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Sopping Up Heavy Metal: Hybrid material removes mercury from water

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12:03pm, September 3, 2003
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Endowed with resourceful molecular machinery, microbes can adapt to environments as unappealing as oil slicks and toxic-waste dumps. Some bacteria can even bind to heavy metals such as mercury–a trick that researchers at the University of California, Riverside are now exploiting for treating contaminated water.

When exposed to mercury, the bacterium Escherichia coli synthesizes a protein that binds specifically to the heavy metal. Wilfred Chen and his colleagues set out to take advantage of that protein for cleanup operations.

First, the scientists genetically engineered bacteria to create a molecule containing both the bacterial protein and an artificial form of the muscle protein elastin. This form tends to clump when heated. The scientists next extracted the resulting compound.

When added to a sample of contaminated water, the compound bound the mercury. And when the temperature was raised to 35C, the complex clumped into aggregates that were easily separated from the water with a short spin in a centrifuge.

The Riverside team tested the material on samples of water doped with mercury and hundredfold-higher concentrations of other heavy metals, including zinc, nickel, and cadmium, which the compound doesn't bind. Not only did the new material reduce the amount of mercury to concentrations permitted in drinking water, but its remedial action wasn't hindered by the other heavy metals. The findings are slated to appear in an upcoming Environmental Science & Technology.

"This is a very exciting and important advance," says Anne Summers of the University of Georgia in Athens.

Chen's technique could offer a safer, cheaper, and more efficient alternative to other mercury-remediation technologies, such as costly filtration systems that employ toxic materials, adds Tamar Barkay of Rutgers University in New Brunswick, N.J. What's more, the complexes are recyclable because the mercury can be easily stripped off.

Using extracted compounds, rather than intact bacterial cells, for remediation avoids the risks associated with genetically modified organisms that could escape into the environment, says Chen.

While other groups are studying elastin-like polypeptides (ELPs) for delivering drugs to the body or for growing new tissues in the lab, "using ELPs for bioremediation is really novel," says Barkay.

Chen's group is currently adapting the technique for use with bacterial proteins that bind other heavy metals, such as arsenic. "It's such a flexible technology," says Chen, adding that the next big challenge is to scale up the process for treating large volumes of water.

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