More than a dozen major sites around the U.S. have uranium-contaminated soil and groundwater. Stanford researchers propose using opal to sequester the uranium for long-term storage. Photo: Doctress Neutopia/Creative Commons |
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 University 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,” says 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.
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.
Radioactive uranium in a piece of natural opal glows bright green under ultraviolet light. Image: Kate Maher/Stanford University |
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 ppm 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 says.
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 says.
“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% of the crust, and in many geologic environments
most of the waters are close to saturation with silica. Maher says 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 says
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 says.
