Rust to the Rescue

David Pacchioli
January 01, 1999

"The problem with technetium is not just that it's radioactive," says Sherman Ponder. "The problem is that it lasts forever."

brown building with skies in background

For places like the Hanford Nuclear Reservation along the Columbia River in Washington State, where the groundwater is contaminated with technetium and other radionuclides, radioactive presence—and permanence—is a big problem. The Hanford site, which produced the plutonium used in the first atomic bombs, opened in 1943. For its first 30 years, Hanford produced radioactive materials; for the last 25, scientists, environmentalists, and government officials have been trying to figure out how to remove the radioactive residue that's leaked into the water and soil.

Ponder, a graduate student in chemistry at Penn State, believes that "ferragels," a material he has invented with his adviser, Thomas Mallouk, and for which Penn State is seeking a patent, may offer a solution—not only for cleaning up this nuclear mess, but for eliminating pollution at sites contaminated by toxic metals and by organic compounds such as pesticides, dioxins, and PCBs.

The main ingredients of ferragels are nanoparticles of iron, each less than a micron in size. Their cleaning ability depends on the simple, ordinary process of rusting. Except for those researchers trying to find ways to stop corrosion, Ponder says, "Most scientists haven't paid attention to what rust actually means." Rusting depends on a reduction/oxidation or "redox" reaction. During a redox reaction, the loss of electrons from one substance is called oxidation, and the addition of electrons to another is called reduction. Since this electron transfer always requires a donor and an acceptor, oxidation and reduction always go together. Ponder explains, "When iron oxidizes, something else must be reduced at the same time. We're harnessing the redox reaction to reduce and thus remove contaminants from our groundwater supply."

Ponder studied iron's ability to eliminate toxins as an undergraduate at the University of South Florida, so when he and Mallouk were trying to find a solution for treating technetium, the redox reaction of iron was a natural choice. When the team first began experimenting, they used a gel to stabilize the iron. Although they now use a resin, they've kept the original name, "ferragels."

Making ferragels, Ponder says, he first puts ferrous salt, which looks like sea-green bath crystals, in a small beaker, mixes it with water, and adds a stabilizing agent the color and consistency of flour. To this mix he adds a touch of a chemical that instantly causes the liquid to turn black, bubble, and gurgle. "You should see it when we make a huge vat of this stuff," says Ponder. "It's very cool."

The metamorphosis, he explains, is caused by nanoparticles of iron binding to the resin; after the mixture is strained, you're left with a small handful of ferragels, which smells like railroad tracks, and feels gritty and oily—not the kind of substance you'd imagine could clean anything. But use that same handful to filter water from a stream contaminated with something like dry-cleaning fluid, and the beauty of ferragels shines through.

"The particles act like nano-bloodhounds," says Ponder. In the case of dry-cleaning fluid, ferragels track down and bind to chlorine molecules, and the result is ethylene, a neutral, non-toxic gas—the same thing used to ripen grocery-store tomatoes. "The toxic contaminant no longer exists, and the ethylene should just percolate out of the soil." Laboratory tests show that the ferragels remain in the water as small amounts of rust.

Ponder and Mallouk are working with a simulated "recipe" of Hanford tank waste in their University Park lab. The mixture contains chemicals like aluminum nitrate, sodium hydroxide, iron, and rhenium (a nonradioactive element used instead of technetium). It looks like pond scum. Ponder says the mix is nasty stuff—it would burn your skin off if you fell into a vat of it. So far, his and Mallouk's studies show that ferragels remove 99.999 percent of the rhenium in this test solution.

On a large scale, radioactive water would be filtered through a column containing ferragels. The technetium would then be reduced to an insoluble solid, which could be collected and mixed into large glass rods which would not degrade like metal barrels do. Says Ponder, "If we can get ferragels to work at Hanford, we can get them to work anywhere."

Sherman Ponder is a graduate student in chemistry in the Eberly College of Science, 317 Chandlee Lab, University Park, PA 16802; 814-863-9791; ponder@chem. His adviser is Thomas E. Mallouk, professor of chemistry, 152 Davey Lab; 863-9637; Graduate student Bettina Schrick and undergraduate James R. Ford are also working on this project. The Hanford research is being conducted in conjunction with Pacific Northwest National Laboratory and is funded by the U.S. Department of Energy.

Last Updated January 01, 1999