From Tech Trends November 1995

In Situ Vitrification Treats Organics and Inorganics

By Teri Richardson, Risk Reduction Engineering Laboratory

The Geosafe Corporation's in situ vitrification (ISV) technology is designed to treat soils, sludges, sediments and mine tailings contaminated with organic, inorganic and radioactive compounds. EPA's SITE (Superfund Innovative Technology Evaluation) Program evaluated the technology at the Parsons Chemical site in Grand Ledge, Michigan; soil at the site was contaminated with low levels of pesticides and mercury. The SITE demonstration results concluded that the cleanup levels were met. The process uses electrical current to heat (melt) and vitrify the soil in place. Organic contaminants are decomposed by the extreme heat into simple gases, which then rise through and escape from the molten soil. Inorganic contaminants are trapped within the molten soil, which cools and solidifies into a glassy monolith. The ISV technology operates by means of four graphite electrodes, arranged in a square and inserted into the soil to be treated. A pattern of electrically conductive graphite containing glass frit is placed on the soil in paths between the electrodes. When power is fed to the electrodes, the graphite and glass frit conducts the current through the soil, heating the surrounding area and melting directly adjacent soil. Molten soil is electrically conductive and can continue to carry the current which heats and melts soil downward and outward. The electrodes are allowed to progress down into the soil as it becomes molten, continuing the melting process to the desired treatment depth. As treatment progresses, a "cold cap" of solidified material forms at the surface. When all of the soil in the treatment area becomes molten, the power to the electrodes is discontinued and the molten mass begins to cool. The electrodes are cut near the soil surface and are allowed to settle into the molten soil to become part of the melt. The organic contaminants in the soil are pyrolyzed (heated to decomposition without oxygen) and are generally reduced to simple gases. The gases migrate through the molten soil and/or the adjacent dry zone to the surface, where they are collected in a stainless steel hood placed over the area being treated. Gases from the hood are treated in an off-gas treatment system.

Inorganic contaminants in the soil are generally encapsulated in the molten soil which hardens to a vitrified mass that is dense and hard, which significantly reduces the possibility of leaching from the mass over the long term. Since the vitrification process removes most of the void space in the soil, as well as destroys the organic contamination, there is a volume reduction of 20 to 50%.

Prior to the SITE evaluation demonstration, treatment at the Parsons site had been ongoing for several months in open concrete trenches designed for nine treatment cell settings. The SITE demonstration focused on cell 8, which was the most contaminated cell. The ISV technology treated the soil as expected, completing the melt in 10 days. The cleanup levels specified by EPA Region V for chlordane, 4,4-DDT; dieldrin; and mercury were met. Pesticide concentrations were reduced to non-detectable levels in the vitrified soil, from initial concentrations of 13,050 micrograms per kilogram (ug/kg) for 4,4-DDT to less than 16 ug/kg and from 4,620 ug/kg dieldrin to less than 16 ug/kg. The solid vitrified material collected was subjected to TCLP for mercury and pesticides. The test results indicated that leachable mercury was well below the regulatory guidelines of 40 CFR Part 261.24; and, no target pesticides were detected in the leachate. There were no target pesticides detected in the stack gas samples; metal emissions were below regulatory requirements; and, total hydrocarbon and carbon monoxide emissions were in compliance with EPA Region V limits.

For more information, call Teri Richardson at EPA's Risk Reduction Engineering Laboratory at 513-569-7949. Key findings from the demonstration will be published in an Innovative Technology Evaluation Report which will be available February 1995. Those involved in cleaning up similar sites across the country will be able to use this report to evaluate the Geosafe ISV technology as a potential alternative technology for their sites. A SITE Technology Capsule and videotape will also be available by January 1995. To get on the mailing list for these reports, contact Peggy Heimbrock at 513-569-7472 by phone or by FAX at 513-569-7566.