Degradation and Phase Partition of Methyl Iodide in Soil

Article abstract of DOI:10.1021/jf960413c

Methyl iodide (CH₃I) was recently proposed as a direct replacement for methyl bromide (CH₃Br) in soil fumigation, but no information exists on its behavior and safety in the environment. In this study, we compared CH₃I and CH₃Br for their transformation and phase partitioning in soil and characterized processes that affect the dissipation of CH₃I from water. In moist soil, the adsorption coefficient Kd of CH₃I is greater, and the Henry's law constant K_H is smaller, than that of CH₃Br. In the same soil, CH₃I was about twice as persistent as CH₃Br, and the persistence decreased with increasing soil organic matter content. Chemical reactions, likely nucleophilic substitutions on soil organic matter, were identified as the predominant pathway through which CH₃I and CH₃Br were degraded under the concentrations studied. In water, CH₃I degraded to I₂ and I^- under 254-nm UV irradiation and dissipated rapidly (t_1/2 = 26 h) through volatilization and photohydrolysis under outdoor conditions.

Full text of DOI:10.1021/jf960413c

J. Agric. Food Chem. 1996, 44, 4001
−4008
4001
Degr a d a tion a n d P h a se P a r tition of Meth yl Iod id e in Soil
J ianying Gan* and Scott R. Yates
Soil Physics and Pesticides Research Unit, U.S. Salinity Laboratory, Agricultural Research Service,
U.S. Department of Agriculture, 450 Big Springs Road, Riverside, California 92507
Methyl iodide (CH3I) was recently proposed as a direct replacement for methyl bromide (CH3Br) in
soil fumigation, but no information exists on its behavior and safety in the environment. In this
study, we compared CH3I and CH3Br for their transformation and phase partitioning in soil and
characterized processes that affect the dissipation of CH3I from water. In moist soil, the adsorption
coefficient Kd of CH3I is greater, and the Henry’s law constant KH is smaller, than that of CH3Br.
In the same soil, CH3I was about twice as persistent as CH3Br, and the persistence decreased with
increasing soil organic matter content. Chemical reactions, likely nucleophilic substitutions on soil
organic matter, were identified as the predominant pathway through which CH3I and CH3Br were
degraded under the concentrations studied. In water, CH3I degraded to I2 and I- under 254-nm
UV irradiation and dissipated rapidly (t1/2 ) 26 h) through volatilization and photohydrolysis under
outdoor conditions.
Keyw or d s: Methyl iodide; methyl bromide; degradation; adsorption; photohydrolysis; volatilization
Ta ble 1. Som e P h ysica l-Ch em ica l P r op er ties of Meth yl
Iod id e a n d Meth yl Br om id e
INTRODUCTION
The impending phase-out of methyl bromide (bro-
momethane, CH3Br) as a soil and commodity fumigant,
if not replaced with equally effective alternatives, will
cause substantial economic damage to the agricultural
communities (Ferguson and Padula, 1994). Intensive
research is currently being conducted for identifying and
developing replacements for CH3Br. The few remaining
fumigants and nematicides, including 1,3-dichloropro-
pene, chloropicrin, and methyl isothiocyanate, all have
narrower spectra of activity than CH3Br (Noling and
Becker, 1994). This implies that combinations of two
or more of these chemicals may have to be used to
achieve a broad control.
Recently, methyl iodide (iodomethane, CH3I) was
proposed as a potential candidate as a direct replace-
ment for CH3Br in soil fumigation (Becker et al., 1995;
Sims et al., 1995; Ohr et al., 1996). In extensive
laboratory and field-plot trials, CH3I was shown to be
equivalent to or more efficient than CH3Br for control-
ling a wide variety of soil-borne pests including weeds,
nematodes, and fungi (Becker et al., 1995; Sims et al.,
1995; Ohr et al., 1996). Methyl iodide has also been
tested on a small scale for controlling various pests in
stored products (Muthu and Srinath, 1974). The main
advantage of CH3I over CH3Br is that it degrades
quickly in the troposphere via photolysis and therefore
is unlikely to contribute to ozone depletion (Rassmussen
et al., 1982; Solomon et al., 1994). The estimated
lifetime for CH3I in the atmosphere is only 4-8 days
compared with 1.5-2 years for CH3Br, and its estimated
ozone depletion potential (ODP) is only 0.016 compared
with 0.6-0.7 for CH3Br (Solomon et al., 1994; Ohr et
al., 1996). However, CH3I is not a registered pesticide,
and most aspects of its environmental behavior are
essentially unknown. To predict the environmental
acceptability of developing this chemical into a com-
mercial fumigant, it is imperative that its transforma-
tion and partitioning behavior in the soil-water-air
property
CH3I
142
2.28 (20 °C)
398 (20 °C)
42.4
CH3Br
molecular weight
95
specific gravity (g mL-1
)
1.73 (0 °C)
1600 (20 °C)
3.6
vapor pressure (mmHg)
boiling point (°C)
solubility in water (%)
1.4
1.34
system be characterized. Understanding of these pro-
cesses is also critical for designing optimum application
techniques that would require minimal chemical input
while maintaining adequate efficacy for controlling
target organisms.
Some of the basic physical-chemical properties of
CH3I are listed along with those of CH3Br in Table 1.
CH3I is structurally analogous to CH3Br, and in com-
parison to CH3Br, CH3I has a similar water solubility,
lower vapor pressure, and higher boiling point and
molecular weight. In this study, we compared CH3I and
CH3Br for their persistence and degradation mecha-
nisms in selected soils under aerobic and sterilized
conditions and their partitioning between air-water
phases and soil-water phases. The inclusion of CH3-
Br in the study allows logical predictions about the
environmental fate of CH3I due to the great similarities
between these two compounds and the fact that abun-
dant data are already available for CH3Br. CH3I
photohydrolysis in and volatilization from water, and
their contributions to its persistence in water, were also
investigated.
MATERIALS AND METHODS
Ch em ica ls a n d Soils. CH3I with a purity of 99.5% was
purchased from Chem Service (West Chester, PA), and CH3-
Br with a purity of 99.5% in a lecture bottle was purchased
from Matheson Gas Products Inc. (East Rutherford, NJ ).
Before use, CH3Br was introduced into a Teflon sampling bag,
and CH3Br in the bag was used as the stock gas. At room
temperature and 1.0 atmosphere, the vapor density of CH3Br
in the sampling bag was determined to be 3.8 mg mL-1
Aniline, N-methylaniline, and N,N-dimethylaniline used in the
degradation study were purchased from Aldrich Chemical Co.
(St. Louis, MO).
.
* Author to whom correspondence should be ad-
dressed
[fax
(909) 342-4964; e-mail
jgan@
ussl.ars.usda.gov].
S0021-8561(96)00413-X
This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society

4002 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Gan and Yates
Ta ble 2. Som e P h ysica l-Ch em ica l P r op er ties of Soils
tion of the soil-water solution was made under vacuum
through a Bu¨chner funnel. The extracts were measured for
I- concentrations on an Accumet 25 pH meter (Fisher Scien-
tific, Pittsburgh, PA), with an iodide-specific electrode (Fisher
Scientific), or for Br- concentrations on a QuikChem AE
automated flow injection ion analyzer (LaChat, Milwaukee,
WI). A correction factor based on the degraded fraction was
then introduced to adjust the calculated Cs. The adsorption
coefficient Kd (milliliters per gram) was then obtained using
the expression
Used in Th is Stu d y
soil type
OM (%)
clay (%)
pH (H2O)
Greenfield sandy loam
Carsetas loamy sand
Linne clay loam
0.92
2.51
2.99
9.60
9.5
11.0
25.1
4.0
7.4
7.3
7.2
6.2
nursery potting mix
The physical-chemical properties of the four soils are given
in Table 2. The Greenfield sandy loam (Moreno Valley Field
Station, University of California, Riverside) was used in
previous field and laboratory studies on CH3Br (Gan et al.,
1994; Yates et al., 1996a,b), and the nursery potting mix (1:1
mix of topsoil and fir sawdust) and Carsetas loamy sand
(Coachella Valley Field Station, University of California,
Riverside) were used in some of the experiments for testing
the efficacy of CH3I (Becker et al., 1995; Ohr et al., 1996).
Before use, moist Greenfield and Carsetas soils and the potting
mix were passed through a 2-mm sieve without air-drying to
protect the microbial activity. Air-dried Linne clay loam
(Santa Barbara, CA) was passed through a 2-mm sieve and
then wetted to 18% water content and incubated at room
temperature for 4 weeks before use. The final water content
was readjusted to 12% for Greenfield and Carsetas soils and
to 18% for Linne clay loam and the potting mix. The higher
water content used for Linne clay loam and the potting mix
was necessary because of their higher clay or organic matter
contents.
P a r tition Coefficien ts KH a n d Kd . A headspace method
modified from Rao et al. (1989) was used for measuring the
air-water partition coefficient KH and the adsorption coef-
ficient Kd. Blank headspace vials (121 ( 0.5 mL, Supelco Co.,
Bellefonte, PA), vials containing 20.0 mL of deionized water,
and vials containing 55.0 g of soil with known water content
were spiked with 200 µL of saturated CH3I vapor (over the
surface of liquid CH3I in a closed 1-L flask) or 500 µL of CH3-
Br gas and immediately capped with Telfon-faced butyl rubber
septa and aluminum seals. The vials were kept in the dark
and at room temperature (21 ( 1 °C) for 24 h. During the
equilibration, all of the vials were occasionally shaken by hand.
Preliminary experiments showed that the 24 h equilibration
under the above conditions was adequate for achieving equi-
librium. At equilibrium, an aliquot of the headspace (500 µL)
was withdrawn using a gastight syringe (Supelco) and trans-
ferred to the bottom of a 8.7-mL headspace vial (Supelco), and
the vial was immediately capped. Preliminary tests showed
that the sample transfer was quantitative and reproducible.
The sealed vials were then analyzed for CH3I and CH3Br
content on a Tekmar 7000 headspace autosampler (Tekmar
Co., Cincinnati, OH) in tandem with an HP5890 GC (Hewlett-
Packard, San Fernando, CA) equipped with an electron
capture detector. Headspace autosampler conditions were 90
°C equilibration temperature, 10-min equilibration time, and
100-µL sample loop. The GC parameters were REX-624
capillary column (Restek Co., Bellefonte, PA), 40 °C oven
temperature, 170 °C injection port temperature, 240 °C
detector temperature, and 1.1 mL min-1 column flow rate
(helium). Calibration curves were made by analyzing blank
vials spiked with known quantities of CH3I or CH3Br under
the described conditions.
Kd ) Cs/Cw
(2)
Degr a d a tion in Soil a n d Rea ction w ith An ilin e. Deg-
radation of CH3I and that of CH3Br were compared in the four
selected soils under the same conditions. CH3Br bromide
degradation in the Greenfield, Carsetas, and Linne soils was
studied previously under unsterilized conditions (Gan et al.,
1994) but was determined again in the current study under
the presented concentrations to provide a comparison for CH3I.
To elucidate the degradation mechanism, degradation kinetics
of CH3I and CH3Br was followed in both sterilized and
unsterilized soils. For unsterilized treatments, 8.7-mL head-
space vials were filled with 11.2 g of Greenfield sandy loam
or Carsetas loamy sand (with 12% water content) or 9.4 g of
the potting mix or the Linne clay loam (with 18% water
content). Less soil was used for the latter two soils because
of their low density. Five microliters of acetone solution
containing 46 µg µL-1 CH3I or CH3Br was injected into the
soil using a syringe, and the vials were immediately sealed
with septa and caps. The initial concentrations, on a dry soil
weight basis, were 23 mg kg-1 for the Greenfield and Carsetas
soils and 29 mg kg-1 for the potting mix and Linne clay loam.
These concentrations approximate the concentration during
the first few days in top soil layers following a typical CH3Br
fumigation under tarped conditions (Abdalla et al., 1974;
Kolbezen et al., 1974).
For sterilized treatments, soils were autoclaved twice at 121
°C for 30 min and the soil water content was readjusted to
the same values as used in the unsterilized group. Small
headspace vials filled with the sterilized soil (11.2 g for
Greenfield or Carsetas soil and 9.4 g for Linne soil or potting
mix, wet weight) were spiked with the same amount of CH3I
or CH3Br and then sealed with septa and caps. All of the
treated sample vials were incubated in a constant temperature
room (24 ( 1 °C) in the dark. At different time intervals after
treatment, three replicate samples were removed, and after
5.0 mL of ethyl acetate was added into the sealed vial through
the septum using a syringe, the soil-solvent mixture was
mechanically shaken for 1.0 h. An aliquot of the ethyl acetate
extract was then quickly transferred into a GC vial containing
a small amount of sodium sulfate salt, and 2 µL of the sample
was injected into the GC from an autoinjector. The same GC
conditions as described above were used in the analysis. The
recovery of CH3I or CH3Br from freshly spiked soil samples
using these sample preparation procedures was 95-101%.
To elucidate the degradation pathways, CH3I and CH3Br
reactions with a model nucleophilic compound, aniline, were
studied. Solutions containing 5 mM aniline and 0.5 mM CH3I
or 5 mM aniline and 0.5 mM CH3Br were prepared in
deionized water and filled into 125-mL serum bottles. Bottles
containing deionized water were treated with the same amount
of CH3I or CH3Br and used as controls. All of the bottles were
sealed with Teflon-faced butyl rubber septa and aluminum
caps and kept at 24 °C in the dark. Four replicates were used
for each chemical-substrate combination. At different time
intervals, 100 µL of the incubated solutions was withdrawn
using a gastight syringe and transferred into 8.7-mL head-
space vials for analysis on the headspace GC. During the
experiment, the bottles were inverted to eliminate loss due to
gaseous diffusion through the septa. To identify reaction
products, at the end of the 25-day incubation, 10 mL of the
aniline sample was extracted with 10 mL of ethyl acetate in a
separatory funnel, and an aliquot of the organic phase was
injected into a HP 5890GC-5971 MSD (Hewlett-Packard). The
GC/MS spectra of eluted peaks were compared to that of
aniline, N-methylaniline, and N,N-dimethylaniline. The GC
The concentration in the water phase (Cw) was calculated
from the difference between the measured headspace concen-
trations (Ca) in the blank vials and vials containing water. The
dimensionless KH was then obtained using
KH ) Ca/Cw
(1)
The Cw for vials containing soil and water was obtained using
Ca and the calculated KH in eq 1, and the concentration
adsorbed on soil, Cs, was calculated from the Ca, Cw, and
volume of the three phases. Since degradation during the
equilibration process may cause an overestimation of Kd, the
production of I- and Br- was measured for the vials containing
soil samples at the end of the equilibration. The soil was
mixed thoroughly with 25 mL of deionized water, and extrac-

Degradation of CH3I in Soil
J. Agric. Food Chem., Vol. 44, No. 12, 1996 4003
Ta ble 3. Mea su r ed KH a n d Kd for CH3I a n d CH3Br (21 (
1 °C)
conditions were HP-5MS (30 m × 0.25 mm × 0.25 µm, Hewlett-
Packard), 250 °C injector temperature, and 1.04 mL min-1 flow
rate (helium). The oven temperature was initially 50 °C (0.01
min) and then ramped at 4 °C min-1 to 190 °C.
parameter
CH3I
CH3Br
KH (n ) 8)
0.21 ( 0.01
0.30 ( 0.02
P h otoh yd r olysis. The photohydrolysis of CH3I was car-
ried out under artificial UV irradiation using a system similar
to that of Castro and Belser (1981). A 2300-mL flask (28 cm
in height) was filled with deionized water containing 1.0 mM
CH3I, and a quartz tube [bottom-sealed, 1 cm (i.d.) × 22 cm
(l)] was suspended in the solution, with the bottom of the tube
hanging about 4 cm from the bottom of the flask. Adhesive
aluminum tape was used to carefully seal the flask opening
around the quartz tube. A shortwave pencil lamp emitting
UV at 254 nm (4500 µW/cm2, 5-cm lighted tube, Fisher
Scientific) was then inserted into the quartz tube, with the
tip of the lighted tube positioned about 5 cm from the flask
bottom. After the UV lamp was lit, a stream of air was
directed into the quartz tube to dissipate some of the heat
generated by the lighted lamp. To differentiate photohydroly-
sis and normal hydrolysis, a flask filled with 1.0 mM CH3I
solution but not exposed to UV irradiation was used as the
control. Both flasks were kept at room temperature (21 ( 1
°C) and wrapped with aluminum foil. At different time
intervals, 0.2 mL of the UV-irradiated or the control solution
was sampled using a gastight syringe and analyzed on the
headspace GC. Ten milliliters of the solution was simulta-
neously removed using a long-needle syringe and measured
for I- concentration using the electrode method. During a
preliminary experiment, formation of I2 from CH3I was visually
observed after exposure to the UV source. To quantify the
fraction of I2 in the solution, an excessive quantity of sodium
bisulfite (NaHSO3) was added into the same sample to reduce
I2 to I- (Moran et al., 1995), and a measurement of I- was
repeated. The increase in I- concentration before and after
the reduction reaction was assumed to be due to I2. Precau-
tions were used in handling samples to avoid any significant
loss of I2 due to volatilization. After each sampling, to prevent
volatilization loss of CH3I and I2 from the flask, deionized
water was added back to fill the flask (to eliminate any
headspace), and the needle puncture was carefully sealed with
adhesive aluminum tape.
Kd(measured) (n ) 6) (mL g-1
Greenfield sandy loam
Linne clay loam
)
0.09
0.15
0.16
0.55
0.07
0.10
0.09
0.29
Carsetas loamy sand
potting mix
Kd(corrected) (n ) 6)a (mL g-1
Greenfield sand loam
Linne clay loam
)
0.08
0.13
0.12
0.46
0b
0
0
0.20
Corsetas loamy sand
potting mix
a
b
Corrected for degradation during equilibration. 0 indicates
adsorption below measurable.
solubility, the dimensionless KH was calculated to be
0.21 at 25 °C (Mutziger, 1996). The reported KH for
CH3Br is 0.244 at 20 °C (Goring, 1962). Our measured
KH values for both CH3I (0.21) and CH3Br (0.30) at 21
°C agreed well with the calculated or reported values
(Table 3). With a KH of 0.30 at 21 °C, CH3Br transport
in soil is predominantly driven by its vapor phase
diffusion, which is responsible for its deep penetration
and rapid volatilization consistently observed in numer-
ous field and laboratory studies (Abdalla et al., 1974;
Kolbezen et al., 1974; Van Wambeke et al., 1980; Yates
et al., 1996a,b; Gan et al., 1996). With a KH of 0.21 at
21 °C, the movement of CH3I in soil can also be expected
to be dominated by gas phase diffusion and, therefore,
to be very fast.
The distribution coefficient between soil and water
phases, or the adsorption coefficient Kd, has long been
established as an important parameter in governing
pesticide movement in soil profiles. Assuming CH3I
moves predominantly from diffusion in the vapor phase,
its transport can be described by the model (J in and
J ury, 1995)
Vola tiliza tion fr om Wa ter . Volatilization of CH3I from
open-surface deionized water was measured under indoor and
outdoor conditions. In the indoor experiment, four bottom-
sealed glass cylinders [12.5 cm (i.d.) × 32 cm (h)] were filled
with 0.2 mM CH3I water solution to 30 cm depth and placed
in a vacuum hood. Two of the cylinders were kept static, and
the other two were continuously stirred at a low speed with a
magnetic bar on a stirrer. Samples of solution (0.5 mL) were
removed with a pipet and transferred into 8.7-mL headspace
vials for analyzing CH3I content on the headspace GC. In the
outdoor experiment, two such cylinders filled with 0.2 mM
CH3I were placed directly under sunlight, and the dissipation
of CH3I in the water was followed as in the indoor study. The
average temperature during the 6-day outdoor experiment was
14 °C, with the highest 26 °C and the lowest 4 °C. At the end
of each experiment, samples were also analyzed for I- con-
centration. In both studies, the unaccounted fraction of CH3I
was assumed to be lost by volatilization.
∂Cg
∂t
∂2Cg
∂Z2
Rg
) Ds
- µRgCg
(3)
where Cg is the concentration in soil air, t is time, µ is
the first-order degradation rate constant, Z is the
distance, Ds is the effective gas diffusion coefficient in
soil, and Rg is the retardation factor for gas phase
diffusion and is given by
Rg ) FbKd/KH + θ/KH + a
(4)
where a is soil air porosity, Fb is soil bulk density, and
θ is volumetric water content.
The measured Kd values for CH3Br on the selected
soils were from 0.07 to 0.39, increasing with soil organic
matter content (Table 3). However, measuring Br- at
the end of the adsorption study revealed that a small
fraction of the added CH3Br was degraded during the
24-h equilibration, and more degradation was found in
the potting mix than in the regular soils. After correc-
tion for the degraded fraction, the amount adsorbed on
the Greenfield, Carsetas, and Linne soils was within the
range of experimental errors, and Kd thus became
negligible (Table 3). Degradation of CH3I during the
24-h equilibration was less than that of CH3Br. After
correction for degradation, Kd for CH3I on the tested
soils ranged from 0.08 to 0.46. Among all of the soils
used, adsorption on the potting mix was the strongest
for both CH3Br and CH3I. This may be attributed to
its very high organic matter content (9.6%).
RESULTS AND DISCUSSION
P h a se P a r tition . The partition of a chemical be-
tween air and water phases, represented by its Henry’s
law constant, KH, is extremely important in determining
the pathway over which a chemical moves in the soil-
water-air environment. As a rough rule, at 50% water
saturation, the transport of compounds with KH . 10-4
is vapor phase diffusion dominated and those with KH
, 10-4 is liquid diffusion dominated (J ury et al., 1991).
The vapor phase diffusion of a chemical is approxi-
mately 104 times as fast as its liquid phase diffusion
(J ury et al., 1991). Reported KH values for CH3I are
not directly available. From its vapor pressure and

4004 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Gan and Yates
Small, but non-zero, Kd and Koc values have been
reported for CH3Br by other workers (Arvieu, 1983;
Goring, 1962). However, in these measurements, the
contribution from degradation during the equilibration
to the adsorption was apparently ignored. In numerous
adsorption studies of pesticides using batch equilibra-
tion techniques based on analysis of chemical remaining
in the liquid phase only, degradation has been found to
result in overestimated Kd values (Koskinen et al.,
1979). As illustrated in this study, correction for
degradation is particularly important for compounds
that are weakly adsorbed on soil.
Stronger adsorption and smaller KH of CH3I combine
to indicate that under the same conditions, less CH3I
will be in the soil gas phase than CH3Br. For instance,
in the Greenfield sandy loam, using the estimated Kd
and KH values and assuming a volumetric soil water
content (θ) of 0.2 cm3 cm-3, an air content of 0.2 cm3
cm-3, and a soil bulk density of 2.6 g cm-3, at equilib-
rium, about 23% of the total CH3Br will be in the soil
air and 77% in the soil water. In this soil under the
same conditions, the distribution of CH3I in the soil air,
water, and solid phases will be 11, 55, and 34% respec-
tively. The effect of Kd and KH on the movement of CH3I
or CH3Br in soil via gas phase diffusion may be seen
from the interactions of these two coefficients with the
retardation factor Rg. For example, substituting the
estimated Kd and KH and the assumed soil parameters
in eq 4, the retardation factor Rg for gas phase diffusion
in Greenfield sandy loam was calculated to be 2.14 for
CH3I but only 0.87 for CH3Br. This suggests that under
the same soil conditions, CH3I is likely to diffuse more
slowly than CH3Br, or less volatilization loss of CH3I
will occur if the two chemicals are degraded at the same
rate.
It must be noted that despite the differences between
the two chemicals, the very high KH and insignificant
Kd of CH3I still warrant significant volatilization losses
of this chemical following a subsoil injection if the soil
surface is not covered. Very high emission rates have
been reported for CH3Br under both field and laboratory
conditions (Yagi et al., 1993, 1995; Majewski et al., 1995;
J in and J ury, 1995; Yates et al., 1996a,b; Gan et al.,
1996). However, since CH3I has a very short photolysis
life time in the air, the emitted CH3I is not expected to
be carried to the stratosphere, where it would contribute
to ozone depletion. From this perspective, the rapid
movement of CH3I into the air may be desirable since
it would reduce phytotoxicity of CH3I to plants and
potential groundwater contamination.
F igu r e 1. Degradation of methyl iodide in soil.
exception for the degradation of CH3I in the Greenfield
soil (Table 4). At typical field concentrations, it appears
that the degradation of both CH3I and CH3Br was not
drastically affected by sterilization. This implies that
under these concentrations these two compounds were
mainly degraded chemically in soil.
From their structures, both CH3I and CH3Br are
typical electrophiles and may undergo bimolecular
nucleophilic substitution (SN2) with H2O:
CH3X + H2O f CH3OH + X- + H+
(I)
X represents I or Br. The measured t1/2 for CH3Br
hydrolysis in water is 20-50 days (Mabey and Mill,
1978; Arvieu, 1983; Herzel et al., 1984; Gentile et al.,
1989), and the reported t1/2 for CH3I hydrolysis in water
at neutral pH is 50-110 days (Mabey and Mill, 1978;
Schwazenbach et al., 1993). The fact that the degrada-
tion of both CH3I and CH3Br in the organic matter-rich
Linne soil and potting mix was considerably faster than
their hydrolysis suggests that other reactions were
apparently involved. In a few studies, it is established
that CH3Br reacts with nucleophilic functional groups
(e.g., -NH2, -NH, -SH, -OH) of soil organic matter
in which -Br acts as a leaving group and the organic
matter is methylated (Arvieu, 1983; Gan et al., 1994):
Degr a d a tion in Soil a n d Rea ction w ith An ilin e.
As the only irreversible process, degradation of CH3I
and CH3Br in soil is important in determining the
fraction available for volatilization to the air and/or
diffusing to the groundwater. As found for 1,2-dibromo-
3-chloropropane, CH3Br, and many other volatile com-
pounds, limited degradation in soil consistently leads
to significant volatilization or downward movement of
the chemical in the soil (Wagenet et al., 1989; Yates et
al., 1996a,b). CH3Br volatilization losses were found to
be directly affected by its degradation rate in soil
columns (Gan et al., 1996). Significantly less volatiliza-
tion was observed in the organic matter-rich Linne soil
in which CH3Br was rapidly degraded to Br-.
CH3Br + OM-NH2 f OM-NH-CH3 + Br- + H+
Degradation of CH3I and CH3Br in the four soils was
determined by measuring the decrease of parent com-
pounds with or without a sterilization pretreatment
(Figures 1 and 2). Degradation of both chemicals fits
first-order kinetics model in most treatments, with the
(II)
Similar mechanisms should also control the degradation
of CH3I in soil.

Degradation of CH3I in Soil
J. Agric. Food Chem., Vol. 44, No. 12, 1996 4005
Ta ble 4. F ir st-Or d er Degr a d a tion Ra te Con sta n t (k) a n d Ha lf-Life (t1/2) of CH3I a n d CH3Br in Soils
k (day-1
t1/2 (days)
unsterilized unsterilized
CH3I
)
r2
sterilized
sterilized
unsterilized
sterilized
Greenfield sand loam
Carsetas loamy sand
Linne clay loam
potting mix
0.016
0.064
0.053
0.052
0.011
0.069
0.073
0.040
43
11
13
13
63
10
9
0.86
0.96
0.98
0.97
0.72
0.98
0.98
0.96
17
CH3Br
Greenfield sandy loam
Carsetas loamy sand
Linne clay loam
0.031
0.116
0.123
0.118
0.034
0.133
0.156
0.103
22
6
6
20
5
4
0.97
1.00
0.99
1.00
0.97
1.00
1.00
0.99
potting mix
6
7
F igu r e 3. Dissipation of methyl iodide (a) and methyl bromide
(b) in deionized water (b) and 5 mM aniline solution (O) at 24
F igu r e 2. Degradation of methyl bromide in soil.
°C.
Close dependence on soil organic matter content was
observed in this study in the degradation of CH3I and
CH3Br under both sterilized and unsterilized conditions
(Figures 1 and 2). In the relatively organic matter-rich
Linne and Carsetas soils, t1/2 values of 9-13 and 4-6
days were obtained for CH3I and CH3Br, respectively,
while in the Greenfield sandy loam (0.92%), the corre-
sponding t1/2 was 43-63 and 20-22 days. It appears
that the nature of the soil organic matter is also
important in determining the degradation; even though
the potting mix contained more organic matter, degra-
dation of CH3I or CH3Br in this matrix was not more
rapid than that in the Linne and Carsetas soils. It is
likely that in the potting mix the organic matter is
mostly undegraded plant residues that are less reactive
to CH3I or CH3Br. The influence of soil organic matter
content on CH3Br degradation has been reported (Brown
and J enkinson, 1971; Brown and Rolston, 1980; Arvieu,
1983; Gan et al., 1994). After measuring CH3Br deg-
radation in eight soils, Arvieu (1983) found that the t1/2
generally decreased with increasing organic matter
content in the soil. In a Mandelieu soil with 0.23%
organic matter, the t1/2 was 49 days, but in a Chevigny
soil with 5.11% organic matter, the t1/2 was shortened
to only 3.6 days. In studying CH3Br degradation in soils
from different depths, Gan et al. (1994) found that the
CH3Br degradation rate constant was highly correlated
with soil organic matter content, and the measured
correlation coefficient ranged between 0.95 and 1.00.
Reaction of CH3I and CH3Br with a model nucleo-
philic compound provides direct evidence for the pro-
posed soil degradation pathways (Figure 3). In com-
parison to their hydrolysis, both CH3I and CH3Br
reacted rather rapidly with aniline, yielding a pseudo-
first-order t1/2 of 4.9 and 2.9 days for CH3I and CH3Br,

4006 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Gan and Yates
F igu r e 4. GC/MS profiles of reacted methyl bromide-aniline solution: (a) total ion chromatogram; (b) spectrum of N-methylaniline
in sample; (c) spectrum of N,N-dimethylaniline in sample.
respectively. In water at the same temperature (24 °C),
the t1/2 was about 74 days for CH3I and 22 days for CH3-
Br. As the end products of nucleophilic substitution
with aniline, N-methylaniline and N,N-dimethylaniline
were positively identified on GC/MS by comparing the
spectra of the degradation products with those of the
standards (Figure 4). The following reaction equations
could thus be presumed:
tion of CH3Br in fumigated fields may be suppressed
and chemical degradation is predominant.
Compared to CH3Br, in the same soil, CH3I degraded
more slowly (at a significance level of 0.95 for all
treatments). The calculated t1/2 for CH3I is 1.8-3.1
times that for CH3Br in the same soil (Table 4). In the
Greenfield sandy loam, a t1/2 as long as 43 days was
obtained for CH3I in the unsterilized soil. The slower
degradation of CH3I in soil may be controlled by its
intrinsic reactivity, as reflected in its slower reactions
with aniline and water (Figure 3). In soil fumigation,
if application techniques similar to those used in CH3-
Br fumigations are adopted for CH3I, a higher propor-
tion of CH3I than CH3Br is likely to volatilize into the
air due to this prolonged persistence. The slower
degradation of CH3I in soil may also result in signifi-
cantly more downward movement, particularly if the
soil surface is tarped.
Dissip a tion of CH3I in Wa ter . In this study, two
pathways, i.e., photohydrolysis and volatilization, were
examined under simulated and outdoor conditions to
assess their importance in governing the fate of CH3I
in water. CH3I in water was rapidly photohydrolyzed
under UV at a wavelength of 254 nm to produce I- and
I2 (Figure 5). Formation of I2 was first noticed visually
and later quantified by reduction with NaHSO3 to I-
and measuring the difference in I- concentration. Ad-
ditional confirmation of I2 was accomplished by reacting
the samples with water-soluble starch. The ratio of I2
to I- in the irradiated solution was about 2.5 (Figure
5). It is known that I- may be oxidized under light to
form I2. However, irradiating a KI solution (containing
200 ppm I-) with the same pen-ray lamp for 52 h only
converted about 1.6% of the I- to I2. Therefore, kineti-
cally, oxidation of I- to I2 could not be the main pathway
by which I2 was formed during the exposure study.
CH3X + C6H5-NH2 f C6H5-NH-CH3 + X- + H+
(III)
CH3X + C6H5-NH-CH3 f
C6H5-N-(CH3)2 + X- + H+ (IV)
After 18 days, CH3Br was completely reacted in the
aniline solution, and about 80% N-methylaniline and
20% N,N-dimethylaniline were formed. While in the
aniline-CH3I mixture, after 25 days about 3% of CH3I
was left and 84 and 13% were converted to N-methy-
laniline and N,N-dimethylaniline, respectively.
It is also likely that under the treated concentrations,
original microbial activity in the soil was inhibited due
to toxicity of the fumigants. At 5-14 ppb(v), bacterial
degradation was believed to cause rapid dissipation of
CH3Br in surface soils (Shorter et al., 1995). A few
isolated bacteria, including nitrifying bacteria Nitrosomo-
nas europaea, Nitrosolobus multiformis, and Nitroso-
coccus oceanus (Rasche et al., 1990) and methane-
oxidizing bacteria Methylomonas methanica and
Methylococcus capsulatus (Oremland et al., 1994a), were
reported to degrade CH3Br. In methanotrophic soils,
CH3Br degradation was inhibited at 10 000 ppm, but
not at 1000 ppm. Since initial CH3Br concentrations
during a typical agricultural fumigation are 104-105
ppm (Kolbezen et al., 1974), it is likely that biodegrada-

Degradation of CH3I in Soil
J. Agric. Food Chem., Vol. 44, No. 12, 1996 4007
could be higher. Though not examined in this study, it
must be noted that in waters that are rich in nucleo-
philes, dissipation of CH3I may be further accelerated.
Reaction with HS- was suggested as an important
pathway for removal of both CH3Br and CH3I from the
solution of a saltmarsh (Oremland et al., 1994b).
Con clu sion s. Because of the similarities to CH3Br,
after application into soil as a fumigant, CH3I could be
expected to behave similarly to CH3Br. However, the
distribution of CH3I and CH3Br between the soil-
water-air phases is slightly different, and the difference
will result in slower movement of CH3I in the soil profile
and less volatilization from the soil surface on the same
time scale. Also, the degradation of CH3I is consider-
ably slower than that of CH3Br. The slower degradation
of CH3I determines that, under similar application and
soil conditions, the ultimate volatilization loss of CH3I
would likely be greater than that of CH3Br, and the risk
for CH3I to enter the groundwater would likely be
higher. Since volatilized CH3I is quickly decomposed
in the air under light, it will not contribute to ozone
destruction. Governed by its high air-water partition
ratio (0.22 at 22 °C) and its high photoinduced hydroly-
sis rate in water, once in water, CH3I may dissipate
rapidly if the water is exposed to air and sunlight. The
dissipation half-life for CH3I in water under exposed,
outdoor conditions is in the scale of about 1 day.
F igu r e 5. Dissipation of methyl iodide (b) and formation of
iodide (4) and iodine (2) in deionized water under 254-nm UV
irradiation.
ACKNOWLEDGMENT
We thank C. Taylor and Q. Zhang for obtaining some
of the data. Property data for Carsetas loamy sand and
the potting mix were provided by H. Ohr of the
University of California, Riverside. We also thank W.
F. Spencer, J . J . Sims, H. Ohr, and J . O. Becker of the
University of California, Riverside, for helpful discus-
sions.
F igu r e 6. Dissipation of methyl iodide from open-surface
water under disturbed, indoor conditions (b), static, indoor
conditions (O), and static, outdoor conditions (2).
LITERATURE CITED
Abdalla, N.; Raski, D. J .; Lear, B.; Schmitt, R. V. Distribution
of methyl bromide in soils treated for nematode control in
replant vineyards. Pestic. Sci. 1974, 5, 259-269.
Arvieu, J . C. Some physico-chemical aspects of methyl bromide
behavior in soil. Acta Hortic. 1983, 152, 267-276.
Becker, J . O.; Grech, N. M.; Ohr, H. D.; McGriffin, M.; Sims,
J . J . Efficacy of methyl iodide against root-knot nematodes
in bucket and field trials. In Proceedings of the Second
Annual International Research Conference on Methyl Bro-
mide Alternatives and Emissions Reductions, San Diego,
Nov 6-8, 1995; Methyl Bromide Alternatives Outreach:
Fresno, CA, 1995; p 47.
Brown, B. D.; Rolston, D. E. Transport and transformation of
methyl bromide in soils. Soil Sci. 1980, 130, 68-75.
Brown, G.; J enkison, D. S. Bromide in wheat grown in soil
fumigated with methyl bromide. Soil Sci. Plant Anal. 1971,
2, 45-54.
Castro, C. E.; Belser, N. O. Photohydrolysis of methyl bromide
and chloropicrin. J . Agric. Food Chem. 1981, 29, 1005-1008.
Fahr, A.; Nayak, A. K.; Kurylo, M. J . The Ultraviolet absorp-
tion cross sections of CH3I temperature dependent gas and
liquid phase measurements. Chem. Phys. 1995, 197, 195-
203.
Ferguson, W.; Padula, A. Economic effects of banning methyl
bromide for soil fumigation. USDA Economic Research
Service, Agricultural Economic Report 677, Washington,
DC, 1994.
From these observations, two independent reactions are
likely responsible for the photolytic degradation of CH3I
in water:
CH3I + hν f (CH3I)* + H2O f CH3OH + H+ + I-
(IV)
CH3I + hν f I• f I2
(V)
Castro and Belser (1981) reported reaction IV as the
only pathway by which CH3Br was photodegraded in
water under UV light. Reaction V is similar to the
pathway through which CH3I is decomposed via pho-
tolysis in the air (Fahr et al., 1995).
Dissipation of CH3I from open-surface water was
measured under indoor and outdoor conditions (Figure
6). In the laboratory, CH3I was lost rapidly from the
30-cm deep cylinders, with a first-order t1/2 of 29 h under
static conditions and 6.5 h under disturbed conditions.
At the end of the 6-day indoor experiment, <1% of the
spiked CH3I was detected as I-, indicating volatilization
from the open surface was the main cause for the
dissipation. Under outdoor conditions with sunlight
irradiation, the measured t1/2 was 26 h. At the end of
the 6-day exposure, 3.1% of the spiked CH3I was
recovered as I-, but no I2 was detectable. Since I2
produced from photohydrolysis might have escaped
during the experiment due to its volatility, the actual
contribution from photodegradation in water of CH3I
Gan, J . Y.; Yates, S. R.; Anderson, M. A.; Spencer, W. F.; Ernst,
F. F. Effect of soil properties on degradation and sorption
of methyl bromide in soil. Chemosphere 1994, 29, 2685-
2700.

4008 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Gan and Yates
Gan, J . Y.; Yates, S. R.; Wang, D.; Spencer, W. F. Effect of
soil factors on methyl bromide volatilization from soil.
Environ. Sci. Technol. 1996, 30, 1629-1636.
Rasche, M. E.; Hyman, M. R.; Arp, D. J . Biodegradation of
halogenated hydrocarbon fumigants by nitrifying bacteria.
Appl. Environ. Microbiol. 1990, 56, 2568-2571.
Gentile, I. A.; Ferraris, L.; Sanguinetti, M.; Tiprigan, M.;
Fisichella, G. Methyl bromide in natural waters: hydrolysis
and volatilization. Pestic. Sci. 1992, 34, 297-301.
Goring, C. A. I. Theory and principles of soil fumigation. Adv.
Pest Control Res. 1962, 5, 47-84.
Herzel, F.; Schmidt, G. The persistence of the fumigant methyl
bromide in soil and water. Wasser Boden 1984, 36, 589-
591.
Rassmussen, R. A.; Khalil, M. A. K.; Gunawardena, R.; Hoyt,
I). J . Geophys. Res.
S. D. Atmospheric methyl iodide (CH3
1982, 87, 3086-3090.
Schwazenbach, R. P.; Gschwend, P. M., Imboden, D. M., Eds.
Environmental Organic Chemistry; Wiley: New York, 1993;
p 366.
Shorter, J . H.; Kolb, C. E.; Crill, P. M.; Kerwin, R. A.; Talbot,
R. W.; Hines, M. E.; Harris, R. C. Rapid degradation of
atmospheric methyl bromide in soils. Nature 1995, 377,
717-719.
J in, Y.; J ury, W. A. Methyl bromide diffusion and emission
through soil under various management techniques. J .
Environ. Qual. 1995, 24, 1002-1009.
J ury, W. A.; Gardner, W. R.; Gardner, W. H. Transport of
volatile organic compounds through soil. In Soil Physics;
J ury, W. A., Gardner, W. R., Gardner, W. H., Eds.; Wiley:
New York, 1991; pp 234-242.
Kolbezen, M. J .; Munnecke, D. E.; Wilbur, W. D.; Stolzy, L.
H.; Abu-El-Haj, F. J .; Szuskiewicz, T. E. Factors that affect
deep penetration of field soils by methyl bromide. Hilgardia
1974, 42, 465-492.
Koskinen, W. C.; O’Connor, G. A.; Cheng, H. H. Characteriza-
tion of hysteresis in the desorption of 2,4,5-T from soils. Soil
Sci. Am. Soc. J . 1979, 43, 871-874.
Mabey, W.; Mill, T. Critical review of hydrolysis of organic
compounds in water under environmental conditions. J .
Phys. Chem. Ref. Data 1978, 7, 383-415.
Sims, J . J .; Grech, N. M.; Ole Becker, J .; McGiffen, M., J r.;
Ohr, H. D. Methyl iodide: a potential alternative to methyl
bromide. In Proceedings of the Second Annual International
Research Conference on Methyl Bromide Alternatives and
Emissions Reductions, San Diego, Nov 6-8, 1995; Methyl
Bromide Alternatives Outreach: Fresno, CA, 1995; p 46.
Solomon, S.; Garcia, R. R.; Ravishankara, A. R. On the role of
iodine in ozone depletion. J . Geophys. Res. 1994, 99,
20 491-20 499.
Van Wambeke, E.; Vanachter, A.; Van Assche, C. Distribution
of methyl bromide after fumigation and bromide uptake by
lettuce under practical glass house conditions. Meded. Fac.
Landbouww. Rijksuniv. Gent 1980, 45, 889-893.
Wagenet, R. J .; Hutson, J . L.; Biggar, J . W. Simulating the
fate of a volatile pesticide in unsaturated soil: a case study
with DBCP. J . Environ. Qual. 1989, 78, 247-264.
Yagi, K.; Williams, J .; Wang, N. Y.; Cicerone, R. J . Agricultural
soil fumigation as a source of atmospheric methyl bromide.
Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 8420-8423.
Yagi, K.; Williams, J .; Wang, N. Y.; Cicerone, R. J . Atmospheric
methyl bromide (CH3Br) from agricultural soil fumigations.
Science 1995, 267, 1979-1981.
Yates, S. R.; Gan, J .; Ernst, F. F.; Mutziger, A.; Yates, M. V.
Methyl bromide emissions from a covered field: I. Experi-
mental conditions and degradation in soil. J . Environ. Qual.
1996a , 25, 184-192.
Yates, S. R.; Ernst, F. F.; Gan, J .; Gao, F.; Yates, M. V. Methyl
bromide emissions from a covered field: II. Volatilization.
J . Environ. Qual. 1996b, 25, 192-202.
Majewski, M. S.; McChesney, M. M.; Woodrow, J . E.; Prueger,
J . H.; Seiber, J . N. Aerodynamic measurements of methyl
bromide volatilization from tarped and nontarped fields. J .
Environ. Qual. 1995, 24, 431-444.
Moran, J . E.; Teng, R. T. D.; Rao, U.; Fehn, U. Detection of
iodide in geologic materials by high-performance liquid
chromatography. J . Chromatogr. A 1995, 706, 215-220.
Muthu, M.; Srinath, D. Toxicity of methyl iodide to the adults
of five beetles from stored products. J . Stored Prod. Res.
1974, 10, 65-66.
Mutziger, A. Methyl bromide issues. The temperature depen-
dence of the Henry’s constants, and
a comparison of
measured and simulated diffusion in soil. Thesis, Depart-
ment of Soil and Environmental Sciences, University of
California, Riverside, CA.
Noling, J . W.; Becker, J . O. The challenge of research and
extension to define and implement alternatives to methyl
bromide. J . Nematol. 1994, 26, 573-586.
Ohr, H. D.; Sims, J . J .; Grech, N. M.; Ole Becker, J .; McGiffen,
M. E., J r. Methyl iodide, an ozone-safe alternative to methyl
bromide as a soil fumigant. Plant Dis. 1996, 80, 731-735.
Oremland, R. S.; Miller, L. G.; Culbertson, C. W.; Connell, T.
L.; J ahnke, L. Degradation of methyl bromide by methan-
otrophic bacteria in cell suspensions and soils. Appl.
Environ. Microbiol. 1994a , 60, 3640-3646.
Oremland, R. S.; Miller, L. G.; Strohmaier, F. E. Degradation
of methyl bromide in anaerobic sediments. Environ. Sci.
Technol. 1994b, 28, 514-520.
Received for review J une 10, 1996. Revised manuscript
received October 7, 1996. Accepted October 9, 1996.X This
study was supported by USDA Cooperative State Research
Service Agreement 92-34050-8152. Reference to a company
name or product does not imply the approval or recommenda-
tion of the product by the U.S. Department of Agriculture to
the exclusion of others that may be suitable.
J F960413C
Rao, P. S. C.; Ogwada, R. A.; Rhue, R. D. Adsorption of volatile
organic compounds on anhydrous and hydrated sorbents:
equilibrium adsorption and energetics. Chemosphere 1989,
18, 2177-2191.
X Abstract published in Advance ACS Abstracts, No-
vember 15, 1996.