Fighting Pollution: ALS Studies Unravel Molecular Environmental Interactions

June 26, 1998

By Paul Preuss, [email protected]

At the Workshop on Scientific Directions at the Advanced Light Source held at Berkeley Lab last March, one of the eight working-group sessions was devoted to the challenging new field of molecular environmental science.

"One of the Department of Energy's principal charges is to clean up its legacy wastes, plus other contamination of all kinds, including heavy metals, radioactive substances, and chlorinated hydrocarbons," says David Shuh of the Lab's Chemical Sciences Division. Shuh and Geraldine Lamble of the Earth Sciences Division acted as facilitators of the molecular environmental science (MES) working group at the ALS workshop, which was chaired by Gordon Brown, Jr. of the Stanford Synchrotron Radiation Laboratory and Stanford University.

"To do this job effectively, we have to understand the fundamental interactions of contaminants with their environments at the molecular level, as well as with the macroscopic environment," Shuh says, "which means addressing chemical, physical, biological, geochemical, and other kinds of problems. It's a very complex, interdisciplinary scientific field. Synchrotron radiation techniques are essential to gain an understanding of the basic science."

MES aims to identify the chemical and physical forms of contaminants, known as their speciation, and to understand the way different species in contaminated materials react in the soil and water table. These complex molecular-scale biological and chemical processes affect the stability of contaminant species, their mobility, their toxicity, and their transformations into different species.

Researchers contend with a number of interesting experimental complications. Many environmental processes take place at the surfaces of natural solids and involve aqueous solutions. A sample is often contaminated by several different species, and the con-taminants may be present in such forms or low concentrations that imaging methods like nuclear magnetic resonance are not useful. Furthermore, the reactions are often between different phases--liquid and gas, liquid and solid, or solid and gas. These complex interactions are made even more difficult because of the involvement of living things. All these intricacies make high intensity, tunable synchrotron light an indispensable research tool.

One of the most useful probes of molecular species and their reactions is x-ray absorption fine structure spectroscopy, or XAFS. A synchrotron light source like the ALS can produce photon beams with the brightness and high flux necessary for XAFS to detect dilute levels of metal-ion contaminants in solution or at surfaces by displaying their characteristic individual spectra.

As an example, Stanford's Gordon Brown, Jr. cites a study that used very short-wavelength XAFS to show why wastes from a uranium processing plant in Ohio were not being adequately remediated. The traditional clean-up method was to wash the soil with a carbonate solution, but the synchrotron study revealed that uranium (VI) solid, a species not readily soluble in the carbonate solution, remained in the soil as uranyl phosphate after washing. An alternate method of remediation had to be used.

Shuh, whose own research focuses on actinide chemistry, notes that problematic storage tanks at Hanford, Wash., where waste from production of weapons-grade plutonium has been stored for decades, contain numerous actinide species. "If you want to sequester the actinides from other wastes, for example, you need to understand complex actinide chemistry, things that people couldn't get at directly before the advent of synchrotron studies," he says.

Some schemes for permanent storage of radioactive waste contemplate encapsulating them in cement or glass, such as borosilicate glass. "Although glass has only short-range order, it forms networks," says Shuh. "Other elements compete with some boron species and can interfere with network formation in borosilicates, which could affect the leaching of stored wastes. The synchrotron radiation produced by the ALS is ideal for studying the chemical properties of the light-element species in glass."

Other metal-ion contaminants are often found in soil and groundwater, and synchrotron studies have revealed surprising facts about their environmental interactions. X-ray microscopy done by Brian Tonner, now of the University of Central Florida, working at an ALS beamline built and operated by the Center for X-Ray Optics, has yielded micrographs of bacteria that have absorbed manganese from water; the ingested manganese is oxidized to form manganite, an insoluble mineral. Bacteria that can oxidize or reduce metal ions and minerals offer many opportunities for waste clean-up.

Plants can also be active in transforming contaminant species, especially at the "root zone," where most uptake of chemicals occurs. Geraldine Lamble has studied fungi from Norwegian forest soils contaminated by metals from refineries in the former Soviet Union. Using extended XAFS at the ALS, Lamble established the role of the species zinc oxalate in the uptake, retention, and conversion of zinc by fungi that are symbiotic with the trees via their root network. In other work, XAFS has revealed the transformation of chromate and selenate ions inside the nucleus and cell walls of some plants.

The surfaces of soil particles are active sites of chemical reaction. A flake of clay a few micrometers across can harbor numerous distinct species of a contaminant, each of which can undergo different reactions. Metal oxide surfaces--such as the iron rust studied by Satish Myneni of the Earth Sciences Division--are important reactants in soils and sediments; bright light at wavelengths produced by the ALS is an unequaled tool for studying these compounds.

Recent surveys of MES researchers have identified over a dozen separate fields of study in which fundamental discoveries have recently been made or are soon expected. Already most of the nation's synchrotron beamlines that can be applied to MES are fully subscribed--in some cases badly oversubscribed. The growing discipline needs more facilities, and proposals have been made for construction of new facilities dedicated to MES at the ALS and elsewhere.

"The trademark of MES is complexity," says Shuh. "It's clear that for a multidisciplinary science of such fundamental importance and far-reaching practical implications, the near-term potential for valuable research will be limited only by the tools that can be made available."

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