Sieves make catch of the day
04/01/2001
Mark A. DeSorbo
ALBUQUERQUE, NM—Researchers at Sandia National Laboratories say the same technology that is used to trap lobsters off the coast of Maine can also capture radioactive and other volatile molecules lurking in hazardous waste.
Like a lobster trap, the Sandia Octahedral Molecular Sieves (SOMS), microporous materials that form molecular cages, allow certain species in while keeping others out. Researchers believe SOMS can be useful in microelectronics fabrication as well as other industries where purification of, or extraction from, liquid processes or waste streams is a significant or costly problem. It also aids in the recycling of materials such as chromium, cobalt and nickel from industrial effluents.
"This can be used to rinse out trace metals that can be left over in wash solutions," says Tina M. Nenoff, Sandia's principal project investigator. "Semiconductor fabs have to remove metals in order to reuse them or remove them before wastewater is released in to [sewerage]."
Sandia, she says, developed SOMS with the help of researchers at the University of California-Davis; Pacific Northwest National Laboratory; the University of Michigan; the State University of New York-Stony Brook; and Lawrence Livermore National Laboratory. Research and development was also sponsored by the U.S. Department of Energy's (DOE) Environmental Management Science Program, which funds projects that mitigate DOE-site cleanups.
"The real discovery was done here at Sandia, but we needed all of our collaborators to understand the full description of the material, namely the crystalline structure and radioactive stability," Nenoff says.
Picky pores
SOMS are essentially networks of tiny sponges that slurp up divalent cations, atoms with a +2 charge, into its microscopic pores and ensnares them at negatively charged bonding sites vacated by ions with weaker charges, a process called ion exchange that works just like a tap water filter to remove iron and other minerals.
SOMS, however, are rather fussy about which ions they imbibe, for the size of the openings on crystalline surfaces can be adjusted by altering the same formula to make them. By varying these openings from four to 15 angstroms, one ten-millionth of a millimeter, scientists are able to control which size of ions or molecules can get in or out.
The microporous material was extremely selective for strontium-90, a byproduct of plutonium processing—a step in nuclear warhead production. It is also one of the two most rampant radioactive chemicals of liquid hazardous waste that was inside 177 underground storage tanks at the DOE's Hanford, WA, environmental remediation site. In lab tests, SOMS trapped 99.8 percent of the strontium-90 ions in ppm concentrations from solutions containing chemically similar and highly abundant sodium ions.
Heat 'em up and throw 'em away
So what happens when SOMS become saturated with strontium-90?
Nenoff says that when heated to about 500 degrees Celsius, the SOMS collapse into perovskite, a dense, glass-like material. Its shrunken pores have cations under proverbial lock and key within its crystalline structure. Bricks made from SOMS are resistant to leaching and remain stable at high pH, radiation and heat, making them ideal for disposal.
"You can bury it with the confidence that it won't leak," she adds. "The other way is to flush it with an acid wash to rinse out the metals that you've captured and then you can recycle or dispose of the metal, depending on what particular industries require."
Commercial partner wanted
Although the SOMS is still being tested, Nenoff says Sandia is presently seeking a commercial partner to bring its molecular lobster trap to the industries that would benefit from it.
Nenoff has also played an integral role in the development of another microporous material, which extracts cesium from acid and other solutions. That microporous material is now marketed by UOP Corp., a process-technology-focused company based in Des Plaines, IL.
According to the company Web site (www.uop.com), scientists and engineers at UOP were intrigued by the material's ion exchange properties, especially its ability to seek out only cesium, another byproduct of plutonium processing.
"This material offered a significant improvement over UOP's current materials," the company indicates.
The collaboration between UOP, Sandia and Texas A&M, yielded IONSIV IE-911, which has been used at Oak Ridge National Labs in Tennessee to treat thousands of gallons of dangerous radioactive liquids. Once treated, the water can be returned to the environment, while radioactivity once processed as a solid, is ready for further processing and ultimately burial in a remote government location, according to UOP. An IONSIV IE-910 powder by UOP has been used experimentally to remove cesium and strontium from contaminated milk.