ust as electricity has been used to promote clean and efficient lifestyles, it is now being investigated a s a method for cleaning u p some of the country’s most stubborn hazardous waste sites. At a March workshop at the Electric Power Research Institute (EPRI) in Palo Alto, CA, more than 60 representatives of the private and public sectors discussed the use of electric currents to clean contaminated soils and groundwater in situ. “Participants, including EPRI, needed an assessment of the field and a comparison with other remediation technologies,” explained Fritz Will, professor of chemical engineering at the University of Utah and visiting scientist at EPRI. Although electrochemical remediation generally is still in its infancy, several presentations suggested that the process is inching toward commercialization in certain applications. “Right now its strongest suit is the removal of inorganic contaminants,” said Will. Although studies of electrokinetic remediation of organics are lagging behind those of inorganics, Will pointed out that EPRI has a strong interest in the removal of organics, including petroleum products. Approximately nine field demonstrations are being launched in the United States this summer to analyze the effectiveness of electroremediation in treating a wide variety of pollutants. The field demonstrations follow on the heels of laboratory tests in which scientists reported having achieved removal efficiencies ranging from 44% to 99% for a wide variety of contaminants, including sulfates, nitrates, chromium, uranium, mercury, soluble organics such as phenol and benzene-toluene-xylene, and polycyclic aromatic hydrocarbons. Meanwhile, companies in Europe and Russia have begun to use the process commercially to remove heavy metal contaminants including uranium, mercury, and mixed metals. Electrochemical remediation has garnered considerable interest from U S . scientists and business representatives because it is the only in situ method that could work in lowpermeability clays and silt-laden soil-onditions that characterize at
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least half of the toxic waste sites in the country. “Except for soil vapor extraction, which works only on certain contaminants in unsaturated soils, in situ remediation technologies are for the most Dal only i n the deveioF m e n t stage,” sai’ Jonathan Herrmann, sr pervisory environmer tal engineer at EPA. As a n i n situ ap proach, electroremediz tion holds an advantag over standard “pumF and-treat’’ systems bt cause of its high level of control over flow direction. Economics are also a driving force. Some researchers reported costs of $90-$130 per ton, which are comparable to, if not lower
than, conventional remediation methods. Efforts i n Europe have led the race to apply t h e technology. Geokinetics. based in The Netherlands (see ESbT, Dec. 1993, p. is 26481 holds some of the earliest European patLISon electroremediain of soil. Geokinetics currently stripping ixed toxic metals Rhine sludges in Rotterdam, one of Europe’s most environentally damaged anufacturing areas. While work is being done under confidentiality agreements on a variety of mixed pollutants, Geokinetics has started removing cyanide from gas works sites, reports Robert Clarke, director of technology for the Electro Remediation Group, w h i c h has teamed up with Geokinetics in EU-
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remediation I inching toward commercialization in certain applications.
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0013-936X194/0927-289A$O4.50/0 0 1994 American Chemical Society
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rope and the United States. Several other collaborations are under way, which suggests that companies and researchers are positioning themselves to pursue this rapidly developing technology. But against the backdrop of promising tests and limited overseas commercialization, some scientists have expressed concern about moving too quickly into field demonstrations. “If you’re going to go into a messy mixed waste s i t e with limited knowledge, you’re going to fail, and electroremediation is going to gain a bad name,” said Ronald Probstein, professor of mechanical engineering at the Massachusetts Institute of Technology (MIT). Probstein stressed the need to couple laboratory models with limited field trials, especially at sites where the contaminants are not well characterized and soils are not uniform. With U S . Department of Energy funding, Probstein is developing a mathematical model that determines how best to treat a site by analyzing the effects of varying conditions such as the background chemistry of the groundwater, the contaminants and soil characteristics, and the existence of other contaminants. “It currently uses a Cray computer, but we are adapting the model for use on a PC so that it can be brought to the field sites like a mobile laboratory,” said Probstein. Fritz Will agreed that lab and field work should co-evolve, adding that more soil chemists should be in-
volved because varying soil conditions can affect outcome. As civil and mechanical engineers, electrochemists, biochemists, and soil chemists team up on electrochemical remediation efforts, the process gains more names. It has been called “electroremediation,” “electroreclamation,” “electrorestoration,” and “electrokinetic remediation and restoration.” Although different mechanisms guide the process, electroremediation occurs when low-voltage direct-current electric fields are applied to contaminated sites by means of electrode arrays. Electric current flows between pairs of positive anodes and negative cathodes placed in the ground. Whether situated horizontally in trenches or vertically in drilled tunnels around the marked site, the electrodes serve as goal posts to migrating heavy metals, soluble organics and, in some cases, insoluble organics. Once the charged ionic contaminants or s o h bilized pollutants have accumulated at the electrode, they are more easily extracted in their concentrated form for treatment by conventional methods or remediated i n contained treatment zones. Electrochemical remediation is characterized by electromigration, electroosmosis and, less commonly, electrophoresis. Electromigration is the movement of ions in an electric field from one electrode to another. Because most organics aren’t ionized, this process works primarily
Electroosmosis is one of the primary electrochemical transport mechanisms whereby water moves through charged soil. Electroosmosis has shown good results in removin soluble organics. (Adapted from U.S. Patent No. 5 074 986, Dece-$4,. ,-s* i 19& .) j_i
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on highly soluble ionized inorganics including alkali metals, chlorides, nitrates, and phosphates in moist soil environments. Additionally, heavy metals such as lead, mercury, cadmium, and chromium have responded favorably, some with the aid of complexing agents. In some instances, it is necessary to treat the area around the cathode to adjust pH levels to prevent precipitation of the metals in the soil. For example, the removal of zinc w o u l d require a low pH level, whereas chromium- and uraniumtainted soils would be treated to yield a high-pH solution. Most electromigration tests have taken place in low-permeability, nearly saturated soils, but Eric Lindgren of Sandia National Laboratories (Albuquerque, NM) is working on removing chromate from unsaturated sandy soils. Dry soils, however, require more electricity, which is likely to increase remediation costs. Electroosmosis is the other primary transport mechanism whereby water moves through charged soil. Water molecules are essentially pried away from minute soil particles in low-permeability environments, and the water moves the pollutants with it. Electroosmosis works efficiently only with finegrained soils and clays. The process has shown good results in removing soluble organics. Dale Schultz of DuPont reported positive results at a site in the eastern United States contaminated with an unnamed soluble organic. Contaminant concentrations were reduced below detection levels by electroosmotically driving about 1.5 pore volumes of water through the soil. Moreover, neutralizing pH at the anode with lime or sodium hydroxide significantly increased the efficiency of the process. Results from a field trial planned this summer should reveal more details, Schultz said. Even though insoluble organics such as heavy hydrocarbons pose more of a challenge for electroosmosis, Sibel Pamukcu, professor at Lehigh University (Bethlehem, PA), reported removal efficiencies of 4470% for some components of coal tar-contaminated soil with the aid of surfactant injection. The material tested was excavated from a former Illinois Power Company manufactured gas plant site in Champaign. Given preliminary promising results of electroremediation for treating heavy metals and organics, both the Office of Environmental Restoration and Waste Management at
the U.S. Department of Energy (DOE) and EPA’s Risk Reduction Engineering Laboratory in Cincinnati have been actively involved in electrochemical cleanup demonstrations. “Out of the 3700 hazardous waste sites on DOE’S list, there are 213 sites where electrokinetics may be applicable,” reported Dennis Kelsh of Science Applications International Corporation, who is working on t h e DOE program. “We’re giving electrokinetics five years to prove its validity in the field,” said Kelsh. Facing mandated cleanup requirements in the year 2019, DOE is focusing on cost-effective in situ approaches. In what may become one of the quickest demonstrations of electroremediation technology emerging from laboratory to field testing, Monsanto, DuPont, a n d General Electric, in cooperation with DOE and EPA, are working together to test the “lasagna” process, named for the application of layered treatment zones. Developed by Philip Brodsky, director of corporate research and environmental technology for Monsanto, the lasagna process takes the concept of migrating pollutants one step farther. “By combining electrokinetics with other proven remediation approaches, this process cleans u p hazardous wastes as they are moving,” said Brodsky. Instead of merely attracting toxins to one spot via electric charge, the lasagna process takes advantage of electroosmotic movement by luring stubborn organic chemical deposits through multiple layers of remediation treatment zones that are established through hydrofracturing and sheet piling. First, low-voltage electrical current draws contaminated water by electroosmosis out of tiny pores in silt and clay where it may be trapped. The water is slowly pulled, typically, toward the cathode, moving a few inches per day through treatment zones that utilize existing remediation methods such as biodegradation, catalytic dechlorination, and absorption. The lasagna consortium has just started to demonstrate a vertical electroosmotic system at a DOE site in Paducah, KY. A horizontal lasagna process, more applicable to deeper sites, is being tested concurrently by EPA near Cincinnati, where the agency is already conducting a hydraulic fracturing experiment. Several other collaborations under way are moving the technology
into field tests. New Orleans-based ISOTRON~ has ~ collaborated with Russian scientists on a process to clean up heavy metals. According to Henry Lomasney, president of ISOTRONTM,the Russians have successfully remediated six sites in Uzbekistan and Kazakhstan where acid leachates from mines have contaminated the groundwater. Despite crude techniques, such as using electrodes made of steel instead of more advanced materials available in the United States, Lomasney reported achieving extraction efficiencies of 99.5% for heavy metals, including 97% for uranium and 69% for sulfates. I S O T R O N ~is~ working w i t h HAZWRAP, a subsidiary of Martin Marietta, to replicate the Russian process at an Oak Ridge, TN, site for DOE. ISOTRONTMhas also developed a patented system utilizing cylindrical electrodes that is being used at a DOE-funded project conducted by Westinghouse Savannah River Company at the Savannah River site’s old TNX Seepage Basin, from which results are expected this year. DuPont Environmental Remediation Services has teamed up with Corrpro Companies, Inc., which has worked in the cathodic protection industry for decades. Corrpro licensed MIT’s patented electroosmotic purging process, and the team is developing a panel electrode array to achieve more uniform electric fields. Both research and field tests are receiving support from the federal government. At a total investment of more than $2 million, DOE has sponsored six projects in 1993, including the lasagna project, the development of Probstein’s mathematical m o d e l , a n d t h e d r y - s o i l experiments at Sandia. EPA is also cosponsoring demonstrations, but instead of funding research, it is focusing its limited resources on field demonstrations. By folding electrokinetics into one of its own field tests, EPA will be conducting electroosmosis tests at a hydrofracturing project under way near Cincinnati. Particular attention is being paid to time-dependent flow conditions, nonuniform treatment of soils, the costs associated with chemical a d d i t i v e s a n d aboveground treatment, a n d efficient electrode configurations. In addition, EPRI and Southern California Edison have been funding research in electroremediation for a number of years and jointly
sponsored the recent workshop. Ishwar Murarka and Ammi Amarnath, managers of EPRI’s electroremediation projects, report that EPRI has so far invested approximately $500,000 in three university studies and plans to continue sponsoring research in this area. Several concerns about future research strategies were raised by participants at the workshop. Site characterization factors need to be identified so that sites best suited for electrokinetic treatment can be determined. Knowing whether large metallic objects are in the soil is important because pipes or other metal objects can slow down or even defeat the process. Likewise, poor in situ performance can result from the presence of other chemicals in the soil such as humic substances and naturally occurring electrolytes. Potential side effects such as acidification of the soil and the migration of pollutants into unpolluted areas need to be identified and completely understood. One participant pointed out that surface structures such as power lines and fences can conduct the electricity away. In one experiment at a national laboratory, an electric charge was carried to a nearby fence that was undergoing repair. Just as significant is the need to accurately estimate the costs of electroremediation, even though such estimates are site-specific. Some presenters included rough cost data-General Electric cleaned up chromate for $130 a ton, DuPont cleaned up organics for $90 a tonbut cost estimates were generally not included in bench-scale test results. One participant expressed reservations about the cost advantage of using electricity to treat groundwater because aquifer flow rates can be extremely slow and conventional pump-and-treat methods are relatively inexpensive. “Electrokinetics might be a little ahead of demand,” said Schultz. Regulators have had little choice but to accept pumpand-treat remediation systems because no other practical methods have been available. “Once the effectiveness of electroremediation has been demonstrated, regulators may start demanding it, and create more of a market” he predicted. Jeanne Trombly is a San Franciscobased freelance science writer specializing in science a n d environment.
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