While some separations leave you lonely, other separations leave you glad: they clean up contaminated water, reduce hazardous waste and refine useful chemicals and gases. Many of these separations depend on the help of membranes that allow one substance to pass through and hold back the rest.
Researchers at the Department of Energy's Idaho National Engineering and Environmental Laboratory are creating durable membranes that can be specially tailored to separate different chemicals from water. Fred Stewart, a chemist at the INEEL, presented his group's work at the 218th National Meeting of the American Chemical Society on Aug. 24 in New Orleans, La.
Membranes are used often in both the DOE and in industrial applications. While the DOE is mainly concerned with removing hazardous and radioactive contamination from waste and other liquids, industries delve into water treatment, production of industrial gases and pharmaceutical manufacturing. Many applications require membranes that are tougher, more efficient and more versatile than current membranes.
Membranes decontaminate by filtering substances selectively. Stewart said, "Let's say I've got this thousand gallon drum of something nasty in water, and 5 percent of it is the nasty stuff. If I can pull out the 50 gallons of hazardous substance and recover 950 gallons of water, I've drastically reduced the amount of waste I have to dispose of. Industries are very interested in waste minimization."
The DOE is also interested in waste minimization, but the substances that contaminate DOE sites are problematic for conventional membranes. "The state of the art in membranes is organic membranes, all carbon- or silicon-based. They have very limited utility within the DOE," said Stewart. "These membranes generally can't withstand the chemical and thermal stresses that are placed on them. They also have insufficient stability when exposed to high radiation fields."
Industrial applications require membranes that are stable at high temperatures as well. "Most membranes disintegrate starting at about 150 degrees Celsius, which is low for industrial uses," said Stewart. The membranes Stewart and his group work on are stable up to 300 degrees Celsius, three times the temperature of boiling water.
Stewart's membrane is a hybrid of organic and inorganic molecules. The organic molecules allow water and dissolved substances to interact with the membrane, and the inorganic molecules provide more stability than what most organic-only membranes have.
The organic molecules are chosen based on what dissolved substance will be separated from the water. The inorganic molecules -- alternating phosphate and nitrogen molecules -- form a polymer backbone to which the organic molecules are attached. Single lengths of the backbone molecules, which are akin to wooden slats on a deck, can be linked to each other in a method called cross-linking. This creates a sturdy, micro-sized lattice (the finished deck) that is then placed on a support (the deck's feet).
The holes within the cross-linked polymer lattice allow molecules to flow through. "A Ziplock baggy is a cross-linked polymer," said Stewart. "We want something as durable as a Ziplock baggy -- then maybe we can dispense with the support. Without cross-linking, we think the polymer gets sucked into the support, because the membrane stops working after a few days."
These membranes -- called polyphosphazene membranes -- have two advantages for the researchers. The number of cross-links in the backbone can be varied, and they can add whatever organic molecules they want to the polymer backbone. These qualities allow the researchers to tailor the membranes' selectivity and to withstand temperature, pH, and radiation.
To deal with this, Stewart and his colleagues are developing computer-based models of polymer chemistry. "There are limitations to what a polymer will do," he said. "We're learning how to theoretically predict what a polymer will do so when a customer comes in and says, 'This is what I need,' we'll be able to look at the basic characteristics of their feed stream -- what they'll be separating from what -- and match that up with the right polymers."
Although the membranes being developed at the INEEL might be too expensive for large-scale water treatment, they are just what the DOE needs to clean up the nuclear weapons and nuclear energy research legacy. Stewart said, "One problem DOE has with cleaning up is nothing already available works all that well."
This work has been funded by INEEL discretionary research funds and DOE programs for Environmental Systems Research and Environmental Management. INEEL celebrates its 50 year anniversary in 1999. The national laboratory is operated for the U.S. Department of Energy by Lockheed Martin Idaho Technologies Company.
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