Better and cheaper porous carbon filters.

Although activated carbon has been used for more than half a century to filter contaminants from air and water, scientists have only now imaged the twisting and turning pores that enable the adsorbent to do its job.

Christian L. Mangun and James Economy of the University of Illinois at Urbana-Champaign and their colleagues at Southern Illinois University at Carbondale used scanning tunneling microscopy to look at cross sections of carbon fibers that had been activated by heat to make them porous. By understanding the microstructure, they can tailor the fibers' properties to filter specific contaminants. "We can not only control pore size and shape but also pore chemistry," Economy says. The group's findings will appear in the October Carbon.

Although low-cost carbon granules, not fibers, are used in most applications, no one has yet sliced granules into thin sections that would show the tiny pores, which are often only a few tenths of a nanometer wide, Mangun says.

That pore size is small enough to filter out contaminants like sulfur dioxide, butane, and trichloroethylene. The researchers have changed the chemistry of the pores by lining them with different compounds, thus improving further the fibers' ability to trap passing molecules. For example, acidic pores remove ammonia gas, and basic pores filter out hydrochloric acid, they find.

The researchers' analysis also showed that conventional wisdom about greater surface area leading to greater adsorption wasn't completely correct, Mangun says. At high contaminant concentrations, total pore surface area matters most. But at low concentrations, measured in parts per million, pore size takes precedence.

The activated carbon fibers, invented by Economy in the early 1970s, perform 10 times better than granules, he says, but at $100 a pound, they are 100 times as expensive. Only one company, Nippon Kynol, located in Osaka, Japan, manufactures them.

To address this cost difference, the group developed a cheaper fiber that performs as well as the original. Currently, the carbon fibers are made by heating a weave of plastic fibers to about 800#161#C with steam. The heat turns the plastic into a fabric of pure, porous carbon strands. The new process begins with much cheaper glass fibers, coats them with plastic resin, and then activates the resin carbon by heating it. The product costs only a few dollars a pound, which "brings the cost into a range competitive with activated granules" and other molecules designed for filtering, Economy says. The group filed for a patent last summer.

Economy envisions many applications for these fabrics. Made into face masks, they could protect city dwellers from pollution or civilians from chemical warfare. About 10 companies have expressed interest in the new fibers, he says, as components of air filtration systems for cars, home water purification devices, and industrial waste filtration equipment.