Characterization of uranium- and plutonium-contaminated soils by electron microscopy Page: 5 of 10
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Carbonate leaching has been selected as the most appropriate method for
removing the uranium from Fernald soils. However, the U(IV) phases (uraninite
and uranium metaphosphate) in bench-scale tests were not removed. Uraninite
can be leached by carbonate according to the scheme (9);
UO2 + 1/202 + 3C032 + H20 - [UO2(COl)3]4 + 20H (1)
In other words, as long as an oxidizing agent is present, uraninite will be
attacked. Effective oxidation of tetravalent uranium can be achieved with
molecular oxygen in carbonate solution, with the rate of oxidation being
proportional to the oxygen partial pressure. Chemical methods for adding this
oxygen such as hydrogen peroxide and potassium permanganate are either
expensive and/or increase the amount of pollution. Permanganate, however, was
used in the bench-scale tests, after TEM analysis had shown that uranium(IV)
phases were still present in soils that had been treated with carbonate.
Addition of the oxidizer was found to improve uranium extraction during
carbonate leaching.
The technique used in the mining industry for the recovery of metals, termed
"Heap leaching," may be an attractive alternative for introducing oxygen. The
nature of the Heap, where soil is heaped (or piled) onto an impermeable pad,
can allow some aeration. In the Heap-treated samples (see Fig. 1), there was
evidence from TEM of uraninite dissolution (although some uraninite was still
present in the treated samples); however, no evidence was found of dissolution
of the uranium metaphosphate phase, and a number of uranyl phosphate phases
were still present.
Characterization of "Hot Particles" from Johnston Island
Johnston Island, located in the Pacific Ocean, 1330 km southwest of Honolulu,
became contaminated in 1962 when the Island was used for launching missiles to
test the effects of high-altitude nuclear bursts (10). In a number of
instances problems occurred with the Thor missiles and the nuclear devices
were intentionally destroyed by chemical explosives. One intentional
destruction 59 sec after launching deposited plutonium- and uranium-
contaminated debris throughout the atoll, while a second destruct of a missile
on the launch pad contaminated a smaller land area but to much higher levels.
Plutonium was dispersed by the explosive high temperatures and pressures
generated by the explosion. The growth of the plutonium daughter product
41Am has permitted isolation of "hot spots" by gamma detection.
Tier I size sieving studies and radiochemical analysis by Wolf et al. (11)
demonstrated that 96.5% of the activity was located in the 2 to 0.063 mm
range. Most of the activity was localized in small "hot particles" in the
coral sand. Bramlitt has also indicated that some of these "hot particles"
were magnetic. This suggests that they might be closely associated with iron
(10). A mechanical soil sorting method has been developed by Moroney et al.
(12), termed the segmented gate system, that screens out "hot particles"
automatically by using NaI gamma detectors. Improvements in this system have
helped to reduce the contaminated volume of soil by 98%. The clean soil has a
total alpha radioactivity from the plutonium and americium of less than
500 Bq/Kg.
A Tier II study was undertaken to describe the nature of the plutonium and
uranium contamination in the "hot particles" present in the contaminated
soils, so that the movement of plutonium at Johnston Island can be explained
and further dispersion into the environment predicted. In some sites
plutonium has migrated to depths over 1.5 m, and the mechanism by which this
has occurred is unknown. In addition, the fate of uranium is unclear.
The AEM examination showed that plutonium is not in direct contact with the
coral but is present in "sols" of plutonium and uranium-(50-200 nm in
diameter) within a micro-crystalline (partially amorphous) aluminum oxide, as
well as with other components such as iron. Plutonium-bearing iron particles
1- 7--
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Buck, E. C.; Dietz, N. L.; Fortner, J. A.; Bates, J. K. & Brown, N. R. Characterization of uranium- and plutonium-contaminated soils by electron microscopy, article, March 1, 1995; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc681188/m1/5/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.