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June 2, 2004 — Millions of villagers in Bangladesh were exposed to unhealthy levels of arsenic in drinking water in the 1980s and ’90s after the naturally occurring poison seeped from bedrock into groundwater supplying wells.
After a decade of exposure, the Bangladeshis began to show skin abnormalities and other signs of toxicity, including cancers. The contamination still plagues Bangladesh, India and several South Asian nations.
The United States is not immune, either. The U.S. Geological Survey found high concentrations of arsenic in water in parts of California, Texas, Massachusetts and other states in a 2000 analysis. Soon after, the U.S. Environmental Protection Agency revised its standards for the allowable level, from 50 parts per billion to 10 parts per billion. The more stringent standard goes into effect in 2006, giving utilities less than two years to find effective and affordable treatments.
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A handful of companies and research labs are working to provide nanotechnology-based solutions for these domestic and foreign markets. One platform is proving versatile enough to capture not only arsenic in its various forms but also mercury.
“The opportunity for this kind of activity is excellent,” said David Reisner, chief executive of Inframat Corp. (Profile, News, Web) in Farmington, Conn. The nanomaterials company received funding through the Air Force to develop what it calls nanofibrous bird’s nest materials for removing arsenic from water.
Inframat uses manganese dioxide, a common and inexpensive chemical that oxidizes a highly toxic form of arsenic into a form that is easier to remove from water. The chemistry of manganese dioxide is well understood, saving Inframat’s researchers the time and labor associated with designing novel compounds. “This is a textbook molecule,” Reisner said. “It’s been studied for 100 years.”
Inframat focuses on designing interwoven nanofiber structures using manganese dioxide. The porous “bird’s nest” structure has a much larger surface area, allowing more arsenic to come in contact with it and be converted. Lead researcher Amy Chen described initial results as outstanding, but she doesn’t intend to stop at that.
She now is attempting to develop a filtration process that first turns arsenic into its less potent form and then absorbs it. Her tests show the process outperforms a benchmark material on the market today and meets the 10-ppb standard.
Researchers at Pacific Northwest National Laboratory (PNNL) also are devising a cleanup system that combines a porous structure offering high surface areas and nanomaterials that interact with toxic metals. Their Self-Assembled Monolayers on Mesoporous Supports, or SAMMS, provide a platform for removing arsenic, mercury and other health hazards from water.
Like Inframat, the PNNL scientists begin with a material that has been well researched and is commercially available. They use mesoporous ceramics first produced by Mobil Technology Co. in the early 1990s. They then design self-assembled monolayers that bind to the ceramic substrate, exposing the other side to the contaminants.
“It’s like a carpet; the molecules are firmly linked and laid on the floor and the functioning end sticks out,” said Shas Mattigod, a PNNL staff scientist who has worked in the SAMMS program for six years. He devised many different types of monolayers, including one that captures mercury and a more complicated system for absorbing arsenic.
The challenge for Mattigod and his team is finding the right chemical architecture for each specific application. The monolayers must self-assemble, a bottom-up process that helps reduce labor and cost. One end must bind to the surface and the other to the contaminant. And once the poison is captured, SAMMS must hold on tight to prevent any reintroduction of the pollutant to the environment.
Mattigod reported that the mercury-absorbing SAMMS captured almost 99 percent of mercury in a solution. He envisions incorporating the material into a cartridge that can then be removed and disposed in a landfill.
“We’ve done preliminary cost comparison,” he said. “For industry, this would be a significant cost savings. You don’t need a whole lot of material … and the disposal costs are less.”
The arsenic-absorbing SAMMS relies on a more complicated lock-and-key system, Mattigod said. The monolayer binds to the substrate, and when certain forms of arsenic pass by, the exposed end reacts or “unlocks.” The arsenic molecule then slips into the “keyhole” and is absorbed. “It’s designed so that only the key of the right type will bind,” he said. “It has high selectivity.”