Uranium - Toxicology of Metals - page 861

and physical properties of uranium and several of its
most important compounds.
The renowned French physicist, Antoine Henri Bec-
querel (1852-1908), was awarded the 1903 Nobel Prize
in Physics for discovering the radioactive nature of
uranium in 1896. All 22 of its currently recognized iso-
topes ( 217-219, 222-240, 242 U) are radioactive. The three that
occur naturally ( 234, 235, and238 U) are part of two decay
chains, headed by the latter two. In these, an atom of
235 U transforms 11 times into different nuclides and
fi nally becomes 207 Pb, whereas 238 U transforms 14 times
(one of which produces 234 U) and eventually becomes
206 Pb. Table 2 shows the order by which a uranium
atom transforms through intermediate species (includ-
ing radium and radon) to become an atom of lead.
The ratio of 234, 235, and238 U in the undisturbed crust is
0.000055:0.0072:0.99275 by mass and 0.489:0.022:0.489
by radioactivity. These ratios change imperceptibly
over a lifetime and slowly over millennia because of
differences in the long radioactive half-lives of the 235 U
(7.04 × 10 8 years) and 238 U (4.468 × 10 9 years) parents
of the radioactive decay chains. Despite the relatively
short half-life of 234 U (2.457 × 10 5 years), this isotope
and 238 U contribute equally to the radioactivity ratio
because they are in secular equilibrium. These ratios
can change signifi cantly in either direction when the
rock (or especially soil) is in contact with water, as
in an aquifer. 234 and 238 U are in secular equilibrium in
undisturbed rock, but their activity equivalence can
vary from 0.8-10 in drinking water, 0.8-8 in precipita-
tion, and 0.5-1.2 in soil (EPA, 1996; 2005a). The process
has not been fully explained. It may be associated with
an increased rate of thorium dissolution over uranium
at the surface of rock or soil grains, as well as with
the ejection of uranium atoms whose alpha emissions
directed inward might recoil the atom from the surface
and into interstitial water. Both processes increase 234 Th
and its progeny 234 U in surrounding water relative to
the solid, and environmental factors could further alter
the ratio. The consequence of nonequilibrium ratios to
conversions between mass and radioactivity units is
addressed at the end of Section 2.

Some radiological information that distinguishes
the natural uranium isotopes is shown in Table 3.

2 ANALYTICAL METHODS

Uranium can be analyzed chemically, physically,
or radiologically, depending on the media, analyti-
cal equipment, regulatory requirement, and other
needs. Conversion between radiological and mass
units requires knowledge of the relative proportions
of the three natural isotopes. The standard 0.67 pCi/ µ g

(0.025 Bq/ g) conversion factor for undisturbed crustal

rock may not be appropriate for a given sample because
of a normal disequilibrium between 234 U and 238 U, for
which the ratio has been found to have a broad range
(e.g., 1.2-40 for drinking water). As a result, the con-
vention used in this chapter is to fi rst display the units
used in the source document followed by the conver-
sion value when considered appropriate.
Several analytical methods are available for determin-
ing the presence, concentration, or quantity of uranium
or its compounds in a range of media. The methods are
generally characterized as photometric (fl uorometry or
kinetic phosphorescence analysis [KPA]), radiometric
(alpha spectroscopy, gamma spectroscopy, liquid scin-
tillation spectroscopy, gas fl ow proportional counting,
X-ray fl uorescence, or neutron activation analysis), and
mass spectroscopic (MS). Any of these methods can be
used for in vitro analyses of biological samples (urine,
feces, blood, hair/nails, or tissues) or environmental
samples (water, soil, sediment, air, fl ora, or fauna). In
vivo analysis requires the detection of emitted gamma
radiation by gamma spectroscopy, because available
alpha and beta particles are absorbed within the organ-
ism. The goals (regulatory and detection limits, gross
versus isotopic analysis, and cost) determine the suit-
able methods, required sample preparation steps, and
counting time. Detection limits have been published for
KPA as 0.05

µ

g/L for serum (Ejnik
et al ., 2000), and for mass spectroscopy may currently
be <3 ng/L of urine for individual uranium isotopes.

µ g/L for urine to 0.6 µ

TABLE 2 Order by Which a Uranium Atom Transforms into Lead

1 a 2 3 4 5 b 6 7 8 9 b 10 11 b 12 13 14 b 15

238 U 234 Th 234 Pa 234 U 230 Th 226 Ra 222 Rn 218 Po 218 At 214 Bi 214 Po 210 Pb 210 Bi 210 Po 206 Pb

(parent)

214 Pb 210 Tl 206 Tl

(fi nal)

235 U 231 Th 231 Pa 227 Ac 227 Th 223 Ra 219 Rn 215 Po 215 At 211 Bi 211 Po 207 Pb

(parent)

223 Fr 211 Pb 207 Tl

(fi nal)

a The parent nucleus transforms sequentially to the right until it becomes the fi nal stable nuclide.
b Double entries in a cell indicate predecessor nucleus exercises two transformation options.