Defenders of the earth

Zap! And another pollutant bites the dust

Published: Sunday 15 March 1998

-- (Credit: Sanjay Ghosh)they stand ready to zap, vaporise or otherwise transmogrify the "bad" chemicals into good ones and they are not a bunch of cartoon heroes. They are the National Science Foun-dation(nsf)-funded next generation of pollution fighters.

Take plasma generators. At the University of Illinois, usa , engineer Mark Kushner and his team are busy developing tools to make diesel fuel's toxic chemicals harmless. The device will pass the post-combustion fumes through a plasma, an ionised gas with free electrons. Unlike conventional combustion, which relies on heat of atoms and molecules to break mole-cular bonds and initiate a chemical reaction, plasma chemistry is a kind of cold combustion. Applying an electric field excites the charged electrons without directly affecting plasma's neutral atoms (Frontiers , January 98).

Because the electric field moves the light-weight electrons - Kushner compares them to Ping-Pong balls - and not the heavier atoms ("think bowling balls," says Kushner), the gas does not get very hot while the energetic electrons break bonds more selectively than their atomic cousins. As a result, scientists have more control over the reaction.

The idea, still being optimised on the computer's drawing boards, is to design a plasma generator, about as big as a normal automobile silencer, in which the electrons collide with the nitrous oxides in the exhaust and convert them to harmless nitrogen and oxygen molecules.

Another group of pollutants targeted for change is toxic metals. Soils filled with these have to be carted away and stored as hazardous waste. But at the University of Georgia in Athens, Greece, geneticist Richard Meagher and his colleagues have genetically engineered a variety of plants that soak up mercury through their roots and convert metals to less toxic elemental mercury, used in tooth fillings. The trick is performed by a bacterial gene that produces an enzyme - MerA - hungry for toxic mercury. So far, the team has placed this gene into mustard plants, tobacco, canola and even yellow poplar trees. "And they all thrive on mercury," he says.

The researchers are not sure whether the converted mercury transpires up to the leaves, where it vaporises, or simply diffuses through the roots, "but in any case, we don't find the metal in the plant itself." With the help of their vast root system, these transgenic plants could, in theory at least, clean up hundreds of acres of mercury-contaminated land at a fraction of what it would cost using the conventional methods.

Meagher hopes that this approach - called phytoremediation - will prove helpful in water as well as soil. He has found another version of the bacterial enzyme - MerB - that gobbles up methyl mercury, the kind that shows up in fish from polluted waters and can cause severe neurological damage. Meagher's team has also begun experimenting with MerA to see if they can "teach" it through test-tube evolution to soak up other toxic metals as well, including copper, cadmium and nickel.

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