Accelerated degradation of methyl iodide by agrochemicals

Article abstract of DOI:10.1021/jf020820q

The fumigant methyl iodide (Mel, iodomethane) is considered a promising alternative to methyl bromide (MeBr) for soil-borne pest control in high-cash-value crops. However, the high vapor pressure of Mel results in emissions of a significant proportion of the applied mass into the ambient air, and this may lead to pollution of the environment. Integrating the application of certain agrochemicals with soil fumigation provides a novel approach to reduce excessive fumigant. emissions. This study investigated the potential for several agrochemicals that are commonly used in farming operations, including fertilizers and nitrification inhibitors, to transform Mel in aqueous solution. The pseudo-first-order hydrolysis half-life (t_1/2) of Mel was ～ 108 d, while the transformation of Mel in aqueous solutions containing selected agrochemicals was more rapid, with t_1/2 < 100 d (t_1/2 < 0.5 d in some solutions containing nitrification inhibitors). The influence of these agrochemicals on the rate of Mel degradation in soil was also determined. Adsorption to soil apparently reduced the availability of some nitrification inhibitors in the soil aqueous phase and lowered the degradation rate in soil. In contrast, addition of the nitrification inhibitors thiourea and allylthiourea to soil significantly accelerated the degradation of Mel, possibly due to soil surface catalysis. The t_1/2 of Mel was <20 h in thiourea- and allylthiourea-amended soil, considerably less than that in unamended soil (t_1/2 > 300 h).

Full text of DOI:10.1021/jf020820q

J. Agric. Food Chem. 2003, 51, 673−679 673
Accelerated Degradation of Methyl Iodide by Agrochemicals
WEI ZHENG, SHARON K. PAPIERNIK, MINGXIN GUO, AND SCOTT R. YATES*
George E. Brown Jr. Salinity Laboratory, Soil Physics and Pesticide Research Unit, USDA-ARS,
450 West Big Springs Road, Riverside, California 92507
The fumigant methyl iodide (MeI, iodomethane) is considered a promising alternative to methyl bromide
(MeBr) for soil-borne pest control in high-cash-value crops. However, the high vapor pressure of MeI
results in emissions of a significant proportion of the applied mass into the ambient air, and this may
lead to pollution of the environment. Integrating the application of certain agrochemicals with soil
fumigation provides a novel approach to reduce excessive fumigant emissions. This study investigated
the potential for several agrochemicals that are commonly used in farming operations, including
fertilizers and nitrification inhibitors, to transform MeI in aqueous solution. The pseudo-first-order
hydrolysis half-life (t1/2) of MeI was ∼108 d, while the transformation of MeI in aqueous solutions
containing selected agrochemicals was more rapid, with t1/2 < 100 d (t1/2 < 0.5 d in some solutions
containing nitrification inhibitors). The influence of these agrochemicals on the rate of MeI degradation
in soil was also determined. Adsorption to soil apparently reduced the availability of some nitrification
inhibitors in the soil aqueous phase and lowered the degradation rate in soil. In contrast, addition of
the nitrification inhibitors thiourea and allylthiourea to soil significantly accelerated the degradation of
MeI, possibly due to soil surface catalysis. The t1/2 of MeI was <20 h in thiourea- and allylthiourea-
amended soil, considerably less than that in unamended soil (t1/2 > 300 h).
KEYWORDS: MeI; methyl iodide; iodomethane; degradation; transformation; nitrification inhibitor;
fertilizer; soil; agrochemical
INTRODUCTION
may be hazardous to workers or nearby residents due to its
moderate toxicity and suspected carcinogenicity. Massive
exposure of MeI can lead to pulmonary edema (8).
The fumigant methyl bromide (MeBr) has been used exten-
sively in California and other parts of the world to control plant
pathogens such as nematodes, soil-borne diseases, and weeds
in economically important crops such as strawberries and
nursery stock (1). Because of its stratospheric ozone depletion
potential, MeBr has been scheduled for elimination in the United
States and other developed countries by the year 2005 (2). The
impending phase-out has resulted in an intensive search for
alternative fumigants and the development of other integrated
pest management strategies to replace MeBr. Methyl iodide
(iodomethane, MeI) is often referred to as the “drop-in replace-
ment” because its fate and transport characteristics and ef-
fectiveness as a biocide are similar to those properties for MeBr
(3). MeI has a distinguished advantage over MeBr in that its
atmospheric lifetime is only 4-8 days, compared with 0.4-
0.9 years for MeBr (4). Therefore, it is unlikely that MeI will
reach the stratosphere and contribute to ozone depletion (5, 6),
although the volatilization of MeI may be similar to that of
MeBr. In laboratory soil columns, cumulative MeI emissions
ranged from ∼30 to 70% of the applied MeI for polyethylene-
tarped soil application (7). Although MeI is not considered an
ozone depleter, excessive MeI emission into the ambient air
Generally, the most important environmental problem as-
sociated with the use of soil fumigants is atmospheric emissions.
This is due to their high vapor pressure that allows them to
readily volatilize and enter the atmosphere. Further compound-
ing the problem is that fumigants are often applied at levels
that exceed that required to control the target organisms, to
compensate for losses due to volatilization and degradation. This
ensures adequate pest control but increases the risk of air, surface
water, and groundwater contamination. To minimize the nega-
tive impact of fumigants on the environment, many management
strategies have been proposed to reduce fumigant emissions.
For instance, covering the soil surface with impermeable plastic
films and sealing the soil surface with water has been shown to
reduce fumigant volatilization (9, 10). However, use of imper-
meable films may cause unintended problems such as worker
exposure for those removing a film after a relatively short
fumigation (i.e., cover) period. If a film is removed from a field
before extensive fumigant degradation has occurred, the gas
trapped under the film could be released at high concentrations
(11, 12). Other methods of fumigation have also been explored
to reduce fumigant emissions, such as deep injection or
application via drip irrigation, both of which increase the soil
residence time by increasing the travel distance to the soil
* To whom correspondence should be addressed [telephone (909) 369-
4803; fax (909) 342-4964; e-mail syates@ussl.ars.usda.gov].
10.1021/jf020820q This article not subject to U.S. Copyright. Published 2003 by the American Chemical Society
Published on Web 01/01/2003

674 J. Agric. Food Chem., Vol. 51, No. 3, 2003
Zheng et al.
For each nucleophilic agrochemical, a 1.0 mM solution was prepared
in deionized water and placed in a 55-mL serum bottle. All bottles
were spiked with 1.0 mM MeI and then immediately capped with a
Teflon-faced butyl rubber septum and an aluminum seal. At various
times, 0.5-mL aliquots were withdrawn from triplicate bottles and
transferred to sealed glass vials containing 5.0 mL of ethyl acetate and
3.0 g of anhydrous sodium sulfate. The vials were vortexed for 2 min,
and an aliquot of the ethyl acetate extract was quickly transferred to a
GC vial for MeI analysis.
Transformation of MeI by Na-DEDTC and NH4-DEDTC was studied
in aqueous solutions with molar ratios (DEDTC:MeI) of 0:1, 0.25:1,
0.5:1, 1:1, 2:1, and 4:1. Samples were prepared, incubated, sampled,
and analyzed as described above. At each sampling time, an additional
0.5-mL aliquot of solution was withdrawn from the 1:1 treatment for
analysis of iodide ion. The samples were diluted with deionized water
and immediately analyzed by ion chromatography (IC).
Soil Incubation Experiment. The kinetics of MeI degradation in
soils amended with agrochemicals was investigated. Briefly, separate
samples of soil were each premixed with a different agrochemical at
1.0 mmol kg-1. Ten grams of the amended soil (dry weight, gravimetric
soil water content 12.5%) was weighed into a 21-mL headspace vial
and then spiked with MeI at 0.5 mmol kg-1. Unamended soils were
also prepared and spiked in the same way. The vials were sealed with
Teflon-faced butyl rubber septa and aluminum seals immediately after
spiking and incubated at 30 ( 1 °C in the dark. Triplicate samples
were removed at different times and chilled at -21 °C for 3-4 h, and
then their seals were removed, 10 g of anhydrous sodium sulfate and
10 mL of ethyl acetate were added, and the vials were resealed. The
samples were vigorously shaken for 1 h and vortexed for 2 min at
room temperature in order to attain complete recovery. An aliquot of
the ethyl acetate extract was transferred to a GC vial and stored in a
freezer (-21 °C) until the end of the experiment, when all samples
were analyzed for MeI using gas chromatography. Preliminary experi-
ments showed >95% recovery for unamended soil using the above
extraction procedures.
The rate of MeI transformation was evaluated in soils containing
different initial concentrations of the NIs Na-DEDTC and ATS. Soils
treated with Na-DEDTC and ATS at 0, 0.5, 1.0, 2.0, and 4.0 mmol
kg-1 were spiked with 0.5 mmol kg-1 MeI, resulting in initial Na-
DEDTC or ATS-to-MeI molar ratios of 0:1, 1:1, 2:1, 4:1, and 8:1. The
procedures described above were used for treatment, incubation, and
extraction of samples. The rate of MeI degradation in soil was
determined from the concentration of MeI remaining as a function of
time.
surface or by reducing gas-phase diffusion (13, 14). Soil
conditions (such as soil temperature and soil moisture content)
and physical-chemical properties (such as soil type and organic
matter and clay contents) generally play a significant role in
determining the fate of fumigants in soil by influencing the rate
of abiotic and biological degradation reactions (15, 16). Previous
studies investigated the potential for application of organic waste
at the soil surface to reduce fumigant volatilization. Incorpora-
tion of composted manure into soil was found to significantly
stimulate the growth and activity of fumigant-degrading organ-
isms, causing accelerated fumigant dissipation (17-19). Ap-
plication of nucleophilic compounds such as ammonium thio-
sulfate (ATS) at the soil surface has been shown to reduce
fumigant volatilization by rapid transformation via abiotic
reaction with halogenated fumigants (20, 21). Currently, very
little information is available concerning the effect of other
agrochemicals on the rate of fumigant degradation in soil and
water.
Agrochemicals such as fertilizers are applied worldwide to
enhance agricultural production, and their use is increasing.
Nitrification inhibitors (NIs) are a group of agrochemicals used
to limit the rate of fertilizer nitrification and increase fertilizer
efficiency. They help to reduce nitrate leaching to groundwater
and decrease N loss as N2O or N2 (22). Use of NIs with N
fertilizers has been proposed as a management technique to
reduce N leaching and improve N uptake by plants (23-25).
The use of these agrochemicals may alter the soil chemical and
biological conditions, which may also influence fumigant
degradation.
In this study, we evaluated the effects of several agrochemi-
cals on MeI degradation. The agrochemicals selected included
fertilizers, such as urea, oxamide, and calcium cyanamide, and
NIs, such as dicyandiamide (DCD), thiourea, 1-allyl-2-thiourea,
ammonium thiosulfate (ATS), sodium diethyldithiocarbamate
(Na-DEDTC), and ammonium diethyldithiocarbamate (NH4-
DEDTC). These agrochemicals were chosen because they are
nucleophiles and were expected to react abiotically with MeI.
The specific objectives of this study were to (i) determine the
rate of MeI transformation by these agrochemicals in aqueous
solution and propose their reaction mechanism and (ii) assess
the rate of MeI degradation in soils amended with these
agrochemicals to determine their effectiveness in enhancing the
MeI transformation rate. This information will be useful in
developing effective management practices to prevent atmo-
spheric emissions and remediate soils contaminated by pesti-
cides.
The rate of MeI (0.5 mmol kg-1) degradation was determined in
soils that were pretreated with Na-DEDTC or ATS (1.0 mmol kg-1) at
various times to evaluate the influence of aging of the NIs in soil. Soils
were amended with Na-DEDTC or ATS and mixed thoroughly and
then stored in the dark at 30 ( 1 °C. At different times (0-14 d) after
incubation, a fraction of the amended soil was removed, placed in a
serum bottle, and spiked with MeI. The same processes as given above
were used for spiking, incubation, sampling, and extraction of residual
MeI.
MATERIALS AND METHODS
Instrumental Analysis. A Hewlett-Packard HP 6890 gas chromato-
graph (GC) equipped with a micro-electron capture detector was used
to analyze the extracted MeI. A DB-VRX capillary column (30 m ×
250 µm × 1.4 µm, J&W Scientific, Folsom, CA) was used with the
following conditions: 1.0 mL min-1 carrier gas flow rate, 220 °C inlet
temperature, and 280 °C detector temperature. The oven was held at
50 °C for 1 min and then increased at 15 °C min-1 to 140 °C and kept
at 140 °C for 1 min. Under these conditions, the retention time of MeI
was 3.96 min.
Soil and Chemicals. The soil used in this study, an Arlington sandy
loam (coarse-loamy, mixed, thermic, Haplic Durixeralf), was collected
from the University of California, Riverside Agricultural Experiment
Station; the plot has no history of fumigant application. Soil was
collected 15 cm from the surface. Moist soils were passed through a
2-mm sieve without complete air-drying and stored at low temperature
before use. The soil had a pH of 7.2 and consisted of 0.92% organic
matter, 64% sand, and 7% clay.
MeI standard (>99% purity) was purchased from Chem Service
(West Chester, PA). Thiourea (99% purity), allylthiourea (98% purity),
Na-DEDTC, NH4-DEDTC, DCD, oxamide, calcium cyanamide, and
hydroxylamine (99% purity) were all obtained from Aldrich Chemical
Co. (Milwaukee, WI). Urea was purchased from Sigma Chemical Co.
(St. Louis, MO). ATS (99.0% purity) was purchased from Fluka
Chemical Co. (Ronkonkoma, NY).
A Dionex DX-100 ion chromatograph (IC) equipped with an AS5
column (Dionex, Sunnyvale, CA) and AS40 automated sampler was
used to determine iodide ion. The mobile phase consisted of 2 mM
NaOH, 4.5 mM Na2CO3, and 2% acetonitrile (v/v), and the flow rate
was 1.0 mL min-1
.
RESULTS AND DISCUSSION
Aqueous-Phase Kinetics Experiments. The reaction of MeI with
different agrochemicals was first studied in aqueous solutions at 20 (
1 °C. In these experiments, the disappearance of MeI was measured.
Degradation Kinetics of MeI in Aqueous Solution Con-
taining Fertilizers. Figure 1 shows the disappearance of MeI

Accelerated Degradation of MeI
J. Agric. Food Chem., Vol. 51, No. 3, 2003 675
Figure 1. Disappearance of MeI (1.0 mM) in aqueous solutions containing
different fertilizers (1.0 mM). Error bars represent standard deviation of
triplicate samples.
(1.0 mM) in aqueous solution containing the nitrogen fertilizers
urea (NH2CONH2) and oxamide (NH2COCONH2). During the
25 d incubation, the disappearance of MeI in aqueous solutions
containing these fertilizers was slightly accelerated compared
to the control. For instance, after 25 d, about 86% of applied
MeI remained in the control samples, while less than 77%
remained in solutions containing 1.0 mM oxamide or urea
solution (Figure 1), indicating a possible reaction between MeI
and these fertilizers in aqueous solution. Urea and oxamide
contain electron-rich groups (:NH2) which are nucleophiles
susceptible to nucleophilic substitution reaction with halogenated
fumigants, but little enhancement in the rate of MeI degradation
was observed in the two fertilizer solutions. It is surmised that
the electron-withdrawing character of the carboxyl (CdO) group
reduced the nucleophilic activity of the amino group in aqueous
solution. Even though the fertilizers urea and oxamide did not
drastically accelerate degradation of MeI, the common use of
these fertilizers in agricultural systems suggests that residual
material may have some effect on the fate of the fumigant in
soil.
Figure 2. Disappearance of MeI (1.0 mM) in aqueous solutions containing
different nitrification inhibitors (1.0 mM). Error bars represent standard
deviation of triplicate samples.
attacks MeI by breaking the C-I bond and forming a C-S
bond:
In previous research it was reported that hydroxylamine
(NH2OH), a product of the catalytic oxidation of ammonia in
soil, could degrade and oxidize MeBr to formaldehyde (26,
27). Figure 1 shows that MeI disappeared more rapidly in
1.0 mM hydroxylamine aqueous solution than in the fer-
tilizer solutions and control samples. More than 30% of the
applied MeI was degraded in 25 d of incubation, due to a
combination of hydrolysis and chemical reaction with hydroxy-
lamine.
Degradation Kinetics of MeI in Aqueous Solution Con-
taining Nitrification Inhibitors. The disappearance of MeI in
aqueous solutions of selected NIs is shown in Figure 2. With
the exception of dicyandiamide (DCD), the rates of disappear-
ance of MeI were considerably higher in solutions con-
taining these NIs compared to the controls. In the 1.0 mM
thiourea and allylthiourea aqueous solutions, MeI disappeared
rapidly, with less than 13% remaining after 25 d of incubation
(Figure 2a). After deducting the MeI loss via hydrolysis (14%
in the corresponding control sample), the majority of MeI
disappearance in thiourea and allylthiourea aqueous solutions
was attributed to reaction between MeI and these two NIs.
According to the reaction of an alkyl halide converted to an
isothiuronium salt by treatment with thiourea (28), it can be
surmised that the sulfur atom of thiourea and allylthiourea
Extremely rapid dissipation of MeI was found in aqueous
solution containing 1.0 mM Na-DEDTC, NH4-DEDTC, or
ATS (Figure 2b) compared to the other agrochemicals (Fig-
ures 1 and 2a; Table 1). Complete dissipation of MeI occurred
within 24 h in the Na-DEDTC and NH4-DEDTC solution, and
less than 20% remained in ATS solution. The MeI degradation
half-life, based on a pseudo-first-order rate coefficient, was
drastically decreased in Na-DEDTC, NH4-DEDTC, and ATS
aqueous solutions (respective t1/2 of 0.13, 0.14, and 0.35 d)
compared to degradation by hydrolysis (t1/2 ) 108.47 d) (Table
1). MeI was dissipated more rapidly in the Na-DEDTC or
NH4-DEDTC solution than in ATS solution, with t1/2 2-3 times
less than that of ATS (Table 1). These results suggest that MeI
would be quickly transformed in aqueous systems treated with
Na-DEDTC or NH4-DEDTC, which may substantially reduce
the risk of off-site transport.
The reaction mechanism between MeI and ATS is postu-
lated to involve conversion to the Bunte salts (29) which would

676 J. Agric. Food Chem., Vol. 51, No. 3, 2003
Zheng et al.
Table 1. First-Order and Second-Order Degradation Rate Constant
and Half-Life of Methyl Iodide (1.0 mM) in Aqueous Solution
Containing Different Agrochemicals (1.0 mM)a
first-order kinetics
second-order kinetics
k × 10-1
t1/2
(d)
µ × 10-1
(mM-1 d-1
t1/2
(d)
f
agrochemical
Na-DEDTC
(d-1
)
)
53.93 ± 1.34 0.13
(1.00)
118.32 ± 9.55
(0.99)
0.08
NH -DEDTC
4
49.39 ± 1.45 0.14
114.00 ± 8.30
0.09
(1.00)
(0.99)
ATS
19.85 ± 0.37 0.35
(1.00)
34.08 ± 1.75
(0.99)
0.29
thiourea
allylthiourea
DCD
0.93 ± 0.03
(1.00)
1.07 ± 0.04
(1.00)
0.08 ± 0.01 83.12
(0.88)
0.14 ± 0.02 50.71
(0.90)
0.11 ± 0.01 61.18
(0.98)
7.47
1.54 ± 0.01
(0.98)
2.03 ± 0.01
(0.99)
0.14 ± 0.02
(0.71)
0.24 ± 0.3
(0.80)
0.15 ± 0.01
(0.97)
6.51
6.47
4.92
69.49
41.15
65.79
63.69
130.65
hydroxylamine
oxamide
urea
0.10 ± 0.01 71.96
(0.89)
0.06 ± 0.01 108.47
0.16 ± 0.02
(0.80)
0.08 ± 0.01
control
(0.91)
(0.91)
a Values in parentheses are correlation coefficient r.
be relatively stable in neutral aqueous solution:
CH3I + S2O32- f CH3-S-SO3- + I-
(2)
In Na-DEDTC and NH4-DEDTC aqueous solutions, (C2H5)2-
NSCS- exists as a thiolate ion (RS-), which is a strong
nucleophile that is susceptible to SN2 nucleophilic substitution
reactions with halogenated hydrocarbons. The following reaction
between MeI and diethyldithiocarbamate ion in aqueous solution
is thus proposed:
Figure 3. First-order degradation rate constant k (h-1) of fumigants (1.0
mM) in aqueous solutions containing varying initial concentrations of
f
Na-DEDTC (a) and NH -DEDTC (b).
4
CH3I + (C2H5)2NSCS- f (C2H5)2NSCS-CH3 + I- (3)
For completeness, the second-order reaction rate coefficients,
µ, may be found from the following second-order relationship
(31, 32):
As shown in Figure 2b, I- was quantitatively produced as the
MeI was consumed in Na-DEDTC aqueous solution.
Fitting a pseudo-first-order kinetic model to the MeI dis-
appearance data showed that as the initial Na-DEDTC or
NH4-DEDTC-to-MeI ratio increased, the degradation rate
constant of MeI increased (Figure 3). For instance, the half-
life of MeI in aqueous solution containing 0.25 mM Na-DEDTC
or NH4-DEDTC was ∼20 h, whereas the half-life was re-
duced to <0.75 h when the concentration of Na-DEDTC or
NH4-DEDTC was increased to 4.0 mM. Wang et al. (29) plotted
apparent first-order rate constants vs the initial concentration
of ATS to attain second-order reaction constants of fumigants
in aqueous solutions of ATS; good correlation suggested that
the reaction followed second-order kinetics typically exhibited
in SN2 reactions. In these experiments, pseudo-first-order
degradation constants of MeI were linearly correlated with the
initial concentration of Na-DEDTC or NH4-DEDTC in aqueous
solution (Figure 3), implying that the reaction mechanism
between MeI and Na-DEDTC or NH4-DEDTC is an SN2
nucleophilic substitution reaction. Because of the mathematical
simplicity of the integrated rate equation for first-order kinetics
in comparison with second-order kinetics, a first-order kinetic
model is often utilized for comparison purposes in pesticide
degradation experiments (30).
dC
dt
) -µXC ) - µ(C - C0 + X0)C
(4)
where C and X are the MeI and NI or fertilizer concentrations,
respectively, C0 and X0 are their initial values, and µ is the
second-order rate constant. After integrating, the appropriate
solution to eq 4 is described as follows:
(X0 - C0) exp[-µ(X0 - C0)t]
C(t) ) C0
(5)
X0 - C0 exp[-µ(X0 - C0)t]
When the experiment is arranged so that the initial concentra-
tions of MeI and NI or fertilizer are equal (X0 ) C0), eq 5
becomes
C0
C(t) )
(6)
1 + µC0t
When half of the original concentration MeI is consumed, we
may insert C0/2 for C(t) and thereby obtain the second-order
half-life equation, which is

Accelerated Degradation of MeI
J. Agric. Food Chem., Vol. 51, No. 3, 2003 677
Table 2. First-Order Degradation Rate Constant k (h-1) and Half-Life
1
µC0
f
T1/2
)
for X0 ) C0
(7)
(t1/2) of Methyl Iodide (0.5 mmol kg-1) in Arlington Sandy Loam with
Different Agrochemical (1.0 mmol kg-1) Amendment
Table 1 contains the second-order reaction rate coefficients
(µ) and half-lives (t1/2) together with relevant pseudo-first-order
kinetics for MeI (1.0 mM) in agrochemical aqueous solution
(1.0 mM). Both the first-order and second-order kinetic models
produced a good fit to the data, especially for some NIs such
as Na-DEDTC, NH4-DEDTC, ATS, thiourea, and allylthiourea
that rapidly reacted with MeI in aqueous solution. Because of
the occurrence of MeI hydrolysis and the possible influence of
degradation products on these reaction systems, it is difficult
to verify that the reaction between MeI and agrochemicals in
aqueous solution follows second-order kinetics. Moreover, the
rate of a second-order reaction is dependent on the initial reactant
concentrations, whereas the rate of a first-order reaction is not.
A more complex equation (e.g., eq 5) for calculating rate
coefficients is necessary if the initial concentrations of fumigant
and agrochemical in aqueous solution are not equal (32).
Therefore, for purposes of comparing fumigant degradation
under different experimental conditions described herein, the
first-order kinetic model is appropriate and relatively easy to
use (29, 33).
agrochemical
k × 10-3 (h-1
)
t1/2 (h)
r
f
oxamidea
3.01 ± 0.34
3.44 ± 0.78
5.28 ± 0.54
3.87 ± 0.79
3.90 ± 0.40
3.83 ± 0.74
39.8 ± 3.9
42.9 ± 4.4
44.4 ± 2.7
2.24 ± 0.55
226.5
195.8
131.3
179.1
177.7
181.0
17.4
16.2
15.6
309.4
0.91
0.75
0.93
0.87
0.93
0.79
0.99
0.98
0.99
0.88
ureaa
calcium cyanamidea
NH -DEDTC
4
Na-DEDTC
DCD
ATS
thiourea
allylthiourea
control
a Concentration 150 µg of N/g of soil.
secondary amine according to the following reaction:
+
-
H3O or OH
2CH3I + NCN2- f (CH3)2-NCN
8 (CH3)2NH
(8)
Accelerated MeI Degradation in Soil Amended with
Nitrification Inhibitors. The degradation of MeI was further
studied in Arlington soil amended with several NIs. The
disappearance of MeI in these soils was well-described by
first-order kinetics (Table 2). While Na- and NH4-DEDTC
showed strong nucleophilic substitution activity with MeI in
aqueous solution (Table 1), amending soil with Na-DEDTC
and NH4-DEDTC had a relatively small impact on the rate of
MeI degradation in comparison with the degradation in un-
amended soil (Table 2). Further, the level of enhancement in
the rate of MeI degradation in amended soil (ATS .
Na-DEDTC > NH4-DEDTC) was significantly different from
the trend in aqueous solution (Na-DEDTC > NH4-DEDTC >
ATS) (Tables 1 and 2). This difference may be attributable to
sorption of Na-DEDTC and NH4-DEDTC to soil, which would
reduce the availability of the amendment for reaction. In these
experiments, MeI was injected into the soils, where it partitioned
between the air, water, and solid phases of soil. Generally,
reactions between halogenated fumigants and nucleophilic
amendments take place only in the aqueous phase. As sorption
decreases aqueous concentrations, the overall reaction rate is
decreased. Contrary to the reduced MeI degradation rate in the
Na-DEDTC- and NH4-DEDTC-amended soils, MeI degradation
in ATS-amended soils remained similar to the degradation rate
in aqueous solution. This occurs because ATS is an inorganic
chemical and its reactive group (thiosulfate anion, S2O32-) exists
in the aqueous phase of the soil as the ion. This distinguishes
ATS from nucleophilic organic chemicals such as Na-DEDTC
and NH4-DEDTC, where a large portion of the chemical may
be distributed in the sorbed phase.
As the initial Na-DEDTC:MeI and ATS:MeI ratios increased,
the degradation rate of MeI was increased in amended soils
(Figure 4). Increasing the molar ratio of Na-DEDTC to MeI
did not proportionally increase the degradation rate of MeI.
However, the rate of MeI degradation in ATS-amended soils
depended on their molar ratios: when the molar ratio of ATS
to MeI doubled, the degradation rate of MeI also doubled. These
results further support observations that MeI transformation in
ATS-amended soil is abiotic and that the degradation rate
depends on the ATS concentration (i.e., second-order reaction).
Sorption of diethyldithiocarbamates to soil may have reduced
the availability of Na-DEDTC and NH4-DEDTC in the aqueous
phase, suppressing the MeI degradation rate in soil.
Accelerated MeI Degradation in Soil Amended with
Fertilizers. The potential influence of agrochemical application
on the rate of the MeI degradation was further studied in soil.
Accelerated MeI degradation occurred in soils amended with
all agrochemicals tested. Table 2 shows the first-order rate
coefficients kf (h-1) and corresponding half-lives for MeI (0.5
mmol kg-1) in Arlington sandy loam amended with different
agrochemicals.
Incorporation of the fertilizers oxamide, urea, and calcium
cyanamide (150 µg of N/g of soil) into Arlington soil was found
to moderately increase the rate of MeI degradation. The
degradation was about 1.3, 1.5, and 2.4 times faster in oxamide-,
urea-, and calcium cyanamide-amended soil, respectively, than
in unamended soil. Generally, both microbial and chemical
degradation may be involved in fumigant degradation in soil.
However, for short-chained halogenated hydrocarbons such as
MeBr and MeI, abiotic degradation is often the major factor
(26, 34), whereas biological degradation has been shown to be
important for other fumigants such as MITC (35), 1,3-D (33),
and chloropicrin (11). The application of fertilizers to soil can
alter the biological and chemical conditions and affect the rate
of MeI degradation in soil. Ou et al. (26) reported that MeBr
degradation was accelerated by liming and applying an ammonia
fertilizer as well as through inoculation with an ammonia
oxidizing bacterium. When the N fertilizers oxamide and urea
are incorporated into soil, they should stimulate the activity of
nitrifiers, which may lead to an increased capacity to degrade
MeI in a manner similar to that reported for the oxidization of
MeBr (producing formaldehyde and bromide ion) by terrestrial
and marine nitrifiers (27, 36). Moreover, the urea and oxamide
dissolved in soil water may react with MeI, so a slight increase
in the abiotic degradation of MeI in soils may also be observed.
Compared to the degradation rate of MeI in soil amended
with urea and oxamide, calcium cyanamide-amended soil
showed a higher potential to rapidly degrade MeI. Calcium
cyanamide (CaNCN) is also a nitrogen fertilizer, though it is
less commonly used than urea. Though calcium cyanamide is
essentially insoluble in water, it may undergo partial hydroly-
sis to the soluble calcium hydrogen cyanamide, which is a
source of cyanamide ion. In the presence of cyanamide ion,
MeI may be transformed to a disubstituted methyl cyanamide,
which may then be hydrolyzed and decarboxylated to a

678 J. Agric. Food Chem., Vol. 51, No. 3, 2003
Zheng et al.
Therefore, to achieve optimum control of fumigant emissions,
ATS application should be carried out concurrently with
fumigant application because volatilization of fumigants from
soil is generally most rapid shortly after fumigation.
A dramatic acceleration in MeI degradation in soil amended
with thiourea and allylthiourea was observed. The enhancement
was greater than that for the other agrochemicals tested in these
experiments (Table 2). For instance, the first-order half-life (t1/2
)
of MeI in thiourea- and allylthiourea-amended soil was ∼16 h,
significantly less than that in unamended soil (>300 h). Soil
application of thiourea and allylthiourea may offer an alternative
approach to accelerate the degradation of MeI and reduce the
emission of halogenated fumigants into the atmosphere.
Degradation of MeI was greatly enhanced in soil amended
with thiourea and allylthiourea relative to the rate of degradation
in aqueous solution (Tables 1 and 2). Apparently, the rate of
MeI degradation in the amended soils did not depend solely on
reactions that occurred in the aqueous solutions, but also
depended on other soil reaction processes (i.e., catalysis). Owing
to the negative charge characteristic of the soil surface, thiourea
could be converted to the corresponding thiuronium ions in soil,
which would react with MeI to give isothiuronium salt. This
would further cleave to mercaptan in alkali soil or in the
presence of high-molecular-weight amines in soil. The proposed
reaction for thiourea is as follows:
Figure 4. Degradation of MeI (0.5 mmol/kg) in Arlington soil amended
with Na-DEDTC (a) and ATS (b) at different molar ratios. Error bars
represent standard deviation of triplicate samples.
The degradation product, mercaptan, would be available to
degrade additional MeI through an SN2 nucleophilic reaction.
Thus, the electrostatic interaction of thiuronium ion with the
soil may lower the energy requirements for the reaction with
MeI and result in a higher degradation rate of MeI in soil
compared to that in aqueous solution. Similarly, enhanced MeI
degradation in allylthiourea-amended soil could follow a similar
catalytic mechanism in soil. Previous research has indicated that
the application of thiosulfate salts to soils may substantially
decrease volatilization of halogenated fumigants from soil, and
surface application has been promoted as a promising emission
control and soil remediation measure (20, 21). Soil application
of thiourea and allythiourea to increase the rate of MeI
degradation may provide an effective tool to prevent and reduce
fumigant emissions. To further develop this approach, fumigant
degradation and emissions reduction in soils amended with
thiourea and allylthiourea needs to be further evaluated under
different conditions and for other soils.
Figure 5. Influence of amended soil (1.0 mmol/kg) incubation time on
the degradation half-life of MeI (0.5 mmol/kg).
The degradation of MeI in soils was determined in amended
soils held for different incubation periods prior to addition of
MeI. The MeI degradation half-life was not significantly af-
fected by aging Na-DEDTC in soil for a 0-14-d period be-
fore fumigation (Figure 5). This implies that the timing of
Na-DEDTC application would not affect the degradation of MeI
in soil amended with Na-DEDTC. In contrast, the degradation
half-life of MeI in ATS-amended soil greatly increased with
incubation time (Figure 5). This demonstrates that ATS is
unstable in soil, because the thiosulfate ion can be easily
oxidized to sulfate (37), which is not reactive with MeI.
ABBREVIATIONS USED
MeI, methyl iodide; Na-DEDTC, sodium diethyldithiocar-
bamate; NH4-DEDTC, ammonium diethyldithiocarbamate; ATS,
ammonium thiosulfate; DCD, dicyandiamide; NI, nitrification
inhibitor.
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Received for review July 24, 2002. Revised manuscript received
November 1, 2002. Accepted November 4, 2002.
JF020820Q