By Heather Rock Woods
A new technology that acts like a giant underground filter is
successfully beginning to mop up the uranium contaminating an aquifer in a
remote Utah canyon. Uranium contamination in groundwater is a serious
problem because the toxic metal can travel long distances in underground
aquifers, which are vital sources of fresh water for people, animals and
Installation of the apatite PRB at Fry Canyon, Utah.
courtesy of John Bargar)
Recent research at SSRL showed that the filters—called PRBs for
permeable reactive barriers—intercept uranium in an unexpected way. This
fundamental knowledge has important implications and serves as the latest
example that many environmental cleanup ideas work differently in reality
than in theory.
"We knew that the barriers worked to stop uranium, now we know how they
work. We can use this information to predict how long they will work and
what the costs will be," said John Bargar (ESRD), molecular environmental
scientist at SSRL. "This information is necessary to compare this concept
to other technologies and to select and engineer new designs."
Originally, scientists thought uranium would react with a
mineral—called apatite—in the filter to form an inert mineral that would
effectively remove uranium from the water. This general concept has worked
well for lead and cadmium contaminated soils. Apatite is also the mineral
that makes up the teeth and bones in all vertebrate animals.
To verify this hypothesis, Bargar and two colleagues, Christopher
Fuller and James Davis from the US Geological Survey in Menlo Park, used
x-ray diffraction and EXAFS spectroscopy, both of which are
synchrotron-based techniques. They were surprised to find that uranium
adsorbs, or sticks, to the surfaces of apatite, rather than chemically
reacting with it to make the new mineral.
The team studied samples created in a lab as well as samples from Fry
Canyon, Utah, where several government agencies (USGS, EPA, DOE and BLM)
are collaborating to demonstrate PRB technology in a shallow aquifer
contaminated by an abandoned uranium-ore processing plant. Numerous sites
throughout America, particularly in the Four Corners area and Wyoming, are
contaminated with uranium and other radionuclides as a result of mining,
milling and other industrial processes.
"It’s really unacceptable to have polluted watersheds. This is a clear
example of how synchrotron techniques can be used to solve a very
practical problem regarding the clean up of uranium contamination in
aquifers," said Bargar.
The field demonstration also shows that PRBs will need to be monitored
over time to ensure they are still working. Apatite was the best hope yet
for encapsulating uranium through chemical reaction into a mineral,
providing a way to permanently remove uranium’s threat. Still, scientists
are happy that apatite does trap uranium, with the advantage that there is
no new mineral precipitate that could clog up a PRB.
One key area to investigate now is how much uranium the PRBs can trap
and for how long before it gets re-released under certain conditions (e.g.
a change in groundwater acidity or saturation of the barrier).
"Field tests are really the only way to evaluate the useful lifetime of
any PRB," said Fuller. "A number of kinds of barriers are being studied
around the country. However, knowledge of the contaminant removal process
is critical to designing an effective PRB with sufficient lifetime
necessary for real world applications."
Monitoring the apatite PRB at Fry Canyon will continue for at least
three more years.
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