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The Case for Particle Transport
Expectations based on actinide chemistry
Migration of actinides in the environment takes place within the context of physical and chemical processes. Chemical reactions, particularly redox reactions within soil and ponds, were and are often hypothesized to explain actinide mobility. At one extreme, the actinide(s) may react chemically along with the surrounding materials, to create soluble (dissolved) and mobile components. At the other extreme, the actinide(s) might remain unchanged at the molecular and atomic scale while the associated materials react to create mobile and immobile components.
The great contrast between actinide solubilities—plutonium and americium have very low solubilities, whereas uranium has relatively higher solubility—drove consideration of colloidal and particulate transport processes and required careful evaluation of evidence that could distinguish solubility and redox process results. For example, actinide chemists have recognized the preferred stability of plutonium in the form of plutonium dioxide (PuO2) and as colloidal-sized materials, but detailed knowledge of reactivity and mobility of such materials in the environment is limited at concentrations of picocuries per liter (pCi/L) in waters and picocuries per gram (pCi/g) to nanocuries per gram (nCi/g) in soils.
Extensive field observations and research have been conducted internationally on the environmental behavior of actinide elements in diverse sets of environments over the past 30 to 40 years. This research has provided a good base for understanding the major types of species and their transport mechanisms in soils and natural waters.
In natural waters plutonium solubility is limited by the formation of amorphous Pu(OH)4 or polycrystalline PuO2, which provides an upper limit on the amount of dissolved (i.e., ionic/molecular) plutonium that can be present. PuO2 has a laboratorymeasured solubility range of 10-10-10-13 mole per liter (mol/L) and is limited by the formation of amorphous Pu(OH)4.
Because of the very low solubility and the tendency of Pu(IV) compounds to adhere to organic and mineral particles, the primary path of plutonium transport is usually migration of colloidal particles. Indeed, when concentrations above fallout levels of plutonium have been investigated in detail, they have been linked to colloids and particulates. The problem with plutonium is that it is a very emotional issue with concerned citizen and stakeholder groups. There
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Steven Conradson of Los Alamos inspects the low-temperature cryostat and sample positioner at the Stanford Synchrotron Radiation Laboratory. Researchers used X-ray absorption spectroscopy to gather information about the chemical form of plutonium in RFETS soils and concretes.
A comparison of synchrotron based XANES spectra for plutonium in environmental samples collected from 903 Pad soil and smoke exposed concrete from building fi res. The spectra labeled Pu(III), Pu(IV), Pu(V), and Pu(VI) are oxidation- state standards. The spectra labeled RFETS soil and RFETS concrete are environmental samples. From the XANES it is clear that the RFETS environmental samples are nearly identical to the Pu(IV) standard. The inset shows the second derivative, which is another way of comparing and analyzing XANES spectra, and is commonly used to determine the energy of the absorption edge.
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