). They possess an electrochemically or chemically active moiety. These moieties could be any of the following: redox or ligand sites and ion-exchange sites, which possess certain functionalities or donor groups (Rajawat and Satsangee ).
Coconut shell has been widely used as an agricultural waste material for the sorption of Cd (II) from aqueous solution (Pino et al. ; Okafor et al. ). In continuation of our previous research work on modified carbon paste electrodes (Rajawat et al. [2012, 2013a]) and keeping the above views in mind, the powder of coconut shell (Cocos nucifera) was used to modify the carbon paste electrode with the main goal of using it as a modifier material for the development of a sensor for the determination of cadmium.
Chemicals and reagents
For DPASV study, first, accumulation was done under open-circuit potential by placing the electrode in a metal solution with stirring for a certain time, rinsed with deionized distilled water followed by medium exchange for stripping analysis.
All chemicals were of analytical reagent grade. A 1,000-ppm stock solution of Cd (II) was prepared by dissolving an appropriate amount of cadmium nitrate (Merck & Co., Inc., Whitehouse Station, NJ, USA). The working solution was prepared daily by the dilutions from the stock solution. Graphite powder (<20 μm) and mineral oil Nujol (light, density 0.838) were obtained from Aldrich (Wyoming, IL, USA). Triply distilled water (ELGA, Millipore Co., Billerica, MA, USA) was used throughout the experiment.
All quantitative measurements were carried out in anodic stripping voltammetry using differential pulse (DP) to achieve the sensitivity required for trace analysis. Each DPASV run was made up of two steps: accumulation under open circuit where the modified electrode is immersed in metal solution for a certain time. The electrode was then rinsed with deionized distilled water, followed by medium ex-change for stripping analysis. All measurements were carried out at room temperature (24 ± 1°C). Finally, the calibration curves were plotted and the influence of various substances as potential interference compounds on the determination of Cd(II) ions was studied under the optimum conditions.
Preparation of coconut shell powder-modified carbon paste electrode
Coconut was purchased from the local market of Agra. The coconut shell between the outer layer and inner layer (i.e., mesocarp) was separated. It was properly washed with water and dried in an oven at 50°C. The dried material was grounded and passed through the sieve. Fraction of the particles with size less than 150 μm was selected for electrode preparation. Unmodified carbon paste electrode was prepared by mixing the graphite powder with the mineral oil (80:20 w/w ratio) using mortar and pestle. MCPEs of different proportions (5%, 10%, 15%, 20%, 25%, and 30% w/w) were prepared by substituting the corresponding amount of graphite powder with coconut shell powder. The mixture is thoroughly hand-mixed in a mortar and pestle. The paste was pressed in a glass tube with an inner diameter of 3 mm and a depth of 4 cm to form a target electrode surface. A copper wire was inserted from the backside for electrical contact. A smooth and fresh electrode surface was obtained by pushing the electrode material from the backside, removing a small amount of paste from the electrode tip, and polishing the electrode surface on a photo paper.
Results and discussion
Characterization of the coconut shell powder-modified carbon paste electrode
All the electrochemical experiments were performed using a μAutolab Type III potentiostat (Eco Chemie, Utrecht, Netherlands) controlled by a PC using the NOVA 1.8 software. A three-electrode system containing the modified carbon paste electrode as working, an Ag/AgCl (3.0 mol L−1 KCl) as reference, and a platinum wire as auxiliary electrodes was used.
Electrochemical impedance study
Electrochemical studies for metal determination
Mechanism of accumulation
Based on our experimental findings and pertinent information available on the relevant topic, a mechanism for metal binding at the modified electrode surface is proposed. Coconut shell powder contains oxygen-containing functional groups in lignins and cellulose. These groups may constitute a physiologically active group to interact with the Cd (II) ions. The mechanism of the accumulation at the modified electrode is as follows:
Different parameters affecting the voltammetric determination of Cd (II) such as amount of modifier, accumulation media, accumulation time, and stripping media were investigated.
Amount of modifier
The accumulation of cadmium was examined in supporting electrolytes such as acid solution, base solution, and different buffers like acetate buffer, phosphate buffer, and Britton-Robinson buffer. Voltammetric peaks were observed in most of these electrolytes; however, in acetate buffer solution, the anodic peak current was higher, and better defined peak shape was observed for Cd (II). A baseline for the determination of metals in acetate buffer is comparatively low, suggesting acetate buffer as the best accumulating medium.
The influence of accumulation time on the stripping peak currents of 0.1, 1, and 10 ppm of Cd (II) in 1 mM sodium acetate buffer solution was investigated. An increase in the current response was observed with increasing pre-concentration time initially, which indicates that cadmium was rapidly adsorbed on the modified electrode surface while further prolonged accumulation did not improve the peak height. In comparison to the three concentrations of metal ions selected for this experiment, the electrode surface gets saturated early for higher concentration compared to the low concentration. For further experiment, 1 ppm Cd (II) solution was used with accumulation time of 10 min.
Determination of metals in the presence of surfactants
The effect of the presence of other metal ions present in the solution on the anodic peak currents of Cd (II) was evaluated. A 10% change in the current response is assumed as a constant current response. No effect on the determination of Cd (II) was observed up to 250-fold of Pb (II), 5-fold of Cu (II), 25-fold Ni (II), and 30-fold Cr (VI). The results of interference study can be justified by hard-soft acid base theory. The main binding sites in these modifiers are COOH and OH groups, which are hard bases, whereas the metals Pb (II) and Cu (II) are intermediate; Cd (II), Hg (I), and Zn (II) are soft acids. According to hard-soft acid base theory, hard acid tends to form complexes with hard bases, and soft acids tend to form complexes with soft bases. Since copper is strongly bonded on electrode surface, so it is the most interfering metal.
The detection limit is evaluated to be about 105 ppb (S/N = 3) after a 15-min accumulation. The limit of detection of the prepared electrodes was compared with the previously prepared electrodes; it is comparatively better with some of the previously reported electrodes (Beltagi et al. ; Roa et al. ). The stability of the prepared electrode was determined using the DPASV for the same modified electrode with an interval of 2 weeks over 6 months, and it was found 6 months, assuming 5% change in the current response as a constant current response.
The present paper demonstrates a simple, ecofriendly, and sensitive electrochemical method for the determination of cadmium based on the coconut shell powder-modified carbon paste electrode. Cyclic voltammetry and electrochemical impedance spectroscopy study results confirm the incorporation of coconut shell powder at the electrode surface. Open-circuit accumulation, followed by anodic stripping voltammetry, was used for the determination of cadmium. An enhancement in the current response was observed in the presence of anionic surfactants. Different factors affecting the sensitivity of the prepared electrode were optimized. The optimized conditions for the determination of Cd (II) using CS-MCPE are acetate buffer of pH = 5 as accumulating solvent, 15 min accumulation time and hydrochloric acid as the stripping solvent. Despite some mutual interference effects, cadmium ions can be reliably determined with low detection limits using the standard addition procedure. The utilization of plant-based electrode in place of mercury-based electrodes is an attempt to perform environment friendly electrochemical determination of cadmium.
carbon paste electrode:
coconut shell powder-modified carbon paste electrode:
differential pulse anodic stripping voltammetry:
Fourier transform infrared:
modified carbon paste electrode:
sodium dodecyl sulfate:
scanning electron microscope:
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