PLUTONIUM
171
6. POTENTIAL FOR HUMAN
EXPOSURE
al. 2006). Oxidation states (III)–(VII) can be prepared and stabilized in solution under appropriate
conditions. The lower oxidation states (III and IV) are more stable under acid conditions; the higher
oxidation states (VI and VII) are more stable under alkaline conditions. Pu(IV) is the most stable and
most studied oxidation state, followed by (III) and (VI). Pu(V) and Pu(VI) cations are strong Lewis acids
and hydrolyze in solution
to form trans dioxo cations, PuO
2
+
and PuO
2
2+
, which are commonly referred to
as plutonyl ions. The reduction potentials that couple the four most common oxidation states of
plutonium (III, IV, V, VI) in acidic solution are very close in magnitude and are close to 1 volt vs. the
standard hydrogen electrode. In addition the kinetics oxidation-reduction reactions between these
oxidation states are such that multiple oxidation states of plutonium can exist in aqueous solution under
appropriate conditions (Clark et al. 2006; EPA 2006b).
The important chemical transformation process in surface water is the oxidation
or reduction of
plutonium. In waters with low suspended solids, plutonium is generally found in oxidized forms,
dissolved in the water. In waters with high suspended solids, plutonium is generally reduced and sorbed
onto either suspended solids or sediments (DOE 1987a, 1987h; Higgo and Rees 1986).
Plutonium behaves differently than many other inorganic elements in that it can exist simultaneously in
four oxidation states over a range of pH values. Under acidic conditions, the nature of the complexing
ligands present in solution will influence the oxidation state of plutonium. The presence of fulvic acid (a
naturally occurring organic acid) facilitates the reduction of plutonium(IV) to plutonium(III), especially
below pH 3.1. The reduction of the higher oxidation states appears to
be even less dependent on pH,
especially below pH 6 (IAEA 1976d).
6.3.2.3 Sediment and Soil
Plutonium found in soils may undergo the same oxidation/reduction reactions described for surface
waters in places where soil contacts water. In addition to oxidation/reduction reactions, plutonium can
react with other ions in soil to form complexes. These complexes may then be absorbed by roots and
move within plants; however, the relative uptake by plants is low.
In plants, the complex can be degraded
but the elemental plutonium will remain.
6.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT
Reliable evaluation of the potential for human exposure to plutonium depends in part on the reliability of
supporting analytical data from environmental samples and biological specimens. Concentrations of
PLUTONIUM
172
6. POTENTIAL FOR HUMAN EXPOSURE
plutonium in unpolluted atmospheres and in pristine surface waters are often
so low as to be near the
limits of current analytical methods. In reviewing data on plutonium levels monitored or estimated in the
environment, it should also be noted that the amount of chemical identified analytically is not necessarily
equivalent to the amount that is bioavailable. The analytical methods available for monitoring plutonium
in a variety of environmental media are detailed in Chapter 7.
Monitoring studies typically report combined
239,240
Pu concentrations because
239
Pu and
240
Pu are not
readily distinguishable by alpha spectroscopy (Eisenbud and Gesell 1997).
6.4.1
Air
Atmosphere nuclear weapons testing, which ended in 1980, is the major source of plutonium
contamination. Since this time, essentially all fallout
239
Pu has been removed from the atmosphere,
allowing for measurement of baseline measurement of plutonium in air.
Air
concentrations of
239
Pu at the Pacific Northwest National Laboratory near Richland, Washington
averaged 1.3x10
-7
Bq/m
3
(3.5x10
-6
pCi/m
3
). During 2004, a network of 85 continuously operating air
samplers were used to monitor radioactive material in air near the Hanford Site (DOE 2005c). In 2004,
238
Pu was detected in 4 of 40 site-wide composite air samples, with average and maximum concentrations
of 5x10
-7
and 1.3x10
-5
pCi/m
3
(2x10
-8
and 4.8x10
-7
Bq/m
3
), respectively. From 1999 to 2003,
238
Pu was
detected in 10 site-wide air samples, with a maximum concentration of 5.3x10
-6
pCi/m
3
(2.0x10
-7
Bq/m
3
).
Only 7 of the 40 site-wide air samples had detectable amounts of
239,240
Pu in 2004 with
an average
concentration of 1.5x10
-6
pCi/m
3
(5.6x10
-8
Bq/m
3
). One of the 28 perimeter samples had a detectable
amount of
239,240
Pu at a concentration of 2.5x10
-6
pCi/m
3
(9.3x10
-8
Bq/m
3
). In 2004, none of the nearby or
distant communities site air samples contained detectable amounts of
238
Pu or
239,240
Pu. From 1999 to
2003,
238
Pu was found in one air sample each from perimeter site and nearby communities sites;
239,240
Pu
was detected in seven, four, and
one of the perimeter, nearby, and distant communities air samples,
respectively (DOE 2005c).
Concentrations measured at the Argonne National Laboratory-East near Chicago, Illinois ranged from
6x10
-8
to 1.4x10
-7
Bq/m
3
(1.6x10
-6
–3.8x10
-6
pCi/m
3
) (DOE 1999a).
239,240
Pu concentrations in airborne particulate matter collected during 2005 from five locations at the site
boundary of the Rocky Flats Environmental Technology Site (RFETS), Colorado, a former nuclear