Toxic Sponge

Transmission electron microscope image of one chalcogel. Image courtesy of M. Kanatzidis

Tuesday, August 7th, 2007--

A new line of materials, dubbed chalcogels (pronounced calco-gels), has an unusual molecular make-up that allows the material to suck mercury, cadmium and other heavy elements out of polluted water, according to a recent study in the journal Science.

Gels are solvent-based and most are made of carbon and oxides--compounds containing oxygen. Gels are "very much like Jello. In fact Jello is a gel," says Mercouri Kanatzidis, Charles E. and Emma H. Morrison professor of chemistry at Northwestern University in Evanston, IL and senior author on the study.

Researchers can freeze dry gels to make a three-dimensional, porous structure that filters out some molecules like a sieve. A freeze-dried gel is called an aerogel. But traditional oxide-based aerogels cannot absorb heavy metals: "The elements go through the oxide material and they come out the other side," Kanatzidis says.

Making non-oxide based gels is tricky: precipitates usually form when oxides aren't used and this ruins the gel, Kanatzidis says. But Kanatzidis and his colleagues discovered that by combining platinum with sulfide compounds, a non-oxide gel will form without precipitation. Plus, the sulfide ions in this new aerogel make strong bonds with heavy metals: "A mercury ion in solution will hit the surface of the porous material and get stuck," Kanatzidis explains.

The chalcogel was remarkably effective at removing mercury from water. 10 milligrams of chalcogel almost completely purified water that had toxic levels of mercury. The concentration of mercury dropped from 645 parts per million (ppm) to ~0.04 ppm after purification. "The 600 plus ppm of water represents a highly contaminated stream. This is a dramatic purification effect, to the point where perhaps the water could become safe to drink," Kanatzidis says. (For a rough comparison, the FDA sets the mercury limit in fish at 1 ppm for a fish to be safe to eat.)

Should we expect to see chalcogels in every polluted waterway? "Not so fast," Kanatzidis says. "The materials contain platinum. It would be ludicrous to suggest that we should dump this stuff everywhere because it would be extremely expensive." Kanatzidis is already working on generation of cheaper chalcogels, but, he cautions: "We don't exactly know in what form something like this could be used. It's early yet."

-Flora Lichtman

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--Flora Lichtman