Caustic soils at Hanford may lock up contaminants fast

Soil particles lock up contaminants hundreds to thousands of times faster under the caustic conditions found beneath leaking toxic waste tanks at the Hanford Nuclear Reservation than under normal soil conditions, researchers report.

Understanding more about how contaminants such as radioactive cesium and strontium move through the soil under real-world conditions will help cleanup efforts at Hanford and other sites contaminated with nuclear waste. Previous research on the movement of soil contaminants had not replicated the extreme conditions found at the Hanford Site in Washington state.

“There’s a lot of interest in trying to determine the best approach to dealing with the massive subsurface waste at Hanford,” said research team leader Jon Chorover, an associate professor of environmental chemistry at the University of Arizona in Tucson. “Our work helps predict the migration of this stuff. Understanding how these contaminants move in the soil can help with remediation.”

Sunkyung Choi, a research associate at UA, will present the team’s finding at the 227th national meeting of the American Chemical Society.

Choi’s presentation, “Cesium and strontium uptake to clay minerals and their weathering products in a caustic waste,” will be given at 4:15 p.m Pacific time on Monday, March 29, in Grand Ballroom D of the Anaheim Marriott, 700 West Convention Way, Anaheim, Calif.

Other members of the team include UA research specialist Mary Kay Amistadi and UA professor of materials science and engineering Supapan Seraphin. The research is funded by the U.S. Department of Energy.

At the Hanford Site, high-level nuclear waste from the manufacture of weapons is stored in 177 tanks buried in the soil. The waste contains toxic and highly radioactive elements, including plutonium, cesium 137 and strontium 90, mixed in with other materials that make the liquids in the tanks extremely caustic.

Weapons production began at the Hanford Site in 1944. Since then, 67 of the tanks have leaked, releasing highly radioactive liquid waste into the soil. Some of the contaminants have been found in the groundwater. Cost estimates for cleaning up the site run into the tens of billions of dollars.

Figuring out exactly how the wastes move through the soil is difficult because the tanks’ contents are complex mixtures of chemicals. But the contaminated soils and toxic wastes materials are too dangerous for the scientists to work with directly.

“If we were working with the concentrations of radioactivity that are out there, it would be lethal doses,” Chorover said. “It would kill everyone in the lab.”

He and his team are doing the next best thing. They have created non-radioactive chemical mixtures that match those in the Hanford tanks and are studying how those chemicals move through uncontaminated Hanford soil collected near the tanks.

In addition, to better understand how the different components of soil interact with the various chemicals, the team created some model soils to test with the Hanford-like chemical mixtures.

Even so, geochemical processes in soils occur over years, so the research, too, must be conducted over years, not days or weeks.

So to test how various liquid-and-soil mixtures interact over time, the researchers put the mixtures in small chemical-resistant plastic bottles and loaded the bottles onto mixers that look like mini-Ferris wheels.

The wheels, each of which holds more than 100 bottles, turn the bottles end-over-end. The wheels keep turning day, after day, after day. One of the experiments has been going on more than 2 years.

At set times, the researchers test the liquid and solid portions of bottles’ contents to see how chemicals from the liquid have reacted with the soil particles.

To the researchers’ surprise, over time the contaminants in the experimental set-ups were bound by newly formed clays in the soil, forming unusual minerals called zeolites and feldspathoids. The mineral particles, about a thousand times smaller than a grain of sand, don’t dissolve easily and therefore keep the contaminants trapped in the soil.

“We find the contaminants are remarkably slow to redissolve and appear to be more stable than initially thought,” said Chorover. That stability may keep the contaminants from leaching deeper into the soil and the groundwater.

The team also found that the various clays from the soil and the various contaminants react differently with one another.

Although learning that contaminants get sequestered in solid particles seems like good news, Chorover is cautious.

“We really don’t know the lifetimes of these particles. We’ve shown it in the lab, but we don’t know what’s happening in the field,” he said. “We do know contaminants are migrating through the soil in the field, so these laboratory results don’t explain all of what we see at Hanford.”

The team’s next step is figuring out how stable the solids are and how long they last in the environment.