Environ. Sci. Technol. 1997, 31, 467-471
Physical and Chemical Characterization of Actinides in Soil from Johnston Atoll STEPHEN F. WOLF,* JOHN K. BATES, EDGAR C. BUCK, NANCY L. DIETZ, JEFFREY A. FORTNER, AND NEIL R. BROWN† Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4837
Characterization of the actinide content of a sample of contaminated coral soil from Johnston Atoll, the site of three non-nuclear destructs of nuclear warhead-carrying THOR missiles in 1962, revealed that >99% of the total actinide content is associated with discrete bomb fragments. After removal of these fragments, there was an inverse correlation between actinide content and soil particle size in particles from 43 to 0.4 µm diameter. Detailed analyses of this remaining soil revealed no discrete actinide phase in these soil particles, despite measurable actinide content. Observations indicate that exposure to the environment has caused the conversion of relatively insoluble actinide oxides to the more soluble actinyl oxides and actinyl carbonate coordinated complexes. This process has led to dissolution of actinides from discrete particles and migration to the surrounding soil surfaces, resulting in a dispersion greater than would be expected by physical transport of discrete particles alone.
Introduction The study of mechanisms responsible for the transport of actinides in the environment is complicated by their low concentrations and by difficulties of deconvoluting multiple competing physical and chemical processes (1). Ecosystems that have been contaminated with large amounts of actinides provide perhaps the best opportunity for evaluating these processes as they occur in nature. Johnston Atoll, a group of four primarily man-made coral islands located 1330 km WSW of Honolulu, was the launch site of atmospheric nuclear weapons tests in the late 1950s and early 1960s. In 1962, three non-nuclear destructs of nuclear warhead-carrying THOR missiles, one on the launch pad and two others at altitudes of 9 and 33 km, deposited Pu- and U-containing particles throughout the atoll with a higher concentration of activity near the launch pad (2, 3). Initial cleanup operations removed the debris that was detected either visually or by R-particle detection. Growth of the 241Pu decay product, 241 Am, has since permitted isolation of a majority of the contamination as “hot particles” (discrete anomalously radioactive particles) by γ-ray detection (4). However, many areas contain a low level of continuum activity, and this activity has migrated to subsurface areas (2, 3). Determination of the mechanism(s) responsible for actinide dispersion and fixation, which is required for ultimate site remediation, * Author to whom correspondence should be addressed; telephone: 630-252-6497; e-mail:
[email protected]. † Present address: U.S. DOE Richland Operations, P.O. Box 550, Mail Stop K6-51, Richland, WA 99352.
S0013-936X(96)00295-7 CCC: $14.00
1997 American Chemical Society
requires a thorough characterization of the physical and chemical nature of actinide source(s) and contaminated soil. We have characterized contaminated soil from Johnston Atoll to investigate how prolonged environmental exposure has affected the disposition of actinides. We delineate two types of contamination: (i) discrete hot particles that are fragments from an aborted nuclear weapon comprising >99% of the total activity of our sample and (ii) contaminated carbonate soil exhibiting low level continuum activity. We find direct evidence for dissolution of U/Pu-rich material contained in hot particles along with in situ formation of (U/Pu)O2CO3. We conclude that environmental exposure has led to actinide oxidation and formation of actinyl carbonate coordinated complexes that enhanced actinide solubility and led to dissolution from discrete particles and migration to surrounding soil. In turn, this has led to the spread of actinide contamination greater than would be expected by physical transport of discrete particles alone.
Materials and Methods To investigate mechanism(s) responsible for actinide migration from the putative source(s) of actinide contamination, i.e., discrete hot particles to soil, we examined a 100-g sample of contaminated soil from Johnston Atoll. This sample is not representative of Johnston Atoll soil as a whole but represents a biased sampling of contaminated soil that had been exposed to the environment on Johnston Atoll for approximately 30 years. Our first objective was to identify, separate, and characterize discrete hot particles in this sample. We measured the actinide elemental and isotopic content in several hot particles to determine whether a distinct actinide signature could establish a direct link from hot particles to contaminated soil. To achieve this, we surveyed the sample with a high-resolution n-type γ-ray spectrometer having a relative efficiency of 104.4% and a resolution (fwhm) of 2.28 keV at 1.33 MeV. Discrete hot particles were separated manually from the whole soil sample, weighed, counted individually in our γ-spectrometer, and set aside for further characterization. After non-destructive γ-spectrometry, one particle was dissolved and analyzed using R-spectrometry and inductively coupled plasma-mass spectrometry (ICPMS) to further characterize elemental and isotopic composition of the actinides. Three hot particles were characterized using optical microscopy, scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), energy dispersive X-ray spectroscopy (EDS), and electron diffraction. We analyzed material from surfaces and interiors of hot particles with SEM/EDS and ATEM/EDS after ultramicrotomy to characterize the actinide distribution and to identify environmentally caused alteration (5). Convergent beam electron diffraction was used to determine the crystal structure of actinide-rich material. Our second objective was to characterize the macroscopic and microscopic distribution of actinides in the remaining soil to determine the nature of the actinide contamination. We characterized the macroscopic actinide composition by analyzing multiple random samplings of soil with R-spectrometry, γ-spectrometry, and ICP-MS. The soil sample was divided into ∼10-g aliquots and analyzed using γ-spectrometry. After counting, one of the aliquots was dissolved, and the actinide content was further characterized with R-spectrometry and ICP-MS subsequent to radiochemical separation of the actinides (6, 7). Characterization of the actinide distribution on a microscopic scale was achieved both by analyzing individual soil particles using SEM/EDS and by analyzing soil particles that had been size-sorted by sequential ultrafiltration using R-spectrometry or SEM/ATEM/EDS.
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Sequential ultrafiltration was performed by filtering contaminated coral soil through a series of polycarbonate filters of decreasing mesh size. This procedure was performed in triplicate. Each time, a ∼0.5-g aliquot of coral soil that had been previously analyzed by γ-spectrometry was slurried with water and filtered sequentially through preweighed 43-, 21-, 12-, 5-, 1-, and 0.4-µm filters. Particles were disaggregated in an ultrasonic bath. Filters were dried under an IR lamp overnight and reweighed. The filtered soil from two of the trials was dissolved and electrodeposited onto stainless steel disks and counted in R-spectrometers. Total R counts in the 3-6-MeV energy range were measured. The filters from trial 3 were analyzed using SEM/EDS. Another ∼3-g aliquot representative of the variety of particles in our sample was reserved for microscopic analysis with SEM/ATEM/EDS.
Results Hot Particles. Nine hot particles, designated HP1-HP9, ranging in mass from 2000 to 0.2 mg, contained >99% of the activity in our bulk sample. γ-Spectrometry of HP1-HP9 showed that these particles range from 2 to 0.2 wt % 239Pu. Analyses of HP3 with γ-spectrometry and ICP-MS gave 241Am/ 239Pu and 240Pu/239Pu of 0.0035 ( 0.0002 and 0.0598 ( 0.0006, respectively. The mean 241Am/239Pu in HP1-HP9 is 0.004 ( 0.001. This ratio is consistent with a Pu purification age of >30 years with no significant Am-Pu fractionation (2). The exteriors of the hot particles were covered with CaCO3, thus causing them to appear similar to soil particles. However, analysis of hot-particle cross sections with SEM/EDS typically revealed that this CaCO3 surface surrounded a heterogeneous mixture of oxidized metals. Figure 1A shows a SEM image of HP1, the largest hot particle with a diameter of approximately 1 cm. This particle is composed of a metallic core surrounded by two concentric layers. The outer layer (Figure 1B, region i) is CaCO3. The inner layer is a heterogeneous mixture of oxidized Al, Si, Mg, Fe, Ti, Cr, Cu, Ni, U, and Pu (Figure 1B, region ii) measuring
