In the Laboratory The Microscale Laboratory
Laboratory Experiments on Electrochemical Remediation of the Environment. Part 4: Color Removal of Simulated Wastewater by Electrocoagulation–Electroflotation Jorge G. Ibanez Departamento Ing. y C. Quimicas, Universidad Iberoamericana, Prol Reforma 880, 01210 Mexico, D.F. Mexico M. M. Singh and Z. Szafran Department of Chemistry, Merrimack College, 315 Turnpike St., North Andover, MA 01845
Billions of liters of aqueous waste streams from the world’s textile mills and dye production plants are generated every day. Per liter, these wastes can contain up to 20 g of dyes and intermediates that can cause environmental problems by absorbing light and interfering with fundamental aquatic biological processes (1). In addition, some of these dyestuffs are toxic and can also have negative aesthetic impact (which is of immediate concern to the general public) (2–6 ). Several kinds of treatments have been used to remove color from these effluents, including physical, chemical, and biological treatments and hybrid processes (2, 4,6–8). Physical treatments include adsorption on activated carbon, reverse osmosis, and ultra filtration, which are limited by the low concentration ranges that can be treated coupled with the high concentrations in reject streams. Chemical treatments include oxidation with O3, Cl2, H2O2, HOCl, or NaOCl; reduction with Na2S2O4, Na2SO3, or NaHSO3; and precipitation/flocculation/coagulation. Their main drawbacks are the addition of further chemicals, and for chlorine-containing moieties, the possibility of forming toxic chlorinated organic substances. In addition, the precipitates formed are normally somewhat voluminous. Biological treatment (bioremediation) is often the process of choice when treating large volumes of effluents. Unfortunately, these processes are not very effective in the case of textile effluents and are often sensitive to the chemicals present as well as to variations in pH and composition. Hybrid processes involve combining two of the processes described earlier to optimize their individual advantages (e.g., chemical coagulation and electrolysis) (6, 8). Electrochemical processes have been under intense study for this application. The idea here is to take advantage of a combined effect due to the production of electrolysis gases (H2 and O 2) and the production of polyvalent cations from the oxidation of corrodible anodes (such as Fe and Al). The gas bubbles would carry the pollutant to the top of the solution where it can be more easily concentrated, collected, and removed. The metallic ions can react with the OH᎑ ions produced at the cathode during the evolution of H2 gas, to yield insoluble hydroxides that will adsorb pollutants out of the solution and also contribute to coagulation by neutralizing any negatively charged colloidal particles that might be present. The solid sludge thus produced has been reported to be more compact than sludges obtained by chemical methods (9–11). In the case of iron or steel anodes, two mechanisms for the production of the metal hydroxide have been proposed (2, 3, 5, 7). 1040
Mechanism 1 Anode:
4 Fe (s) = 4 Fe2+(aq) + 8 e᎑ 4 Fe 2+ (aq) + 10 H2O (l) + O2 (g) = 4 Fe (OH)3(s) + 8 H+(aq)
Cathode: 8 H+(aq) + 8 e᎑ = 4 H2 (g) Overall: 4 Fe (s) + 10 H2O (l) + O 2(g) = 4 Fe (OH)3(s) + 4 H2(g)
Mechanism 2 Anode:
Fe (s) = + 2 e᎑ Fe 2+ (aq) + 2 OH᎑(aq) = Fe (OH) 2(s) Fe2+(aq)
Cathode: 2 H2O (l) + 2 e᎑ = H2 (g) + 2 OH᎑(aq) Overall:
Fe(s) + 2 H2O (l) = Fe(OH)2(s) + H2(g)
Once the insoluble metal hydroxide (iron, in this case) is produced, it can remove pollutants by surface complexation or electrostatic attraction (2, 12). In surface complexation it is assumed that the pollutant can act as a ligand (L) to bind a hydrous iron moiety: L–H(aq) + (HO)OFe(s) → L–OFe(s) + H2O(l) In electrostatic attraction, the hydrated iron oxide particles contain regions of apparent positive and negative charge, which attract the opposite regions of the polluting species and remove them from solution. In previous experiments we have addressed different ways in which electrochemistry can help achieve environmental remediation (13–15). In this experiment, an iron electrode (paper clip) is used to provide metallic ions for the formation of an insoluble hydroxide that will adsorb a dye present in the solution. In addition, gas bubbles produced at the cathode attach some of the flocs formed by the hydroxide and help the separation stage of this dye removal procedure. The dyes used in the experiment are pH indicators, so spectacular color changes will be noticed. Experimental Procedure Place 4 mL of H2O in a 10-mL beaker with 100 mg of Na2SO4 as an inert electrolyte and a few drops of thymol blue indicator (pH 1.2, red; 2.8, yellow; 9.2, blue) as a waste dye surrogate. (Ten milligrams of thymol blue in 10 mL of isopropyl alcohol diluted with water to 50 mL gives enough indicating solution for a large class.) Add 0.01 M H2SO4 dropwise with stirring until the pink endpoint is reached. Stop at this point, since too much acid in the solution will delay the desired electrocoagulation effect. Take 1 mL of this pink solution and set it aside as a color reference. (NOTE: Universal indicator [e.g., Fisher Universal Indicator] may also
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
In the Laboratory
Plastic Sleeve
Paper Clip Electrodes
containing Whatman #1 filter paper, collect the filtrate, and measure its pH. For comparison, take the initial color reference solution, adjust its pH to the pH value of the filtrate, and observe if there is any difference. This qualitative experiment can be made quantitative by measuring the corresponding absorbances in a spectrophotometer with the aid of a standard calibration plot. Conclusions
Alligator Clips Beral Pipet
DC Source –
+
Figure 1. Microelectrochemical cell for color removal in dye wastewater.
be used in place of thymol blue with similarly spectacular results. It gives several color changes in the pH range 4–10. To use this, place 4 mL of 0.01 M H2SO4 in a 10-mL beaker and add 40 drops of the indicator. Take 1 mL of the resulting solution and set it aside as a color reference). Fabricate a microelectrochemical cell by inserting two steel paper clips in opposite sides of a Beral pipet (14) as shown in Figure 1. These will later be connected to a dc power source (e.g., a 9-V battery). A little twist can be made on each wire so as to fit the wires somewhat tightly around the stem of the pipet and keep them from touching each other and shorting the circuit. A 1-cm piece of the stem also can be cut and preinserted on one of the wires for this purpose, as shown. A simpler cell can be made by inserting bent paper clips in a 10-mL beaker and connecting them with alligator clips to the dc power source. Stirring may be provided with a homemade microstirring bar (seal a piece of paper clip in a glass capillary tube) and a magnetic stirring plate. Draw the waste dye surrogate solution into the microelectrochemical cell. Connect the wires with alligator clips to the dc power source. At this point, the anode of the cell starts dissolving slowly while the cathode produces hydrogen bubbles. The pH increases around the cathode, as represented by the equations given previously. The indicator dye will immediately start changing color around the cathode and a sludge (containing iron hydroxide) will start to form. In 15– 30 minutes there will be enough hydroxide sludge to adsorb most of the dye present in the solution and the experiment can be discontinued. Shake the cell and its contents well, expel the solution through the pipet stem into a microfunnel
An electrochemical cell equipped with a corrodible anode (Fe) that produces polyvalent metal cations can be used for the production of a metal hydroxide sludge capable of removing significant quantities of pollutants from a waste solution. This principle is used here for the removal of waste dye surrogates from an aqueous solution. Acknowledgments Experimental assistance by Rocio Sanchezarmas, Alejandro Alatorre, and Elizabeth García (U. Iberoamericana) and helpful comments from Ronald Pike (Merrimack College) are gratefully acknowledged. Financial assistance was provided by the International Business Office of CONACYT (Mexico), the Division of Sciences and Engineering of the Universidad Iberoamericana, and the National Microscale Chemistry Center (Merrimack College). Literature Cited 1. Lin, S. H.; Peng, C. F. Water Res. 1994, 28, 277–282. 2. Wilcock, A. Textile Chemist Colorist 1992, 24(11), 29–36. 3. McClung, S. M.; Lemley, A. T. Preprinted Extended Abstracts, 204th Meeting of the American Chemical Society, Washington, DC, August 1992; Vol. 32, No. 2, p 376. 4. Wilcock, A.; Brewster, M.; Tincher, W. Am. Dyestuff Rep. 1992, 81(8), 15–23. 5. Wilcock, A. E.; Hay, S. P. Can. Text. J. 1991, 108(4), 37–44. 6. Youchun, Z.; Dunwen, L.; Yongqi, Z.; Jianmin, L.; Meiqiang, L. Water Treat. 1991, 6, 227–236. 7. Endyuskin, P. N.; Selezenkin, S. V.; Dyumaev, K. M. J. Appl. Chem. USSR 1983, 56, 1100–1102. 8. Endyuskin, P. N.; Selezenkin, S. V.; Dyumaev, K. M.; Ultrivanova, M. A. J. Appl. Chem. USSR 1979, 52, 558–562. 9. Sorkin, E. I.; Kucheryavykh, E. I.; Vykhovanets, V. V. J. Appl. Chem. USSR 1983, 56, 63–66. 10. Bersier, P. M. Electrochemical Decolorization of Dye Effluents; presented at 6th International Forum on Electrolysis: Environmental Applications of Electrochemical Technology; The Electrosynthesis Co., Miami, FL, Nov 1992. 11. Persin, F.; Rumeau, M. Trib. Eau 1989, 82(3), 45–56. 12. Rajeshwar, K.; Ibanez, J. G. Environmental Electrochemistry; Academic: San Diego, 1997; Chapter 5. 13. Ibanez, J. G.; Takimoto, M. M.; Vasquez, R. C.; Basak, S.; Myung, N.; Rajeshwar, K. J. Chem. Educ. 1995, 72, 1050–1052. 14. Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem. Educ. 1997, 74, 1449–1450. 15. Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem. Educ. 1998, 75, 634–635.
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