Laboratory Experiments on Electrochemical Remediation of The Environment. Part 3: Microscale Electrokinetic Processing of Soils Jorge G. Ibanez Departamento de Ing. y C. Quimicas, Universidad Iberoamericana, Prol. Reforma 880, 01210 Mexico, D.F. Mexico M. M. Singh, R. M. Pike, and Z. Szafran Department of Chemistry, Merrimack College, 315 Turnpike St., North Andover, MA 01845
In previous papers in this Journal we have addressed the electrochemically assisted cleanup of an oil-in-water emulsion (1) and the indirect oxidation of organic hazardous materials (2). Electric fields have also been used for the decontamination of soils containing unwanted organic or inorganic substances (3–9). In the following experiment, students will be able to observe this phenomenon with the aid of colored species that move along an electric field (or are produced by it) as follows: (i) demonstration of ionic pollutant migration, (ii) demonstration of the creation and movement of an acidic front and a basic front, and (iii) demonstration of water movement though the soil. Theory The remediation of contaminated subsurfaces (groundwater and soil) is frequently the most time- and moneyconsuming task of any site cleanup (10). For the remediation of a piece of land, suitable anodes and cathodes can be strategically placed in the ground and an electric field from a dc source applied. Soil particles in contact with aqueous media are frequently negatively charged owing to chemical and physical adsorption phenomena and lattice imperfections. Then, a double layer naturally forms inside a charged soil pore when cations from the liquid tend to neutralize this charge, and as a result the outer layer of the liquid typically becomes positively charged. The applied electric field produces a movement of this outer layer and a drag interaction between this layer and its bulk inside the soil pore results. The liquid then moves along the potential field to wells where it can be collected and removed (3). This phenomenon is called electroosmotic transport. In addition, chemical reactions at the electrodes produce H2(g) and OH {(aq) (cathode) and O2(g) and H+(aq) (anode), as in the electrolysis of water. These charged species (H+ and OH {), along with other ions encountered in the medium, are attracted to the oppositely charge electrodes and migrate, creating an acidic and a basic front, respectively ( 5). The movement of these fronts is aided by concentration gradients that promote diffusion. However, this movement is dominated by the transport of H+ ions, which are smaller and move faster than OH { or other ions, making the acidic front move faster than the basic front. The acidic front, which can also be produced by direct injection of a suitable acid, can be used to introduce acidity to soils, solubilize basic metal hydroxides, salts or adsorbed species, and protonate electronrich organic functional groups to give molecules a more
cationic character and promote their migration across the electric field, thus facilitating their removal (11). On the other hand, the basic front can have a focusing effect in that, when it encounters a metal ion front, the corresponding metal hydroxide is formed and precipitates in a narrow band of soil. A purging solution (typically containing buffering compounds or weak, nonpolluting acids such as acetic acid) is usually fed into the soil (3, 5, 6 ) to aid in the removal of undesired species, flushing them out by virtue of the electroosmotic effect (which occurs even in soils with low hydraulic conductivities). The purging solution also helps to stabilize the soil, preventing nonhomogeneous spots that would create sites with very high electrical resistivities, thus rendering this technique ineffective. The combined effect of electric, chemical, and hydraulic potentials is known as electrokinetic remediation or electrokinetic processing of soils. It is more powerful than the hydraulic pressure alone, which is used for in situ bioremediation or chemical remediation and which require the transport of nutrients or chemicals through the soil (10). This transport of nutrients or chemicals is particularly difficult to achieve in fine-grained solids with low hydraulic conductivities; the electrokinetic approach does not encounter such a limitation, since the electroosmotic flow is independent of pore size (10). Experimental Procedure and Discussion
Demonstration of Metal Ion Migration The components of a mixture of positive Cu 2+ and negative Cr2O72{ ions are separated by migration under an electric field (12, 13). C AUTION: Cr( VI) compounds are highly toxic and can be carcinogenic. This part should be performed by the instructor as a demonstration. Prepare a simulated soil sample by weighing approximately 1 g of fine silica gel (e.g., of the type used for preparative layer chromatography) in a small weighing dish. Similar dishes will be used as soil containers throughout this paper. Insert a piece of Pb wire or foil (Pt also works well) as the anode in the simulated soil on one side of the dish and a paper-clip cathode on the opposite side, as shown in Figure 1. Cathodes made of iron or stainless steel also work well. Then, add dropwise (and evenly, throughout the entire simulated soil) 2 mL of a green solution 0.1 M in CuSO4 and 0.01 M in K2Cr2O7. Connect the Pb electrode to the positive terminal of a dc power source (e.g., a 9-V battery) and the paper clip to the negative terminal with alligator clips. Within
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a few minutes, some bubbling will be noted at both electrodes due to the evolution of H2 and O2; a yellow-orange spot will be noted at the anode and a blue spot at the cathode, showing the electromigration of the anion and cation, respectively. (Note: this demonstration can also be done using KMnO4 in place of K2Cr2O7 to lower the toxicity of the sample. However, the experiment will take more time and the results are not as spectacular.) For disposal, the Cr(VI) in the soil sample can be reduced to Cr(III) by adding sodium sulfite and then immobilized as the insoluble hydroxide by the addition of NaOH.
Demonstration of Creation and Movement of an Acidic and a Basic Front Weigh approximately 1.5 g of fine silica gel in a weighing dish (as in the previous experiment). Insert the two electrodes as described above. Add dropwise (and evenly) 2–3 mL of 0.1 M Na2SO4 throughout the entire simulated soil as a supporting electrolyte to increase electrical conductivity. In the same manner, add enough drops of a thymol blue indicator solution (1 mg of indicator dissolved in 1 mL of isopropyl alcohol, then diluted to 10 mL with water) so the entire surface is slightly yellow. Connect the electrodes to the dc source. Within a few minutes, a red spot should appear near the anode and a blue color around the cathode. (It may be necessary at this point to add more indicator to improve the visibility of the colors.) The color changes (end points) of thymol blue are pH 1.2, red; 2.8, yellow; 9.2, blue. Demonstration of Water Movement through Soil Weigh approximately 1.5 g of fine silica gel in a weighing dish. Add 2–3 mL of a 0.1 M Na2SO4 solution. (For more dramatic effects, kaolin [hydrated aluminum silicate] may be used. In this case, add 1 mL of the 0.1 M Na 2SO4 solution.) Stir the resulting mixture with a glass rod or a paper clip to form a homogeneous paste. Insert the electrodes as described above, with the paste covering the immersed portion of the electrodes, and connect them to the dc power source. Within a few minutes, water can be noted on the surface around the cathode owing to the electroosmotic migration discussed above. To make the effect more dramatic, a pH indicator can be added at this point. Since the water coming out of the cathode is basic, owing to the production of OH { during water electrolysis, an indicator such as phenolphthalein yields a strongly red solution here (other suitable indicators may be used as well). At the same time, the area surrounding the anode will appear dryer. Electroosmosis has been used to remove water from clays, silts, and fine sands in geochemical engineering (3 ). More recently, it has also been used for the dewatering of solid wastes (14 ).
7, 19), benzene, toluene, trichloroethylene, and m-xylene (4, 7 ), and petroleum and polycyclic aromatic hydrocarbons (18, 19). Removal of up to 90–99% of some of these pollutants has been observed under optimum conditions. Acknowledgments Experimental assistance by Rocio Sanchezarmas, Elizabeth Garcia, and Alejandro Alatorre is gratefully acknowledged. Financial assistance was provided by the National Microscale Chemistry Center, the International Business Office of CONACYT (Mexico), and the Division of Sciences and Engineering of the Universidad Iberoamericana. Literature Cited 1. Ibanez, J. G.; Takimoto, M. M.; Vasquez, R. C.; Basak, S.; Myung, N.; Rajeshwar, K. J. Chem. Educ. 1995, 72, 1050 –1052. 2. Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem. Educ. 1997, 74, 1449–1450. 3. Probstein, R. F.; Hicks, R. E. Science 1993, 260, 468–503. 4. Segall, B.A.; Bruell, C. J. J. Environ. Eng. 1992, 118(1) 84–100. 5. Acar, Y. B.; Alshawabkeh, A. N. Environ. Sci. Technol. 1993, 27, 2638–2647. 6. Shapiro, A. P.; Probstein, R.F. Environ. Sci. Technol. 1993, 27, 283–291. 7. Acar, Y. B.; Li, H.; Gale, R. J. J. Geotech. Eng. 1992, 118, 1837–1851. 8. Hamed, J.; Acar, Y. B.; Gale, R. J. J. Geotech. Eng. 1991, 117, 241–271. 9. Rajeshwar, K.; Ibanez, J. G. Environmental Electrochemistry; Academic: San Diego, 1997. 10. Cabrera-Guzman, D.; Swartzbaugh, J. T.; Weisman, A. W. J. Air Waste Manag. Assoc. 1990, 40, 1670–1676. 11. Rajeshwar, K.; Ibanez, J. G.; Swain, G. J. Appl. Electrochem. 1994, 24, 1077–1091. 12. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook For Teachers of Chemistry, Vol. 4. The University of Wisconsin Press: Madison, WI, 1992; p 150. 13. Little, J. G. J. Chem. Educ. 1990, 67, 1063–1064. 14. Smollen, M.; Kafaar, A. Water Environ. Technol. 1995, 7(11), 13–14. 15. Lageman, R. Environ. Sci. Technol. 1993, 27, 2648–2650. 16. Mattson, E. D.; Lindgren, E. R. In Emerging Technologies in Hazardous Waste Management V; Tedder, D. W.; Pohland, F. G., Eds.; ACS Symposium Series 607; American Chemical Society: Washington, DC, 1995; Chapter 2. 17. Thornton, R. F.; Shaphiro, A. P. In Emerging Technologies in Hazardous Waste Management V; Tedder, D. W.; Pohland, F. G., Eds.; ACS Symposium Series 607; American Chemical Society: Washington, DC, 1995; Chapter 4. 18. Trombly, J. Environ. Sci. Technol. 1994, 28, 289A–291A. 19. Krishnan, R.; Parker, H. W.; Tock, R. W. J. Hazard. Mater. 1996, 48, 111–119.
Conclusions Three phenomena occurring during the electrokinetic processing of soils are demonstrated by treating simulated contaminated soil with an electric field: (i) the migration of ionic pollutants, (ii) the creation and movement of an acidic and a basic front, and (iii) the movement of water under flow direction control. Studies and real site applications in this area include removal of metal ions (e.g., ions of Cd, Cr, Pb, Hg, Ni, Cu, Zn, and U) (4, 5, 7, 8, 15–17 ), arsenic (15), sulfates and nitrates (18), acetate (6 ), phenol and pentachlorophenol (6,
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Alligator Clips
Paper Clip Electrode
Pb Wire Electrode
+ Simulated Soil Weighing Boat
DC Source
Figure 1. Microscale electrokinetic processing of soils.
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