RESEARCHERS USE SUPERCRITICAL FLUIDS FOR ENVIRONMENTAL CLEANUPS
COLUMBUS, Ohio -- A process currently used to decaffeinate coffee may also be useful in removing organic waste from soil, according to researchers at Ohio State University.
Manufacturers now use a supercritical fluid to remove caffeine from coffee beans. Using a similar technique, scientists have been able to remove contaminants such as DDT from soil, said David Tomasko, an assistant professor of chemical engineering.
A supercritical fluid is created when the pressure and temperature of a compound are increased beyond the critical point where the gas and liquid physical states can no longer co-exist. At that point, the compound is in a single state and will not boil or condense.
For example, water boils at 100 degrees Celsius at normal atmospheric pressure (14.7 pounds per square inch). At these conditions, water changes from a liquid to a gas. But the critical point of water is 374 degrees Celsius and 3209 psi. Any
increase in temperature or pressure above this makes a phase change impossible.
"When any compound, usually a gas, is heated and compressed above its critical temperature and pressure, it becomes a solvent capable of dissolving organic materials," Tomasko said.
Tomasko's research looked at how well supercritical carbon dioxide could act as a solvent and dissolve different types of organic waste in soil. One study found the CO2 was capable of removing almost 60 percent of DDT from a sample of activated carbon, a common charcoal absorbent that acts as a model for soil.
While the use of supercritical fluids has been studied since the late 1970s, industry hasn't capitalized on the technology for environmental cleanup because of the high cost of building a pressure vessel, Tomasko said. But the operating cost is competitive with other contaminant removal technologies, and Tomasko is looking at ways to make the technology even more cost efficient.
"At this point, environmental cleanup is still driven by regulation, not by profit, and industry wants to follow those regulations as inexpensively as possible," he said.
This research was published in a recent issue of the journal Separation Science and Technology. Co-authors were Stuart Macnaughton and Neil Foster from the University of New South Wales in Australia and Charles Eckert from the Georgia Institute of Technology in Atlanta.
In Tomasko's research, CO2 is compressed by a pump to a specific setting and then flows through a heating coil immersed in a water bath. When the CO2 reaches a supercritical stage -- 40 degrees Celsius and 2900 psi in this work -- it is exposed to the contaminated material in a stainless steel pressure vessel.
In a supercritical phase, the CO2 retains some of the properties of a gas, but has a higher density, similar to that of a liquid. It flows through the contaminated material, extracting the contaminants, and into a separation vessel.
The density of the fluid is then lowered, either by reducing the pressure or increasing the temperature, and the contaminant, or solute, falls out of the CO2 and is removed.
While the technique is able to remove all of some types of organic waste, other waste, such as DDT, is only partially removed.
"One thing we're looking at is how to increase the solvent power of the CO2 so it is able to dissolve contaminants better," Tomasko said. "We're doing that by adding co-solvents such as methanol or acetone to the supercritical fluid, which increases the density of the fluid and increases its solvent power."
Using supercritical fluids to extract contaminants from soil has advantages over other cleanup techniques, Tomasko said.
"Here, the solute is easily and completely removed from the solvent via a drop in pressure," said Tomasko. "In conventional liquid extraction methods, the contaminant cannot be completely separated from the solvent that easily."
Tomasko is looking at the potential use of supercritical fluids as a replacement for toxic organic solvents used by companies that manufacture products such as pharmaceuticals.
"The big push in environmental research is to move the focus from cleaning up problems to preventing the problems," Tomasko said. "If companies can replace a hazardous organic solvent with an environmentally-friendly solvent, such as supercritical CO2, then they've prevented a pollution problem."
The research is now funded by the National Science Foundation and the Environmental Science and Engineering Key Technology Program at Ohio State.
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Contact: David Tomasko (614) 292-4249; e-mail: email@example.com
Written by Kelli Whitlock, (614) 292-9475