Scientists try using natural process to get rid of dangerous chlorine solvents
By ALANNA MITCHELL
December 13, 2004
How do you get rid of the most common toxic contaminants in the ground water of
the industrialized world? Until recently, you didn't. Dumped in open waste pits
for decades and left to evaporate (instead of being safely recycled, as they
mainly are now), these chlorine-based chemicals sank beneath the ground,
collected in toxic blobs and then seeped into the water table to persist for
centuries.
If people consume even minute amounts, or breathe tiny volumes of the gases they give off, the legacy can be nerve damage or cancer.
Now scientists, led by University of Toronto chemical engineer Elizabeth Edwards, are trying a novel approach to get rid of the toxins.
They are injecting naturally occurring bacteria into underground sludge to consume the toxins, leaving behind benign substances. When the toxins are gone, the bacteria go to sleep, to be awakened only if more sludge turns up.
It's a process scientists call "bioremediation" or "bio-cleanup," meaning making a polluted site clean again using natural processes instead of synthetic chemicals.
A different type of bioremediation has been used for years to clean up the petrochemicals that have accumulated at abandoned gas stations.
Typically, that's done by fencing off the tainted land, piling up polluted dirt and letting microbes in the air consume the petroleum products.
But the group of chlorine-based solvents is a far more difficult and more common foe.
This class of synthetic chemicals was developed after the Second World War and became the most widely used, employed in such industries as cosmetics, pharmaceuticals, dry cleaning and electronics. They were once even used to take the caffeine out of coffee.
"They were thought to be wonderful new chemicals, and inert," Prof. Edwards said.
They include: tetrachloroethene (PCE), known as perc and used as a dry-cleaning solvent; trichloroethene (TCE), widely used as a degreaser in industry; dichloroethene (DCE); and vinyl chloride, used in the manufacture of plastic polyvinyl chloride, known in hardware stores as PVC.
Each is a chemical sister of the other, with PCE having four molecules of chlorine, TCE having three, DCE having two and vinyl chloride having just one. Once all the chlorine molecules are stripped off, all that's left is ethene, a harmless and abundant gas that ripens apples and pears.
The chlorine solvents are ubiquitous in the polluted sites that the U.S. Environmental Protection Agency has designated for urgent cleanup, known as "superfund sites," and are considered equally common in Canada.
TCEs are so commonly used in the making of computers that all the shallow wells in California's Silicon Valley have been closed for drinking water because they are contaminated with the degreaser, Prof. Edwards said.
"There isn't enough money even in rich countries like the United States to clean them up," she said. "And it's from an era when we didn't know better."
The concern is not only the rural residents who drink untreated water that may be contaminated, but the toxic vapours that can seep into homes, businesses and factories through a contaminated water table, said David Major, a biologist who now works at the biotech consultancy GeoSyntec in Guelph, Ont.
Enter the strange little group of bacteria called dehalococcoides. Only recently discovered living underground, these ancient and simple organisms eat hydrogen and breathe chlorine. Oxygen, the life-giver for most known organisms, kills these fellows.
Using material from a highly contaminated transfer station in north Toronto, Prof. Edwards isolated a culture of these bacteria that she calls KB-1 (after her mother's red Toyota pickup truck, nicknamed Kick Butt).
It's programmed by evolution to eat every last bit of chlorine in the toxic solvent, leaving behind only the harmless ethene.
The problem is that there haven't been enough of these bacteria to eat quickly enough all the chlorine solvents collecting underground.
Prof. Edwards figured that the answer was to farm the bacteria with colleagues at the biotech company SiREM (an offshoot of GeoSyntec) and add them in big batches to the underground blobs.
When she did that in 2002 at a Texas air force base, the dangerous chlorine-based chemicals diminished, while the benign ethene replaced them.
"We proved that it matters that we added bacteria and the bacteria really did grow," she said.
Now the toxin-gobbling bacteria have been blessed by the Environmental Protection Agency and have been injected in 22 sites throughout the United States, Dr. Major said. Plans are to inject them in sites in Denmark and the U.K. as well. SiREM has eight full-time employees and $2.5-million (U.S.) of proposals for work, he said.
Meanwhile, Prof. Edwards is still working with students on the microbe at her U of T lab. But the innovation is a year or two away from being approved by Environment Canada for use here because Canada has strict rules on substances being added to groundwater, including ones that nature makes.
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