BY DAWN LEVY
When chromium atoms are surrounded by six oxygen atoms, chromium typically takes the trivalent form, which is harmless. When surrounded by four oxygen atoms, however, it tends to assume the hexavalent form, which -- as anyone who's seen the movie Erin Brockovich knows -- can cause cancer.
Whether chromium is benign or malignant comes down to miniscule differences in the distance between atoms and the number and types of atoms surrounding it. The same is true of other contaminants, including arsenic, lead, uranium and plutonium. The distance between atoms helps define a molecule's structure and subsequently its toxicity in the environment and its behavior in living cells.
Scientists now have a powerful tool for determining a molecule's form and function. For almost three decades, biologists, chemists and geologists have made use of a tool of physicists -- an oval-shaped subatomic particle accelerator called a synchrotron. Siphoning ultra-narrow, ultra-bright X-ray beams from synchrotrons, they use the light to take "snapshots" of biological molecules or of charged atoms in water or on the surfaces of minerals. Now, environmental scientists have joined their ranks by using synchrotron-based spectroscopy to examine environmental contaminants in greater detail than ever before.
Stanford's Gordon E. Brown Jr., who is a geochemist, has used synchrotrons to approach environmental science at the level of the molecule. In December, he gave an overview of synchrotron applications in geochemistry and environmental science to about 200 scientists attending a three-day course in Monterey and at the American Geophysical Union meeting in San Francisco. The course was sponsored by the Geochemical Society and the Mineralogical Society of America.
"The bottom line is we can use synchrotron-based spectroscopy to determine rather quickly what the chemical form of mercury or chromium or other heavy metals is in different environmental settings and therefore determine potential danger -- whether these metals are potentially toxic or potentially bioavailable," says Brown, the Dorrell William Kirby Professor of Geology in the School of Earth Sciences. He also chairs the Stanford Synchrotron Radiation Laboratory (SSRL) Faculty at the Stanford Linear Accelerator Center (SLAC).
The work is revealing some surprises. For example, some contaminants occur in the environment in inert forms -- so it may be better to leave them alone than clean them up. And geological features may create impermeable barriers to other contaminants. These findings may guide cleanup of some of the 1,235 hazardous waste sites identified as priorities for remediation under the Superfund Program.
Taking the 'fingerprint' of a contaminant
"The X-rays from a synchrotron are several million times brighter than the X-ray source used in a dentist's office to X-ray your teeth," explains Brown. Intense X-ray beams enable the detection of elements and phases at much lower concentrations than are possible with conventional X-ray techniques, such as X-ray diffraction, which requires samples to be highly ordered at the atomic level. In contrast, synchrotron-based spectroscopy can analyze powders, glasses and other amorphous materials. Plus, scientists can analyze samples without having to prepare them for chemical assays that are expensive and time-consuming and alter the samples.
Scientists can tune synchrotron beams throughout a range of energies, from a few electron-volts to about 100,000 electron-volts. They can deliver X-rays of a specific energy and focus on objects as small as 50 nanometers at the lower energies -- about five times the thickness of the metal film on a bag of potato chips. This is the size scale of colloids, particles suspended in water that can transport contaminants.
Brown places a sample in a holder and shines X-rays on it, gradually increasing the energy until a big jump in absorbed light -- called the absorption edge -- signals the excitation of a specific electron in an atom. That jump and associated fine structure form a pattern -- an atomic fingerprint -- that reveals the local structure around the atom within a molecule or solid. The technique, called X-ray absorption fine structure (XAFS) spectroscopy, reveals an atom's oxidation state, what other atoms that atom is bound to locally, and what sort of a solid the atom occurs in. XAFS is only one of many synchrotron techniques that have given environmental science and other disciplines new analytical teeth.
To clean or not to clean
In late 1999, Brown received a four-year, $780,000 grant from the Environmental Protection Agency to characterize mercury and arsenic from abandoned mines in California using synchrotron methods. This also was the thesis topic of one of his graduate students, Christopher Kim, now a postdoctoral scholar at the University of California-Berkeley, and is the current subject of study by graduate student Aaron Slowey and postdoc Stephen Johnson in Brown's group.
Not all mercury needs to be cleaned up. Whether mercury is lethal or lethargic depends on the company it keeps. Bound to sulfur atoms, it forms mercury sulfide, or cinnabar, a harmless, insoluble mineral.
At the Cinnabar Winery, built in south San Jose near a former mercury mine, mercury is dominantly in the form of cinnabar. "It's extremely insoluble," Brown says. "You can eat this stuff and it won't hurt you."
But mercury in a different form -- combined with one or more methyl groups -- can kill. In 1997, dimethylmercury caused the death of Dartmouth chemist Karen Wetterhahn, whose lab gloves failed to protect her from small but lethal amounts.
In San Francisco Bay muds, bacteria can methylate mercury to create monomethyl mercury, which can pass up the food chain and accumulate in the tissues of swordfish, marlin, tuna and other game fish.
"You and I both have methylmercury in our bodies because we eat fish and we live in this area," Brown says. "If you get enough of the mercury concentrated in you, or you are exposed to enough of it, then you can start having serious health problems."
Mercury is especially toxic to the brain, kidneys and the developing fetus, and it is the reason pregnant women are cautioned not to consume tuna.
With graduate student Jeff Catalano and other scientists, Brown is also determining the form of chromium and uranium in sediments sent to SSRL from underneath the Hanford Tank Farm in Washington state, where about half of the nation's high-level radioactive waste is stored. Chromium, used in rods from nuclear reactors, has been released from spills during overfilling and from leaks in the deteriorating tanks. It is seeping into the groundwater and into the Columbia River. But is it the good trivalent or the bad hexavalent form?
The majority is hexavalent, Brown says. "It's carcinogenic, teratogenic and mutagenic -- you don't want to be around it. It's also extremely mobile in groundwater and very soluble, which means that it can be transported great distances in flowing groundwater."
To make it into the water table, chromium has to travel through a porous layer of sediment called the vadose zone. Brown's group found that some bad chromium was transformed to good chromium in the vadose zone -- but not enough to prevent some of the bad stuff from making it to the water table. Even "sticky" iron- or manganese-oxide particles in the vadose zone were not able to sequester the poison.
"It turns out it's not as disastrous as you think because the natural background level of radiation coming down the Columbia River is greater than the stuff introduced into it by the leaking Hanford tanks," Brown says. "We're exposed to radioactivity in many places anyway because rocks contain radioactive elements. We're also exposed to potentially toxic elements like arsenic and mercury because rocks in some areas, such as the California Coast Range, naturally contain those as well."
For cleanup, environmental engineers can sometimes exploit nature to change contaminants to forms that are no longer bioavailable or toxic. For example, the roots of the aquatic fern Salvinia rotundiflora convert the toxic form of chromium to the benign form.
Another solution is to create barriers to toxin movement. One of Brown's former students, John Bargar, now of SSRL, used meal from ground-up cow bones to sequester uranium from old mining sites in Utah. Says Brown: "What they found is that uranium sorbed to, or reacted with, the outside of the bone meal and stuck, and therefore this is an effective way of removing uranium from groundwater."
Synchrotron-based spectroscopy conducted at SSRL already has saved millions of dollars in cleanup costs. A case in point is the former Rocky Flats plutonium processing plant in Colorado. A group from Los Alamos National Laboratory came to SSRL in the late '90s to determine the form of plutonium in contaminated soil and concrete from Rocky Flats. Synchrotron-based spectroscopy revealed the form to be extremely insoluble.
"That knowledge, coupled with how plutonium is distributed, saved the contractor -- and that means the U.S. taxpayer -- millions of dollars because they didn't have to do nearly as much as they thought they would in terms of cleanup," says Brown.
Environmental consulting firms rarely do the level of analysis that Brown and his students do. The reason is the high cost of synchrotrons -- roughly a billion dollars for a third-generation facility -- relegates them to the category of research tools and keeps them out of the commercial service sector. Says Brown: "There's no way that you could commercially build a synchrotron -- you'd never pay for it. That's where the U.S. Department of Energy comes in. They build synchrotrons."
Emanating from the oval-shaped synchrotron ring at Stanford are sophisticated "pipes" -- beamlines -- that guide electromagnetic radiation to 27 stations at the end of the pipes. It is at these stations that scientists conduct their experiments, and the facility runs practically 24/7. Each beamline costs $6 million to $10 million to build. The DOE funded the beamline Brown uses. It would cost a commercial firm about $8,000 a day to run a beamline, Brown estimates. And that cost doesn't include training of the operator.
"You can't really put a dollar value to running a sample. I've been working in this area for almost 27 years now ... and it took me several years to learn how to do stuff, analyze and interpret data, and so forth, and I'm still learning."
Bright past, brighter future
SSRL was the first light source built in the United States for general users. Now, about 75 synchrotron facilities are in operation, under construction, funded or in advanced planning in 23 countries.
SSRL is also the first place in the world where synchrotron beams were applied to a material of importance to environmental and earth science.
The lab has a bright future, as it will complete an upgrade in December that will turn it into a third-generation facility. Its brighter, tighter beam will allow scientists to explore questions that they simply could not address with the larger beams of second-generation light sources, Brown says.
Beyond that, the next decade is likely to see light sources at
SLAC with X-ray pulses as short as a quadrillionth of a second.
That speed will make it possible for scientists to watch chemical
bonds as they break and reform and enable researchers to discern
the mechanisms of chemical reactions. They will be able to
effectively do "freeze-frame photography" of biological molecules,
which will elucidate the dynamics of molecules as they interact
during biological processes.
Gordon Brown and his Jack Russell terrier, Timmy, take a break in Brown’s office in the Green Earth Sciences Building. In December, Brown published a 115-page "chapter" to accompany a talk he delivered about applications of synchrotron radiation in geochemistry and environmental science. Photo: L.A. Cicero
Stanford Report, January 15, 2003