In the Laboratory
Electrochemical Removal of Metal Cations from Wastewater Monitored by Differential Pulse Polarography
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Delphine Bruce, Alexander Kuhn, and Neso Sojic* Laboratoire d’Analyse Chimique par Reconnaissance Moléculaire, Ecole Nationale Supérieure de Chimie et de Physique de Bordeaux, 16, Avenue Pey-Berland, 33607 Pessac, France; *
[email protected] The experiments described in this article have been chosen to demonstrate that electrochemistry can not only be used for analytical purposes but also to solve environmental issues. Electrochemical processes can contribute to a cleaner environment through the treatment of wastewater by elimination or recycling of pollutants (1–3). In this Journal, Ibanez et al. have reported various interesting applications of electrochemical techniques for wastewater and effluent treatment (4–8). In the present case the wastewater contaminants are electrochemically active metal cations and therefore electrodeposition is a possible route for their elimination. The combination of this approach with electroanalytical techniques makes electrochemistry even more attractive. Identification and quantification of ion mixtures in solution are general goals in analytical chemistry and differential pulse polarography (DPP) is a sensitive and extremely useful technique in this context (9–11). The experiments use DPP combined with the electrodeposition of several metal cations. A solution containing Cd2+, Cu2+, and Zn2+ simulates an industrial wastewater. The goal is to eliminate metal cations from the effluent down to micromolar concentration levels. The electrolytic elimination step consists of the electrodeposition of metal ions at a three-dimensional high surface area cathode. DPP is used to monitor the kinetics of the electrochemical treatment. The students become familiar with techniques and concepts such as DPP, mass transport, three-dimensional electrodes, and the thermodynamic and kinetic aspects of an electrochemical process. The experiment is suitable for a four-hour project in an analytical or environmental chemistry laboratory course. It may be the occasion to illustrate and discuss general concepts and techniques of electrochemistry and to attract the student’s attention to environmental problems.
pump (Iwaki Magnet Pump, model MD-10-230GS01) allows convection of the solution in the reactor and operates at a 10 L兾min flow rate. The advantages of this reactor are the ease of operation and the low price of the material used. It is commercially available ($1300), but in principle the experiment can also be performed in a simple 3-L beaker with a stirring system. The cathode could be homemade using plastic grids containing the carbon particles and a stainless steel electrode to make an electric contact with this material of high electroactive area. The electrodes are connected to a dc power source (AFX5510A) that is operated at a fixed current of 2 A. The electrolysis is conducted for about three hours for 2-L solution volume. The concentration of metal cations in the reactor was monitored electrochemically from aliquots collected every 10 or 20 minutes. The aliquot volume is very small (about 5 mL). Therefore, we assume that this volume modification is negligible.
Electroanalysis The polarographic experiments were performed with a polarographic stand (Radiometer MDE 150) using a dropping mercury electrode, a platinum auxiliary electrode, and
A
C
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Experimental Procedure The solution simulating an industrial wastewater is a mixture of CuSO4 (10᎑2 M), ZnSO4 (10᎑2 M), and Cd(NO3)2 (5 × 10᎑3 M) in the presence of a supporting electrolyte K2SO4 (2 × 10᎑1 M). The pH of the solution is adjusted to three by adding few drops of 3 M sulfuric acid. This pH value was chosen to avoid the formation of metal cation hydroxides.
Electrodeposition The reactor (Socem-Elec) is a pilot-scale test system developed for the recovery of precious metals but can also be used for the electrolytic treatment of effluents. It is formed by a three-dimensional cathode and two oxide-covered titanium anodes (Figure 1). The cathode consists of a bed of about 1-mm diameter carbon particles (Socem-Elec). The
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pump Figure 1. Simplified schematic drawing of the electrodeposition reactor with the three-dimensional cathode (C) and the anodes (A). The arrows indicate the flow direction.
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Ag兾AgCl reference electrode. It was coupled to a polarographic analyzer (Radiometer POL150), which is controlled by a software TraceMaster5 (Radiometer). The drop time was set at 1 s and the scan rate at 5 mV兾s. With DPP, the voltage pulse and the pulse time were 25 mV and 40 ms, respectively. All the samples were degassed for 100 s with N2 to avoid interferences with the cathodic reactions studied. The samples collected after 10, 20, and 30 minutes were 10-fold diluted in K2SO4 (2 × 10᎑1 M) prior to the measurements. The following samples, collected after 30 minutes, were not diluted and analyzed as collected.
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Hazards Mercury is highly toxic by inhalation, in contact with skin, and if swallowed. Because of its low vapor pressure (0.002 mm Hg at 20 ⬚C), mercury waste should be stored under water. Sulfuric acid is corrosive. The electrolytic solution should be prepared in a well-ventilated hood since concentrated acid is used. Also, care should be taken during the electrolysis to prevent accidental contact with electrodes as the current applied through the reactor is as high as 2 A. Results
Comparison of Classic Polarography and Differential Pulse Polarography One important application of polarography is the determination of metal ions in environmental analysis. DPP is one of the most widely used polarographic methods today (9, 11–13). In comparison with classic polarography, this technique does not produce a wave, but a peak-shaped signal with the current maximum at roughly the half-wave potential (E1兾2) of the classic polarograms (Figure 2). Essentially the DPP curve can be considered as the differential of the classic wave form (10). In solution, several cations can be detected sequentially depending on their formal potential (E°′) as a result of the cathodic reaction : Mn+ + ne− → M Figure 2 shows classic and DPP polarograms of the starting (t = 0) solution that simulates an industrial wastewater. We can observe three peaks corresponding to the reduction of Cu2+, Zn2+, and Cd2+. At this point of the experiment, the students are asked to identify the peaks with the related cation reductions using standard tables. In order to assign the different peaks, we compare the standard potentials (14) and the reduction potentials of the species involved in the redox process (Table 1). We can find an acceptable agreement between the theoretical and experimental data. Starting from the most positive potential and scanning in a negative direction, Cu2+, Cd2+, and Zn2+ are successively reduced at the mercury drop.
Calibration Curves The detection limit of DPP is orders of magnitude lower than that of classic polarography (11). We therefore used this differential method to monitor the cation concentrations in the samples collected during the electrochemical wastewater
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Figure 2. Comparison of the classic (dashed line) and differential pulse (solid line) polarograms obtained in a deaerated solution containing Cu2+ (10᎑3 M), Zn2+ (10᎑3 M), Cd2+ (5x10᎑4 M) in the presence of a supporting electrolyte at pH = 3.
Table 1. Comparison of the Half-Wave Potentials (E1/2) and Peak Potentials (Ep) and the Standard Potentials (E°) of the Redox Couples Potential
Cu(II)/Cu
Cd(II)/Cd
Zn(II)/Zn
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Ep
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᎑990
E°
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᎑960
E1/2
NOTE: The data are in millivolts versus Ag/AgCl.
treatment. Calibration curves were obtained from polarograms recorded with several dilutions of the starting wastewater solution. Plotting the peak current (IP) as a function of the corresponding cation concentration gives a straight line (Figure 3). This linear dependence verifies the theoretical relation (10):
IP = nFA
D π tm
σ = exp
σ −1 C = KC σ +1 nF ∆E RT 2
where n is the number of electrons exchanged, F is Faraday constant (C mol᎑1), A is the electrode area (cm2), C is the concentration of the electroactive species (mole cm᎑3), D is the diffusion coefficient of the electroactive species (cm2 s᎑1), tm is the pulse time (s), σ is the dimensionless parameter related to voltage pulse, ∆E is the voltage pulse (V), R is the gas constant (J mol᎑1 K᎑1), and T is the absolute temperature (K). As shown in Figure 3, the linear plots obtained were perfectly adjusted to the experimental points. These calibration curves were used in the following section to calculate the unknown concentrations of the different cations during the electrochemical treatment.
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Cu Zn Cd
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The authors would like to thank the Ecole Nationale Supérieure de Chimie et Physique de Bordeaux, the Conseil Régional d’Aquitaine, and Novelect for financial support.
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t = 0 min t = 30 min t = 60 min t = 120 min
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Figure 3. Calibration curves for three cations obtained by DPP from the diluted starting solution. The copper concentrations were: 1 x 10᎑3, 6 x 10᎑4, 3 x 10᎑4, 1 x 10᎑4, 3 x 10᎑5, 10᎑5, and 3 x 10᎑6 M.
Conclusion
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The objective of the experiment is to eliminate polluting metals contained in industrial effluents by reducing them directly at the cathode of an electrolytic reactor. Using a cathode with a high electroactive area, metal cations were removed down to micromolar concentration levels. Similar results might be reached with a classic two-dimensional cathode but a much longer electrodeposition time is needed, which is not suitable for an industrial process or a chemistry laboratory course. The cation concentration followed by differential pulse polarography gives qualitative and quantitative information about the elimination process. At the end of the experiment, students should be familiar with some simple electrochemical equipment and understand a few basic concepts and techniques. Furthermore they learn that electrochemistry can be used for analytical purposes and also to solve some environmental problems.
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Thermodynamic and Kinetic Aspects of the Electrochemical Treatment The starting solution, which simulates an industrial wastewater, contains three metal cations: Cu2+, Zn2+, and Cd2+ at different concentrations. We studied the kinetics of electrodeposition of the different cations during three hours of electrolysis. At regular time intervals, aliquots of the reaction mixture were collected in order to follow by DPP the evolution of the concentration of each electroactive species. Figure 4A shows the polarograms obtained for different characteristic reaction times. The ion concentrations were determined from the measurement of the polarographic peak height and by using the calibration curves mentioned above. Plotting the concentrations against reaction time yields the kinetics of the electrochemical treatment. These results are shown in Figure 4B. We notice that Cu2+ is removed first from the solution, in less than 60 minutes. The copper concentration is then below the detection limit. During this initial stage, the concentration of the other two cations is roughly constant. This means that one can selectively electrodeposit this metal ion from the mixture and get almost pure copper at the cathode. After 100 minutes, more than 70% of the cadmium is consumed while 50% of the zinc remains in solution. The latter ion can be totally removed after 180 minutes. At that time, all the metal ion concentrations are below the micromolar range. Cu2+, Cd2+, and Zn2+ are successively eliminated from the solution. The students can verify that the ion characterized by the most positive standard potential is deposited preferentially onto the electrode. The process using a three-dimensional cathode is fast and efficient. It allows the removal of metal ions down to micromolar concentration levels even in the case of solutions containing several metal cations. Furthermore, in an industrial environment, this procedure allows the recycling of some of the metals.
Zn2ⴙ Cd 2ⴙ Cu2ⴙ
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t / min Figure 4. (A) Differential pulse polarograms obtained in a deaerated cation solution: Cu2+ (10᎑2 M), Zn2+ (10᎑2 M), Cd2+ (5 x 10᎑3 M) in presence of supporting electrolyte at pH = 3 and corresponding to different reaction times (t = 0, 30, 60, and 120 min). (B) Plot of the kinetics of electrochemical removal of metal cations followed by DPP as a function of electrolysis time.
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Supplemental Material
Information for the instructor, photos of the equipment, and student report forms are available in this issue of JCE Online. Literature Cited 1. Bersier, P. M.; Carlsson, L.; Bersier, J. Electrochemistry for a Better Environment; Springer-Verlag: Berlin, 1994; Vol. 170, pp 113–229. 2. Osella, D.; Ravera, M.; Soave, C.; Scorza, S. J. Chem. Educ. 2002, 79, 343–344. 3. Rajeshwar, K.; Ibanez, J. G. Environmental Electrochemistry; Academic: San Diego, CA, 1997. 4. Ibanez, J. G.; Takimoto, M. M.; Vasquez, R. C.; Basak, S.; Myung, N.; Rajeshwar, K. J. Chem. Educ. 1995, 72, 1050–1052. 5. Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem.
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Educ. 1997, 74, 1449–1450. 6. Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem. Educ. 1998, 75, 634–635. 7. Ibanez, J. G.; Singh, M. M.; Szafran, Z. J. Chem. Educ. 1998, 75, 1040–1041. 8. Ibanez, J. G. J. Chem. Educ. 2001, 78, 778–779. 9. García-Armada, P.; Losada, J.; de Vicente-Pérez, S. J. Chem. Educ. 1996, 73, 544–546. 10. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2001. 11. Riley, T.; Watson, A. Polarography and Other Voltammetric Methods; Wiley: New York, 1987. 12. Uslu, B.; Özkan, S. A.; Aboul-Enein, H. Y. Electroanalysis 2002, 14, 736–740. 13. López-de-Alba, P. L.; López-Martínez, L.; De-León-Rodríguez, L. M. Electroanalysis 2002, 14, 197–205. 14. Handbook of Chemistry and Physics, 68th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1987.
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