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Columnists
Sustainability
and Sustainable Development
Larry
Olson, PhD Professor ASU
Polytechnic
Larry
Olson, PhD., Professor, Arizona State University Environmental
Technology Management Program. Dr. Olson holds a Ph.D. in Chemistry from
the University of Pennsylvania, and is an environmental chemist with
interests in remediation technologies and international environmental
management. He can be reached at 480-727-1499 or by email at Larry.Olson@asu.edu.
Perchlorate
-- An Emerging Contaminate
Feb/Mar
2009
EPA
defines an "emerging contaminant" as a chemical or material that
is a perceived, potential, or real threat to human health or the
environment where new sources or pathways of human exposure have been
discovered and where health standards are non-existent or evolving. One
such chemical is perchlorate and it has been much in the news lately.
The
perchlorate ion, ClO4-, may occur naturally in soils, particularly in the
Atacama Desert in Chile and other arid environments such as the
southwestern U.S. It is also produced commercially for use in fireworks,
signal flares, and as a solid propellant in rockets and missiles. For
example, the solid rocket boosters on the space shuttle produce thrust
through a reaction where Al is oxidized, Cl is reduced, and hot gases are
produced:
10
Al(s) + 6 NH4ClO4(s) g
4 Al2O3(s) + 2 AlCl3(s) + 12 H2O(g) + 3 N2(g)
Once
ignited, the reaction can’t be stopped. The temperature rises to about
5800 oF which causes a rapid expansion of the gases and provides about 71%
of the lift needed to get the shuttle into orbit.
Almost
all manufactured perchlorate consists of four compounds: ammonium, sodium,
and potassium perchlorate and perchloric acid. Perchlorate salts are water
soluble and so they can migrate quickly from soil to ground water, where
plumes can be extensive. Even though perchlorate is a strong oxidizer, it
is relatively unreactive at room temperature, not volatile, and not
subject to biodegradation. Thus, perchlorates are considered persistent
contaminants. Perchlorate has been detected at nearly 270 sites in the
U.S., primarily in areas associated with the manufacture and use of
ammunition and rocket fuels.
The
thyroid gland synthesizes hormones involved in the body’s metabolism,
reproduction, cardiovascular system and nervous system. Iodide is crucial
to the production of key thyroid hormones and perchlorate, like nitrates
or thiocyonates from cigarette smoke, can interfere with the uptake of
iodide by the thyroid. For this reason potassium perchlorate was used to
treat hyperthyroidism (too much thyroid activity) in the 1950s and 60s
with good results. But the practice was discontinued after suspicions
about a link to aplastic anemia in some patients. Hypothyroidism is more
common and results in decreased metabolism, tiredness, impairment in
movement, vision, and intelligence. The effects of low levels of thyroid
hormones can be more even more devastating for pregnant women and for a
developing fetus or infant.
The
major routes of human exposure to perchlorate are through drinking water
and food. Perchlorate concentrations of less than 4 µg/L have been
detected in the Colorado River downstream from Hoover Dam. Contaminated
water is used in some areas for irrigation and studies have shown
perchlorate to accumulate in leafy greens, alfalfa, cattle, and milk.
But
what levels would be expected to cause measurable effects on thyroid
function? Epidemiological studies have suggested an association with
perchlorate exposure, but since individual exposures were not
characterized, definitive cause and effect links are hard to establish.
In
2005, the National Research Council recommended a perchlorate reference
dose (RfD), also adopted by EPA, of 0.7 µg/kg of body weight per day –
a level that would not be expected to cause deleterious effects over a
lifetime of exposure. If we assume that all perchlorate exposure comes
from drinking water, the RfD equates to a Drinking Water Equivalent Level
(DWEL) of 24.5 µg/L. In October 2008, EPA published in the Federal
Register a preliminary regulatory determination that it would not
establish a national drinking water standard for perchlorate because there
was not "a meaningful opportunity for health risk reduction…"
Over 32,000 comments were received on this decision and EPA announced in
December 2008 that it would ask the National Research Council to look
again at the effect of perchlorate exposure on sensitive populations.
Until these results are received, EPA has established an Interim Drinking
Water Health Advisory level of 15 µg/L. Stay tuned – this story isn’t
over.
2008/1234
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