Water comes clean - A nasty fuel additive can now be removed from drinking supplies

Sacramento

BY WORKING out how to extract aromas from foods, two scientists in California
may have solved one of America’s most vexing environmental problems: cleaning up
groundwater contaminated with a potentially carcinogenic fuel additive.

To improve the efficiency of combustion and to minimise air pollution, many
US states have added methyl tertiary-butyl ether (MTBE) to vehicle
fuel. But MTBE dissolves readily in water and is building up in reservoirs of
groundwater used for drinking, especially in California.

Chemical engineer Marc Sims, who runs a company in Berkeley called Marc Sims
Supercritical Fluid Extraction, and molecular biologist Jim Robinson, who has a
company in Livermore called Setec, have developed a powerful yet simple device
for separating molecules using liquid carbon dioxide. Their contraption sends
fluid through a tube made of polypropylene membranes suspended in a chamber of
liquid CO₂. The CO₂, but not the fluid, can flow back and
forth across the membranes. It collects molecules that prefer to dissolve in
liquid CO₂. These molecules gradually disappear from the fluid, which
is left slightly carbonated.

The researchers originally used their device, called PoroCrit, to extract the
chemicals responsible for the aromas in foods such as orange, lemon, apple,
garlic, onion and butter. But while they were busy purifying these essences,
problems with MTBE in groundwater hit the news in California. “It took a little
while to sink in,” says Sims. “It’s an ether; exactly the sort of compound that
we’ve been extracting all along.”

Sims and Robinson began to experiment with MTBE-contaminated water about
three months ago. They found that just one pass through their device reduced
MTBE levels up to one million-fold. They are now testing a larger model that can
purify water at a rate of 10 litres per minute.

Existing methods of decontamination do not work very well for MTBE. Sims and
Robinson are confident that PoroCrit will be the most effective way to clean up
MTBE-tainted water. The researchers have been contacted by the California state
government, fuel associations and the federal Environmental Protection
Agency.

Although the US government classifies MTBE as a “possible carcinogen” only at
concentrations above 20 parts per billion, water smells of turpentine when the
chemical is present at just 15 ppb. “People can smell it when they’re in the
shower,” says Sims. However, the slightly fizzy liquid produced by the PoroCrit
device has no aroma at all.