STANFORD UNIVERSITY —Across the United States there are more than a dozen major sites where soil and groundwater are contaminated with substantial amounts of uranium — a highly mobile, radioactive element. Most of the contamination is from poor disposal practices at mines or plants that processed uranium-rich ore for power plants or nuclear weapons, or reprocessed spent or decommissioned uranium.
Cleaning up such sites is a problem that has bedeviled remediation efforts for decades. There has been no simple, reliable, cost effective way to do it. Now a team of researchers led by Stanford geochemist Kate Maher is proposing to imitate nature by using amorphous silica — also known as the precious gemstone opal — to sequester the uranium. Once ensconced inside opal, the uranium molecules would be rendered immobile and chemically inert.
“We have looked at opaline silica in deposits across the western U.S. and almost universally we find very high uranium concentrations,” said Maher.
“From dating these deposits, we have found that they have been stable, closed systems for hundreds of thousands — and in some cases millions — of years.”
Whether in soils, hydrothermal deposits, hot spring or cold spring deposits, when enfolded in an opaline embrace, uranium seems about as active as a bug trapped in amber.
According to computer modeling studies that the researchers have done using their data from natural opal deposits, opaline silica may offer a faster, cheaper, more enduring way to sequester uranium than other current or proposed methods.
Maher, an assistant professor of geological and environmental sciences at Stanford, is presenting the team’s research at the 2011 annual meeting of the American Geophysical Union in San Francisco on Dec. 6.
The sequestering process would involve pumping a solution rich in dissolved silica into the subsurface through injection wells, effectively flooding the contaminated areas with it. As the solution moved through the soil or rock, chemically interacting with its surroundings, amorphous silica would precipitate out and latch on to dissolved uranium.
Various methods for remediating uranium-contaminated zones have been tried. Excavating and hauling contaminated soil elsewhere for treatment and permanent disposal is an expensive way to go, so cheaper on-site, or in situ, remediation is preferable. The most common approach has been “pump and treat,” which is exactly what the name implies – clean water is flushed through the system to displace the uranium-contaminated water, which is pumped out for treatment.
Approaches for in situ remediation generally involve reducing the electrical charge of the uranium atoms — and thus their chemical reactivity — by means of various biological or chemical agents. Certain microbes have had some success in reducing uranium to a stable state, and some chemical additives, such as certain forms of iron and sulfur, also have demonstrated some promise. Introducing phosphate into contaminated soil or sediment, where it would chemically bond with uranium to form a new mineral, also has been proposed.
But all of those methods rely on creating and maintaining an environment in which the agents of reduction are always present. If conditions change and those agents diminish in abundance, either through biodegradation or physically washing out of the contaminated area, the uranium could return to a more mobile – and dangerous – state.
Opaline silica, on the other hand, is not only a demonstrably long lasting host, it is also much more welcoming than other potential mineral hosts such as the calcite that is often precipitated along with the opal. Maher said that, on average, the enrichment of the uranium into the opaline silica tends to be “many orders of magnitude greater” than what the researchers found in the calcite.
“We see up to 1,000 parts per million of uranium in some natural opal deposits compared with a few parts per billion levels in calcite that often precipitates along with the opal,” she said.
Opaline silica is also stable over a wider range of pH conditions than calcite and other minerals that often precipitate with opal, further enhancing opal’s relative durability.
On top of its striking capacity and stability, opal also incorporates uranium into its amorphous form at a relatively rapid rate, according to the researchers’ modeling of different sequestration scenarios.
“From our modeling analysis, within 10 years of flooding a contaminated area with sodium silicate, nearly the whole aquifer has been decontaminated,” Maher said.
“The uranium has been sequestered to levels far below the maximum contaminant level allowed by federal law, while with the traditional pump and treat approach, less than half of the aquifer is beneath that level.”
Once uranium has been incorporated into opal, about the only way for it to get back out would be if fluids that contained very low amounts of silica began circulating through the zone in which the uranium was sequestered. If the silica content of the fluid was low enough, the amorphous silica could start dissolving and set the uranium free to roam and contaminate its surroundings.
But silicate minerals are the most abundant class of rock-forming minerals in the crust of the Earth, composing about 90 percent of the crust, and in many geologic environments most of the waters are close to saturation with silica. Maher said that makes the researchers confident that opaline silica will be stable over long time scales.
Silica is also relatively inexpensive, making it an affordable method for storing uranium in situ in the subsurface.
So far the researchers’ work has been focused on sampling and analyzing naturally occurring deposits of opal and using that data to model the reactivity and transport of uranium under different scenarios. They are particularly interested in how iron oxides, which are commonly present in soil and sediment, might affect the incorporation of uranium into opal.
But Maher said they hope to try the method at the experimental scale in the laboratory within the next few months and then run a trial at a contaminated site.
“Our initial feasibility study suggests that this is a potentially more reliable and more effective strategy than trying to create reducing conditions in the subsurface environment,” Maher said.
Michael Massey and Joseph Nelson, graduate students in the departments of Environmental Earth System Science and Geological and Environmental Sciences, respectively, contributed to this research, as did Craig Bethke, a visiting professor from the University of Illinois, and Scott Fendorf, a professor of environmental Earth system science.
— Louis Bergeron
Also on Stanford Knowledgebase: