Scientists looking for ways to clean up a common,
persistent type of organic pollutant have developed an
approach that not only restores the power of a naturally
occurring pollution buster but also boosts it to levels of
effectiveness that they can't currently explain.
"It's safe to say that we don't fully understand why
this approach works so well, but we'll take it and develop
it and figure out the details as we go," Gerald Meyer,
professor of chemistry in the Krieger
School of Arts and Sciences at Johns Hopkins, said with a
The targets of the new technique, developed by Sherine
Obare, a postdoctoral fellow in Meyer's lab, are
organohalides, a class of compounds used in pesticides,
pharmaceuticals and manufacturing. They pose health risks
to humans and have been linked to environmental problems
like ozone depletion and climate change.
Obare's new approach combines an extremely thin film
of titanium dioxide with a compound found in life known as
hemin. After exposure to ultraviolet light, the hemin and
titanium dioxide can break up organohalides at surprisingly
high rates. Obare and Meyer presented results of tests of
the new approach last week at the 226th national meeting of
the American Chemical Society, held in the Javits
Convention Center in New York.
Seventeen of the top 25 organic groundwater
contaminants in urban areas are organohalides, according to
a 1997 Environmental Protection Agency report.
Organohalides are a class of organic compounds that include
a halogen, a group of elements comprised of bromine,
fluorine, iodine and chlorine. The compounds are very
difficult to break down chemically. Some instances of
organohalides in the environment today, for example, can be
traced back to the dry cleaning industry of the 1920s and
Meyer is director of the National Science
Foundation-funded CRAEMS Center (Collaborative Research
Activities in Environmental Molecular Sciences) at Johns
Hopkins, which is dedicated to finding ways to deal with
the environmental effects of organohalides. "These
compounds play many important and beneficial roles in the
chemical and pharmaceutical industries, so they're not
going away soon, and it's important that we find ways to
minimize their environmental effects," he said.
According to Meyer, scientists have known for decades
that hemes, a naturally occurring group of compounds that
contain iron atoms, can break up organohalides. The most
well-known heme is hemoglobin, a compound in red blood
cells that carries oxygen.
"There's a lot of speculation that hemes in proteins
are what cells use to defend themselves from
organohalides," Meyer explained. "We can buy hemes —
we don't have to extract them from protein or anything
— but when you remove them from their naturally
occurring environment, you tend to oxidize them."
In their oxidized state, hemes are no longer useful
for breaking down organohalides. Hemes can be reactivated
using chemical or electrochemical techniques, but Obare
wanted to try using a practical, easily available energy
source to power the reactivation: sunlight. She decided to
try to take advantage of titanium dioxide's abilities as a
photocatalyst, meaning it promotes chemical reactions in
other nearby materials when exposed to light.
"I anchored hemin on porous thin films of
nanocrystalline titanium dioxide, and when I exposed the
system to light, the hemin was activated to a reduced state
where it reacted rapidly with organohalides, producing much
better results than I expected," Obare explained. "I've
even been able to recycle and reactivate the thin films for
further organohalide degradation."
Meyer noted that there's still a lot of development
work to be done, not the least of which is figuring out
exactly how the chemistry of the new system works. But he
speculated that scientists might someday be able to insert
a similar system in drinking water — down a well, for
example — and power the removal of organohalides with