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Electrically moving nontoxic chemicals or even clean water into contaminated soil can greatly enhance contamination-removal techniques that make use of electric fields, researchers at MIT have discovered.
The enhancement technique, for which MIT has been issued a patent, holds great promise of lowering the cost of cleaning Superfund sites and other hazardous waste locations where organic contaminants and metals have been dumped, says Ronald F. Probstein, Ford Professor of Engineering at MIT. Professor Probstein, a member of the Department of Mechanical Engineering, is the lead author of a paper in the April 23 issue of Science which reports the findings. The co-author is Senior Lecturer R. Edwin Hicks of the Department of Mechanical Engineering.
Professor Probstein is a pioneer in developing techniques for the removal of contaminants from soils by electric fields.
Introducing direct-current electric fields into the ground induces the "transport" of contaminants to either positive or negative electrodes where they are readily pumped out. Unwanted material may also be concentrated in a few locations thereby making removal easier than having to truck tons of soil from an entire site.
But how efficiently and at what cost this could be done has been a question.
In laboratory experiments with lead and zinc, the MIT engineers have now shown that using reagents-chemicals that produce known reactions-can enhance removal rates to such an extent that clean-up costs could be $20 to $30 a ton, compared with $150 a ton using current methods, Professor Probstein said. Reagents act in various ways, including keeping a contaminant from being absorbed by the soil, or enhancing the solubleness of a contaminant.
"We have found that, depending on what we do at the electrodes, we can either remove metals at the electrodes or accumulate them in very specific locations of the soil where they can be removed efficiently," Professor Probstein said.
In a laboratory test using contaminants dispersed throughout a cylinder of clay soil, the MIT researchers have found that they can "focus" metals at specific locations or remove them at one of the electrodes by the extent to which they "wash" one or both of the electrodes.
"The reagents cause a change in the electrode chemistry that leads to enhanced removal rates," Professor Probstein said. The recent experiments have developed data on reagent-induced changes in soil characteristics, pH level and liquidity, all of which have a bearing on how contaminants are transported in soil.
Limited field tests and computer modeling support the projections of low-cost, effective contaminant transport, Professor Probstein said, "but the ability to make quantitative predictions of expected contaminant removal and costs at an actual hazardous waste site is at an early stage."
To assess the method's potential, extensive field tests will be needed in conjunction with the development of additional laboratory data, he said.
The use of electrical current to "dewater" land for construction and other purposes has a long history. Using electricity for removal of contaminants is a recent effort that Professor Probstein, a noted expert in fluid dynamics, and his colleagues have pioneered.
Electromigration, electroosmosis and electrophoresis are the principal methods used to achieve contaminant transport. Electromigration is the transport of a charged ion in a solution. In electroosmosis, a liquid containing ions moves in response to a stationary charged surface. In electrophoresis, the reverse occurs: a charged particle moves in response to a stationary liquid.
Professor Probstein's group is examining all of these methods.
A version of this article appeared in the April 28, 1993 issue of MIT Tech Talk (Volume 37, Number 30).