RICHLAND, Wash. - Tiny grains of ceramic material inhabited by hungry molecules are looking like enormously effective options for cleaning up contaminated waterways and recovering precious metals.
SAMMS -- Self-Assembled Monolayers on Mesoporous Supports -- has been developed by researchers at the Department of Energy's Pacific Northwest National Laboratory. SAMMS integrates mesoporous ceramics technology first created by Mobil Oil Corp. with an innovative method for attaching "monolayers" (single layers of densely packed molecules) to the pore surfaces throughout the ceramic material. The molecules are custom designed to seek out mercury, lead, chromium and other toxic or precious metals.
"SAMMS can be tailored chemically to selectively bind a wide range of contaminant types, including radionuclides," said Jun Liu, a staff scientist at Pacific Northwest who directed the fundamental research. "And SAMMS can be used effectively in water, non-aqueous solutions or gas phase waste streams."
According to Nick Lombardo, a commercialization manager at Pacific Northwest, DOE is interested in exploring the use of SAMMS for soil and water cleanup activities at sites where mercury contamination is prevalent, and for the removal of mercury from radioactive and hazardous wastes. Mercury, released from a number of natural and man-made sources, can cause serious health effects if inhaled or ingested. "In addition to being able to clean mercury-contaminated sites, we believe SAMMS also has applications in industry, particularly mining and metal finishing, where it could be used to clean the water used for processing and even recover valuable metals present in waste streams," Lombardo said.
SAMMS is produced in bead or powder form. Each grain of ceramic material, (in this case a type of silicate) is only five to 15 micrometers in diameter and contains a densely ordered array of cylindrical caverns or pores, giving the material a honeycomb appearance. The chemically tailored monolayers reside within the pores, with the molecules strongly binding at one end to the ceramic material. The free ends of the tethered molecules then are available for binding to a targeted metal species passing through the pore.
"Although difficult to imagine, these pores provide a large surface area for selective trapping of metal ions in solution," Liu said. "In fact, a mere tablespoon of this material in powder form has the surface area equivalent to that of a football field."
Upon release in water, demonstrations have shown that SAMMS quickly immobilizes the targeted metal, reducing the concentration to far below drinking water standards. The small pore size also precludes the metal from leaving and resolubilizing into a more toxic and/or mobile form.
"The SAMMS material has demonstrated the highest metal-loading capacity reported by anyone so far," Liu said. "Part of the reason is we have found an effective way to create a seating chart, so to speak, for the molecules, and have established a proficient method for getting the molecules into their proper seats."
Liu credits Pacific Northwest chemist Glen Fryxell with developing a process for attaching the monolayers to the mesoporous supports so that the density of the functional molecules can be optimized without blocking the tiny pore channels.
"I think the most exciting thing about SAMMS is that we not only have the ability to change pore size but also to custom design the molecular properties on the surface so that the molecules can recognize certain species and reject others," said Liu. "That kind of molecular recognition has tremendous potential."
SAMMS' versatility is reflected in the fact that it also can serve as a waste storage medium following absorption of the metal. Essentially, the metal is encapsulated within the ceramic material. When subjected to the Toxic Leaching Characteristic Test, a regulatory benchmark which measures release under environmental conditions, researchers found that the bound metal contaminants remain in the solid and do not leach into solution.
Presently, Pacific Northwest is gathering data on the commercialization potential of SAMMS. "We're trying to make sure that the technical and economic issues associated with getting the technology into the marketplace are being addressed," Lombardo said. Current research is evaluating the performance of SAMMS in different forms (dispersed powders, solid pellets, membranes or filters) to determine the effectiveness of each material for a given application. "Companies such as Mobil and 3M are working on the engineered form of the material with us," Lombardo added.
Liu points out that the laboratory is one of many research organizations exploring and advancing the integration of mesoporous ceramics and functional monolayers. This discipline also is being pursued actively within DOE's Center of Excellence for Synthesis and Processing of Advanced Materials, a collaborative, multi-laboratory effort.
SAMMS was named by R&D magazine as one of the 100 most significant innovations of 1997, and was a finalist for the 1998 Discover magazine Award for Technological Innovation.
Pacific Northwest National Laboratory is one of DOE's nine multiprogram national laboratories, conducting basic and applied research to solve problems in environmental, energy, health and national security areas. The laboratory has been operated by Battelle of Columbus, Ohio, since 1965.
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