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Heavy Metal Filter Made Largely from Air

New material proves an adept filter for heavy metals as well as an efficient semiconductor, once freeze-dried into an aerogel

A pinkish brown gel with the brittle consistency of astronaut ice cream and composed mostly of air has the ability to filter nearly all of the heavy metal mercury, a potent neurotoxin, out of drinking water. This new aerogel may also be able to efficiently harvest the energy in sunlight and rapidly purify hydrogen fuel, among other things, chemists report this week in Science.

"We have a material that presents a very high surface area and the surface itself is made out of something with different reactivities than oxides," says chemist Mercouri Kanatzidis of Northwestern University and Argonne National Laboratory, whose team discovered the gels. "That gives rise to properties that oxides don't have."

The researchers discovered seven gels in all—made in solution from chalcogens (the column of elements on the periodic table that begins with oxygen) and platinum. More common aerogels are made from oxides, such as silicon dioxide (or sand). The latter chemical compounds, in addition to being ubiquitous in Earth's crust, are used for a variety of purposes, from sunscreen to antirust coatings. But the so-called chalcogels—made from sulfur or selenium—offer a host of new features.

For example, researchers discovered that when water contaminated with the heavy metal mercury (which can cause nerve and brain damage in fetuses and children) was run through a sulfur-germanium chalcogel, the amount of mercury dropped from 645 parts-per-million to just 0.04 ppm. "These heavy elements love to bind to sulfur atoms," Kanatzidis says. The metal-bearing water passes through the "torturous, porous network" of the chalcogel and "sooner or later, these heavy elements will encounter sulfur. It's sticky to them."

The mechanism for that stickiness "is unknown," says chemist Stephanie Brock of Wayne State University in Detroit, who has created similar gels with a more rigid internal structure. "It may either be a sorption phenomenon (in which the [positive] ions attach to vacant sites on the chalcogenide [negative ions]) or an ion-exchange phenomenon (in which platinum is released and mercury is picked up)."

The key is the expanded surface area of the chalcogel, an extremely low-density solid made up primarily of air. "A cubic centimeter of this stuff could have 1,500 square meters—almost a football field—inside of it," Kanatzidis notes. That expanded surface is a widely spaced version of the amorphous structure that forms the initial gel, but it's a mystery why the gel forms in the first place instead of becoming a solid precipitate that drops out of solution. "To make a gel, you have to have a growth of the framework that is three-dimensional so that it extends in space, kind of like a tree, only a lot faster," Kanatzidis says. "Why it happens like this with platinum is not clear to me."

Regardless, the different elemental components offer other nifty properties that can then be multiplied over the tremendous internal surface area. "The optical absorbance of these chalcogenide aerogels corresponds nicely to the solar spectrum, so these could be promising platforms for photovoltaic solar cells and photocatalysts," Brock says. In other words, such chalcogels might provide the basis for more efficient and faster conversion of incoming light into electricity or to break down water into hydrogen.

Membranes made from these chalcogels could also potentially help purify hydrogen mixed with other elements. "If people are interested in using hydrogen as a fuel, it has to be purified at a rapid pace," Kanatzidis says, another process aided by the large amount of inner space. The gels may also be useful in processing more prosaic fuels, such as removing sulfur contamination from fossil fuels.

To do that, though, the chalcogels will need to withstand higher temperatures; they begin to collapse at temperatures ranging from 150 degrees Celsius (302 degrees Fahrenheit) to 300 degrees C (572 degrees F). They will also have to become less brittle, perhaps by resting on a stronger substrate. In addition, they will have to be cheaper: requiring the precious metal platinum would make them expensive. But "the replacement of platinum" in the manufacture of such chalcogels "has been successful," Kanatzidis says. "These aerogels are not going to be limited to platinum."

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