Chemistry Reference
In-Depth Information
enter the fi reball and be environmentally distributed.
This should be a small contributor to the overall radi-
ation dose that is dominated by the fi ssion products
produced in such an event. On the basis of the pre-
ceding data, a 50-kg bomb might leave an amount of
unfi ssioned uranium found in approximately 10
14
m
3
of natural air or 10
4
m
3
of natural soil.
supply. The least expensive estimate was for stabilizing
the tailings in place (US $249M). Signifi cantly higher
cost estimates were reported for relocating the tailing
to each of three distant sites by each of three modes
(truck, rail, or pipeline). Costs ranged from a low of US
$407M for a truck haul to the closest site to US $543M
for shipment by pipeline to the most distant site (DOE,
2005a). A Record of Decision was signed to move the
tailings by truck 30 miles away (DOE, 2005b).
Uranium can be removed from tailings and deposits
by use of chemicals. One of these is Fenton's reagent,
which is a mixture of iron and hydrogen peroxide.
When the liquid solution is blended or injected into
soil, it produces hydroxyl radicals that nonselectively
oxidixe metals and produce solubilized metal radi-
cals that are extracted with the liquid through suction
or pumping production wells. This method has been
demonstrated to be practical for uranium and other
environmental contaminants (e.g., pentachlorophenol
and polychlorinated biphenyls) (Lin and Luong, 2004).
Hydrogen peroxide was found to remove 60-80% of
depleted uranium from soils at U.S. Army sites and
was more effective than either citric acid (
4.1.5 Other
Uranium is ubiquitous and is present in many
items, including the paper used to make books. It has
been estimated that the range of radiation dose rates to
the eyes from reading books is 0.29-4.19 × 10
−4
Sv/h
from the
238
U. That same paper contained other radio-
nuclides such that the dose rate from
137
Cs was roughly
half, from
232
Th was comparable, and from
40
K was
approximately 50 times higher (Imtiaz
et al.,
2005).
µ
4.2 Working Environment
Occupational exposure to uranium primarily in-
volves inhalation of mixtures of uranium compounds
with solubilities in water ranging from <1 mgU/L (for
oxides) to >300,000 mgU/L (for uranyl nitrate) (Har-
rington and Rhuele, 1959). Particle size is a key feature
for initial lung retention. The most highly exposed
workers have typically been engaged in extraction
and processing operations of the uranium fuel cycle or
weapons production (BEIR IV, 1988). Uranium concen-
trations in occupational settings are regulated to prevent
overexposures. Other groups with relatively unregu-
lated and potentially high exposure include those who
produce or use phosphate fertilizers (Tadmor, 1986) or
make glass or glazed pottery (Rossol, 1997). The regula-
tion of such technologically enhanced naturally occur-
ring radioactive material (TENORM) is increasing.
≤
40%) or
sodium bicarbonate (
60%) (Choy
et al
., 2005).
Another chemical extractant is D2EHPA (di[2- ethyl-
hexyl] phosphoric acid), which has reportedly been used
to commercially recover environmental metals (beryl-
lium, cobalt, iron, uranium, zinc, and rare earth elements).
The molecule is prepared in an organic phase (e.g., tolu-
ene) in which it deprotonates. When the organic solution
mixes with an aqueous phase containing metals, those
metals are chelated into the organic phase with a pH-
dependent effi ciency, and the organic phase is retrieved.
The effi ciency is enhanced through the addition of other
extractants (dialkylphosphorus extractants plus neutral
phosphorus extractants) (IAEA, 2001). Tri-
n
-butyl phos-
phate has been used to precipitate uranium from acid
mine wastewater (Thomas and Macaskie, 1998).
Plants being studied for their potential to concen-
trate uranium and other heavy metals by phytoextrac-
tion and hydroponic rhizofi ltration include Indian
mustard (
Brassica juncea
), Russian thistle (
Salsola tragus
),
and purple amaranth (
Amaranthus blitum
). Uptake
rates have been enhanced 14-fold over controls by
treating the soil with citric acid (Ebbs
et al
., 2001; Frey
et al
., 2004) or chelating agents, or by increasing stro-
matal transpiration rate (Gleba
et al
., 1999).
≤
4.3 Remediation
Sites contaminated with uranium can be controlled
and remediated, but this has proven costly. Because
of the large volumes involved, regulators are consid-
ering allowing disposal in landfi lls as an option to
radiological repositories (EPA, 2003). Most remedia-
tion has involved drying contaminated water, physi-
cally removing contaminated soil, or immobilization
in place by surrounding the tailings with a clay, com-
posite, or capillary barrier. The focus is on limiting ura-
nium migration and radon emission by selecting the
appropriate barrier, for which computer modeling has
been used (Leoni
et al
., 2004). An environmental impact
statement was prepared to address potential options for
dealing with 12 million tons of tailings piles along the
Colorado River to eliminate the potential for fl ood ero-
sion of these tailings into a signifi cant drinking water
5 TOXICOKINETICS
The absorption, distribution, and excretion kinetics
of uranium and its compounds depend on the chemical
form, and for inhaled particles, also their physical aer-
odynamic and thermodynamic properties.
