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Methyl Iodide: A Single Chemical Replacement for Methyl
Bromide
James J. Sims, H.D. Ohr, N.M. Grech, Department of Plant Pathology;
J. Ole Becker,
Department
of Nematology; M.E. McGriffen, Jr., Department of Botany and
Plant Sciences, University of California, Riverside, CA 92521.
Methyl bromide is the most widely used and universally effective
fumigant in the world. However, it has been subject to two major
regulatory actions in the past few years. The first was the
United
Nations' Montreal Protocol on Substances that Deplete the Ozone
Layer which eliminates the use of methyl bromide in industrialized
countries in 2010. Independently, the U.S. Clean Air Act will bring about
a ban on production or importation in the USA after January 1, 2001. The
reason for these actions is the current scientific opinion that methyl
bromide emissions contribute significantly to the destruction of the ozone
layer in the stratosphere.
We started this work about two years ago when we recalled that methyl
iodide decomposes in light. This photolability of methyl iodide results in
a very short residence time in the atmosphere of only a few days. This
suggests that methyl iodide will not reach the stratosphere under normal
circumstances and, therefore, will have an insignificant ozone depletion
potential. The lifetime of methyl bromide in the atmosphere has been
estimated to be l.5-2 years.
Both methyl iodide and methyl bromide have well known chemistry being
used as alkylating agents. Because of this similarity in reactivity, one
would expect that methyl iodide would replace methyl bromide as a biocide.
The literature shows very clearly that both compounds are considered toxic
to animals. The documented chemical reactivity of both compounds is so
similar that we expected that methyl iodide should act as a drop-in
replacement for methyl bromide.
When we checked the biological literature for evidence of our
conjecture, we found very little. Methyl iodide was tested as a potential
fumigant of insects as long as 50 years ago and was shown to have good
activity in a couple of subsequent studies. No literature appears to exist
describing the use of methyl iodide in soil fumigation. It is known that
the activity of many fumigants can be modified by their use i soil.
One further property of methyl iodide gives it an advantage over
methyl bromide. Methyl iodide is a low boiling liquid with a boiling point
of 42.5 oC (108 oF) while methyl bromide is a gas at
ambient temperature and pressure. The ease of handling a liquid over a gas
should increase worker safety.
Our testing of methyl iodide has been in soil against a broad range of
organisms. Most of the tests have been done in comparison with methyl
bromide (at the same molar concentration) and no treatment as controls.
The organisms tested were: the fungi Phytophthora citricola,
P. Cinnamomi, P. parasitica and Rhizoctonia solani; the
nematode Heterodera schachtii; and the weeds Abutilon
theophrastii, Chenopodium album, C. murale, Convolvulus arvensis, Cyper
us rotundus, Poa annua, Portulaca oleracea, and Sisymbrium
irio. In both laboratory and field trials, when compared to equivalent
molar rates, methyl iodide was equal to or better than methyl bromide in
controlling the tested organisms.
Other environmental concerns are worth considering. Both chemicals
have short half lives in water. Methyl bromide is hydrolyzed in 20 to 40
days and methyl iodide in 50 to 100 days.
Both are converted by this hydrolysis to methanol and, respectively,
bromide ion or iodide ion. It has been estimated that methyl bromide has
a half life of 14 days in the soil after a typical fumigation. This will
be dependent on soil type and organic content. Some microbial consumption
of methyl bromide in the soil has recently been reported. No such studies
have been done for methyl iodide, but similar behavior would be expected.
The fate of iodide in the environment is not well known, but unlike
bromide, iodide is a recognized plant and human nutrient. The addition of
iodide to salt is well known in this country. Lack of iodide is a major
cause of mental illness in the world.
Based on its chemical, physical, and biological properties, methyl
iodide is a logical candidate to replace methyl bromide.
[April 1996 Table of Contents]
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Last Updated: October 16, 1996
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Last Modified: 01/30/2002
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