Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

stabilise the +4 oxidation state [11]. A great number of Plutonium compounds have been
studied up to the present [12], but here Plutonium compounds of potential importance from a
radiation protection point of view will be mainly considered.
The oxidized form is probably the most important chemical form. Examples are the
dioxide, PuO 2, which is the most common, the monoxide, PuO, and the sesquioxide, Pu 2O 3.
They can occur in pure form or in non-stoichiometric mixtures with other metal oxides
(Uranium, Potassium, Sodium), and they may have been formed at either very high
temperature (above 1500 °C) or relatively low temperatures up to ambient ones [11]. The
composition and formation temperature of Plutonium oxides determines their solubility and
thus their metabolic behaviour. In relation to the former aspect, oxides of other metals that
can occur in mixed Plutonium oxides are often more soluble, enhancing the solubility of
Plutonium oxides itself. The latter aspect is of interest with regard to the Plutonium in the fallout
from nuclear weapon testing. This represents the main source of public exposure, in
comparison with other anthropogenic sources [13, 14]. The fall-out Plutonium is partially
composed by oxides which were formed at very high temperatures during the explosion
phase, whereas the greatest fraction is due to the reaction 238 U(n,γ) 239 U and subsequent decay
of 239 U through 239 Np to 239 Pu. The latter is a low temperature process generating Plutonium
oxides of high solubility, if compared to oxides produced at high temperatures [11].
Other possible forms of Plutonium are halogen compounds, nitrates, nitrides,
sulphates, phosphates, oxalates but they are less important from a radiation protection point of
view.
In aqueous solution Plutonium ions, particularly Pu(IV) undergo significant hydrolysis
owing to their high ionic charge and relatively small radius [10]. Hydrolytic reactions depend
on many factors, such as concentration of the metal in the solution, temperature, pH and
presence of anions and cations, the latter producing the competing phenomena of complex
formation. Particularly hydrolytic reactions occur more readily at high pH. As in mammalian
systems the pH is about 7, it is quite unlikely that Plutonium ions can exist without
undergoing hydrolytic reactions.
Complex formation too plays an important role in the solution chemistry of Plutonium,
particularly for the Pu(IV) oxidation state, e.g. Plutonium complexes with inorganic ions
(sulphate, nitrate, chloride, carbonate), with acetate, lactate and citrate. The latter is
particularly interesting, because it is the form used in biokinetic studies with intravenously
injected Plutonium in human subjects and animals. Plutonium complexes with citrate are
characterized by very stable binding [10]. Complex formation between Plutonium and heavier
molecules are also important, e.g. with polyaminopolycarboxylic acids for the recovery of
Plutonium from process waste streams [12] and for the treatment of contaminated subjects.
Particularly diethylenetriaminepentaacetic acid (DTPA) is known as one of the most
important chelating agents [15]. This compound firmly binds to circulating Plutonium
enhancing the normal excretion from the body, but toxic effects can accompany its action
[16].
Summarising with regard to the ionic forms of Plutonium entering human body, three
main types of reaction may be expected to occur:
• Hydrolysis yielding polymeric or colloidal species;
• Complexing with proteins or other biological macromolecules;
• Complexing with small molecular weight components, as acid organic and inorganic
anions.
In case of an intravenous injection of Plutonium, the second process is likely the
predominant reaction. At the other hand, if Plutonium enters directly in contact with a tissue,
such as muscle in case of a contamination through a wound, or through an intramuscular
injection, the first or the third process is expected to be occur with higher probability [10].
12