ELECTROCHEMICAL REMEDIATION OF THE ENVIRONMENT: FUNDAMENTALS AND MICROSCALE LABORATORY EXPERIMENT

Jorge G. Ibanez

Centro Mexicano de Quimica en Microescala, Universidad Iberoamericana, D. F. Mexico

jorge.ibanez@uia.mx

The degree of toxicity of a wide range of pollutants can be substantially decreased by oxidation or reduction processes.  A large number of such processes can be performed by electron transfer at electrode surfaces.  Electrochemical techniques offer in specific cases some advantages relative to other technologies for different environmental remediation schemes: environmental compatibility, versatility, energy efficiency, selectivity, amenability to automation and cost effectiveness.  We have developed a series of microscale laboratory experiments aimed at demonstrating some of the applications of electrochemical techniques to the remediation of hazardous wastes, oil/water emulsions, textile dyeing wastes, contaminated soils, etc.  The main electrochemical strategies used for environmental remediation involve the following:

Direct Electrolysis

Pollutants capable of undergoing direct electrochemical oxidation or reduction at an electrode can in principle be transformed and/or removed from water streams or reservoirs by the application of appropriate potentials in electrochemical reactors.  Here, oxidation or reduction processes occur directly on inert electrodes without the involvement of other substances (e.g. electron mediators, biocidal species).  Direct Electrolysis methods include: 1. Anodic Processes. 2. Cathodic Processes.

Indirect Electrolysis

Direct processes are often limited by diffusion and thus low current densities need to be used if high efficiencies are desired.  In addition, the high overpotentials required often necessitate noble metals or similarly expensive compounds to operate.  Alternatively, homogeneous or heterogeneous, cathodic or anodic redox mediated processes have been envisaged to overcome such difficulties, usually at low temperatures and pressures.  The idea here is to use an electrochemically generated redox reagent as a chemical reactant (or catalyst) to convert pollutants to less harmful products.  The redox reagent acts then as an intermediary for shuttling electrons between the pollutant substrate and the electrode.  These indirect processes can be performed as: 1. Reversible Processes. 2. Irreversible Processes.

Electrokinetic Remediation of Soils

Electric fields as well as electron transfer processes have been used for the decontamination of soils and underground water containing unwanted organic or inorganic substances.  The main phenomena involved here are: electroosmosis, electrophoresis, and electromigration.  When suitable anodes and cathodes are strategically buried in the ground or placed in contact with a slurry and an electric field from a DC source is applied, one or more of these phenomena occur and the resulting effect is used for the removal of polluting substances.  The technique has also been called electroreclamation, electroosmotic purging, electroremediation, electrorestoration and electrokinetic processing.

Electroremediation of Gases

Polluting gases must generally be transferred by absorption or reaction to the liquid phase (normally aqueous media) before they can undergo electrochemical oxidation or reduction.  This conversion can be effected in two absorption modes: a) the gas is directly absorbed in an electrochemical cell for treatment (inner-cell process), or b) the gas is absorbed in a separate reservoir and then transferred to the electrochemical cell (outer-cell process).

Electrochemical Disinfection of Water

Potent disinfecting agents are produced electrochemically and then added to water to kill unwanted microorganisms.  A key advantage of this approach involves the possibility of producing them in situ.

Membrane-based Processes and/or ionic exchange

The reagents, products and electrode materials involved in the cathodic and anodic electrode reactions of an electrochemical cell can sometimes interact in an undesired manner as to foster short circuits or side reactions that decrease the figures of merit of the system or can even lead to accidents.  Electrochemists have devised ways to prevent such interactions by using special separators like ion selective membranes (IEMs) which are thin polymeric materials that incorporate ionic centers within their structure capable of attaching ions of opposite charge; such ions can then selectively and directionally migrate under the influence of an electric field and be separated from their counter ions (permselectivity).

 

Laboratory Experiments On Electrochemical Remediation Of The Environment. Part 1. Electrocoagulation Of Oily Wastewater

 

Voluntary or involuntary spilling of oil in different kinds of waters is an issue of relevant environmental concern.  Oil can significantly alter the properties of water and produce: optical changes like color and opacity (with the concomitant absorption of the light necessary for photo biological cycles), esthetic impact, foul smell, bad taste, viscosity, conductivity, etc.  Unfortunately, there are many sources of water-polluting oil such as mills, refineries, off-shore platforms, cutting machines, oil transportation, distribution and storage facilities that undergo spills yielding several millions of tons than end up in water reservoirs and the sea every year; about half this amount contaminates fresh water.  An additional piece of information is that we use as an average a little less than four liters (about one gallon) of hydrocarbons per person each day in the world.

 

The objective of this paper is to describe a laboratory experiment in which an electrical current is passed through an oil/water emulsion using a stable cathode (stainless steel) and an oxidizable anode (iron).  This process leads to a break-up of the emulsion, forming an oil layer which can be easily removed by mechanical means.

 

Bibliography

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Hamed, J.; Acar, Y.B.; Gale, R.J. J. Geotech. Eng., 1991, 117 , 241-271.

Ibanez, J. G.; Takimoto, M. M.; Vasquez, R. C.; Basak, S.; Myung, N.; Rajeshwar, K. J. Chem. Educ., 1995, 72, 1050-1052.

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Krishnan, R.; Parker, H. W.; Tock, R. W. J. Haz. Mat., 1996, XLVIII, 111-119.

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Probstein, R.F.; Hicks, R.E. Science, 1993, 260, 468-503.

Rajeshwar, K.; Ibanez, J. G., Environmental Electrochemistry, Academic Press: San Diego (in press).

Rajeshwar, K.; Ibanez J.G.; Swain, G., J. Appl. Electrochem., 1994, 24, 1077-1091.

Segall, B.A.; Bruell, C.J., J. Envir. Eng., 1992, 118 (1) 84-100.

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Laboratory Experiments On Electrochemical Remediation Of The Environment. Part 2. Microscale Indirect Electrolytic Destruction Of Organic Wastes

 

The objective of this experiment is to destroy, at the microscale level, a sample of surrogate organic waste by generating a powerful oxidizer at the anode of an electrochemical cell.  This generated species oxidizes the waste to harmless products.  The oxidizer can then be regenerated and recycled.  Specifically, this experiment utilizes a redox mediator with a high standard potential (i.e., the Co (III/II) couple, E = 1.82 V) to destroy a surrogate organic waste (e.g., glycerin or acetic acid) by converting it into CO2 and water.  Students can observe the end of the reaction signaled by a color change of the electrolytic medium (from pink to gray-light purple) as well as the evolution of CO2 which precipitates CaCO3 from a Ca(OH)2 solution.  The Co(II) solution and the electrodes can then be reused.

 

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Laboratory Experiments On Electrochemical Remediation Of The Environment. Part 3. Microscale Electrokinetic Processing Of Soils

 

Electrochemical remediation of the environment is gaining widespread acceptance due to the mild conditions used, the cleanliness of the electron as a reagent, the easiness for automation, its versatility, etc.  In this paper three phenomena are presented at the microscale level, originating from the application of an electric field to a simulated soil sample: a) Demonstration of metal ion migration, b) Demonstration of the creation and movement of an acidic and a basic front, and c) Demonstration of water movement through soil.

 

Bibliography

Acar, Y.B.; Li, H.; Gale, R.J. J. Geotech. Eng., 1992, 118, 1837-1851.

Acar, Y.B.; Alshawabkeh, A.N. Envir. Sci. Tech., 1993, 27, 2638-2647.

Cabrera-Guzman, D.; Swartzbaugh, J.T.; Weisman, A.W. J. Air Waste Manag. Assoc., 1990, 40, 1670-1676.

Hamed, J.; Acar, Y.B.; Gale, R.J. J. Geotech. Eng., 1991, 117, 241-271.

Ibanez, J. G.; Singh, M. M.; Szafran, Z.; Pike, R. M. J. Chem. Educ., 1998, 75 (5) 634-635.

Ibanez, J. G.; Takimoto, M. M.; Vasquez, R. C.; Basak, S.; Myung, N.; Rajeshwar, K. J. Chem. Educ., 1995, 72, 1050-1052.

Ibanez, J. G.; Singh, M. M.; Pike, R. M.; Szafran, Z. J. Chem. Educ. (in press).

Krishnan, R.; Parker, H. W.; Tock, R. W. J. Haz. Mat., 1996, XLVIII, 111-119.

Lageman, R. Envir. Sci. Tech., 1993, 27, 2648-2650.

Little, J. G. J. Chem. Educ., 1990, 67, 1063-1064.

Mattson, E. D.; Lindgren, E. R., Electrokinetic Extraction of Chromate from Unsaturated Soils, Chapter 2. In: Tedder, D. W.; Pohland, F. G. (Eds.), Emerging Technologies in Hazardous Waste Management V. American Chemical Society Symposium Series # 607: Washington, 1995.

Probstein, R.F.; Hicks, R.E. Science ,1993, 260, 468-503.

Rajeshwar, K.; Ibanez, J. G. Environmental Electrochemistry, Academic Press: San Diego (in press).

Rajeshwar, K.; Ibanez, J.G.; Swain, G. J. Appl. Electrochem., 1994, 24, 1077-1091.

Shakhashiri, B.Z. Chemical Demonstrations: A Handbook For Teachers of Chemistry, Vol. 4. The University of Wisconsin Press: Madison, WI, 1992, p150.

Segall, B.A.; Bruell, C.J. J. Envir. Eng., 1992, 118 (1) 84-100.

Shapiro, A.P.; Probstein, R.F. Envir. Sci. Tech., 1993, 27, 283-291.

Smollen, M.; Kafaar, A. Wat. Envir. Technol., 1995, VII (11) 13-14.

Thornton, R. F.; Shaphiro, A. P., Modeling and Economic Analysis of In Situ Remediation of Cr(VI)-Contaminated Soil by Electromigration, Chapter 4. In: Tedder, D. W.; Pohland, F. G. (Eds.), Emerging technologies in Hazardous Waste Management V. American Chemical Society Symposium Series # 607: Washington, 1995.

Trombly, J. Envir. Sci. Tech., 1994, 28, 289A-291A.

 

Laboratory Experiments On  Electrochemical Remediation Of The Environment. Part 4. Color Removal of Simulated Wastewater by Electrocoagulation-Electroflotation

 

Due to the large production of aqueous waste streams from textile mills and dye production plants, several processes have been under intense study.  Electrochemical processes offer some distinctive advantages,  including effects due to: 1) the production of electrolysis gases, and 2) the production of polyvalent cations from the oxidation of corrodible anodes (like Fe and Al).  The gas bubbles can 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. In this experiment an iron electrode (paper clip) is used in conjunction with pH indicator dyes, so dramatic color changes will be noticed.

 

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Laboratory Experiments On Electrochemical Remediation Of The Environment. Part 5. Indirect H2S removal

 

Many polluting gases can be transformed to non-polluting or at least, less polluting - forms by changing oxidation states of one or more of their constituent atoms.  This can often be achieved by transferring electrons to/from the pollutant from/to an electrified interface.  Since electrochemical remediation methods require an ion-conducting medium to perform their function, this can be accomplished either by using an absorption medium in an electrochemical cell (inner-cell process) or by absorbing the gas first and then transferring the absorption medium into the electrochemical cell to be treated (outer-cell process).  Also, in such methods the polluting species can either undergo electron transfer on an electrode surface (direct method), or electrons can be shuttled to/from the electrode by an electron carrier (indirect method).  We report here an experiment to demonstrate one way in which the electrochemical treatment of H2S can be accomplished by bubbling it through an iodine solution.  A dramatic change in color occurs due to the oxidation of S2- ions by I2, producing a suspension of yellow elemental S plus colorless iodide ions.  The resulting solution is then electrolyzed and the iodine regenerated.  This cycle can be repeated many times with the same solution.

 

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