Sop story: new porous gel soaks up heavy metal

Science News, July 28, 2007 by Alexandra Goho

A team of chemists has created a new porous material that's extremely effective at sopping up mercury. Called a chalcogenide aerogel or simply a chaleogel, the material could be used as a filter for cleaning contaminated drinking water. The material's versatility also makes it a good candidate for a wide range of other applications, including the production of hydrogen fuel.

Mercouri Kanatzidis of Northwestern University in Evanston, III., and his colleagues have created several chalcogels by, in each case, combining two ingredients. One ingredient is always a chalcogenide--a compound containing at least one of the elements sulfur, selenium, or tellurium, which lie directly below oxygen in the periodic table, and at least one positively charged element. The second ingredient includes platinum, which links the chalcogenide molecules.

In this case, the researchers combined a platinum compound with a chalcogenide containing sulfur and the semiconductor germanium. After dissolving the two ingredients in water, the researchers poured the mixture onto a petri dish and let it sit for 2 days. The liquid turned into a dark-brown gel. Careful washing and drying preserved the gel's highly porous structure.

"It's very simple chemistry," says Stephanie Brock, a chemist at Wayne State University in Detroit. "And that's one of the things that's very elegant about it."

To test the new material's ability to clean water, the team passed solutions highly contaminated with mercury through the chalcogel. Because mercury likes to bind to sulfur, explains Kanatzidis, the heavy metal accumulated on the surfaces of the numerous pores inside the material. In fact, the chalcogel removed up to 99.9 percent of the mercury in tainted solutions, the researchers report in the July 27 Science.

The team's initial findings indicate that the chalcogel performs just as well as, and sometimes even better than, commercially available water-filtration materials, says Kanatzidis. Unfortunately, the presence of platinum makes the new material too expensive to use in a commercial setting, he adds. His lab's next goal is to replace the platinum with a cheaper alternative.

Because the material also has interesting electronic and optical properties--due in part to the presence of germanium--such a chalcogel could be used for a range of applications beyond water remediation. Chalcogels can absorb both visible and infrared light, making them good candidates to act as light-triggered catalysts for a variety of reactions, including those that split water into hydrogen and oxygen.

Materials that use solar energy to produce hydrogen could one day generate the large amounts of hydrogen fuel needed to support a hydrogen economy (SN: 10/30/04, p. 282). That's one of many applications that Kanatzidis' group is interested in pursuing, he says.

"A lot of people are working on making hydrogen fuel," says Brock. "It's not a simple problem, and I don't think anyone has hit on a solution yet" The chalcogel, she says, could offer a new way forward.