Solvated Electron Treatment
Description
Solvated electron chemistry technology uses a solution of ammonia and an ÒactiveÓ metal to create a powerful reducing agent that can chemically transform toxic contaminants, such as polychlorinated biphenyls (PCBs) and pesticides, into relatively benign substances. In the Solvated Electron Treatment (SET) process, a mixture of anhydrous ammonia and sodium metal creates solvated electrons. In general, solvated electrons are formed when certain alkaline earth metals (e.g., sodium, calcium, lithium, and potassium) are dissolved in ammonia and form metal ions and free electrons. These free electrons produce a strong reducing agent that removes halogens (primarily chlorine) from organic molecules and reduces other contaminants.
In the SET process, contaminated soil is excavated, screened to remove debris, and dewatered. The soil is then placed into a sealed treatment vessel. At room temperature liquid ammonia is added to the vessel, where it is mixed into a slurry. After mixing, elemental calcium or sodium is added to the slurry, and mixing continues until the reaction is complete. The mixture is then transferred to an ammonia/soil separation vessel where liquid ammonia is separated from the soil. The separator is then rotated, warming the soil and driving off the remaining ammonia as vapor. The vapor is collected, along with the liquid, in the ammonia/water separator. Water is separated from the ammonia for return to the cleaned soil. The ammonia is returned to the main ammonia storage tank for reuse.
Limitations and Concerns
Since the process utilizes anhydrous ammonia and sodium, both of which are highly reactive, commercial operations of SET require large quantities of these materials at the site, posing serious health and safety issues and concerns, all of which need to be addressed when using this technology.
The reduction of halogenated materials from soils, oily wastes, sludge, and sediments requires removal of moisture through pre-drying. Liquid ammonia reacts quickly with water to form ammonium hydroxide, which inhibits the production of solvated electrons.
The solvated electron reaction is highly exothermic (releases heat to the surrounding environment). It is especially exothermic when water is present. Using this technology for treating wet soil or sludge on a large scale raises safety, health, and environmental issues.
The highly corrosive solvated electron solution could impact the structural integrity of the SET reactor and other components.
Insufficient amounts of reagents such as sodium or ammonia in the reactor, or the presence of reactive metals, such as iron, in the soil being treated, could lead to only partial decomposition of target contaminants.
When treating large volumes of wastes containing TCE and other chlorinated hydrocarbons, pressure relief valves and permits for the accidental venting of ammonia into the air may be required.
The SET process may create toxic byproducts.
Applicability
The ex-situ SET process has been tested for treating halogenated semi-volatile organic compounds (SVOCs) including polychlorinated biphenyls (PCBs), volatile organic compounds (VOCs), pesticides, and ordnance compounds. The Department of Energy (DOE) is also considering it as a treatment for the organic portion of mixed radiological waste.
Technology Development Status
The solvated electron solution dates back to 1865, when scientists first discovered that alkaline earth metals react with dissolved ammonia. The SET technology has been demonstrated on a pilot scale. EPA issued a nationwide permit to the SET process for PCB destruction in soil.
Web Links
http://www.environment.gov.au/settlements/publications/chemicals/scheduled-waste/swtt/solvated.html
Other Resources and Demonstrations
Also, see Report of the Secretary of Energy Advisory BoardÕs Panel on Emerging Technological Alternatives to Incineration, December 2000, Secretary of Energy Advisory Board, U.S. Department of Energy.