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 The Johns Hopkins University, said with a laugh.
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.
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 1930s.
Meyer is director of the National Science Foundation-funded Collaborative Research Activities in Environmental Molecular Sciences (CRAEMS) Center 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 re-activated using chemical or electrochemical techniques, but Obare wanted to try using a practical, easily available energy source to power the re-activation: sunlight. She decided to try to take advantage of titanium dioxide's abilities as a photocatalyst, a substance that 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 sunlight.