BERKELEY, CA --
A surprising alternative to microorganisms for immobilizing selenium
contamination in soil and sediment has been identified by researchers at the
U.S. Department of Energy's Lawrence Berkeley National Laboratory. Green rust,
a harmless natural iron oxide, was shown to chemically react with toxic
selenium, converting it to a safer elemental form.
Selenium is a trace mineral that can be highly toxic or carcinogenic to
humans and wildlife. The poisoning deaths of wild birds at the Kesterson
Reservoir in the San Joaquin Valley in the early 1980s have been attributed to
selenium in drainage from irrigation water. The incident was a graphic
demonstration of how agricultural development can result in the accumulation of
abnormally high and potentially lethal concentrations of selenium and other
trace contaminants in soils and sediments.
At the edge of the Kesterson National Wildlife
Selenium's fate in contaminated soils has long been linked to the
decomposition of plant material and other microbial activity, which was thought
to be the primary means by which soluble, chemically active forms of selenium
could be reduced to an elemental state. Elemental selenium is insoluble, which
means it is less of a threat to move up through the soil into the food chain,
or down through the soil into the groundwater.
Contrary to this past belief, however, a laboratory study led by Satish
Myneni of Berkeley Lab's Earth Sciences Division, has revealed that green rust
has the same effect as microorganisms on soluble forms of selenium.
"We have shown that the selenium transformation reaction in sediments and
soils reduction can take place without the presence of the bacteria, via a
different mechanism," says Myneni.
Joining him in this study were Tetsu Tokunaga, also with Berkeley Lab's Earth Sciences Division, and Gordon Brown, Jr., at the
Stanford Synchrotron Radiation Laboratory. Their results were reported in a
recent issue of the journal Science (11/7/97).
Although Myneni and his colleagues are not proposing any remediation
strategy for selenium contaminated sites based on green rust, future cleanups
and environmental management efforts depend upon a thorough understanding of
selenium's basic chemistry and geochemical cycling. Furthermore, the green
rust transformation reactions they have identified in selenium should also
apply to other trace contaminants as well, such as chromium, and chlorinated
The researchers analyzed their reactions using various x-ray beam
techniques, including x-ray absorption near edge structure (XANES), and
extended x-ray absorption fine structure spectroscopy (EXAFS). A key to their
findings was that the selenium transformation reactions take place under
conditions of oxygen-depletion such as in the sediment beneath ponded water.
These are the same conditions under which green rust is formed.
"Other researchers have shown that elemental iron and ferrous oxides can
reduce soluble selenium to a less active state, but, unfortunately, these two
forms of iron oxides do not occur in nature," says Myneni. "On the other hand,
recent thermodynamic and kinetics studies show that green rust may be an
important mineral in anoxic sediments."
Myneni and his colleagues are now in the process of analyzing samples of
soils and sediments collected from selenium-contaminated sites for the presence
of green rust. For this work, they will use the x-ray microscopy beamline at
Berkeley Lab's Advanced Light Source.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.