Synchrotron Radiation Studies Aid Environmental Cleanup

Los Alamos Study Offers the First Spectroscopic Confirmation of the Speciation of Plutonium in Soils at Rocky Flats

The Rocky Flats plant was a top-secret production site 15 miles northwest of downtown Denver. From 1952 to 1989, the primary mission was to manufacture parts for the U.S. nuclear weapons stockpile. These operations involved fabricating components out of plutonium, uranium, and beryllium. Nearly 40 years of nuclear weapons production left a legacy of nuclear waste at the site, including contaminated facilities, process waste lines, and buried wastes. Major dispersal of plutonium contamination into the immediate environment resulted from fires in production buildings and leakage of contaminated waste oil stored outdoors.

In 1992, the Rocky Flats mission changed to closure and cleanup. Today, Rocky Flats is in the process of deactivating, decontaminating, decommissioning, and demolishing the weapons production facilities and buildings in the industrial area. The purpose of the final closure phase is remediation of the environmental legacy of nuclear weapons production and transition to long-term stewardship as a wildlife refuge.

The Rocky Flats Environmental Technology Site (RFETS) is an environmental cleanup site located about 15 miles northwest of downtown Denver. Formerly known as the Rocky Flats Plant, this site made components for nuclear weapons using various radioactive and hazardous materials until December 1989, when plant operations were shut down.

Nearly 40 years of nuclear weapons production left behind a legacy of contaminated facilities, soils, and surface water. Two decades of routine monitoring have shown that the environment around RFETS is contaminated with actinide elements (uranium, plutonium, and americium) from site operations. More than 2.5 million people live within a 50-mile radius of the site; 300,000 of those live in the Rocky Flats watershed. The Environmental Protection Agency designated the site a Superfund cleanup site and a massive accelerated cleanup effort began in 1995.

The key priority of site management and surrounding community leaders is the safe, accelerated closure of Rocky Flats. Kaiser-Hill, the company in charge of the cleanup, and the Department of Energy (DOE), in close coordination with Rocky Flats stakeholders, are working aggressively to substantially complete the cleanup and closure of Rocky Flats by 2006. The price tag for the closure is estimated to be between $6 billion and $8 billion.

Chemical speciation studies

Researchers at Los Alamos are assisting the cleanup effort by studying contaminated soils from the site using x-ray absorption spectroscopy at Stanford Synchrotron Radiation Laboratory. Earlier studies of the site have shown that plutonium is present in surface soils and that there is a clear west-east trend in contamination away from an old drum storage site known as the 903 Pad. More than 90 percent of the plutonium is contained within the upper 10 to 12 centimeters of soils downwind of the 903 Pad.

The Los Alamos team has been studying the application of synchrotron radiation techniques to plutonium environmental behavior, and their most recent study has resulted in the first spectroscopic confirmation of the chemical speciation of plutonium in soils at RFETS. The speciation of contaminants (i.e., the elemental identities of the contaminants, their physical states, oxidation states, host-phase identities, molecular structures, and compositions) controls their toxicity, bioavailability, transport, and fate in the environment. The data acquired from the Los Alamos study (combined with other site-specific studies) are key to choosing proper remediation strategies, the correct model for assessing public health risks, and aiding decisions for future land configuration and management.

The probability of the release of plutonium from RFETS soils to the surrounding environment depends on the solubility of its compounds in groundwater and surface waters, the tendency of plutonium compounds to be adsorbed onto mineral phases in soil particles, and by the probability that the colloidal forms of plutonium will be filtered out by the soil or rock matrices, or adsorb or settle out during transport.

The 903 Pad was used in the 1950s and 1960s for storage, on bare ground, of approximately 4,000 drums of plutonium-contaminated solvents and oils. A major release of plutonium to the environment occurred when plutonium-contaminated waste oil leaked from these drums. The drums were removed in 1967 and 1968 after radioactive contamination was detected. Plutonium-contaminated soil was dispersed by the wind during remediation activities, and an asphalt pad was installed in 1969 to control the spread of plutonium contamination. The area around the 903 Pad continues to be one of the major sources of plutonium contamination at the Site. Los Alamos researchers used synchrotron x-ray absorption spectroscopy on plutonium-contaminated soils from the 903 Pad to identify the chemical form (or speciation) of plutonium in these soils to assess its environmental behavior and to assist the site in assessing cleanup strategies. These factors are largely governed by the chemical oxidation state and its associated chemistry. Plutonium in lower oxidation states tends to form complexes with extremely low solubilities and stronger sorption to mineral surfaces under most environmental conditions. Plutonium in the higher oxidation states tends to form complexes with relatively higher solubilities and weak sorption to mineral surfaces.

Scientific techniques that provide information on the nature of plutonium oxidation states in the environment are therefore of great interest. Synchrotron-based methods are extremely powerful for the study of speciation in the environment because they can be used under environmentally relevant conditions, namely, in the presence of water at ambient pressures and temperatures, and at dilute metal ion concentrations.

The chemical oxidation state and electronic properties can be determined from the X-ray Absorption Near Edge Structure (XANES). X-ray Absorption Fine Structure (XAFS) spectroscopy probes the local chemical environment of a material, providing information on the identity of atoms in the first coordination sphere of the central metal ion, the number of neighboring atoms, and their interatomic distances.

Because the samples do not need to be crystalline, XAFS is ideally suited to the study of highly disordered solids and amorphous materials that are likely to be found as a result of accidental environmental contamination.

The Los Alamos team examined x-ray absorption spectroscopy of a series of well-characterized standard compounds followed by samples of contaminated RFETS soils and concrete collected from the site. These studies used Stanford Synchrotron Radiation Laboratory's new Molecular Environmental Science Beam Line.

The XANES measurements on RFETS soil from the 903 Pad and concrete from a contaminated building clearly show that the oxidation state of plutonium is Pu(IV), and the XANES spectral signatures are very similar to that of solid plutonium dioxide (PuO(SUB)2) in both soil and concrete samples. One of the soil samples was concentrated enough that XAFS data could be studied. XAFS data analysis revealed local structure features nearly identical to that of the solid PuO2 standard.

Los Alamos x-ray absorption studies therefore show unambiguously that plutonium in RFETS soils taken from the 903 Pad is in oxidation state (IV) and in the chemical form of insoluble PuO2. For decades it had been presumed that plutonium in RFETS soils existed as PuO2, but this hypothesis had never been proven.

Using XANES Spectroscopy to Determine Oxidation States

When a beam of x-rays passes through matter, it loses intensity via interaction with the matter. A plot of the x-ray absorption as a function of energy (the graphic on the top) shows a decrease in absorption with increasing energy, the presence of a sharp rise at certain energies called edges, and a series of oscillatory wiggles (or fine structure) at energies above these edges. It is these characteristic energy regions where x-rays are strongly absorbed (referred to as absorption edges) that are used in x-ray absorption spectroscopy.

The x-ray absorption near edge structure (XANES) can be used to determine the oxidation state of the target (x-ray absorbing) element in solution or in the solid state. The energy at which an absorption edge appears depends on the ionization potential of the ion. This ionization potential increases with the ion's valence, so in general, the absorption shifts to higher energy with increasing oxidation state.

Los Alamos researchers used this effect to determine the oxidation state of plutonium in contaminated soils and concrete samples from the Rocky Flats Environmental Technology Site. In XANES spectra of plutonium (the graphic on the right), there are distinct differences in the energy of the rising absorption edge, the intensity of the peak (sometimes referred to as the "white line"), and the structure in the absorption features at the higher energies beyond the absorption peak.

All of these features change with the changing oxidation state of plutonium. These differences in the XANES spectra are used to identify the oxidation state of plutonium in RFETS samples.

Using XAFS Spectroscopy to Determine Local Atomic Structure

X-ray Absorption Fine Structure (XAFS) spectroscopy reveals information on the solid-state structure of a sample, even if the sample is amorphous, noncrystalline, or dissolved in solution. XAFS provides information about the number of atoms and their interatomic distance from a central target atom.

In the case of plutonium dioxide (PuO2), the crystalline structure has cubic symmetry and is shown here from the perspective of a central plutonium atom (dark green). If we probe the x-ray absorption of plutonium in this sample, we will extract the local structural environment around a plutonium atom in the sample.

For PuO2, there are eight near-neighbor oxygen atoms (shown in red) that all sit at a 2.33 angstrom distance from the central plutonium atom in the structure. In the cubic PuO2 structure, there are also 12 neighboring plutonium atoms (light green) at an interatomic distance of 3.81 angstroms from the central plutonium atom. Finally, in this example, there is another "shell" containing 24 distant oxygen atoms (shown in pink) at an interatomic distance of 4.66 angstroms.

It is this combination of the number of near-neighbor atoms, their elemental identities, and their interatomic distances that uniquely define the chemical structure using XAFS spectroscopy.

The Los Alamos study is the first spectroscopic confirmation of the speciation of plutonium in soils at RFETS. This finding is consistent with the observed insolubility of plutonium in site-specific waters and supports a growing body of evidence that physical (particulate) transport is the dominant mechanism for plutonium migration at RFETS.

This recognition not only identified the need for the site to develop a soil-erosion model, but also significantly helped in gaining public trust that an erosion model was the correct model for the site and that soluble transport models are inappropriate for plutonium in RFETS soils.

Plutonium XAS measurements have developed into a decision- making tool for Kaiser-Hill LLC, saved the company millions of dollars by focusing site-directed efforts in the correct areas, and will aid the DOE in its effort to clean up and close the RFETS by 2006.

This article was contributed by David L. Clark (NMT-DO); Steven D. Conradson (MST-8); Mary P. Neu, D. Webster Keogh, Pamela L. Gordon, and C. Drew Tait (C-SIC); Wolfgang Runde and Mavis Lin (C-INC); and Craig Van Pelt (NMT-9).

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