384
Trace Elements in Abiotic and Biotic Environments
about 70% deposits in the bones and 4% deposits in the liver, where it is retained,
with a biological half-life of about 22 years and of 700 days, respectively. Its small
proportion, about 16%, is uniformly distributed to all other organs and tissues. Most
of the remaining 10% is directly excreted (Argonne 2007). Thorium binds to aspartic
and glutamic acids, and bone glucoprotein. It inhibits certain digestive enzymes.
Thorium is generally a health hazard. It is odorless and tasteless. Excessive exposure to Th has been linked to diseases of the lung, liver, bone, kidney, pancreas, and
blood. External γ exposure is not a major concern, because Th emits only a small
amount of γ radiation. The main health concern for environmental exposures is generally bone cancer (Argonne 2007). 232 Th and its decay products are classified as
carcinogenic to humans in Group 1 (International Agency for Research on Cancer
[IARC] 2001, 2013). Because Th is radioactive and may be stored in the bone for a
long time, bone cancer is also a potential concern for people exposed to Th. Liver
diseases and effects on the blood occur in people injected with Th in order to take
special X-rays. Several types of cancer developed in people many years after Th was
injected into their bodies. Although some forms of Th can stay in the lungs for long
periods of time, in most cases, small amounts of Th leave the body in the feces and
urine within days (ATSDR 1990b).
58.3 URANIUM [U, 92]
58.3.1 INTRODUCTION
Uranium is a silvery-white metallic element, weekly radioactive, but very stable.
Its most common isotopes are 238 U (99.28% natural abundance) and 235 U (0.72%).
The isotope 234 U is very rare (<0.006%). Its contents in the Earth’s crust are estimated at the mean value of 2.7 mg/kg, and its highest concentrations are in igneous
rocks, within the range of 3–5 mg/kg. It may also be accumulated in some sedimentary rocks: in black shales, up to 1244 mg/kg, and in phosphate rocks, up to 300 mg/kg.
It occurs naturally, in low concentrations of a few mg/kg, in soils, rocks, and waters.
The mobility of U depends highly on the host minerals, which after the dissolution
are likely to be quickly precipitated as oxides, carbonates, phosphates, and other
compounds. Thus, the distribution of U in the lithosphere is highly controlled by
the Eh–pH system. The most common forms of U oxides are U 3 O 8 , UO 2 , and U 3 O 8 .
They are relatively stable over a wide range of environmental conditions. The oxidation state of U may vary from +2 to +6, which is the most common.
Uranium ore, carnotite, contains U, at an average of 11%. Uranium in minerals is associated mainly with Si; these minerals are soddyite [(UO 2 ) 2 SiO 4 ·2H 2 O], kasolite [Pb(UO 2 )
(SiO 4 )·H 2 O], uranosilite (UO 3 ·7SiO 2 ), and sklodovskite [Mg(UO 2 ) 2 (HSiO 4 ) 2 ·H 2 O].
A major commercial mineral, and the primary source of U, is pitchblende (uraninite)
of the composition: UO 2 + UO 3 .
The worldwide production of U in 2009 amounted to about 50 Mt, of which 27%
was mined in Kazakhstan, 20% in Canada, and 16% in Australia (USGS 2011).
The major application of U is in the military sector, as a high-density penetrator. This ammunition consists of depleted U (DU), alloyed with 1%–2% of other
elements.  However, DU contains low amounts of 235 U, and thus the use of DU