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SSRL Seeks Chromate Contamination
Solutions
By Heather Rock Woods
Toxic and carcinogenic chromate has contaminated the groundwater in
Hanford, Washington, one of the unfortunate legacies of producing
plutonium for nuclear weapons. At times, plumes of contaminated
groundwater have reached the Columbia River, posing a risk to spawning
salmon.
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These two graphs show the
typical X-ray Absorption Spectroscopy ‘fingerprints’ for
trivalent chromium, Cr(III) (top) and hexavalent chromium, Cr(VI).
(Image courtesy of John Bargar) |
Existing technologies, such as
pump-and-treat, are largely ineffective for mitigating chromate in the
groundwater at this location due to the large volume (millions of
gallons) and depth of the contaminant plumes.
Recent research conducted at SSRL and at the Pacific Northwest National
Lab (PNNL) has shown that naturally occurring sediments at the Hanford
site are immobilizing some of the chromate. John Zachara (PNNL) and
Gordon Brown and Jeffrey Catalano (both of Stanford University), tested
contaminated and highly radioactive soil samples at SSRL to determine
the chemical forms of the chromate—carcinogenic hexavalent chromium or
the less toxic trivalent chromium—and their relative amounts.
They used X-ray Absorption Spectroscopy, which makes distinctive
patterns that act like fingerprints to identify different chemical
forms. Hanford had previously used hexavalent chromium in the industrial
process to recover plutonium from irradiated nuclear fuels. The
resulting high-level waste corroded its storage tanks and leaked into
the desert subsoils at Hanford.
About 42 percent of the chromate in the contaminant plumes had become an
immobile solid that contained trivalent chromium. Researchers said that
the solid forms when hexavalent chromium reacts with iron-bearing
sediments in the aquifer. Solid trivalent chromium is unlikely to
dissolve—and thus is effectively taken out of circulation in the
groundwater.
“It’s a great synergy,” said John Bargar (SSRL), the molecular
environmental scientist who runs the beam line where the tests were
conducted. “The sediment-hosted ferrous iron prefers to transfer its
electrons to oxidized species such as chromate. The resulting ferric
iron and trivalent chromium want to precipitate together into a mineral.
Once present in solid form, chromium can’t flow in the groundwater, and
it’s very slow to dissolve or won’t dissolve. As a result of these
reactions, about half of the problem is abated.”
Unfortunately, more than half of the chromium in the plumes remains as
the more toxic hexavalent form, which moves readily through the
subsurface sediments. However, the team’s discovery is a necessary and
huge step toward mitigating the problem, Bargar stated. Zachara’s study
is one of many ongoing studies at SSRL looking at contaminated sediments
from Hanford. Developing technical solutions to such largescale
contaminant problems requires key insights into the form of the
contaminant. This information allows engineers to assess the hazards
posed by such extreme chemical and radioactive materials, and to design
the most effective, long-term strategies to deal with them.
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