Electrokinetics, a technology used to recover oil, remove water from sludge and stabilize the ground for construction, is now be-ing used to clean up hazardous waste sites. The technology shoots an electric current between electrodes placed in the soil, which drives the chemicals to one of the electrode sites where they are collected then either disposed of or recycled (see chart).
"The technology has the potential to clean up sites where the type of soil and contaminants present are known," said Sibel Pamukcu, as-sistant professor of civil engineering at Lehigh University. "It's more economical than excavating the contaminated soil and placing it in a landfill, and it also has advantages in certain areas over other methods of treatment such as biological waste treatment or thermal extraction."
Pamukcu began studying electrokinetics in 1989 and is now working to fine-tune the technology. Working in conjunction with Lehigh's Environmental Studies Center and scientists and engineers from various departments, Pamukcu plans to apply the technology with Electro-Petroleum Inc., Wayne, Pa.
Pamukcu, in conjunction with Lehigh's Energy Research Center and Illinois Power Co., used electrokinetics to clean up a coal tar site in a small urban area where excavation was impractical. In this study, the coal tar was transported through clay soil to a collection system.
In a recent U.S. Department of Energy laboratory study, electrokinetics removed up to 99 percent of cesium, strontium, lead, cadmium and chromium; uranium, mercury and arsenic all required further treatment for effective removal. The process also succeeded in a recent Electric Power Research Institute lab study in which most of the hydrocarbons were first swept toward an electrode site, concentrated, and then removed from the site.
In the early 1980s, hazardous wastes from old gas station tanks, former coal gasification plants and heavy metals at industrial sites were commonly landfilled. The traditional extraction methods, which can be more expensive, require large earth-moving equipment to dig up the contaminated soil, while the landfill receiving the waste has to continue to be maintained. Building a pumping station and pumping out a substance like coal tar also has been attempted, as has biological waste treatment and thermal extraction methods. Each method has uses, according to Pa-mukcu, "but the electrokinetic removal method [can be] efficient and economically viable in certain cases."
For example, in clay soil, it's nearly impossible to pump waste out and treat it; however, with the electrokinetic method, the only equipment needed is a portable DC generator, two electrodes and a collection system. While a waste pump creates a hydraulic gradient, the voltage from the generator - less than used to run a hair dryer - creates an electric gradient. As long as the wastes are in a liquid phase, electrokinetics can move them to one spot for collection and removal.
"Electrokinetics does have its limits," she said. If the soil has a high pH, or the ability to neutralize acids, wastes such as heavy metals may never reach the point where they dissolve. Also, trying to move organic material like coal tar may prove to be difficult. Some organics may not be charged and will not react to the current, and will clog in one place if removal at the electrode site is slower than the transport of the material. One solution, according to Pamukcu, would be to liquefy the organics by the chemical treatment, so they would move. Water driven to a collection site along with the waste must be stored, transported and treated, or treated on site. Also, care must be taken not to remove too much water from an electrokinetic site in order to prevent the ground from collapsing.