Plasma Systems Zap Toxic Waste
Dangerous materials change to harmless fill
ACCORDING to the famous bedtime story, Cinderella's fairy godmother waved her magic wand and turned a pumpkin to a gilded coach - but that was only make-believe. Now researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., have devised two real-life methods that can transform hazardous wastes into chemically harmless compounds.
The projects, under way at MIT's Plasma Fusion Center, aim to use plasma - the same kind of electrical streams of ionized gas that are found in lightning bolts - to alter the chemical composition of certain toxic industrial wastes more cheaply, safely, and efficiently than the most common method of waste disposal, incineration.
"Plasma processing is a new approach to waste," says Daniel Cohn, acting assistant director of the MIT lab.
Mr. Cohn and his team of six researchers have built models of two plasma systems that can zap toxic waste. One is an arc of high-temperature or "hot" plasma, and the other is a room-temperature electron beam scientists refer to as "cold" plasma.
"Hot plasma is like a sledgehammer," Cohn says. "Cold plasma is like a scalpel."
"The hot-plasma arc is generated between two graphite electrodes," Cohn says. "It heats solid or liquid wastes to 1,700 degrees Celsius and converts them to a hard, glass-like substance." Cohn says the waste is safe for landfills and might even be used as a construction material.
Matt Schuetze, a graduate-student research assistant, explains that the cold-plasma reactor focuses a beam of electrons onto a stream of gas that contains toxic molecules. "The toxins are hungry for electrons," Mr. Schuetze says. "They capture free electrons from the plasma and form unstable molecules, which break up into less-toxic substances."
The hot- and cold-plasma systems will be operated remotely by computers and robots, decreasing operating costs and human risk, Cohn says. The systems are also relatively mobile: A steel furnace that could be built on a waste site generates the plasma arc, and the electron-beam device fits inside a tractor-trailer.
The projects are funded by the United States Department of Energy (DOE), which allocated $200 million to developing efficient toxic-waste treatments. DOE officials, MIT researchers, Electro-Pyrolysis Inc., and Battelle Pacific Northwest Laboratories joined forces in 1991 to fashion plasma treatment processes for two of the DOE's most notorious dump sites.
THE hot-plasma arc furnace was built to process radioactive waste stored in massive underground tanks at the Idaho National Engineering Laboratory in Idaho Falls, Idaho. The slurry mixture of metals and liquids, which contains traces of plutonium, has leaked through the walls of several of the containers.
The cold-plasma project will be implemented at the Hanford Nuclear Reservation in Richland, Wash., where officials at the site's weapons plant dumped 2 million pounds of industrial solvent into open pits. The solvent, carbon tetrachloride, is considered a carcinogen and threatens to seep into groundwater at the site.
"Carbon tetrachloride can be sucked up to the surface in gaseous form," Schuetze says. "The electron-beam processor would reduce it to carbon dioxide and salt." Cohn adds that the electron-beam process would use 10 to 100 times less energy than an incinerator would use to do the same job.
While the plasma systems were designed specifically for the Idaho and Washington sites, Cohn says the technology has much wider implications.
"Down the road, if it's efficient, reliable, and cost competitive, plasma technology could be used to treat a whole range of substances," he says. "This is not just one gimmick."
Jeff Surma, an engineer at Battelle who oversees the plasma projects, says he expects final tests of both systems to be completed in 1995. At that point, officials from the DOE, like the prince's courtiers in the fairy tale, will knock on the laboratory door to see if the glass slipper fits.
"This technology might not change the way we live," Mr. Surma says, "but it will help in the future with waste treatment. It's one more choice we have in maintaining a clean environment and cleaning up the sins of our past."